Hibernate.org Community Documentation

# HIBERNATE - Relational Persistence for Idiomatic Java

## Hibernate Reference Documentation

### The Hibernate Team

### The JBoss Visual Design Team

4.1.2.Final

Copyright © 2004 Red Hat, Inc.

Legal Notice

2012-04-05

List of Tables

3.1. Hibernate JDBC Properties
3.2. Hibernate Datasource Properties
3.3. Hibernate Configuration Properties
3.4. Hibernate JDBC and Connection Properties
3.5. Hibernate Cache Properties
3.6. Hibernate Transaction Properties
3.7. Miscellaneous Properties
3.8. Hibernate SQL Dialects (`hibernate.dialect`)
3.9. Hibernate Log Categories
3.10. JTA TransactionManagers
10.1. Features of inheritance mappings
12.1. Affect of read-only entity on property types
18.1. Alias injection names
21.1. Cache Providers
21.2. Cache Concurrency Strategy Support
22.1. Summary
22.2. `SchemaExport` Command Line Options
22.3. SchemaExport Connection Properties
22.4. `SchemaUpdate` Command Line Options
22.5. `SchemaValidator` Command Line Options

List of Examples

4.1. Simple POJO representing a cat
4.2. Disabling proxies in `hbm.xml`
4.3. Disabling proxies in annotations
4.4. Proxying an interface in `hbm.xml`
4.5. Proxying an interface in annotations
4.6. Specify custom tuplizers in annotations
4.7. Specify custom tuplizers in `hbm.xml`
5.1. @NotFound annotation
5.2. @OnDelete annotation
5.3. @ForeignKey annotation
5.4. One to One association
6.1. Defining and registering the custom Type
6.2. Defining the custom UserType
6.3. Defining the custom CompositeUserType
6.4. Overriding the standard `StringType`
6.5. Snippet from BasicType.java
7.1. Hibernate uses its own collection implementations
7.2. Collection mapping using @OneToMany and @JoinColumn
7.3. Collection mapping using @OneToMany and @JoinTable
7.4. Mapping a Set using <set>
7.5. options of <one-to-many> element
7.6. Elements of the <map> mapping
7.7. Ordered lists using `@OrderBy`
7.8. Explicit index column using `@OrderColumn`
7.9. index-list element for indexed collections in xml mapping
7.10. Use of target entity property as map key via `@MapKey`
7.11. Map key as basic type using `@MapKeyColumn`
7.12. map-key xml mapping element
7.13. map-key-many-to-many
7.14. Collection of basic types mapped via `@ElementCollection`
7.15. @ElementCollection for embeddable objects
7.16. <element> tag for collection values using mapping files
7.17. Sorted collection with @Sort
7.18. Sorted collection using xml mapping
7.19. Sorting in database using order-by
7.20. Sorting via a query filter
7.21. Bidirectional one to many with many to one side as association owner
7.22. Bidirectional associtaion with one to many side as owner
7.23. Bidirectional one to many via Hibernate mapping files
7.24. Many to many association via @ManyToMany
7.25. Default values for `@ManyToMany` (uni-directional)
7.26. Default values for `@ManyToMany` (bi-directional)
7.27. Many to many association using Hibernate mapping files
7.28. Effect of inverse vs. non-inverse side of many to many associations
7.29. Bidirectional association with indexed collection
7.30. Bidirectional association with indexed collection, but no index column
7.31. Ternary association mapping
7.32. Example classes `Parent` and `Child`
7.33. One to many unidirectional `Parent-Child` relationship using annotations
7.34. One to many unidirectional `Parent-Child` relationship using mapping files
7.35. Table definitions for unidirectional `Parent`-`Child` relationship
7.36. One to many bidirectional `Parent-Child` relationship using annotations
7.37. One to many bidirectional `Parent-Child` relationship using mapping files
7.38. Table definitions for bidirectional `Parent`-`Child` relationship
7.39. Enforcing NOT NULL constraint in unidirectional relation using annotations
7.40. Enforcing NOT NULL constraint in unidirectional relation using mapping files
7.41. Many to many `Parent-Child` relationship using annotations
7.42. Many to many `Parent-Child` relationship using mapping files
7.43. Table definitions for many to many releationship
11.1. Defining a named query using `@NamedQuery`
11.2. Defining a named query using `<query>`
11.3. Parameter binding of a named query
11.4. `@OneToMany` with `orphanRemoval`
18.1. Named sql query using the <sql-query> maping element
18.2. Execution of a named query
18.3. Named sql query with association
18.4. Named query returning a scalar
18.5. <resultset> mapping used to externalize mapping information
18.6. Programmatically specifying the result mapping information
18.7. Named SQL query using `@NamedNativeQuery` together with `@SqlResultSetMapping`
18.8. Implicit result set mapping
18.9. Using dot notation in @FieldResult for specifying associations
18.10. Scalar values via `@ColumnResult`
18.11. Custom CRUD via annotations
18.12. Custom CRUD XML
18.13. Overriding SQL statements for collections using annotations
18.14. Overriding SQL statements for secondary tables
18.15. Stored procedures and their return value
19.1. @FilterDef and @Filter annotations
19.2. Using `@FilterJoinTable` for filterting on the association table
19.3. Defining a filter definition via `<filter-def>`
19.4. Attaching a filter to a class or collection using `<filter>`
21.1. Specifying a fetch profile using `@FetchProfile`
21.2. Specifying a fetch profile using `<fetch-profile>` outside `<class>` node
21.3. Specifying a fetch profile using `<fetch-profile>` inside `<class>` node
21.4. Activating a fetch profile for a given `Session`
21.5. Definition of cache concurrency strategy via `@Cache`
21.6. Caching collections using annotations
21.7. `@Cache` annotation with attributes
21.8. The Hibernate `<cache>` mapping element
21.9. Explcitly evicting a cached instance from the first level cache using `Session.evict()`
21.10. Second-level cache eviction via `SessionFactoty.evict()` and `SessionFacyory.evictCollection()`
21.11. Browsing the second-level cache entries via the `Statistics` API
21.12. Enabling Hibernate statistics
23.1. Using custom groups for validation

## Preface

Working with both Object-Oriented software and Relational Databases can be cumbersome and time consuming. Development costs are significantly higher due to a paradigm mismatch between how data is represented in objects versus relational databases. Hibernate is an Object/Relational Mapping solution for Java environments. The term Object/Relational Mapping refers to the technique of mapping data from an object model representation to a relational data model representation (and visa versa). See http://en.wikipedia.org/wiki/Object-relational_mapping for a good high-level discussion.

## Note

While having a strong background in SQL is not required to use Hibernate, having a basic understanding of the concepts can greatly help you understand Hibernate more fully and quickly. Probably the single best background is an understanding of data modeling principles. You might want to consider these resources as a good starting point:

- http://www.agiledata.org/essays/dataModeling101.html

- http://en.wikipedia.org/wiki/Data_modeling

Hibernate not only takes care of the mapping from Java classes to database tables (and from Java data types to SQL data types), but also provides data query and retrieval facilities. It can significantly reduce development time otherwise spent with manual data handling in SQL and JDBC. Hibernate’s design goal is to relieve the developer from 95% of common data persistence-related programming tasks by eliminating the need for manual, hand-crafted data processing using SQL and JDBC. However, unlike many other persistence solutions, Hibernate does not hide the power of SQL from you and guarantees that your investment in relational technology and knowledge is as valid as always.

Hibernate may not be the best solution for data-centric applications that only use stored-procedures to implement the business logic in the database, it is most useful with object-oriented domain models and business logic in the Java-based middle-tier. However, Hibernate can certainly help you to remove or encapsulate vendor-specific SQL code and will help with the common task of result set translation from a tabular representation to a graph of objects.

If you are new to Hibernate and Object/Relational Mapping or even Java, please follow these steps:

1. Read Chapter 1, Tutorial for a tutorial with step-by-step instructions. The source code for the tutorial is included in the distribution in the `doc/reference/tutorial/` directory.

2. Read Chapter 2, Architecture to understand the environments where Hibernate can be used.

3. View the `eg/` directory in the Hibernate distribution. It contains a simple standalone application. Copy your JDBC driver to the `lib/` directory and edit `etc/hibernate.properties`, specifying correct values for your database. From a command prompt in the distribution directory, type `ant eg` (using Ant), or under Windows, type `build eg`.

4. Use this reference documentation as your primary source of information. Consider reading [JPwH] if you need more help with application design, or if you prefer a step-by-step tutorial. Also visit http://caveatemptor.hibernate.org and download the example application from [JPwH].

5. FAQs are answered on the Hibernate website.

6. Links to third party demos, examples, and tutorials are maintained on the Hibernate website.

7. The Community Area on the Hibernate website is a good resource for design patterns and various integration solutions (Tomcat, JBoss AS, Struts, EJB, etc.).

There are a number of ways to become involved in the Hibernate community, including

- Trying stuff out and reporting bugs. See http://hibernate.org/issuetracker.html details.

- Trying your hand at fixing some bugs or implementing enhancements. Again, see http://hibernate.org/issuetracker.html details.

- http://hibernate.org/community.html lists a few ways to engage in the community.

  - There are forums for users to ask questions and receive help from the community.

  - There are also IRC channels for both user and developer discussions.

- Helping improve or translate this documentation. Contact us on the developer mailing list if you have interest.

- Evangelizing Hibernate within your organization.

## Chapter 1. Tutorial

Intended for new users, this chapter provides an step-by-step introduction to Hibernate, starting with a simple application using an in-memory database. The tutorial is based on an earlier tutorial developed by Michael Gloegl. All code is contained in the `tutorials/web` directory of the project source.

## Important

This tutorial expects the user have knowledge of both Java and SQL. If you have a limited knowledge of JAVA or SQL, it is advised that you start with a good introduction to that technology prior to attempting to learn Hibernate.

## Note

The distribution contains another example application under the `tutorial/eg` project source directory.

## 1.1. Part 1 - The first Hibernate Application

For this example, we will set up a small database application that can store events we want to attend and information about the host(s) of these events.

## Note

Although you can use whatever database you feel comfortable using, we will use HSQLDB (an in-memory, Java database) to avoid describing installation/setup of any particular database servers.

### 1.1.1. Setup

The first thing we need to do is to set up the development environment. We will be using the "standard layout" advocated by alot of build tools such as Maven. Maven, in particular, has a good resource describing this layout. As this tutorial is to be a web application, we will be creating and making use of `src/main/java`, `src/main/resources` and `src/main/webapp` directories.

We will be using Maven in this tutorial, taking advantage of its transitive dependency management capabilities as well as the ability of many IDEs to automatically set up a project for us based on the maven descriptor.

    <project xmlns="http://maven.apache.org/POM/4.0.0"
             xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance"
             xsi:schemaLocation="http://maven.apache.org/POM/4.0.0 http://maven.apache.org/xsd/maven-4.0.0.xsd">

        <modelVersion>4.0.0</modelVersion>

        <groupId>org.hibernate.tutorials</groupId>
        <artifactId>hibernate-tutorial</artifactId>
        <version>1.0.0-SNAPSHOT</version>
        <name>First Hibernate Tutorial</name>

        <build>
             <!-- we dont want the version to be part of the generated war file name -->
             <finalName>${artifactId}</finalName>
        </build>

        <dependencies>
            <dependency>
                <groupId>org.hibernate</groupId>
                <artifactId>hibernate-core</artifactId>
            </dependency>

            <!-- Because this is a web app, we also have a dependency on the servlet api. -->
            <dependency>
                <groupId>javax.servlet</groupId>
                <artifactId>servlet-api</artifactId>
            </dependency>

            <!-- Hibernate uses slf4j for logging, for our purposes here use the simple backend -->
            <dependency>
                <groupId>org.slf4j</groupId>
                <artifactId>slf4j-simple</artifactId>
            </dependency>

            <!-- Hibernate gives you a choice of bytecode providers between cglib and javassist -->
            <dependency>
                <groupId>javassist</groupId>
                <artifactId>javassist</artifactId>
            </dependency>
        </dependencies>

    </project>

## Tip

It is not a requirement to use Maven. If you wish to use something else to build this tutorial (such as Ant), the layout will remain the same. The only change is that you will need to manually account for all the needed dependencies. If you use something like Ivy providing transitive dependency management you would still use the dependencies mentioned below. Otherwise, you'd need to grab all dependencies, both explicit and transitive, and add them to the project's classpath. If working from the Hibernate distribution bundle, this would mean `hibernate3.jar`, all artifacts in the `lib/required` directory and all files from either the `lib/bytecode/cglib` or `lib/bytecode/javassist` directory; additionally you will need both the servlet-api jar and one of the slf4j logging backends.

Save this file as `pom.xml` in the project root directory.

### 1.1.2. The first class

Next, we create a class that represents the event we want to store in the database; it is a simple JavaBean class with some properties:

    package org.hibernate.tutorial.domain;

    import java.util.Date;

    public class Event {
        private Long id;

        private String title;
        private Date date;

        public Event() {}

        public Long getId() {
            return id;
        }

        private void setId(Long id) {
            this.id = id;
        }

        public Date getDate() {
            return date;
        }

        public void setDate(Date date) {
            this.date = date;
        }

        public String getTitle() {
            return title;
        }

        public void setTitle(String title) {
            this.title = title;
        }
    }

This class uses standard JavaBean naming conventions for property getter and setter methods, as well as private visibility for the fields. Although this is the recommended design, it is not required. Hibernate can also access fields directly, the benefit of accessor methods is robustness for refactoring.

The `id` property holds a unique identifier value for a particular event. All persistent entity classes (there are less important dependent classes as well) will need such an identifier property if we want to use the full feature set of Hibernate. In fact, most applications, especially web applications, need to distinguish objects by identifier, so you should consider this a feature rather than a limitation. However, we usually do not manipulate the identity of an object, hence the setter method should be private. Only Hibernate will assign identifiers when an object is saved. Hibernate can access public, private, and protected accessor methods, as well as public, private and protected fields directly. The choice is up to you and you can match it to fit your application design.

The no-argument constructor is a requirement for all persistent classes; Hibernate has to create objects for you, using Java Reflection. The constructor can be private, however package or public visibility is required for runtime proxy generation and efficient data retrieval without bytecode instrumentation.

Save this file to the `src/main/java/org/hibernate/tutorial/domain` directory.

### 1.1.3. The mapping file

Hibernate needs to know how to load and store objects of the persistent class. This is where the Hibernate mapping file comes into play. The mapping file tells Hibernate what table in the database it has to access, and what columns in that table it should use.

The basic structure of a mapping file looks like this:

    <?xml version="1.0"?>
    <!DOCTYPE hibernate-mapping PUBLIC
            "-//Hibernate/Hibernate Mapping DTD 3.0//EN"
            "http://www.hibernate.org/dtd/hibernate-mapping-3.0.dtd">

    <hibernate-mapping package="org.hibernate.tutorial.domain">
    [...]
    </hibernate-mapping>

Hibernate DTD is sophisticated. You can use it for auto-completion of XML mapping elements and attributes in your editor or IDE. Opening up the DTD file in your text editor is the easiest way to get an overview of all elements and attributes, and to view the defaults, as well as some comments. Hibernate will not load the DTD file from the web, but first look it up from the classpath of the application. The DTD file is included in `hibernate-core.jar` (it is also included in the `hibernate3.jar`, if using the distribution bundle).

## Important

We will omit the DTD declaration in future examples to shorten the code. It is, of course, not optional.

Between the two `hibernate-mapping` tags, include a `class` element. All persistent entity classes (again, there might be dependent classes later on, which are not first-class entities) need a mapping to a table in the SQL database:

    <hibernate-mapping package="org.hibernate.tutorial.domain">

        <class name="Event" table="EVENTS">

        </class>

    </hibernate-mapping>

So far we have told Hibernate how to persist and load object of class `Event` to the table `EVENTS`. Each instance is now represented by a row in that table. Now we can continue by mapping the unique identifier property to the tables primary key. As we do not want to care about handling this identifier, we configure Hibernate's identifier generation strategy for a surrogate primary key column:

    <hibernate-mapping package="org.hibernate.tutorial.domain">

        <class name="Event" table="EVENTS">
            <id name="id" column="EVENT_ID">
                <generator class="native"/>
            </id>
        </class>

    </hibernate-mapping>

The `id` element is the declaration of the identifier property. The `name="id"` mapping attribute declares the name of the JavaBean property and tells Hibernate to use the `getId()` and `setId()` methods to access the property. The column attribute tells Hibernate which column of the `EVENTS` table holds the primary key value.

The nested `generator` element specifies the identifier generation strategy (aka how are identifier values generated?). In this case we choose `native`, which offers a level of portability depending on the configured database dialect. Hibernate supports database generated, globally unique, as well as application assigned, identifiers. Identifier value generation is also one of Hibernate's many extension points and you can plugin in your own strategy.

## Tip

`native` is no longer consider the best strategy in terms of portability. for further discussion, see Section 28.4, “Identifier generation”

Lastly, we need to tell Hibernate about the remaining entity class properties. By default, no properties of the class are considered persistent:

    <hibernate-mapping package="org.hibernate.tutorial.domain">

        <class name="Event" table="EVENTS">
            <id name="id" column="EVENT_ID">
                <generator class="native"/>
            </id>
            <property name="date" type="timestamp" column="EVENT_DATE"/>
            <property name="title"/>
        </class>

    </hibernate-mapping>

Similar to the `id` element, the `name` attribute of the `property` element tells Hibernate which getter and setter methods to use. In this case, Hibernate will search for `getDate()`, `setDate()`, `getTitle()` and `setTitle()` methods.

## Note

Why does the `date` property mapping include the `column` attribute, but the `title` does not? Without the `column` attribute, Hibernate by default uses the property name as the column name. This works for `title`, however, `date` is a reserved keyword in most databases so you will need to map it to a different name.

The `title` mapping also lacks a `type` attribute. The types declared and used in the mapping files are not Java data types; they are not SQL database types either. These types are called Hibernate mapping types, converters which can translate from Java to SQL data types and vice versa. Again, Hibernate will try to determine the correct conversion and mapping type itself if the `type` attribute is not present in the mapping. In some cases this automatic detection using Reflection on the Java class might not have the default you expect or need. This is the case with the `date` property. Hibernate cannot know if the property, which is of `java.util.Date`, should map to a SQL `date`, `timestamp`, or `time` column. Full date and time information is preserved by mapping the property with a `timestamp` converter.

## Tip

Hibernate makes this mapping type determination using reflection when the mapping files are processed. This can take time and resources, so if startup performance is important you should consider explicitly defining the type to use.

Save this mapping file as `src/main/resources/org/hibernate/tutorial/domain/Event.hbm.xml`.

### 1.1.4. Hibernate configuration

At this point, you should have the persistent class and its mapping file in place. It is now time to configure Hibernate. First let's set up HSQLDB to run in "server mode"

## Note

We do this do that the data remains between runs.

We will utilize the Maven exec plugin to launch the HSQLDB server by running: mvn exec:java -Dexec.mainClass="org.hsqldb.Server" -Dexec.args="-database.0 file:target/data/tutorial" You will see it start up and bind to a TCP/IP socket; this is where our application will connect later. If you want to start with a fresh database during this tutorial, shutdown HSQLDB, delete all files in the `target/data` directory, and start HSQLDB again.

Hibernate will be connecting to the database on behalf of your application, so it needs to know how to obtain connections. For this tutorial we will be using a standalone connection pool (as opposed to a `javax.sql.DataSource`). Hibernate comes with support for two third-party open source JDBC connection pools: c3p0 and proxool. However, we will be using the Hibernate built-in connection pool for this tutorial.

## Caution

The built-in Hibernate connection pool is in no way intended for production use. It lacks several features found on any decent connection pool.

For Hibernate's configuration, we can use a simple `hibernate.properties` file, a more sophisticated `hibernate.cfg.xml` file, or even complete programmatic setup. Most users prefer the XML configuration file:

    <?xml version='1.0' encoding='utf-8'?>
    <!DOCTYPE hibernate-configuration PUBLIC
            "-//Hibernate/Hibernate Configuration DTD 3.0//EN"
            "http://www.hibernate.org/dtd/hibernate-configuration-3.0.dtd">

    <hibernate-configuration>

        <session-factory>

            <!-- Database connection settings -->
            <property name="connection.driver_class">org.hsqldb.jdbcDriver</property>
            <property name="connection.url">jdbc:hsqldb:hsql://localhost</property>
            <property name="connection.username">sa</property>
            <property name="connection.password"></property>

            <!-- JDBC connection pool (use the built-in) -->
            <property name="connection.pool_size">1</property>

            <!-- SQL dialect -->
            <property name="dialect">org.hibernate.dialect.HSQLDialect</property>

            <!-- Enable Hibernate's automatic session context management -->
            <property name="current_session_context_class">thread</property>

            <!-- Disable the second-level cache  -->
            <property name="cache.provider_class">org.hibernate.cache.internal.NoCacheProvider</property>

            <!-- Echo all executed SQL to stdout -->
            <property name="show_sql">true</property>

            <!-- Drop and re-create the database schema on startup -->
            <property name="hbm2ddl.auto">update</property>

            <mapping resource="org/hibernate/tutorial/domain/Event.hbm.xml"/>

        </session-factory>

    </hibernate-configuration>

## Note

Notice that this configuration file specifies a different DTD

You configure Hibernate's `SessionFactory`. SessionFactory is a global factory responsible for a particular database. If you have several databases, for easier startup you should use several `<session-factory>` configurations in several configuration files.

The first four `property` elements contain the necessary configuration for the JDBC connection. The dialect `property` element specifies the particular SQL variant Hibernate generates.

## Tip

In most cases, Hibernate is able to properly determine which dialect to use. See Section 28.3, “Dialect resolution” for more information.

Hibernate's automatic session management for persistence contexts is particularly useful in this context. The `hbm2ddl.auto` option turns on automatic generation of database schemas directly into the database. This can also be turned off by removing the configuration option, or redirected to a file with the help of the `SchemaExport` Ant task. Finally, add the mapping file(s) for persistent classes to the configuration.

Save this file as `hibernate.cfg.xml` into the `src/main/resources` directory.

### 1.1.5. Building with Maven

We will now build the tutorial with Maven. You will need to have Maven installed; it is available from the Maven download page. Maven will read the `/pom.xml` file we created earlier and know how to perform some basic project tasks. First, lets run the `compile` goal to make sure we can compile everything so far:

    [hibernateTutorial]$ mvn compile
    [INFO] Scanning for projects...
    [INFO] ------------------------------------------------------------------------
    [INFO] Building First Hibernate Tutorial
    [INFO]    task-segment: [compile]
    [INFO] ------------------------------------------------------------------------
    [INFO] [resources:resources]
    [INFO] Using default encoding to copy filtered resources.
    [INFO] [compiler:compile]
    [INFO] Compiling 1 source file to /home/steve/projects/sandbox/hibernateTutorial/target/classes
    [INFO] ------------------------------------------------------------------------
    [INFO] BUILD SUCCESSFUL
    [INFO] ------------------------------------------------------------------------
    [INFO] Total time: 2 seconds
    [INFO] Finished at: Tue Jun 09 12:25:25 CDT 2009
    [INFO] Final Memory: 5M/547M
    [INFO] ------------------------------------------------------------------------

### 1.1.6. Startup and helpers

It is time to load and store some `Event` objects, but first you have to complete the setup with some infrastructure code. You have to startup Hibernate by building a global `org.hibernate.SessionFactory` object and storing it somewhere for easy access in application code. A `org.hibernate.SessionFactory` is used to obtain `org.hibernate.Session` instances. A `org.hibernate.Session` represents a single-threaded unit of work. The `org.hibernate.SessionFactory` is a thread-safe global object that is instantiated once.

We will create a `HibernateUtil` helper class that takes care of startup and makes accessing the `org.hibernate.SessionFactory` more convenient.

    package org.hibernate.tutorial.util;

    import org.hibernate.SessionFactory;
    import org.hibernate.cfg.Configuration;

    public class HibernateUtil {

        private static final SessionFactory sessionFactory = buildSessionFactory();

        private static SessionFactory buildSessionFactory() {
            try {
                // Create the SessionFactory from hibernate.cfg.xml
                return new Configuration().configure().buildSessionFactory();
            }
            catch (Throwable ex) {
                // Make sure you log the exception, as it might be swallowed
                System.err.println("Initial SessionFactory creation failed." + ex);
                throw new ExceptionInInitializerError(ex);
            }
        }

        public static SessionFactory getSessionFactory() {
            return sessionFactory;
        }

    }

Save this code as `src/main/java/org/hibernate/tutorial/util/HibernateUtil.java`

This class not only produces the global `org.hibernate.SessionFactory` reference in its static initializer; it also hides the fact that it uses a static singleton. We might just as well have looked up the `org.hibernate.SessionFactory` reference from JNDI in an application server or any other location for that matter.

If you give the `org.hibernate.SessionFactory` a name in your configuration, Hibernate will try to bind it to JNDI under that name after it has been built. Another, better option is to use a JMX deployment and let the JMX-capable container instantiate and bind a `HibernateService` to JNDI. Such advanced options are discussed later.

You now need to configure a logging system. Hibernate uses commons logging and provides two choices: Log4j and JDK 1.4 logging. Most developers prefer Log4j: copy `log4j.properties` from the Hibernate distribution in the `etc/` directory to your `src` directory, next to `hibernate.cfg.xml`. If you prefer to have more verbose output than that provided in the example configuration, you can change the settings. By default, only the Hibernate startup message is shown on stdout.

The tutorial infrastructure is complete and you are now ready to do some real work with Hibernate.

### 1.1.7. Loading and storing objects

We are now ready to start doing some real work with Hibernate. Let's start by writing an `EventManager` class with a `main()` method:

    package org.hibernate.tutorial;

    import org.hibernate.Session;

    import java.util.*;

    import org.hibernate.tutorial.domain.Event;
    import org.hibernate.tutorial.util.HibernateUtil;

    public class EventManager {

        public static void main(String[] args) {
            EventManager mgr = new EventManager();

            if (args[0].equals("store")) {
                mgr.createAndStoreEvent("My Event", new Date());
            }

            HibernateUtil.getSessionFactory().close();
        }

        private void createAndStoreEvent(String title, Date theDate) {
            Session session = HibernateUtil.getSessionFactory().getCurrentSession();
            session.beginTransaction();

            Event theEvent = new Event();
            theEvent.setTitle(title);
            theEvent.setDate(theDate);
            session.save(theEvent);

            session.getTransaction().commit();
        }

    }

In `createAndStoreEvent()` we created a new `Event` object and handed it over to Hibernate. At that point, Hibernate takes care of the SQL and executes an `INSERT` on the database.

A `org.hibernate.Session` is designed to represent a single unit of work (a single atomic piece of work to be performed). For now we will keep things simple and assume a one-to-one granularity between a Hibernate `org.hibernate.Session` and a database transaction. To shield our code from the actual underlying transaction system we use the Hibernate `org.hibernate.Transaction` API. In this particular case we are using JDBC-based transactional semantics, but it could also run with JTA.

What does `sessionFactory.getCurrentSession()` do? First, you can call it as many times and anywhere you like once you get hold of your `org.hibernate.SessionFactory`. The `getCurrentSession()` method always returns the "current" unit of work. Remember that we switched the configuration option for this mechanism to "thread" in our `src/main/resources/hibernate.cfg.xml`? Due to that setting, the context of a current unit of work is bound to the current Java thread that executes the application.

## Important

Hibernate offers three methods of current session tracking. The "thread" based method is not intended for production use; it is merely useful for prototyping and tutorials such as this one. Current session tracking is discussed in more detail later on.

A `org.hibernate.Session` begins when the first call to `getCurrentSession()` is made for the current thread. It is then bound by Hibernate to the current thread. When the transaction ends, either through commit or rollback, Hibernate automatically unbinds the `org.hibernate.Session` from the thread and closes it for you. If you call `getCurrentSession()` again, you get a new `org.hibernate.Session` and can start a new unit of work.

Related to the unit of work scope, should the Hibernate `org.hibernate.Session` be used to execute one or several database operations? The above example uses one `org.hibernate.Session` for one operation. However this is pure coincidence; the example is just not complex enough to show any other approach. The scope of a Hibernate `org.hibernate.Session` is flexible but you should never design your application to use a new Hibernate `org.hibernate.Session` for every database operation. Even though it is used in the following examples, consider session-per-operation an anti-pattern. A real web application is shown later in the tutorial which will help illustrate this.

See Chapter 13, Transactions and Concurrency for more information about transaction handling and demarcation. The previous example also skipped any error handling and rollback.

To run this, we will make use of the Maven exec plugin to call our class with the necessary classpath setup: mvn exec:java -Dexec.mainClass="org.hibernate.tutorial.EventManager" -Dexec.args="store"

## Note

You may need to perform mvn compile first.

You should see Hibernate starting up and, depending on your configuration, lots of log output. Towards the end, the following line will be displayed:

    [java] Hibernate: insert into EVENTS (EVENT_DATE, title, EVENT_ID) values (?, ?, ?)

This is the `INSERT` executed by Hibernate.

To list stored events an option is added to the main method:

            if (args[0].equals("store")) {
                mgr.createAndStoreEvent("My Event", new Date());
            }
            else if (args[0].equals("list")) {
                List events = mgr.listEvents();
                for (int i = 0; i < events.size(); i++) {
                    Event theEvent = (Event) events.get(i);
                    System.out.println(
                            "Event: " + theEvent.getTitle() + " Time: " + theEvent.getDate()
                    );
                }
            }

A new `listEvents() method is also added`:

        private List listEvents() {
            Session session = HibernateUtil.getSessionFactory().getCurrentSession();
            session.beginTransaction();
            List result = session.createQuery("from Event").list();
            session.getTransaction().commit();
            return result;
        }

Here, we are using a Hibernate Query Language (HQL) query to load all existing `Event` objects from the database. Hibernate will generate the appropriate SQL, send it to the database and populate `Event` objects with the data. You can create more complex queries with HQL. See Chapter 16, HQL: The Hibernate Query Language for more information.

Now we can call our new functionality, again using the Maven exec plugin: mvn exec:java -Dexec.mainClass="org.hibernate.tutorial.EventManager" -Dexec.args="list"

## 1.2. Part 2 - Mapping associations

So far we have mapped a single persistent entity class to a table in isolation. Let's expand on that a bit and add some class associations. We will add people to the application and store a list of events in which they participate.

### 1.2.1. Mapping the Person class

The first cut of the `Person` class looks like this:

    package org.hibernate.tutorial.domain;

    public class Person {

        private Long id;
        private int age;
        private String firstname;
        private String lastname;

        public Person() {}

        // Accessor methods for all properties, private setter for 'id'

    }

Save this to a file named `src/main/java/org/hibernate/tutorial/domain/Person.java`

Next, create the new mapping file as `src/main/resources/org/hibernate/tutorial/domain/Person.hbm.xml`

    <hibernate-mapping package="org.hibernate.tutorial.domain">

        <class name="Person" table="PERSON">
            <id name="id" column="PERSON_ID">
                <generator class="native"/>
            </id>
            <property name="age"/>
            <property name="firstname"/>
            <property name="lastname"/>
        </class>

    </hibernate-mapping>

Finally, add the new mapping to Hibernate's configuration:

    <mapping resource="org/hibernate/tutorial/domain/Event.hbm.xml"/>
    <mapping resource="org/hibernate/tutorial/domain/Person.hbm.xml"/>

Create an association between these two entities. Persons can participate in events, and events have participants. The design questions you have to deal with are: directionality, multiplicity, and collection behavior.

### 1.2.2. A unidirectional Set-based association

By adding a collection of events to the `Person` class, you can easily navigate to the events for a particular person, without executing an explicit query - by calling `Person#getEvents`. Multi-valued associations are represented in Hibernate by one of the Java Collection Framework contracts; here we choose a `java.util.Set` because the collection will not contain duplicate elements and the ordering is not relevant to our examples:

    public class Person {

        private Set events = new HashSet();

        public Set getEvents() {
            return events;
        }

        public void setEvents(Set events) {
            this.events = events;
        }
    }

Before mapping this association, let's consider the other side. We could just keep this unidirectional or create another collection on the `Event`, if we wanted to be able to navigate it from both directions. This is not necessary, from a functional perspective. You can always execute an explicit query to retrieve the participants for a particular event. This is a design choice left to you, but what is clear from this discussion is the multiplicity of the association: "many" valued on both sides is called a many-to-many association. Hence, we use Hibernate's many-to-many mapping:

    <class name="Person" table="PERSON">
        <id name="id" column="PERSON_ID">
            <generator class="native"/>
        </id>
        <property name="age"/>
        <property name="firstname"/>
        <property name="lastname"/>

        <set name="events" table="PERSON_EVENT">
            <key column="PERSON_ID"/>
            <many-to-many column="EVENT_ID" class="Event"/>
        </set>

    </class>

Hibernate supports a broad range of collection mappings, a `set` being most common. For a many-to-many association, or n:m entity relationship, an association table is required. Each row in this table represents a link between a person and an event. The table name is decalred using the `table` attribute of the `set` element. The identifier column name in the association, for the person side, is defined with the `key` element, the column name for the event's side with the `column` attribute of the `many-to-many`. You also have to tell Hibernate the class of the objects in your collection (the class on the other side of the collection of references).

The database schema for this mapping is therefore:

        _____________        __________________
       |             |      |                  |       _____________
       |   EVENTS    |      |   PERSON_EVENT   |      |             |
       |_____________|      |__________________|      |    PERSON   |
       |             |      |                  |      |_____________|
       | *EVENT_ID   | <--> | *EVENT_ID        |      |             |
       |  EVENT_DATE |      | *PERSON_ID       | <--> | *PERSON_ID  |
       |  TITLE      |      |__________________|      |  AGE        |
       |_____________|                                |  FIRSTNAME  |
                                                      |  LASTNAME   |
                                                      |_____________|

### 1.2.3. Working the association

Now we will bring some people and events together in a new method in `EventManager`:

        private void addPersonToEvent(Long personId, Long eventId) {
            Session session = HibernateUtil.getSessionFactory().getCurrentSession();
            session.beginTransaction();

            Person aPerson = (Person) session.load(Person.class, personId);
            Event anEvent = (Event) session.load(Event.class, eventId);
            aPerson.getEvents().add(anEvent);

            session.getTransaction().commit();
        }

After loading a `Person` and an `Event`, simply modify the collection using the normal collection methods. There is no explicit call to `update()` or `save()`; Hibernate automatically detects that the collection has been modified and needs to be updated. This is called automatic dirty checking. You can also try it by modifying the name or the date property of any of your objects. As long as they are in persistent state, that is, bound to a particular Hibernate `org.hibernate.Session`, Hibernate monitors any changes and executes SQL in a write-behind fashion. The process of synchronizing the memory state with the database, usually only at the end of a unit of work, is called flushing. In our code, the unit of work ends with a commit, or rollback, of the database transaction.

You can load person and event in different units of work. Or you can modify an object outside of a `org.hibernate.Session`, when it is not in persistent state (if it was persistent before, this state is called detached). You can even modify a collection when it is detached:

        private void addPersonToEvent(Long personId, Long eventId) {
            Session session = HibernateUtil.getSessionFactory().getCurrentSession();
            session.beginTransaction();

            Person aPerson = (Person) session
                    .createQuery("select p from Person p left join fetch p.events where p.id = :pid")
                    .setParameter("pid", personId)
                    .uniqueResult(); // Eager fetch the collection so we can use it detached
            Event anEvent = (Event) session.load(Event.class, eventId);

            session.getTransaction().commit();

            // End of first unit of work

            aPerson.getEvents().add(anEvent); // aPerson (and its collection) is detached

            // Begin second unit of work

            Session session2 = HibernateUtil.getSessionFactory().getCurrentSession();
            session2.beginTransaction();
            session2.update(aPerson); // Reattachment of aPerson

            session2.getTransaction().commit();
        }

The call to `update` makes a detached object persistent again by binding it to a new unit of work, so any modifications you made to it while detached can be saved to the database. This includes any modifications (additions/deletions) you made to a collection of that entity object.

This is not much use in our example, but it is an important concept you can incorporate into your own application. Complete this exercise by adding a new action to the main method of the `EventManager` and call it from the command line. If you need the identifiers of a person and an event - the `save()` method returns it (you might have to modify some of the previous methods to return that identifier):

            else if (args[0].equals("addpersontoevent")) {
                Long eventId = mgr.createAndStoreEvent("My Event", new Date());
                Long personId = mgr.createAndStorePerson("Foo", "Bar");
                mgr.addPersonToEvent(personId, eventId);
                System.out.println("Added person " + personId + " to event " + eventId);
            }

This is an example of an association between two equally important classes : two entities. As mentioned earlier, there are other classes and types in a typical model, usually "less important". Some you have already seen, like an `int` or a `java.lang.String`. We call these classes value types, and their instances depend on a particular entity. Instances of these types do not have their own identity, nor are they shared between entities. Two persons do not reference the same `firstname` object, even if they have the same first name. Value types cannot only be found in the JDK , but you can also write dependent classes yourself such as an `Address` or `MonetaryAmount` class. In fact, in a Hibernate application all JDK classes are considered value types.

You can also design a collection of value types. This is conceptually different from a collection of references to other entities, but looks almost the same in Java.

### 1.2.4. Collection of values

Let's add a collection of email addresses to the `Person` entity. This will be represented as a `java.util.Set` of `java.lang.String` instances:

        private Set emailAddresses = new HashSet();

        public Set getEmailAddresses() {
            return emailAddresses;
        }

        public void setEmailAddresses(Set emailAddresses) {
            this.emailAddresses = emailAddresses;
        }

The mapping of this `Set` is as follows:

            <set name="emailAddresses" table="PERSON_EMAIL_ADDR">
                <key column="PERSON_ID"/>
                <element type="string" column="EMAIL_ADDR"/>
            </set>

The difference compared with the earlier mapping is the use of the `element` part which tells Hibernate that the collection does not contain references to another entity, but is rather a collection whose elements are values types, here specifically of type `string`. The lowercase name tells you it is a Hibernate mapping type/converter. Again the `table` attribute of the `set` element determines the table name for the collection. The `key` element defines the foreign-key column name in the collection table. The `column` attribute in the `element` element defines the column name where the email address values will actually be stored.

Here is the updated schema:

      _____________        __________________
     |             |      |                  |       _____________
     |   EVENTS    |      |   PERSON_EVENT   |      |             |       ___________________
     |_____________|      |__________________|      |    PERSON   |      |                   |
     |             |      |                  |      |_____________|      | PERSON_EMAIL_ADDR |
     | *EVENT_ID   | <--> | *EVENT_ID        |      |             |      |___________________|
     |  EVENT_DATE |      | *PERSON_ID       | <--> | *PERSON_ID  | <--> |  *PERSON_ID       |
     |  TITLE      |      |__________________|      |  AGE        |      |  *EMAIL_ADDR      |
     |_____________|                                |  FIRSTNAME  |      |___________________|
                                                    |  LASTNAME   |
                                                    |_____________|

You can see that the primary key of the collection table is in fact a composite key that uses both columns. This also implies that there cannot be duplicate email addresses per person, which is exactly the semantics we need for a set in Java.

You can now try to add elements to this collection, just like we did before by linking persons and events. It is the same code in Java:

        private void addEmailToPerson(Long personId, String emailAddress) {
            Session session = HibernateUtil.getSessionFactory().getCurrentSession();
            session.beginTransaction();

            Person aPerson = (Person) session.load(Person.class, personId);
            // adding to the emailAddress collection might trigger a lazy load of the collection
            aPerson.getEmailAddresses().add(emailAddress);

            session.getTransaction().commit();
        }

This time we did not use a fetch query to initialize the collection. Monitor the SQL log and try to optimize this with an eager fetch.

### 1.2.5. Bi-directional associations

Next you will map a bi-directional association. You will make the association between person and event work from both sides in Java. The database schema does not change, so you will still have many-to-many multiplicity.

## Note

A relational database is more flexible than a network programming language, in that it does not need a navigation direction; data can be viewed and retrieved in any possible way.

First, add a collection of participants to the `Event` class:

        private Set participants = new HashSet();

        public Set getParticipants() {
            return participants;
        }

        public void setParticipants(Set participants) {
            this.participants = participants;
        }

Now map this side of the association in `Event.hbm.xml`.

            <set name="participants" table="PERSON_EVENT" inverse="true">
                <key column="EVENT_ID"/>
                <many-to-many column="PERSON_ID" class="Person"/>
            </set>

These are normal `set` mappings in both mapping documents. Notice that the column names in `key` and `many-to-many` swap in both mapping documents. The most important addition here is the `inverse="true"` attribute in the `set` element of the `Event`'s collection mapping.

What this means is that Hibernate should take the other side, the `Person` class, when it needs to find out information about the link between the two. This will be a lot easier to understand once you see how the bi-directional link between our two entities is created.

### 1.2.6. Working bi-directional links

First, keep in mind that Hibernate does not affect normal Java semantics. How did we create a link between a `Person` and an `Event` in the unidirectional example? You add an instance of `Event` to the collection of event references, of an instance of `Person`. If you want to make this link bi-directional, you have to do the same on the other side by adding a `Person` reference to the collection in an `Event`. This process of "setting the link on both sides" is absolutely necessary with bi-directional links.

Many developers program defensively and create link management methods to correctly set both sides (for example, in `Person`):

        protected Set getEvents() {
            return events;
        }

        protected void setEvents(Set events) {
            this.events = events;
        }

        public void addToEvent(Event event) {
            this.getEvents().add(event);
            event.getParticipants().add(this);
        }

        public void removeFromEvent(Event event) {
            this.getEvents().remove(event);
            event.getParticipants().remove(this);
        }

The get and set methods for the collection are now protected. This allows classes in the same package and subclasses to still access the methods, but prevents everybody else from altering the collections directly. Repeat the steps for the collection on the other side.

What about the `inverse` mapping attribute? For you, and for Java, a bi-directional link is simply a matter of setting the references on both sides correctly. Hibernate, however, does not have enough information to correctly arrange SQL `INSERT` and `UPDATE` statements (to avoid constraint violations). Making one side of the association `inverse` tells Hibernate to consider it a mirror of the other side. That is all that is necessary for Hibernate to resolve any issues that arise when transforming a directional navigation model to a SQL database schema. The rules are straightforward: all bi-directional associations need one side as `inverse`. In a one-to-many association it has to be the many-side, and in many-to-many association you can select either side.

## 1.3. Part 3 - The EventManager web application

A Hibernate web application uses `Session` and `Transaction` almost like a standalone application. However, some common patterns are useful. You can now write an `EventManagerServlet`. This servlet can list all events stored in the database, and it provides an HTML form to enter new events.

### 1.3.1. Writing the basic servlet

First we need create our basic processing servlet. Since our servlet only handles HTTP `GET` requests, we will only implement the `doGet()` method:

    package org.hibernate.tutorial.web;

    // Imports

    public class EventManagerServlet extends HttpServlet {

        protected void doGet(
                HttpServletRequest request,
                HttpServletResponse response) throws ServletException, IOException {

            SimpleDateFormat dateFormatter = new SimpleDateFormat( "dd.MM.yyyy" );

            try {
                // Begin unit of work
                HibernateUtil.getSessionFactory().getCurrentSession().beginTransaction();

                // Process request and render page...

                // End unit of work
                HibernateUtil.getSessionFactory().getCurrentSession().getTransaction().commit();
            }
            catch (Exception ex) {
                HibernateUtil.getSessionFactory().getCurrentSession().getTransaction().rollback();
                if ( ServletException.class.isInstance( ex ) ) {
                    throw ( ServletException ) ex;
                }
                else {
                    throw new ServletException( ex );
                }
            }
        }

    }

Save this servlet as `src/main/java/org/hibernate/tutorial/web/EventManagerServlet.java`

The pattern applied here is called session-per-request. When a request hits the servlet, a new Hibernate `Session` is opened through the first call to `getCurrentSession()` on the `SessionFactory`. A database transaction is then started. All data access occurs inside a transaction irrespective of whether the data is read or written. Do not use the auto-commit mode in applications.

Do not use a new Hibernate `Session` for every database operation. Use one Hibernate `Session` that is scoped to the whole request. Use `getCurrentSession()`, so that it is automatically bound to the current Java thread.

Next, the possible actions of the request are processed and the response HTML is rendered. We will get to that part soon.

Finally, the unit of work ends when processing and rendering are complete. If any problems occurred during processing or rendering, an exception will be thrown and the database transaction rolled back. This completes the `session-per-request` pattern. Instead of the transaction demarcation code in every servlet, you could also write a servlet filter. See the Hibernate website and Wiki for more information about this pattern called Open Session in View. You will need it as soon as you consider rendering your view in JSP, not in a servlet.

### 1.3.2. Processing and rendering

Now you can implement the processing of the request and the rendering of the page.

            // Write HTML header
            PrintWriter out = response.getWriter();
            out.println("<html><head><title>Event Manager</title></head><body>");

            // Handle actions
            if ( "store".equals(request.getParameter("action")) ) {

                String eventTitle = request.getParameter("eventTitle");
                String eventDate = request.getParameter("eventDate");

                if ( "".equals(eventTitle) || "".equals(eventDate) ) {
                    out.println("<b><i>Please enter event title and date.</i></b>");
                }
                else {
                    createAndStoreEvent(eventTitle, dateFormatter.parse(eventDate));
                    out.println("<b><i>Added event.</i></b>");
                }
            }

            // Print page
           printEventForm(out);
           listEvents(out, dateFormatter);

           // Write HTML footer
           out.println("</body></html>");
           out.flush();
           out.close();

This coding style, with a mix of Java and HTML, would not scale in a more complex application;keep in mind that we are only illustrating basic Hibernate concepts in this tutorial. The code prints an HTML header and a footer. Inside this page, an HTML form for event entry and a list of all events in the database are printed. The first method is trivial and only outputs HTML:

        private void printEventForm(PrintWriter out) {
            out.println("<h2>Add new event:</h2>");
            out.println("<form>");
            out.println("Title: <input name='eventTitle' length='50'/><br/>");
            out.println("Date (e.g. 24.12.2009): <input name='eventDate' length='10'/><br/>");
            out.println("<input type='submit' name='action' value='store'/>");
            out.println("</form>");
        }

The `listEvents()` method uses the Hibernate `Session` bound to the current thread to execute a query:

        private void listEvents(PrintWriter out, SimpleDateFormat dateFormatter) {

            List result = HibernateUtil.getSessionFactory()
                    .getCurrentSession().createCriteria(Event.class).list();
            if (result.size() > 0) {
                out.println("<h2>Events in database:</h2>");
                out.println("<table border='1'>");
                out.println("<tr>");
                out.println("<th>Event title</th>");
                out.println("<th>Event date</th>");
                out.println("</tr>");
                Iterator it = result.iterator();
                while (it.hasNext()) {
                    Event event = (Event) it.next();
                    out.println("<tr>");
                    out.println("<td>" + event.getTitle() + "</td>");
                    out.println("<td>" + dateFormatter.format(event.getDate()) + "</td>");
                    out.println("</tr>");
                }
                out.println("</table>");
            }
        }

Finally, the `store` action is dispatched to the `createAndStoreEvent()` method, which also uses the `Session` of the current thread:

        protected void createAndStoreEvent(String title, Date theDate) {
            Event theEvent = new Event();
            theEvent.setTitle(title);
            theEvent.setDate(theDate);

            HibernateUtil.getSessionFactory()
                    .getCurrentSession().save(theEvent);
        }

The servlet is now complete. A request to the servlet will be processed in a single `Session` and `Transaction`. As earlier in the standalone application, Hibernate can automatically bind these objects to the current thread of execution. This gives you the freedom to layer your code and access the `SessionFactory` in any way you like. Usually you would use a more sophisticated design and move the data access code into data access objects (the DAO pattern). See the Hibernate Wiki for more examples.

### 1.3.3. Deploying and testing

To deploy this application for testing we must create a Web ARchive (WAR). First we must define the WAR descriptor as `src/main/webapp/WEB-INF/web.xml`

    <?xml version="1.0" encoding="UTF-8"?>
    <web-app version="2.4"
        xmlns="http://java.sun.com/xml/ns/j2ee"
        xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance"
        xsi:schemaLocation="http://java.sun.com/xml/ns/j2ee http://java.sun.com/xml/ns/j2ee/web-app_2_4.xsd">

        <servlet>
            <servlet-name>Event Manager</servlet-name>
            <servlet-class>org.hibernate.tutorial.web.EventManagerServlet</servlet-class>
        </servlet>

        <servlet-mapping>
            <servlet-name>Event Manager</servlet-name>
            <url-pattern>/eventmanager</url-pattern>
        </servlet-mapping>
    </web-app>

To build and deploy call `mvn package` in your project directory and copy the `hibernate-tutorial.war` file into your Tomcat `webapps` directory.

## Note

If you do not have Tomcat installed, download it from http://tomcat.apache.org/ and follow the installation instructions. Our application requires no changes to the standard Tomcat configuration.

Once deployed and Tomcat is running, access the application at `http://localhost:8080/hibernate-tutorial/eventmanager`. Make sure you watch the Tomcat log to see Hibernate initialize when the first request hits your servlet (the static initializer in `HibernateUtil` is called) and to get the detailed output if any exceptions occurs.

## 1.4. Summary

This tutorial covered the basics of writing a simple standalone Hibernate application and a small web application. More tutorials are available from the Hibernate website.

## Chapter 2. Architecture

## 2.1. Overview

The diagram below provides a high-level view of the Hibernate architecture:

Unfortunately we cannot provide a detailed view of all possible runtime architectures. Hibernate is sufficiently flexible to be used in a number of ways in many, many architectures. We will, however, illustrate 2 specifically since they are extremes.

### 2.1.1. Minimal architecture

The "minimal" architecture has the application manage its own JDBC connections and provide those connections to Hibernate; additionally the application manages transactions for itself. This approach uses a minimal subset of Hibernate APIs.

### 2.1.2. Comprehensive architecture

The "comprehensive" architecture abstracts the application away from the underlying JDBC/JTA APIs and allows Hibernate to manage the details.

### 2.1.3. Basic APIs

Here are quick discussions about some of the API objects depicted in the preceding diagrams (you will see them again in more detail in later chapters).

SessionFactory (`org.hibernate.SessionFactory`)

  A thread-safe, immutable cache of compiled mappings for a single database. A factory for `org.hibernate.Session` instances. A client of `org.hibernate.connection.ConnectionProvider`. Optionally maintains a `second level cache` of data that is reusable between transactions at a process or cluster level.

Session (`org.hibernate.Session`)

  A single-threaded, short-lived object representing a conversation between the application and the persistent store. Wraps a JDBC `java.sql.Connection`. Factory for `org.hibernate.Transaction`. Maintains a `first level cache` of persistent the application's persistent objects and collections; this cache is used when navigating the object graph or looking up objects by identifier.

Persistent objects and collections

  Short-lived, single threaded objects containing persistent state and business function. These can be ordinary JavaBeans/POJOs. They are associated with exactly one `org.hibernate.Session`. Once the `org.hibernate.Session` is closed, they will be detached and free to use in any application layer (for example, directly as data transfer objects to and from presentation). Chapter 11, Working with objects discusses transient, persistent and detached object states.

Transient and detached objects and collections

  Instances of persistent classes that are not currently associated with a `org.hibernate.Session`. They may have been instantiated by the application and not yet persisted, or they may have been instantiated by a closed `org.hibernate.Session`. Chapter 11, Working with objects discusses transient, persistent and detached object states.

Transaction (`org.hibernate.Transaction`)

  (Optional) A single-threaded, short-lived object used by the application to specify atomic units of work. It abstracts the application from the underlying JDBC, JTA or CORBA transaction. A `org.hibernate.Session` might span several `org.hibernate.Transaction`s in some cases. However, transaction demarcation, either using the underlying API or `org.hibernate.Transaction`, is never optional.

ConnectionProvider (`org.hibernate.connection.ConnectionProvider`)

  (Optional) A factory for, and pool of, JDBC connections. It abstracts the application from underlying `javax.sql.DataSource` or `java.sql.DriverManager`. It is not exposed to application, but it can be extended and/or implemented by the developer.

TransactionFactory (`org.hibernate.TransactionFactory`)

  (Optional) A factory for `org.hibernate.Transaction` instances. It is not exposed to the application, but it can be extended and/or implemented by the developer.

Extension Interfaces

  Hibernate offers a range of optional extension interfaces you can implement to customize the behavior of your persistence layer. See the API documentation for details.

## 2.2. JMX Integration

JMX is the J2EE standard for the management of Java components. Hibernate can be managed via a JMX standard service. AN MBean implementation is provided in the distribution: `org.hibernate.jmx.HibernateService`.

Another feature available as a JMX service is runtime Hibernate statistics. See Section 3.4.6, “Hibernate statistics” for more information.

## 2.3. Contextual sessions

Most applications using Hibernate need some form of "contextual" session, where a given session is in effect throughout the scope of a given context. However, across applications the definition of what constitutes a context is typically different; different contexts define different scopes to the notion of current. Applications using Hibernate prior to version 3.0 tended to utilize either home-grown `ThreadLocal`-based contextual sessions, helper classes such as `HibernateUtil`, or utilized third-party frameworks, such as Spring or Pico, which provided proxy/interception-based contextual sessions.

Starting with version 3.0.1, Hibernate added the `SessionFactory.getCurrentSession()` method. Initially, this assumed usage of `JTA` transactions, where the `JTA` transaction defined both the scope and context of a current session. Given the maturity of the numerous stand-alone `JTA TransactionManager` implementations, most, if not all, applications should be using `JTA` transaction management, whether or not they are deployed into a `J2EE` container. Based on that, the `JTA`-based contextual sessions are all you need to use.

However, as of version 3.1, the processing behind `SessionFactory.getCurrentSession()` is now pluggable. To that end, a new extension interface, `org.hibernate.context.spi.CurrentSessionContext`, and a new configuration parameter, `hibernate.current_session_context_class`, have been added to allow pluggability of the scope and context of defining current sessions.

See the Javadocs for the `org.hibernate.context.spi.CurrentSessionContext` interface for a detailed discussion of its contract. It defines a single method, `currentSession()`, by which the implementation is responsible for tracking the current contextual session. Out-of-the-box, Hibernate comes with three implementations of this interface:

- `org.hibernate.context.internal.JTASessionContext`: current sessions are tracked and scoped by a `JTA` transaction. The processing here is exactly the same as in the older JTA-only approach. See the Javadocs for details.

- `org.hibernate.context.internal.ThreadLocalSessionContext`:current sessions are tracked by thread of execution. See the Javadocs for details.

- `org.hibernate.context.internal.ManagedSessionContext`: current sessions are tracked by thread of execution. However, you are responsible to bind and unbind a `Session` instance with static methods on this class: it does not open, flush, or close a `Session`.

The first two implementations provide a "one session - one database transaction" programming model. This is also known and used as session-per-request. The beginning and end of a Hibernate session is defined by the duration of a database transaction. If you use programmatic transaction demarcation in plain JSE without JTA, you are advised to use the Hibernate `Transaction` API to hide the underlying transaction system from your code. If you use JTA, you can utilize the JTA interfaces to demarcate transactions. If you execute in an EJB container that supports CMT, transaction boundaries are defined declaratively and you do not need any transaction or session demarcation operations in your code. Refer to Chapter 13, Transactions and Concurrency for more information and code examples.

The `hibernate.current_session_context_class` configuration parameter defines which `org.hibernate.context.spi.CurrentSessionContext` implementation should be used. For backwards compatibility, if this configuration parameter is not set but a `org.hibernate.transaction.TransactionManagerLookup` is configured, Hibernate will use the `org.hibernate.context.internal.JTASessionContext`. Typically, the value of this parameter would just name the implementation class to use. For the three out-of-the-box implementations, however, there are three corresponding short names: "jta", "thread", and "managed".

## Chapter 3. Configuration

Hibernate is designed to operate in many different environments and, as such, there is a broad range of configuration parameters. Fortunately, most have sensible default values and Hibernate is distributed with an example `hibernate.properties` file in `etc/` that displays the various options. Simply put the example file in your classpath and customize it to suit your needs.

## 3.1. Programmatic configuration

An instance of `org.hibernate.cfg.Configuration` represents an entire set of mappings of an application's Java types to an SQL database. The `org.hibernate.cfg.Configuration` is used to build an immutable `org.hibernate.SessionFactory`. The mappings are compiled from various XML mapping files.

You can obtain a `org.hibernate.cfg.Configuration` instance by instantiating it directly and specifying XML mapping documents. If the mapping files are in the classpath, use `addResource()`. For example:

    Configuration cfg = new Configuration()
        .addResource("Item.hbm.xml")
        .addResource("Bid.hbm.xml");

An alternative way is to specify the mapped class and allow Hibernate to find the mapping document for you:

    Configuration cfg = new Configuration()
        .addClass(org.hibernate.auction.Item.class)
        .addClass(org.hibernate.auction.Bid.class);

Hibernate will then search for mapping files named `/org/hibernate/auction/Item.hbm.xml` and `/org/hibernate/auction/Bid.hbm.xml` in the classpath. This approach eliminates any hardcoded filenames.

A `org.hibernate.cfg.Configuration` also allows you to specify configuration properties. For example:

    Configuration cfg = new Configuration()
        .addClass(org.hibernate.auction.Item.class)
        .addClass(org.hibernate.auction.Bid.class)
        .setProperty("hibernate.dialect", "org.hibernate.dialect.MySQLInnoDBDialect")
        .setProperty("hibernate.connection.datasource", "java:comp/env/jdbc/test")
        .setProperty("hibernate.order_updates", "true");

This is not the only way to pass configuration properties to Hibernate. Some alternative options include:

1. Pass an instance of `java.util.Properties` to `Configuration.setProperties()`.

2. Place a file named `hibernate.properties` in a root directory of the classpath.

3. Set `System` properties using `java -Dproperty=value`.

4. Include `<property>` elements in `hibernate.cfg.xml` (this is discussed later).

If you want to get started quickly`hibernate.properties` is the easiest approach.

The `org.hibernate.cfg.Configuration` is intended as a startup-time object that will be discarded once a `SessionFactory` is created.

## 3.2. Obtaining a SessionFactory

When all mappings have been parsed by the `org.hibernate.cfg.Configuration`, the application must obtain a factory for `org.hibernate.Session` instances. This factory is intended to be shared by all application threads:

    SessionFactory sessions = cfg.buildSessionFactory();

Hibernate does allow your application to instantiate more than one `org.hibernate.SessionFactory`. This is useful if you are using more than one database.

## 3.3. JDBC connections

It is advisable to have the `org.hibernate.SessionFactory` create and pool JDBC connections for you. If you take this approach, opening a `org.hibernate.Session` is as simple as:

    Session session = sessions.openSession(); // open a new Session

Once you start a task that requires access to the database, a JDBC connection will be obtained from the pool.

Before you can do this, you first need to pass some JDBC connection properties to Hibernate. All Hibernate property names and semantics are defined on the class `org.hibernate.cfg.Environment`. The most important settings for JDBC connection configuration are outlined below.

Hibernate will obtain and pool connections using `java.sql.DriverManager` if you set the following properties:

Table 3.1. Hibernate JDBC Properties

Property name | Purpose
--- | ---
hibernate.connection.driver_class | JDBC driver class
hibernate.connection.url | JDBC URL
hibernate.connection.username | database user
hibernate.connection.password | database user password
hibernate.connection.pool_size | maximum number of pooled connections

Hibernate's own connection pooling algorithm is, however, quite rudimentary. It is intended to help you get started and is not intended for use in a production system, or even for performance testing. You should use a third party pool for best performance and stability. Just replace the hibernate.connection.pool_size property with connection pool specific settings. This will turn off Hibernate's internal pool. For example, you might like to use c3p0.

C3P0 is an open source JDBC connection pool distributed along with Hibernate in the `lib` directory. Hibernate will use its `org.hibernate.connection.C3P0ConnectionProvider` for connection pooling if you set hibernate.c3p0.* properties. If you would like to use Proxool, refer to the packaged `hibernate.properties` and the Hibernate web site for more information.

The following is an example `hibernate.properties` file for c3p0:

    hibernate.connection.driver_class = org.postgresql.Driver
    hibernate.connection.url = jdbc:postgresql://localhost/mydatabase
    hibernate.connection.username = myuser
    hibernate.connection.password = secret
    hibernate.c3p0.min_size=5
    hibernate.c3p0.max_size=20
    hibernate.c3p0.timeout=1800
    hibernate.c3p0.max_statements=50
    hibernate.dialect = org.hibernate.dialect.PostgreSQLDialect

For use inside an application server, you should almost always configure Hibernate to obtain connections from an application server `javax.sql.Datasource` registered in JNDI. You will need to set at least one of the following properties:

Table 3.2. Hibernate Datasource Properties

Property name | Purpose
--- | ---
hibernate.connection.datasource | datasource JNDI name
hibernate.jndi.url | URL of the JNDI provider (optional)
hibernate.jndi.class | class of the JNDI `InitialContextFactory` (optional)
hibernate.connection.username | database user (optional)
hibernate.connection.password | database user password (optional)

Here is an example `hibernate.properties` file for an application server provided JNDI datasource:

    hibernate.connection.datasource = java:/comp/env/jdbc/test
    hibernate.transaction.factory_class = \
        org.hibernate.transaction.JTATransactionFactory
    hibernate.transaction.manager_lookup_class = \
        org.hibernate.transaction.JBossTransactionManagerLookup
    hibernate.dialect = org.hibernate.dialect.PostgreSQLDialect

JDBC connections obtained from a JNDI datasource will automatically participate in the container-managed transactions of the application server.

Arbitrary connection properties can be given by prepending "`hibernate.connection`" to the connection property name. For example, you can specify a charSet connection property using hibernate.connection.charSet.

You can define your own plugin strategy for obtaining JDBC connections by implementing the interface `org.hibernate.connection.ConnectionProvider`, and specifying your custom implementation via the hibernate.connection.provider_class property.

## 3.4. Optional configuration properties

There are a number of other properties that control the behavior of Hibernate at runtime. All are optional and have reasonable default values.

## Warning

Some of these properties are "system-level" only. System-level properties can be set only via `java -Dproperty=value` or `hibernate.properties`. They cannot be set by the other techniques described above.

Table 3.3. Hibernate Configuration Properties

Property name | Purpose
--- | ---
hibernate.dialect | The classname of a Hibernate `org.hibernate.dialect.Dialect` which allows Hibernate to generate SQL optimized for a particular relational database. e.g. `full.classname.of.Dialect` In most cases Hibernate will actually be able to choose the correct `org.hibernate.dialect.Dialect` implementation based on the `JDBC metadata` returned by the JDBC driver.
hibernate.show_sql | Write all SQL statements to console. This is an alternative to setting the log category `org.hibernate.SQL` to `debug`. e.g. `true` | `false`
hibernate.format_sql | Pretty print the SQL in the log and console. e.g. `true` | `false`
hibernate.default_schema | Qualify unqualified table names with the given schema/tablespace in generated SQL. e.g. `SCHEMA_NAME`
hibernate.default_catalog | Qualifies unqualified table names with the given catalog in generated SQL. e.g. `CATALOG_NAME`
hibernate.session_factory_name | The `org.hibernate.SessionFactory` will be automatically bound to this name in JNDI after it has been created. e.g. `jndi/composite/name`
hibernate.max_fetch_depth | Sets a maximum "depth" for the outer join fetch tree for single-ended associations (one-to-one, many-to-one). A `0` disables default outer join fetching. e.g. recommended values between `0` and `3`
hibernate.default_batch_fetch_size | Sets a default size for Hibernate batch fetching of associations. e.g. recommended values `4`, `8`, `16`
hibernate.default_entity_mode | Sets a default mode for entity representation for all sessions opened from this `SessionFactory` `dynamic-map`, `dom4j`, `pojo`
hibernate.order_updates | Forces Hibernate to order SQL updates by the primary key value of the items being updated. This will result in fewer transaction deadlocks in highly concurrent systems. e.g. `true` | `false`
hibernate.generate_statistics | If enabled, Hibernate will collect statistics useful for performance tuning. e.g. `true` | `false`
hibernate.use_identifier_rollback | If enabled, generated identifier properties will be reset to default values when objects are deleted. e.g. `true` | `false`
hibernate.use_sql_comments | If turned on, Hibernate will generate comments inside the SQL, for easier debugging, defaults to `false`. e.g. `true` | `false`
hibernate.id.new_generator_mappings | Setting is relevant when using `@GeneratedValue`. It indicates whether or not the new `IdentifierGenerator` implementations are used for `javax.persistence.GenerationType.AUTO`, `javax.persistence.GenerationType.TABLE` and `javax.persistence.GenerationType.SEQUENCE`. Default to `false` to keep backward compatibility. e.g. `true` | `false`

## Note

We recommend all new projects which make use of to use `@GeneratedValue` to also set `hibernate.id.new_generator_mappings=true` as the new generators are more efficient and closer to the JPA 2 specification semantic. However they are not backward compatible with existing databases (if a sequence or a table is used for id generation).

Table 3.4. Hibernate JDBC and Connection Properties

Property name | Purpose
--- | ---
hibernate.jdbc.fetch_size | A non-zero value determines the JDBC fetch size (calls `Statement.setFetchSize()`).
hibernate.jdbc.batch_size | A non-zero value enables use of JDBC2 batch updates by Hibernate. e.g. recommended values between `5` and `30`
hibernate.jdbc.batch_versioned_data | Set this property to `true` if your JDBC driver returns correct row counts from `executeBatch()`. It is usually safe to turn this option on. Hibernate will then use batched DML for automatically versioned data. Defaults to `false`. e.g. `true` | `false`
hibernate.jdbc.factory_class | Select a custom `org.hibernate.jdbc.Batcher`. Most applications will not need this configuration property. e.g. `classname.of.BatcherFactory`
hibernate.jdbc.use_scrollable_resultset | Enables use of JDBC2 scrollable resultsets by Hibernate. This property is only necessary when using user-supplied JDBC connections. Hibernate uses connection metadata otherwise. e.g. `true` | `false`
hibernate.jdbc.use_streams_for_binary | Use streams when writing/reading `binary` or `serializable` types to/from JDBC. *system-level property* e.g. `true` | `false`
hibernate.jdbc.use_get_generated_keys | Enables use of JDBC3 `PreparedStatement.getGeneratedKeys()` to retrieve natively generated keys after insert. Requires JDBC3+ driver and JRE1.4+, set to false if your driver has problems with the Hibernate identifier generators. By default, it tries to determine the driver capabilities using connection metadata. e.g. `true|false`
hibernate.connection.provider_class | The classname of a custom `org.hibernate.connection.ConnectionProvider` which provides JDBC connections to Hibernate. e.g. `classname.of.ConnectionProvider`
hibernate.connection.isolation | Sets the JDBC transaction isolation level. Check `java.sql.Connection` for meaningful values, but note that most databases do not support all isolation levels and some define additional, non-standard isolations. e.g. `1, 2, 4, 8`
hibernate.connection.autocommit | Enables autocommit for JDBC pooled connections (it is not recommended). e.g. `true` | `false`
hibernate.connection.release_mode | Specifies when Hibernate should release JDBC connections. By default, a JDBC connection is held until the session is explicitly closed or disconnected. For an application server JTA datasource, use `after_statement` to aggressively release connections after every JDBC call. For a non-JTA connection, it often makes sense to release the connection at the end of each transaction, by using `after_transaction`. `auto` will choose `after_statement` for the JTA and CMT transaction strategies and `after_transaction` for the JDBC transaction strategy. e.g. `auto` (default) | `on_close` | `after_transaction` | `after_statement` This setting only affects `Session`s returned from `SessionFactory.openSession`. For `Session`s obtained through `SessionFactory.getCurrentSession`, the `CurrentSessionContext` implementation configured for use controls the connection release mode for those `Session`s. See Section 2.3, “Contextual sessions”
hibernate.connection.<propertyName> | Pass the JDBC property <propertyName> to `DriverManager.getConnection()`.
hibernate.jndi.<propertyName> | Pass the property <propertyName> to the JNDI `InitialContextFactory`.

Table 3.5. Hibernate Cache Properties

Property name | Purpose
--- | ---
`hibernate.cache.provider_class` | The classname of a custom `CacheProvider`. e.g. `classname.of.CacheProvider`
`hibernate.cache.use_minimal_puts` | Optimizes second-level cache operation to minimize writes, at the cost of more frequent reads. This setting is most useful for clustered caches and, in Hibernate3, is enabled by default for clustered cache implementations. e.g. `true|false`
`hibernate.cache.use_query_cache` | Enables the query cache. Individual queries still have to be set cachable. e.g. `true|false`
`hibernate.cache.use_second_level_cache` | Can be used to completely disable the second level cache, which is enabled by default for classes which specify a `<cache>` mapping. e.g. `true|false`
`hibernate.cache.query_cache_factory` | The classname of a custom `QueryCache` interface, defaults to the built-in `StandardQueryCache`. e.g. `classname.of.QueryCache`
`hibernate.cache.region_prefix` | A prefix to use for second-level cache region names. e.g. `prefix`
`hibernate.cache.use_structured_entries` | Forces Hibernate to store data in the second-level cache in a more human-friendly format. e.g. `true|false`
`hibernate.cache.default_cache_concurrency_strategy` | Setting used to give the name of the default `org.hibernate.annotations.CacheConcurrencyStrategy` to use when either `@Cacheable` or `@Cache` is used. `@Cache(strategy="..")` is used to override this default.

Table 3.6. Hibernate Transaction Properties

Property name | Purpose
--- | ---
`hibernate.transaction.factory_class` | The classname of a `TransactionFactory` to use with Hibernate `Transaction` API (defaults to `JDBCTransactionFactory`). e.g. `classname.of.TransactionFactory`
`jta.UserTransaction` | A JNDI name used by `JTATransactionFactory` to obtain the JTA `UserTransaction` from the application server. e.g. `jndi/composite/name`
`hibernate.transaction.manager_lookup_class` | The classname of a `TransactionManagerLookup`. It is required when JVM-level caching is enabled or when using hilo generator in a JTA environment. e.g. `classname.of.TransactionManagerLookup`
`hibernate.transaction.flush_before_completion` | If enabled, the session will be automatically flushed during the before completion phase of the transaction. Built-in and automatic session context management is preferred, see Section 2.3, “Contextual sessions”. e.g. `true` | `false`
`hibernate.transaction.auto_close_session` | If enabled, the session will be automatically closed during the after completion phase of the transaction. Built-in and automatic session context management is preferred, see Section 2.3, “Contextual sessions”. e.g. `true` | `false`

Table 3.7. Miscellaneous Properties

Property name | Purpose
--- | ---
`hibernate.current_session_context_class` | Supply a custom strategy for the scoping of the "current" `Session`. See Section 2.3, “Contextual sessions” for more information about the built-in strategies. e.g. `jta` | `thread` | `managed` | `custom.Class`
`hibernate.query.factory_class` | Chooses the HQL parser implementation. e.g. `org.hibernate.hql.internal.ast.ASTQueryTranslatorFactory` or `org.hibernate.hql.internal.classic.ClassicQueryTranslatorFactory`
`hibernate.query.substitutions` | Is used to map from tokens in Hibernate queries to SQL tokens (tokens might be function or literal names, for example). e.g. `hqlLiteral=SQL_LITERAL, hqlFunction=SQLFUNC`
`hibernate.hbm2ddl.auto` | Automatically validates or exports schema DDL to the database when the `SessionFactory` is created. With `create-drop`, the database schema will be dropped when the `SessionFactory` is closed explicitly. e.g. `validate` | `update` | `create` | `create-drop`
`hibernate.hbm2ddl.import_files` | Comma-separated names of the optional files containing SQL DML statements executed during the `SessionFactory` creation. This is useful for testing or demoing: by adding INSERT statements for example you can populate your database with a minimal set of data when it is deployed. File order matters, the statements of a give file are executed before the statements of the following files. These statements are only executed if the schema is created ie if `hibernate.hbm2ddl.auto` is set to `create` or `create-drop`. e.g. `/humans.sql,/dogs.sql`
`hibernate.hbm2ddl.import_files_sql_extractor` | The classname of a custom `ImportSqlCommandExtractor` (defaults to the built-in `SingleLineSqlCommandExtractor`). This is useful for implementing dedicated parser that extracts single SQL statements from each import file. Hibernate provides also `MultipleLinesSqlCommandExtractor` which supports instructions/comments and quoted strings spread over multiple lines (mandatory semicolon at the end of each statement). e.g. `classname.of.ImportSqlCommandExtractor`
`hibernate.bytecode.use_reflection_optimizer` | Enables the use of bytecode manipulation instead of runtime reflection. This is a System-level property and cannot be set in `hibernate.cfg.xml`. Reflection can sometimes be useful when troubleshooting. Hibernate always requires either CGLIB or javassist even if you turn off the optimizer. e.g. `true` | `false`
`hibernate.bytecode.provider` | Both javassist or cglib can be used as byte manipulation engines; the default is `javassist`. e.g. `javassist` | `cglib`

### 3.4.1. SQL Dialects

Always set the `hibernate.dialect` property to the correct `org.hibernate.dialect.Dialect` subclass for your database. If you specify a dialect, Hibernate will use sensible defaults for some of the other properties listed above. This means that you will not have to specify them manually.

Table 3.8. Hibernate SQL Dialects (`hibernate.dialect`)

RDBMS | Dialect
--- | ---
DB2 | `org.hibernate.dialect.DB2Dialect`
DB2 AS/400 | `org.hibernate.dialect.DB2400Dialect`
DB2 OS390 | `org.hibernate.dialect.DB2390Dialect`
PostgreSQL | `org.hibernate.dialect.PostgreSQLDialect`
MySQL5 | `org.hibernate.dialect.MySQL5Dialect`
MySQL5 with InnoDB | `org.hibernate.dialect.MySQL5InnoDBDialect`
MySQL with MyISAM | `org.hibernate.dialect.MySQLMyISAMDialect`
Oracle (any version) | `org.hibernate.dialect.OracleDialect`
Oracle 9i | `org.hibernate.dialect.Oracle9iDialect`
Oracle 10g | `org.hibernate.dialect.Oracle10gDialect`
Oracle 11g | `org.hibernate.dialect.Oracle10gDialect`
Sybase | `org.hibernate.dialect.SybaseASE15Dialect`
Sybase Anywhere | `org.hibernate.dialect.SybaseAnywhereDialect`
Microsoft SQL Server 2000 | `org.hibernate.dialect.SQLServerDialect`
Microsoft SQL Server 2005 | `org.hibernate.dialect.SQLServer2005Dialect`
Microsoft SQL Server 2008 | `org.hibernate.dialect.SQLServer2008Dialect`
SAP DB | `org.hibernate.dialect.SAPDBDialect`
Informix | `org.hibernate.dialect.InformixDialect`
HypersonicSQL | `org.hibernate.dialect.HSQLDialect`
H2 Database | `org.hibernate.dialect.H2Dialect`
Ingres | `org.hibernate.dialect.IngresDialect`
Progress | `org.hibernate.dialect.ProgressDialect`
Mckoi SQL | `org.hibernate.dialect.MckoiDialect`
Interbase | `org.hibernate.dialect.InterbaseDialect`
Pointbase | `org.hibernate.dialect.PointbaseDialect`
FrontBase | `org.hibernate.dialect.FrontbaseDialect`
Firebird | `org.hibernate.dialect.FirebirdDialect`

### 3.4.2. Outer Join Fetching

If your database supports ANSI, Oracle or Sybase style outer joins, outer join fetching will often increase performance by limiting the number of round trips to and from the database. This is, however, at the cost of possibly more work performed by the database itself. Outer join fetching allows a whole graph of objects connected by many-to-one, one-to-many, many-to-many and one-to-one associations to be retrieved in a single SQL `SELECT`.

Outer join fetching can be disabled globally by setting the property `hibernate.max_fetch_depth` to `0`. A setting of `1` or higher enables outer join fetching for one-to-one and many-to-one associations that have been mapped with `fetch="join"`.

See Section 21.1, “Fetching strategies” for more information.

### 3.4.3. Binary Streams

Oracle limits the size of `byte` arrays that can be passed to and/or from its JDBC driver. If you wish to use large instances of `binary` or `serializable` type, you should enable `hibernate.jdbc.use_streams_for_binary`. This is a system-level setting only.

### 3.4.4. Second-level and query cache

The properties prefixed by `hibernate.cache` allow you to use a process or cluster scoped second-level cache system with Hibernate. See the Section 21.2, “The Second Level Cache” for more information.

### 3.4.5. Query Language Substitution

You can define new Hibernate query tokens using `hibernate.query.substitutions`. For example:

    hibernate.query.substitutions true=1, false=0

This would cause the tokens `true` and `false` to be translated to integer literals in the generated SQL.

    hibernate.query.substitutions toLowercase=LOWER

This would allow you to rename the SQL `LOWER` function.

### 3.4.6. Hibernate statistics

If you enable `hibernate.generate_statistics`, Hibernate exposes a number of metrics that are useful when tuning a running system via `SessionFactory.getStatistics()`. Hibernate can even be configured to expose these statistics via JMX. Read the Javadoc of the interfaces in `org.hibernate.stats` for more information.

## 3.5. Logging

## Important

Completely out of date. Hibernate uses JBoss Logging starting in 4.0. This will get documented as we migrate this content to the Developer Guide.

Hibernate utilizes Simple Logging Facade for Java (SLF4J) in order to log various system events. SLF4J can direct your logging output to several logging frameworks (NOP, Simple, log4j version 1.2, JDK 1.4 logging, JCL or logback) depending on your chosen binding. In order to setup logging you will need `slf4j-api.jar` in your classpath together with the jar file for your preferred binding - `slf4j-log4j12.jar` in the case of Log4J. See the SLF4J documentation for more detail. To use Log4j you will also need to place a `log4j.properties` file in your classpath. An example properties file is distributed with Hibernate in the `src/` directory.

It is recommended that you familiarize yourself with Hibernate's log messages. A lot of work has been put into making the Hibernate log as detailed as possible, without making it unreadable. It is an essential troubleshooting device. The most interesting log categories are the following:

Table 3.9. Hibernate Log Categories

Category | Function
--- | ---
`org.hibernate.SQL` | Log all SQL DML statements as they are executed
`org.hibernate.type` | Log all JDBC parameters
`org.hibernate.tool.hbm2ddl` | Log all SQL DDL statements as they are executed
`org.hibernate.pretty` | Log the state of all entities (max 20 entities) associated with the session at flush time
`org.hibernate.cache` | Log all second-level cache activity
`org.hibernate.transaction` | Log transaction related activity
`org.hibernate.jdbc` | Log all JDBC resource acquisition
`org.hibernate.hql.internal.ast.AST` | Log HQL and SQL ASTs during query parsing
`org.hibernate.secure` | Log all JAAS authorization requests
`org.hibernate` | Log everything. This is a lot of information but it is useful for troubleshooting

When developing applications with Hibernate, you should almost always work with `debug` enabled for the category `org.hibernate.SQL`, or, alternatively, the property `hibernate.show_sql` enabled.

## 3.6. Implementing a `NamingStrategy`

The interface `org.hibernate.cfg.NamingStrategy` allows you to specify a "naming standard" for database objects and schema elements.

You can provide rules for automatically generating database identifiers from Java identifiers or for processing "logical" column and table names given in the mapping file into "physical" table and column names. This feature helps reduce the verbosity of the mapping document, eliminating repetitive noise (`TBL_` prefixes, for example). The default strategy used by Hibernate is quite minimal.

You can specify a different strategy by calling `Configuration.setNamingStrategy()` before adding mappings:

    SessionFactory sf = new Configuration()
        .setNamingStrategy(ImprovedNamingStrategy.INSTANCE)
        .addFile("Item.hbm.xml")
        .addFile("Bid.hbm.xml")
        .buildSessionFactory();

`org.hibernate.cfg.ImprovedNamingStrategy` is a built-in strategy that might be a useful starting point for some applications.

## 3.7. Implementing a PersisterClassProvider

You can configure the persister implementation used to persist your entities and collections:

- by default, Hibernate uses persisters that make sense in a relational model and follow Java Persistence's specification

- you can define a `PersisterClassProvider` implementation that provides the persister class used of a given entity or collection

- finally, you can override them on a per entity and collection basis in the mapping using `@Persister` or its XML equivalent

The latter in the list the higher in priority.

You can pass the `PersisterClassProvider` instance to the `Configuration` object.

    SessionFactory sf = new Configuration()
        .setPersisterClassProvider(customPersisterClassProvider)
        .addAnnotatedClass(Order.class)
        .buildSessionFactory();

The persister class provider methods, when returning a non null persister class, override the default Hibernate persisters. The entity name or the collection role are passed to the methods. It is a nice way to centralize the overriding logic of the persisters instead of spreading them on each entity or collection mapping.

## 3.8. XML configuration file

An alternative approach to configuration is to specify a full configuration in a file named `hibernate.cfg.xml`. This file can be used as a replacement for the `hibernate.properties` file or, if both are present, to override properties.

The XML configuration file is by default expected to be in the root of your `CLASSPATH`. Here is an example:

    <?xml version='1.0' encoding='utf-8'?>
    <!DOCTYPE hibernate-configuration PUBLIC
        "-//Hibernate/Hibernate Configuration DTD//EN"
        "http://www.hibernate.org/dtd/hibernate-configuration-3.0.dtd">

    <hibernate-configuration>

        <!-- a SessionFactory instance listed as /jndi/name -->
        <session-factory
            name="java:hibernate/SessionFactory">

            <!-- properties -->
            <property name="connection.datasource">java:/comp/env/jdbc/MyDB</property>
            <property name="dialect">org.hibernate.dialect.MySQLDialect</property>
            <property name="show_sql">false</property>
            <property name="transaction.factory_class">
                org.hibernate.transaction.JTATransactionFactory
            </property>
            <property name="jta.UserTransaction">java:comp/UserTransaction</property>

            <!-- mapping files -->
            <mapping resource="org/hibernate/auction/Item.hbm.xml"/>
            <mapping resource="org/hibernate/auction/Bid.hbm.xml"/>

            <!-- cache settings -->
            <class-cache class="org.hibernate.auction.Item" usage="read-write"/>
            <class-cache class="org.hibernate.auction.Bid" usage="read-only"/>
            <collection-cache collection="org.hibernate.auction.Item.bids" usage="read-write"/>

        </session-factory>

    </hibernate-configuration>

The advantage of this approach is the externalization of the mapping file names to configuration. The `hibernate.cfg.xml` is also more convenient once you have to tune the Hibernate cache. It is your choice to use either `hibernate.properties` or `hibernate.cfg.xml`. Both are equivalent, except for the above mentioned benefits of using the XML syntax.

With the XML configuration, starting Hibernate is then as simple as:

    SessionFactory sf = new Configuration().configure().buildSessionFactory();

You can select a different XML configuration file using:

    SessionFactory sf = new Configuration()
        .configure("catdb.cfg.xml")
        .buildSessionFactory();

## 3.9. Java EE Application Server integration

Hibernate has the following integration points for J2EE infrastructure:

- Container-managed datasources: Hibernate can use JDBC connections managed by the container and provided through JNDI. Usually, a JTA compatible `TransactionManager` and a `ResourceManager` take care of transaction management (CMT), especially distributed transaction handling across several datasources. You can also demarcate transaction boundaries programmatically (BMT), or you might want to use the optional Hibernate `Transaction` API for this to keep your code portable.

- Automatic JNDI binding: Hibernate can bind its `SessionFactory` to JNDI after startup.

- JTA Session binding: the Hibernate `Session` can be automatically bound to the scope of JTA transactions. Simply lookup the `SessionFactory` from JNDI and get the current `Session`. Let Hibernate manage flushing and closing the `Session` when your JTA transaction completes. Transaction demarcation is either declarative (CMT) or programmatic (BMT/UserTransaction).

- JMX deployment: if you have a JMX capable application server (e.g. JBoss AS), you can choose to deploy Hibernate as a managed MBean. This saves you the one line startup code to build your `SessionFactory` from a `Configuration`. The container will startup your `HibernateService` and also take care of service dependencies (datasource has to be available before Hibernate starts, etc).

Depending on your environment, you might have to set the configuration option `hibernate.connection.aggressive_release` to true if your application server shows "connection containment" exceptions.

### 3.9.1. Transaction strategy configuration

The Hibernate `Session` API is independent of any transaction demarcation system in your architecture. If you let Hibernate use JDBC directly through a connection pool, you can begin and end your transactions by calling the JDBC API. If you run in a J2EE application server, you might want to use bean-managed transactions and call the JTA API and `UserTransaction` when needed.

To keep your code portable between these two (and other) environments we recommend the optional Hibernate `Transaction` API, which wraps and hides the underlying system. You have to specify a factory class for `Transaction` instances by setting the Hibernate configuration property `hibernate.transaction.factory_class`.

There are three standard, or built-in, choices:

`org.hibernate.transaction.JDBCTransactionFactory`

  delegates to database (JDBC) transactions (default)

`org.hibernate.transaction.JTATransactionFactory`

  delegates to container-managed transactions if an existing transaction is underway in this context (for example, EJB session bean method). Otherwise, a new transaction is started and bean-managed transactions are used.

`org.hibernate.transaction.CMTTransactionFactory`

  delegates to container-managed JTA transactions

You can also define your own transaction strategies (for a CORBA transaction service, for example).

Some features in Hibernate (i.e., the second level cache, Contextual Sessions with JTA, etc.) require access to the JTA `TransactionManager` in a managed environment. In an application server, since J2EE does not standardize a single mechanism, you have to specify how Hibernate should obtain a reference to the `TransactionManager`:

Table 3.10. JTA TransactionManagers

Transaction Factory | Application Server
--- | ---
`org.hibernate.transaction.JBossTransactionManagerLookup` | JBoss AS
`org.hibernate.transaction.WeblogicTransactionManagerLookup` | Weblogic
`org.hibernate.transaction.WebSphereTransactionManagerLookup` | WebSphere
`org.hibernate.transaction.WebSphereExtendedJTATransactionLookup` | WebSphere 6
`org.hibernate.transaction.OrionTransactionManagerLookup` | Orion
`org.hibernate.transaction.ResinTransactionManagerLookup` | Resin
`org.hibernate.transaction.JOTMTransactionManagerLookup` | JOTM
`org.hibernate.transaction.JOnASTransactionManagerLookup` | JOnAS
`org.hibernate.transaction.JRun4TransactionManagerLookup` | JRun4
`org.hibernate.transaction.BESTransactionManagerLookup` | Borland ES
`org.hibernate.transaction.JBossTSStandaloneTransactionManagerLookup` | JBoss TS used standalone (ie. outside JBoss AS and a JNDI environment generally). Known to work for `org.jboss.jbossts:jbossjta:4.11.0.Final`

### 3.9.2. JNDI-bound `SessionFactory`

A JNDI-bound Hibernate `SessionFactory` can simplify the lookup function of the factory and create new `Session`s. This is not, however, related to a JNDI bound `Datasource`; both simply use the same registry.

If you wish to have the `SessionFactory` bound to a JNDI namespace, specify a name (e.g. `java:hibernate/SessionFactory`) using the property `hibernate.session_factory_name`. If this property is omitted, the `SessionFactory` will not be bound to JNDI. This is especially useful in environments with a read-only JNDI default implementation (in Tomcat, for example).

When binding the `SessionFactory` to JNDI, Hibernate will use the values of `hibernate.jndi.url`, `hibernate.jndi.class` to instantiate an initial context. If they are not specified, the default `InitialContext` will be used.

Hibernate will automatically place the `SessionFactory` in JNDI after you call `cfg.buildSessionFactory()`. This means you will have this call in some startup code, or utility class in your application, unless you use JMX deployment with the `HibernateService` (this is discussed later in greater detail).

If you use a JNDI `SessionFactory`, an EJB or any other class, you can obtain the `SessionFactory` using a JNDI lookup.

It is recommended that you bind the `SessionFactory` to JNDI in a managed environment and use a `static` singleton otherwise. To shield your application code from these details, we also recommend to hide the actual lookup code for a `SessionFactory` in a helper class, such as `HibernateUtil.getSessionFactory()`. Note that such a class is also a convenient way to startup Hibernate—see chapter 1.

### 3.9.3. Current Session context management with JTA

The easiest way to handle `Sessions` and transactions is Hibernate's automatic "current" `Session` management. For a discussion of contextual sessions see Section 2.3, “Contextual sessions”. Using the `"jta"` session context, if there is no Hibernate `Session` associated with the current JTA transaction, one will be started and associated with that JTA transaction the first time you call `sessionFactory.getCurrentSession()`. The `Session`s retrieved via `getCurrentSession()` in the `"jta"` context are set to automatically flush before the transaction completes, close after the transaction completes, and aggressively release JDBC connections after each statement. This allows the `Session`s to be managed by the life cycle of the JTA transaction to which it is associated, keeping user code clean of such management concerns. Your code can either use JTA programmatically through `UserTransaction`, or (recommended for portable code) use the Hibernate `Transaction` API to set transaction boundaries. If you run in an EJB container, declarative transaction demarcation with CMT is preferred.

### 3.9.4. JMX deployment

The line `cfg.buildSessionFactory()` still has to be executed somewhere to get a `SessionFactory` into JNDI. You can do this either in a `static` initializer block, like the one in `HibernateUtil`, or you can deploy Hibernate as a managed service.

Hibernate is distributed with `org.hibernate.jmx.HibernateService` for deployment on an application server with JMX capabilities, such as JBoss AS. The actual deployment and configuration is vendor-specific. Here is an example `jboss-service.xml` for JBoss 4.0.x:

    <?xml version="1.0"?>
    <server>

    <mbean code="org.hibernate.jmx.HibernateService"
        name="jboss.jca:service=HibernateFactory,name=HibernateFactory">

        <!-- Required services -->
        <depends>jboss.jca:service=RARDeployer</depends>
        <depends>jboss.jca:service=LocalTxCM,name=HsqlDS</depends>

        <!-- Bind the Hibernate service to JNDI -->
        <attribute name="JndiName">java:/hibernate/SessionFactory</attribute>

        <!-- Datasource settings -->
        <attribute name="Datasource">java:HsqlDS</attribute>
        <attribute name="Dialect">org.hibernate.dialect.HSQLDialect</attribute>

        <!-- Transaction integration -->
        <attribute name="TransactionStrategy">
            org.hibernate.transaction.JTATransactionFactory</attribute>
        <attribute name="TransactionManagerLookupStrategy">
            org.hibernate.transaction.JBossTransactionManagerLookup</attribute>
        <attribute name="FlushBeforeCompletionEnabled">true</attribute>
        <attribute name="AutoCloseSessionEnabled">true</attribute>

        <!-- Fetching options -->
        <attribute name="MaximumFetchDepth">5</attribute>

        <!-- Second-level caching -->
        <attribute name="SecondLevelCacheEnabled">true</attribute>
        <attribute name="CacheProviderClass">org.hibernate.cache.internal.EhCacheProvider</attribute>
        <attribute name="QueryCacheEnabled">true</attribute>

        <!-- Logging -->
        <attribute name="ShowSqlEnabled">true</attribute>

        <!-- Mapping files -->
        <attribute name="MapResources">auction/Item.hbm.xml,auction/Category.hbm.xml</attribute>

    </mbean>

    </server>

This file is deployed in a directory called `META-INF` and packaged in a JAR file with the extension `.sar` (service archive). You also need to package Hibernate, its required third-party libraries, your compiled persistent classes, as well as your mapping files in the same archive. Your enterprise beans (usually session beans) can be kept in their own JAR file, but you can include this EJB JAR file in the main service archive to get a single (hot-)deployable unit. Consult the JBoss AS documentation for more information about JMX service and EJB deployment.

## Chapter 4. Persistent Classes

Persistent classes are classes in an application that implement the entities of the business problem (e.g. Customer and Order in an E-commerce application). The term "persistent" here means that the classes are able to be persisted, not that they are in the persistent state (see Section 11.1, “Hibernate object states” for discussion).

Hibernate works best if these classes follow some simple rules, also known as the Plain Old Java Object (POJO) programming model. However, none of these rules are hard requirements. Indeed, Hibernate assumes very little about the nature of your persistent objects. You can express a domain model in other ways (using trees of `java.util.Map` instances, for example).

## 4.1. A simple POJO example

Example 4.1. Simple POJO representing a cat

    package eg;
    import java.util.Set;
    import java.util.Date;

    public class Cat {
    private Long id; // identifier

    private Date birthdate;
    private Color color;
    private char sex;
    private float weight;
        private int litterId;

        private Cat mother;
        private Set kittens = new HashSet();

        private void setId(Long id) {
            this.xml:id=id;
        }
        public Long getId() {
            return id;
        }

        void setBirthdate(Date date) {
            birthdate = date;
        }
        public Date getBirthdate() {
            return birthdate;
        }

        void setWeight(float weight) {
            this.weight = weight;
        }
        public float getWeight() {
            return weight;
        }

        public Color getColor() {
            return color;
        }
        void setColor(Color color) {
            this.color = color;
        }

        void setSex(char sex) {
            this.sex=sex;
        }
        public char getSex() {
            return sex;
        }

        void setLitterId(int id) {
            this.litterId = id;
        }
        public int getLitterId() {
            return litterId;
        }

        void setMother(Cat mother) {
            this.mother = mother;
        }
        public Cat getMother() {
            return mother;
        }
        void setKittens(Set kittens) {
            this.kittens = kittens;
        }
        public Set getKittens() {
            return kittens;
        }

        // addKitten not needed by Hibernate
        public void addKitten(Cat kitten) {
            kitten.setMother(this);
        kitten.setLitterId( kittens.size() );
            kittens.add(kitten);
        }
    }

The four main rules of persistent classes are explored in more detail in the following sections.

### 4.1.1. Implement a no-argument constructor

`Cat` has a no-argument constructor. All persistent classes must have a default constructor (which can be non-public) so that Hibernate can instantiate them using `java.lang.reflect.Constructor.newInstance()`. It is recommended that this constructor be defined with at least package visibility in order for runtime proxy generation to work properly.

### 4.1.2. Provide an identifier property

## Note

Historically this was considered option. While still not (yet) enforced, this should be considered a deprecated feature as it will be completely required to provide a identifier property in an upcoming release.

`Cat` has a property named `id`. This property maps to the primary key column(s) of the underlying database table. The type of the identifier property can be any "basic" type (see ???). See Section 9.4, “Components as composite identifiers” for information on mapping composite (multi-column) identifiers.

## Note

Identifiers do not necessarily need to identify column(s) in the database physically defined as a primary key. They should just identify columns that can be used to uniquely identify rows in the underlying table.

We recommend that you declare consistently-named identifier properties on persistent classes and that you use a nullable (i.e., non-primitive) type.

### 4.1.3. Prefer non-final classes (semi-optional)

A central feature of Hibernate, proxies (lazy loading), depends upon the persistent class being either non-final, or the implementation of an interface that declares all public methods. You can persist `final` classes that do not implement an interface with Hibernate; you will not, however, be able to use proxies for lazy association fetching which will ultimately limit your options for performance tuning. To persist a `final` class which does not implement a "full" interface you must disable proxy generation. See Example 4.2, “Disabling proxies in `hbm.xml`” and Example 4.3, “Disabling proxies in annotations”.

Example 4.2. Disabling proxies in `hbm.xml`

    <class name="Cat" lazy="false"...>...</class>

Example 4.3. Disabling proxies in annotations

    @Entity @Proxy(lazy=false) public class Cat { ... }

If the `final` class does implement a proper interface, you could alternatively tell Hibernate to use the interface instead when generating the proxies. See Example 4.4, “Proxying an interface in `hbm.xml`” and Example 4.5, “Proxying an interface in annotations”.

Example 4.4. Proxying an interface in `hbm.xml`

    <class name="Cat" proxy="ICat"...>...</class>

Example 4.5. Proxying an interface in annotations

    @Entity @Proxy(proxyClass=ICat.class) public class Cat implements ICat { ... }

You should also avoid declaring `public final` methods as this will again limit the ability to generate proxies from this class. If you want to use a class with `public final` methods, you must explicitly disable proxying. Again, see Example 4.2, “Disabling proxies in `hbm.xml`” and Example 4.3, “Disabling proxies in annotations”.

### 4.1.4. Declare accessors and mutators for persistent fields (optional)

`Cat` declares accessor methods for all its persistent fields. Many other ORM tools directly persist instance variables. It is better to provide an indirection between the relational schema and internal data structures of the class. By default, Hibernate persists JavaBeans style properties and recognizes method names of the form `getFoo`, `isFoo` and `setFoo`. If required, you can switch to direct field access for particular properties.

Properties need not be declared public. Hibernate can persist a property declared with `package`, `protected` or `private` visibility as well.

## 4.2. Implementing inheritance

A subclass must also observe the first and second rules. It inherits its identifier property from the superclass, `Cat`. For example:

    package eg;

    public class DomesticCat extends Cat {
            private String name;

            public String getName() {
                    return name;
            }
            protected void setName(String name) {
                    this.name=name;
            }
    }

## 4.3. Implementing `equals()` and `hashCode()`

You have to override the `equals()` and `hashCode()` methods if you:

- intend to put instances of persistent classes in a `Set` (the recommended way to represent many-valued associations); and

- intend to use reattachment of detached instances

Hibernate guarantees equivalence of persistent identity (database row) and Java identity only inside a particular session scope. When you mix instances retrieved in different sessions, you must implement `equals()` and `hashCode()` if you wish to have meaningful semantics for `Set`s.

The most obvious way is to implement `equals()`/`hashCode()` by comparing the identifier value of both objects. If the value is the same, both must be the same database row, because they are equal. If both are added to a `Set`, you will only have one element in the `Set`). Unfortunately, you cannot use that approach with generated identifiers. Hibernate will only assign identifier values to objects that are persistent; a newly created instance will not have any identifier value. Furthermore, if an instance is unsaved and currently in a `Set`, saving it will assign an identifier value to the object. If `equals()` and `hashCode()` are based on the identifier value, the hash code would change, breaking the contract of the `Set`. See the Hibernate website for a full discussion of this problem. This is not a Hibernate issue, but normal Java semantics of object identity and equality.

It is recommended that you implement `equals()` and `hashCode()` using Business key equality. Business key equality means that the `equals()` method compares only the properties that form the business key. It is a key that would identify our instance in the real world (a natural candidate key):

    public class Cat {

        ...
        public boolean equals(Object other) {
            if (this == other) return true;
            if ( !(other instanceof Cat) ) return false;

            final Cat cat = (Cat) other;

            if ( !cat.getLitterId().equals( getLitterId() ) ) return false;
            if ( !cat.getMother().equals( getMother() ) ) return false;

            return true;
        }

        public int hashCode() {
            int result;
            result = getMother().hashCode();
            result = 29 * result + getLitterId();
            return result;
        }

    }

A business key does not have to be as solid as a database primary key candidate (see Section 13.1.3, “Considering object identity”). Immutable or unique properties are usually good candidates for a business key.

## 4.4. Dynamic models

## Note

The following features are currently considered experimental and may change in the near future.

Persistent entities do not necessarily have to be represented as POJO classes or as JavaBean objects at runtime. Hibernate also supports dynamic models (using `Map`s of `Map`s at runtime) and the representation of entities as DOM4J trees. With this approach, you do not write persistent classes, only mapping files.

By default, Hibernate works in normal POJO mode. You can set a default entity representation mode for a particular `SessionFactory` using the `default_entity_mode` configuration option (see Table 3.3, “Hibernate Configuration Properties”).

The following examples demonstrate the representation using `Map`s. First, in the mapping file an `entity-name` has to be declared instead of, or in addition to, a class name:

    <hibernate-mapping>

        <class entity-name="Customer">

            <id name="id"
                type="long"
                column="ID">
                <generator class="sequence"/>
            </id>

            <property name="name"
                column="NAME"
                type="string"/>

            <property name="address"
                column="ADDRESS"
                type="string"/>

            <many-to-one name="organization"
                column="ORGANIZATION_ID"
                class="Organization"/>

            <bag name="orders"
                inverse="true"
                lazy="false"
                cascade="all">
                <key column="CUSTOMER_ID"/>
                <one-to-many class="Order"/>
            </bag>

        </class>

    </hibernate-mapping>

Even though associations are declared using target class names, the target type of associations can also be a dynamic entity instead of a POJO.

After setting the default entity mode to `dynamic-map` for the `SessionFactory`, you can, at runtime, work with `Map`s of `Map`s:

    Session s = openSession();
    Transaction tx = s.beginTransaction();

    // Create a customer
    Map david = new HashMap();
    david.put("name", "David");

    // Create an organization
    Map foobar = new HashMap();
    foobar.put("name", "Foobar Inc.");

    // Link both
    david.put("organization", foobar);

    // Save both
    s.save("Customer", david);
    s.save("Organization", foobar);

    tx.commit();
    s.close();

One of the main advantages of dynamic mapping is quick turnaround time for prototyping, without the need for entity class implementation. However, you lose compile-time type checking and will likely deal with many exceptions at runtime. As a result of the Hibernate mapping, the database schema can easily be normalized and sound, allowing to add a proper domain model implementation on top later on.

Entity representation modes can also be set on a per `Session` basis:

    Session dynamicSession = pojoSession.getSession(EntityMode.MAP);

    // Create a customer
    Map david = new HashMap();
    david.put("name", "David");
    dynamicSession.save("Customer", david);
    ...
    dynamicSession.flush();
    dynamicSession.close()
    ...
    // Continue on pojoSession

Please note that the call to `getSession()` using an `EntityMode` is on the `Session` API, not the `SessionFactory`. That way, the new `Session` shares the underlying JDBC connection, transaction, and other context information. This means you do not have to call `flush()` and `close()` on the secondary `Session`, and also leave the transaction and connection handling to the primary unit of work.

More information about the XML representation capabilities can be found in Chapter 20, XML Mapping.

## 4.5. Tuplizers

`org.hibernate.tuple.Tuplizer` and its sub-interfaces are responsible for managing a particular representation of a piece of data given that representation's `org.hibernate.EntityMode`. If a given piece of data is thought of as a data structure, then a tuplizer is the thing that knows how to create such a data structure, how to extract values from such a data structure and how to inject values into such a data structure. For example, for the POJO entity mode, the corresponding tuplizer knows how create the POJO through its constructor. It also knows how to access the POJO properties using the defined property accessors.

There are two (high-level) types of Tuplizers:

- `org.hibernate.tuple.entity.EntityTuplizer` which is responsible for managing the above mentioned contracts in regards to entities

- `org.hibernate.tuple.component.ComponentTuplizer` which does the same for components

Users can also plug in their own tuplizers. Perhaps you require that `java.util.Map` implementation other than `java.util.HashMap` be used while in the dynamic-map entity-mode. Or perhaps you need to define a different proxy generation strategy than the one used by default. Both would be achieved by defining a custom tuplizer implementation. Tuplizer definitions are attached to the entity or component mapping they are meant to manage. Going back to the example of our `Customer` entity, Example 4.6, “Specify custom tuplizers in annotations” shows how to specify a custom `org.hibernate.tuple.entity.EntityTuplizer` using annotations while Example 4.7, “Specify custom tuplizers in `hbm.xml`” shows how to do the same in `hbm.xml`

Example 4.6. Specify custom tuplizers in annotations

    @Entity
    @Tuplizer(impl = DynamicEntityTuplizer.class)
    public interface Cuisine {
        @Id
        @GeneratedValue
        public Long getId();
        public void setId(Long id);

        public String getName();
        public void setName(String name);

        @Tuplizer(impl = DynamicComponentTuplizer.class)
        public Country getCountry();
        public void setCountry(Country country);
    }

Example 4.7. Specify custom tuplizers in `hbm.xml`

    <hibernate-mapping>
        <class entity-name="Customer">
            <!--
                Override the dynamic-map entity-mode
                tuplizer for the customer entity
            -->
            <tuplizer entity-mode="dynamic-map"
                    class="CustomMapTuplizerImpl"/>

            <id name="id" type="long" column="ID">
                <generator class="sequence"/>
            </id>

            <!-- other properties -->
            ...
        </class>
    </hibernate-mapping>

## 4.6. EntityNameResolvers

`org.hibernate.EntityNameResolver` is a contract for resolving the entity name of a given entity instance. The interface defines a single method `resolveEntityName` which is passed the entity instance and is expected to return the appropriate entity name (null is allowed and would indicate that the resolver does not know how to resolve the entity name of the given entity instance). Generally speaking, an `org.hibernate.EntityNameResolver` is going to be most useful in the case of dynamic models. One example might be using proxied interfaces as your domain model. The hibernate test suite has an example of this exact style of usage under the org.hibernate.test.dynamicentity.tuplizer2. Here is some of the code from that package for illustration.

    /**
     * A very trivial JDK Proxy InvocationHandler implementation where we proxy an
     * interface as the domain model and simply store persistent state in an internal
     * Map.  This is an extremely trivial example meant only for illustration.
     */
    public final class DataProxyHandler implements InvocationHandler {
    	private String entityName;
    	private HashMap data = new HashMap();

    	public DataProxyHandler(String entityName, Serializable id) {
    		this.entityName = entityName;
    		data.put( "Id", id );
    	}

    	public Object invoke(Object proxy, Method method, Object[] args) throws Throwable {
    		String methodName = method.getName();
    		if ( methodName.startsWith( "set" ) ) {
    			String propertyName = methodName.substring( 3 );
    			data.put( propertyName, args[0] );
    		}
    		else if ( methodName.startsWith( "get" ) ) {
    			String propertyName = methodName.substring( 3 );
    			return data.get( propertyName );
    		}
    		else if ( "toString".equals( methodName ) ) {
    			return entityName + "#" + data.get( "Id" );
    		}
    		else if ( "hashCode".equals( methodName ) ) {
    			return new Integer( this.hashCode() );
    		}
    		return null;
    	}

    	public String getEntityName() {
    		return entityName;
    	}

    	public HashMap getData() {
    		return data;
    	}
    }

    public class ProxyHelper {
        public static String extractEntityName(Object object) {
            // Our custom java.lang.reflect.Proxy instances actually bundle
            // their appropriate entity name, so we simply extract it from there
            // if this represents one of our proxies; otherwise, we return null
            if ( Proxy.isProxyClass( object.getClass() ) ) {
                InvocationHandler handler = Proxy.getInvocationHandler( object );
                if ( DataProxyHandler.class.isAssignableFrom( handler.getClass() ) ) {
                    DataProxyHandler myHandler = ( DataProxyHandler ) handler;
                    return myHandler.getEntityName();
                }
            }
            return null;
        }

        // various other utility methods ....

    }

    /**
     * The EntityNameResolver implementation.
     *
     * IMPL NOTE : An EntityNameResolver really defines a strategy for how entity names
     * should be resolved.  Since this particular impl can handle resolution for all of our
     * entities we want to take advantage of the fact that SessionFactoryImpl keeps these
     * in a Set so that we only ever have one instance registered.  Why?  Well, when it
     * comes time to resolve an entity name, Hibernate must iterate over all the registered
     * resolvers.  So keeping that number down helps that process be as speedy as possible.
     * Hence the equals and hashCode implementations as is
     */
    public class MyEntityNameResolver implements EntityNameResolver {
        public static final MyEntityNameResolver INSTANCE = new MyEntityNameResolver();

        public String resolveEntityName(Object entity) {
            return ProxyHelper.extractEntityName( entity );
        }

        public boolean equals(Object obj) {
            return getClass().equals( obj.getClass() );
        }

        public int hashCode() {
            return getClass().hashCode();
        }
    }

    public class MyEntityTuplizer extends PojoEntityTuplizer {
    	public MyEntityTuplizer(EntityMetamodel entityMetamodel, PersistentClass mappedEntity) {
    		super( entityMetamodel, mappedEntity );
    	}

    	public EntityNameResolver[] getEntityNameResolvers() {
    		return new EntityNameResolver[] { MyEntityNameResolver.INSTANCE };
    	}

        public String determineConcreteSubclassEntityName(Object entityInstance, SessionFactoryImplementor factory) {
            String entityName = ProxyHelper.extractEntityName( entityInstance );
            if ( entityName == null ) {
                entityName = super.determineConcreteSubclassEntityName( entityInstance, factory );
            }
            return entityName;
        }

        ...

In order to register an `org.hibernate.EntityNameResolver` users must either:

1. Implement a custom tuplizer (see Section 4.5, “Tuplizers”), implementing the `getEntityNameResolvers` method

2. Register it with the `org.hibernate.impl.SessionFactoryImpl` (which is the implementation class for `org.hibernate.SessionFactory`) using the `registerEntityNameResolver` method.

## Chapter 5. Basic O/R Mapping

## 5.1. Mapping declaration

Object/relational mappings can be defined in three approaches:

- using Java 5 annotations (via the Java Persistence 2 annotations)

- using JPA 2 XML deployment descriptors (described in chapter XXX)

- using the Hibernate legacy XML files approach known as hbm.xml

Annotations are split in two categories, the logical mapping annotations (describing the object model, the association between two entities etc.) and the physical mapping annotations (describing the physical schema, tables, columns, indexes, etc). We will mix annotations from both categories in the following code examples.

JPA annotations are in the `javax.persistence.*` package. Hibernate specific extensions are in `org.hibernate.annotations.*`. You favorite IDE can auto-complete annotations and their attributes for you (even without a specific "JPA" plugin, since JPA annotations are plain Java 5 annotations).

Here is an example of mapping

    package eg;

    @Entity
    @Table(name="cats") @Inheritance(strategy=SINGLE_TABLE)
    @DiscriminatorValue("C") @DiscriminatorColumn(name="subclass", discriminatorType=CHAR)
    public class Cat {

       @Id @GeneratedValue
       public Integer getId() { return id; }
       public void setId(Integer id) { this.id = id; }
       private Integer id;

       public BigDecimal getWeight() { return weight; }
       public void setWeight(BigDecimal weight) { this.weight = weight; }
       private BigDecimal weight;

       @Temporal(DATE) @NotNull @Column(updatable=false)
       public Date getBirthdate() { return birthdate; }
       public void setBirthdate(Date birthdate) { this.birthdate = birthdate; }
       private Date birthdate;

       @org.hibernate.annotations.Type(type="eg.types.ColorUserType")
       @NotNull @Column(updatable=false)
       public ColorType getColor() { return color; }
       public void setColor(ColorType color) { this.color = color; }
       private ColorType color;

       @NotNull @Column(updatable=false)
       public String getSex() { return sex; }
       public void setSex(String sex) { this.sex = sex; }
       private String sex;

       @NotNull @Column(updatable=false)
       public Integer getLitterId() { return litterId; }
       public void setLitterId(Integer litterId) { this.litterId = litterId; }
       private Integer litterId;

       @ManyToOne @JoinColumn(name="mother_id", updatable=false)
       public Cat getMother() { return mother; }
       public void setMother(Cat mother) { this.mother = mother; }
       private Cat mother;

       @OneToMany(mappedBy="mother") @OrderBy("litterId")
       public Set<Cat> getKittens() { return kittens; }
       public void setKittens(Set<Cat> kittens) { this.kittens = kittens; }
       private Set<Cat> kittens = new HashSet<Cat>();
    }

    @Entity @DiscriminatorValue("D")
    public class DomesticCat extends Cat {

       public String getName() { return name; }
       public void setName(String name) { this.name = name }
       private String name;
    }

    @Entity
    public class Dog { ... }

The legacy hbm.xml approach uses an XML schema designed to be readable and hand-editable. The mapping language is Java-centric, meaning that mappings are constructed around persistent class declarations and not table declarations.

Please note that even though many Hibernate users choose to write the XML by hand, a number of tools exist to generate the mapping document. These include XDoclet, Middlegen and AndroMDA.

Here is an example mapping:

    <?xml version="1.0"?>
    <!DOCTYPE hibernate-mapping PUBLIC
          "-//Hibernate/Hibernate Mapping DTD 3.0//EN"
              "http://www.hibernate.org/dtd/hibernate-mapping-3.0.dtd">

    <hibernate-mapping package="eg">

            <class name="Cat"
                table="cats"
                discriminator-value="C">

                    <id name="id">
                            <generator class="native"/>
                    </id>

                    <discriminator column="subclass"
                         type="character"/>

                    <property name="weight"/>

                    <property name="birthdate"
                        type="date"
                        not-null="true"
                        update="false"/>

                    <property name="color"
                        type="eg.types.ColorUserType"
                        not-null="true"
                        update="false"/>

                    <property name="sex"
                        not-null="true"
                        update="false"/>

                    <property name="litterId"
                        column="litterId"
                        update="false"/>

                    <many-to-one name="mother"
                        column="mother_id"
                        update="false"/>

                    <set name="kittens"
                        inverse="true"
                        order-by="litter_id">
                            <key column="mother_id"/>
                            <one-to-many class="Cat"/>
                    </set>

                    <subclass name="DomesticCat"
                        discriminator-value="D">

                            <property name="name"
                                type="string"/>

                    </subclass>

            </class>

            <class name="Dog">
                    <!-- mapping for Dog could go here -->
            </class>

    </hibernate-mapping>

We will now discuss the concepts of the mapping documents (both annotations and XML). We will only describe, however, the document elements and attributes that are used by Hibernate at runtime. The mapping document also contains some extra optional attributes and elements that affect the database schemas exported by the schema export tool (for example, the `not-null` attribute).

### 5.1.1. Entity

An entity is a regular Java object (aka POJO) which will be persisted by Hibernate.

To mark an object as an entity in annotations, use the `@Entity` annotation.

    @Entity

    public class Flight implements Serializable {

        Long id;

        @Id

        public Long getId() { return id; }

        public void setId(Long id) { this.id = id; }

    }

That's pretty much it, the rest is optional. There are however any options to tweak your entity mapping, let's explore them.

`@Table` lets you define the table the entity will be persisted into. If undefined, the table name is the unqualified class name of the entity. You can also optionally define the catalog, the schema as well as unique constraints on the table.

    @Entity
    @Table(name="TBL_FLIGHT",
           schema="AIR_COMMAND",
           uniqueConstraints=
               @UniqueConstraint(
                   name="flight_number",
                   columnNames={"comp_prefix", "flight_number"} ) )
    public class Flight implements Serializable {
        @Column(name="comp_prefix")
        public String getCompagnyPrefix() { return companyPrefix; }

        @Column(name="flight_number")
        public String getNumber() { return number; }
    }

The constraint name is optional (generated if left undefined). The column names composing the constraint correspond to the column names as defined before the Hibernate `NamingStrategy` is applied.

## Tip

Be sure to use the database-level column names for the `columnNames` property of a `@UniqueConstraint`. For example, whilst for simple types the database-level column name may be the same as the entity-level property name, this is often not the case for relational properties.

`@Entity.name` lets you define the shortcut name of the entity you can used in JP-QL and HQL queries. It defaults to the unqualified class name of the class.

Hibernate goes beyond the JPA specification and provide additional configurations. Some of them are hosted on `@org.hibernate.annotations.Entity`:

- `dynamicInsert` / `dynamicUpdate` (defaults to false): specifies that `INSERT` / `UPDATE` SQL should be generated at runtime and contain only the columns whose values are not null. The `dynamic-update` and `dynamic-insert` settings are not inherited by subclasses. Although these settings can increase performance in some cases, they can actually decrease performance in others.

- `selectBeforeUpdate` (defaults to false): specifies that Hibernate should never perform an SQL `UPDATE` unless it is certain that an object is actually modified. Only when a transient object has been associated with a new session using `update()`, will Hibernate perform an extra SQL `SELECT` to determine if an `UPDATE` is actually required. Use of `select-before-update` will usually decrease performance. It is useful to prevent a database update trigger being called unnecessarily if you reattach a graph of detached instances to a `Session`.

- `polymorphisms` (defaults to `IMPLICIT`): determines whether implicit or explicit query polymorphisms is used. Implicit polymorphisms means that instances of the class will be returned by a query that names any superclass or implemented interface or class, and that instances of any subclass of the class will be returned by a query that names the class itself. Explicit polymorphisms means that class instances will be returned only by queries that explicitly name that class. Queries that name the class will return only instances of subclasses mapped. For most purposes, the default `polymorphisms=IMPLICIT` is appropriate. Explicit polymorphisms is useful when two different classes are mapped to the same table This allows a "lightweight" class that contains a subset of the table columns.

- `persister`: specifies a custom `ClassPersister`. The `persister` attribute lets you customize the persistence strategy used for the class. You can, for example, specify your own subclass of `org.hibernate.persister.EntityPersister`, or you can even provide a completely new implementation of the interface `org.hibernate.persister.ClassPersister` that implements, for example, persistence via stored procedure calls, serialization to flat files or LDAP. See `org.hibernate.test.CustomPersister` for a simple example of "persistence" to a `Hashtable`.

- `optimisticLock` (defaults to `VERSION`): determines the optimistic locking strategy. If you enable `dynamicUpdate`, you will have a choice of optimistic locking strategies:

  - `version`: check the version/timestamp columns

  - `all`: check all columns

  - `dirty`: check the changed columns, allowing some concurrent updates

  - `none`: do not use optimistic locking

  It is strongly recommended that you use version/timestamp columns for optimistic locking with Hibernate. This strategy optimizes performance and correctly handles modifications made to detached instances (i.e. when `Session.merge()` is used).

## Tip

Be sure to import `@javax.persistence.Entity` to mark a class as an entity. It's a common mistake to import `@org.hibernate.annotations.Entity` by accident.

Some entities are not mutable. They cannot be updated or deleted by the application. This allows Hibernate to make some minor performance optimizations.. Use the `@Immutable` annotation.

You can also alter how Hibernate deals with lazy initialization for this class. On `@Proxy`, use `lazy`=false to disable lazy fetching (not recommended). You can also specify an interface to use for lazy initializing proxies (defaults to the class itself): use `proxyClass` on `@Proxy`. Hibernate will initially return proxies (Javassist or CGLIB) that implement the named interface. The persistent object will load when a method of the proxy is invoked. See "Initializing collections and proxies" below.

`@BatchSize` specifies a "batch size" for fetching instances of this class by identifier. Not yet loaded instances are loaded batch-size at a time (default 1).

You can specific an arbitrary SQL WHERE condition to be used when retrieving objects of this class. Use `@Where` for that.

In the same vein, `@Check` lets you define an SQL expression used to generate a multi-row check constraint for automatic schema generation.

There is no difference between a view and a base table for a Hibernate mapping. This is transparent at the database level, although some DBMS do not support views properly, especially with updates. Sometimes you want to use a view, but you cannot create one in the database (i.e. with a legacy schema). In this case, you can map an immutable and read-only entity to a given SQL subselect expression using `@org.hibernate.annotations.Subselect`:

    @Entity
    @Subselect("select item.name, max(bid.amount), count(*) "
            + "from item "
            + "join bid on bid.item_id = item.id "
            + "group by item.name")
    @Synchronize( {"item", "bid"} ) //tables impacted
    public class Summary {
        @Id
        public String getId() { return id; }
        ...
    }

Declare the tables to synchronize this entity with, ensuring that auto-flush happens correctly and that queries against the derived entity do not return stale data. The `<subselect>` is available both as an attribute and a nested mapping element.

We will now explore the same options using the hbm.xml structure. You can declare a persistent class using the `class` element. For example:

    <class
            name="ClassName"
            table="tableName"
            discriminator-value="discriminator_value"
            mutable="true|false"
            schema="owner"
            catalog="catalog"
            proxy="ProxyInterface"
            dynamic-update="true|false"
            dynamic-insert="true|false"
            select-before-update="true|false"
            polymorphism="implicit|explicit"
            where="arbitrary sql where condition"
            persister="PersisterClass"
            batch-size="N"
            optimistic-lock="none|version|dirty|all"
            lazy="true|false"                                  (16)
            entity-name="EntityName"                           (17)
            check="arbitrary sql check condition"              (18)
            rowxml:id="rowid"                                  (19)
            subselect="SQL expression"                         (20)
            abstract="true|false"                              (21)
            node="element-name"
    />

| `name` (optional): the fully qualified Java class name of the persistent class or interface. If this attribute is missing, it is assumed that the mapping is for a non-POJO entity.
| `table` (optional - defaults to the unqualified class name): the name of its database table.
| `discriminator-value` (optional - defaults to the class name): a value that distinguishes individual subclasses that is used for polymorphic behavior. Acceptable values include `null` and `not null`.
| `mutable` (optional - defaults to `true`): specifies that instances of the class are (not) mutable.
| `schema` (optional): overrides the schema name specified by the root `<hibernate-mapping>` element.
| `catalog` (optional): overrides the catalog name specified by the root `<hibernate-mapping>` element.
| `proxy` (optional): specifies an interface to use for lazy initializing proxies. You can specify the name of the class itself.
| `dynamic-update` (optional - defaults to `false`): specifies that `UPDATE` SQL should be generated at runtime and can contain only those columns whose values have changed.
| `dynamic-insert` (optional - defaults to `false`): specifies that `INSERT` SQL should be generated at runtime and contain only the columns whose values are not null.
| `select-before-update` (optional - defaults to `false`): specifies that Hibernate should never perform an SQL `UPDATE` unless it is certain that an object is actually modified. Only when a transient object has been associated with a new session using `update()`, will Hibernate perform an extra SQL `SELECT` to determine if an `UPDATE` is actually required.
| `polymorphisms` (optional - defaults to `implicit`): determines whether implicit or explicit query polymorphisms is used.
| `where` (optional): specifies an arbitrary SQL `WHERE` condition to be used when retrieving objects of this class.
| `persister` (optional): specifies a custom `ClassPersister`.
| `batch-size` (optional - defaults to `1`): specifies a "batch size" for fetching instances of this class by identifier.
| `optimistic-lock` (optional - defaults to `version`): determines the optimistic locking strategy.
(16) | `lazy` (optional): lazy fetching can be disabled by setting `lazy="false"`.
(17) | `entity-name` (optional - defaults to the class name): Hibernate3 allows a class to be mapped multiple times, potentially to different tables. It also allows entity mappings that are represented by Maps or XML at the Java level. In these cases, you should provide an explicit arbitrary name for the entity. See Section 4.4, “Dynamic models” and Chapter 20, XML Mapping for more information.
(18) | `check` (optional): an SQL expression used to generate a multi-row check constraint for automatic schema generation.
(19) | `rowid` (optional): Hibernate can use ROWIDs on databases. On Oracle, for example, Hibernate can use the `rowid` extra column for fast updates once this option has been set to `rowid`. A ROWID is an implementation detail and represents the physical location of a stored tuple.
(20) | `subselect` (optional): maps an immutable and read-only entity to a database subselect. This is useful if you want to have a view instead of a base table. See below for more information.
(21) | `abstract` (optional): is used to mark abstract superclasses in `<union-subclass>` hierarchies.

It is acceptable for the named persistent class to be an interface. You can declare implementing classes of that interface using the `<subclass>` element. You can persist any static inner class. Specify the class name using the standard form i.e. `e.g.Foo$Bar`.

Here is how to do a virtual view (subselect) in XML:

    <class name="Summary">
        <subselect>
            select item.name, max(bid.amount), count(*)
            from item
            join bid on bid.item_id = item.id
            group by item.name
        </subselect>
        <synchronize table="item"/>
        <synchronize table="bid"/>
        <id name="name"/>
        ...
    </class>

The `<subselect>` is available both as an attribute and a nested mapping element.

### 5.1.2. Identifiers

Mapped classes must declare the primary key column of the database table. Most classes will also have a JavaBeans-style property holding the unique identifier of an instance.

Mark the identifier property with `@Id`.

    @Entity
    public class Person {
       @Id Integer getId() { ... }
       ...
    }

In hbm.xml, use the `<id>` element which defines the mapping from that property to the primary key column.

    <id
            name="propertyName"
            type="typename"
            column="column_name"
            unsaved-value="null|any|none|undefined|id_value"
            access="field|property|ClassName">
            node="element-name|@attribute-name|element/@attribute|."

            <generator class="generatorClass"/>
    </id>

| `name` (optional): the name of the identifier property.
| `type` (optional): a name that indicates the Hibernate type.
| `column` (optional - defaults to the property name): the name of the primary key column.
| `unsaved-value` (optional - defaults to a "sensible" value): an identifier property value that indicates an instance is newly instantiated (unsaved), distinguishing it from detached instances that were saved or loaded in a previous session.
| `access` (optional - defaults to `property`): the strategy Hibernate should use for accessing the property value.

If the `name` attribute is missing, it is assumed that the class has no identifier property.

The `unsaved-value` attribute is almost never needed in Hibernate3 and indeed has no corresponding element in annotations.

You can also declare the identifier as a composite identifier. This allows access to legacy data with composite keys. Its use is strongly discouraged for anything else.

#### 5.1.2.1. Composite identifier

You can define a composite primary key through several syntaxes:

- use a component type to represent the identifier and map it as a property in the entity: you then annotated the property as `@EmbeddedId`. The component type has to be `Serializable`.

- map multiple properties as `@Id` properties: the identifier type is then the entity class itself and needs to be `Serializable`. This approach is unfortunately not standard and only supported by Hibernate.

- map multiple properties as `@Id` properties and declare an external class to be the identifier type. This class, which needs to be `Serializable`, is declared on the entity via the `@IdClass` annotation. The identifier type must contain the same properties as the identifier properties of the entity: each property name must be the same, its type must be the same as well if the entity property is of a basic type, its type must be the type of the primary key of the associated entity if the entity property is an association (either a `@OneToOne` or a `@ManyToOne`).

As you can see the last case is far from obvious. It has been inherited from the dark ages of EJB 2 for backward compatibilities and we recommend you not to use it (for simplicity sake).

Let's explore all three cases using examples.

##### 5.1.2.1.1. id as a property using a component type

Here is a simple example of `@EmbeddedId`.

    @Entity

    class User {

       @EmbeddedId

       @AttributeOverride(name="firstName", column=@Column(name="fld_firstname")

       UserId id;

       Integer age;

    }

    @Embeddable

    class UserId implements Serializable {

       String firstName;

       String lastName;

    }

You can notice that the `UserId` class is serializable. To override the column mapping, use `@AttributeOverride`.

An embedded id can itself contains the primary key of an associated entity.

    @Entity

    class Customer {

       @EmbeddedId CustomerId id;

       boolean preferredCustomer;

       @MapsId("userId")

       @JoinColumns({

          @JoinColumn(name="userfirstname_fk", referencedColumnName="firstName"),

          @JoinColumn(name="userlastname_fk", referencedColumnName="lastName")

       })

       @OneToOne User user;

    }

    @Embeddable

    class CustomerId implements Serializable {

       UserId userId;

       String customerNumber;

       //implements equals and hashCode

    }

    @Entity

    class User {

       @EmbeddedId UserId id;

       Integer age;

    }

    @Embeddable

    class UserId implements Serializable {

       String firstName;

       String lastName;

       //implements equals and hashCode

    }

In the embedded id object, the association is represented as the identifier of the associated entity. But you can link its value to a regular association in the entity via the `@MapsId` annotation. The `@MapsId` value correspond to the property name of the embedded id object containing the associated entity's identifier. In the database, it means that the `Customer.user` and the `CustomerId.userId` properties share the same underlying column (`user_fk` in this case).

## Tip

The component type used as identifier must implement `equals()` and `hashCode()`.

In practice, your code only sets the `Customer.user` property and the user id value is copied by Hibernate into the `CustomerId.userId` property.

## Warning

The id value can be copied as late as flush time, don't rely on it until after flush time.

While not supported in JPA, Hibernate lets you place your association directly in the embedded id component (instead of having to use the `@MapsId` annotation).

    @Entity

    class Customer {

       @EmbeddedId CustomerId id;

       boolean preferredCustomer;

    }

    @Embeddable

    class CustomerId implements Serializable {

       @OneToOne

       @JoinColumns({

          @JoinColumn(name="userfirstname_fk", referencedColumnName="firstName"),

          @JoinColumn(name="userlastname_fk", referencedColumnName="lastName")

       })

       User user;

       String customerNumber;

       //implements equals and hashCode

    }

    @Entity

    class User {

       @EmbeddedId UserId id;

       Integer age;

    }

    @Embeddable

    class UserId implements Serializable {

       String firstName;

       String lastName;

       //implements equals and hashCode

    }

Let's now rewrite these examples using the hbm.xml syntax.

    <composite-id
            name="propertyName"
            class="ClassName"
            mapped="true|false"
            access="field|property|ClassName"
            node="element-name|.">

            <key-property name="propertyName" type="typename" column="column_name"/>
            <key-many-to-one name="propertyName" class="ClassName" column="column_name"/>
            ......
    </composite-id>

First a simple example:

    <class name="User">
       <composite-id name="id" class="UserId">
          <key-property name="firstName" column="fld_firstname"/>
          <key-property name="lastName"/>
       </composite-id>
    </class>

Then an example showing how an association can be mapped.

    <class name="Customer">
       <composite-id name="id" class="CustomerId">
          <key-property name="firstName" column="userfirstname_fk"/>
          <key-property name="lastName" column="userfirstname_fk"/>
          <key-property name="customerNumber"/>
       </composite-id>

       <property name="preferredCustomer"/>

       <many-to-one name="user">
          <column name="userfirstname_fk" updatable="false" insertable="false"/>
          <column name="userlastname_fk" updatable="false" insertable="false"/>
       </many-to-one>
    </class>

    <class name="User">
       <composite-id name="id" class="UserId">
          <key-property name="firstName"/>
          <key-property name="lastName"/>
       </composite-id>

       <property name="age"/>
    </class>

Notice a few things in the previous example:

- the order of the properties (and column) matters. It must be the same between the association and the primary key of the associated entity

- the many to one uses the same columns as the primary key and thus must be marked as read only (`insertable` and `updatable` to false).

- unlike with `@MapsId`, the id value of the associated entity is not transparently copied, check the `foreign` id generator for more information.

The last example shows how to map association directly in the embedded id component.

    <class name="Customer">
       <composite-id name="id" class="CustomerId">
          <key-many-to-one name="user">
             <column name="userfirstname_fk"/>
             <column name="userlastname_fk"/>
          </key-many-to-one>
          <key-property name="customerNumber"/>
       </composite-id>

       <property name="preferredCustomer"/>
    </class>

    <class name="User">
       <composite-id name="id" class="UserId">
          <key-property name="firstName"/>
          <key-property name="lastName"/>
       </composite-id>

       <property name="age"/>
    </class>

This is the recommended approach to map composite identifier. The following options should not be considered unless some constraint are present.

##### 5.1.2.1.2. Multiple id properties without identifier type

Another, arguably more natural, approach is to place `@Id` on multiple properties of your entity. This approach is only supported by Hibernate (not JPA compliant) but does not require an extra embeddable component.

    @Entity

    class Customer implements Serializable {

       @Id @OneToOne

       @JoinColumns({

          @JoinColumn(name="userfirstname_fk", referencedColumnName="firstName"),

          @JoinColumn(name="userlastname_fk", referencedColumnName="lastName")

       })

       User user;

       @Id String customerNumber;

       boolean preferredCustomer;

       //implements equals and hashCode

    }

    @Entity

    class User {

       @EmbeddedId UserId id;

       Integer age;

    }

    @Embeddable

    class UserId implements Serializable {

       String firstName;

       String lastName;

       //implements equals and hashCode

    }

In this case `Customer` is its own identifier representation: it must implement `Serializable` and must implement `equals()` and `hashCode()`.

In hbm.xml, the same mapping is:

    <class name="Customer">
       <composite-id>
          <key-many-to-one name="user">
             <column name="userfirstname_fk"/>
             <column name="userlastname_fk"/>
          </key-many-to-one>
          <key-property name="customerNumber"/>
       </composite-id>

       <property name="preferredCustomer"/>
    </class>

    <class name="User">
       <composite-id name="id" class="UserId">
          <key-property name="firstName"/>
          <key-property name="lastName"/>
       </composite-id>

       <property name="age"/>
    </class>

##### 5.1.2.1.3. Multiple id properties with with a dedicated identifier type

`@IdClass` on an entity points to the class (component) representing the identifier of the class. The properties marked `@Id` on the entity must have their corresponding property on the `@IdClass`. The return type of search twin property must be either identical for basic properties or must correspond to the identifier class of the associated entity for an association.

## Warning

This approach is inherited from the EJB 2 days and we recommend against its use. But, after all it's your application and Hibernate supports it.

    @Entity

    @IdClass(CustomerId.class)

    class Customer implements Serializable {

       @Id @OneToOne

       @JoinColumns({

          @JoinColumn(name="userfirstname_fk", referencedColumnName="firstName"),

          @JoinColumn(name="userlastname_fk", referencedColumnName="lastName")

       })

       User user;

       @Id String customerNumber;

       boolean preferredCustomer;

    }

    class CustomerId implements Serializable {

       UserId user;

       String customerNumber;

       //implements equals and hashCode

    }

    @Entity

    class User {

       @EmbeddedId UserId id;

       Integer age;

       //implements equals and hashCode

    }

    @Embeddable

    class UserId implements Serializable {

       String firstName;

       String lastName;

       //implements equals and hashCode

    }

`Customer` and `CustomerId` do have the same properties `customerNumber` as well as `user`. `CustomerId` must be `Serializable` and implement `equals()` and `hashCode()`.

While not JPA standard, Hibernate let's you declare the vanilla associated property in the `@IdClass`.

    @Entity

    @IdClass(CustomerId.class)

    class Customer implements Serializable {

       @Id @OneToOne

       @JoinColumns({

          @JoinColumn(name="userfirstname_fk", referencedColumnName="firstName"),

          @JoinColumn(name="userlastname_fk", referencedColumnName="lastName")

       })

       User user;

       @Id String customerNumber;

       boolean preferredCustomer;

    }

    class CustomerId implements Serializable {

       @OneToOne User user;

       String customerNumber;

       //implements equals and hashCode

    }

    @Entity

    class User {

       @EmbeddedId UserId id;

       Integer age;

       //implements equals and hashCode

    }

    @Embeddable

    class UserId implements Serializable {

      String firstName;

      String lastName;

    }

This feature is of limited interest though as you are likely to have chosen the `@IdClass` approach to stay JPA compliant or you have a quite twisted mind.

Here are the equivalent on hbm.xml files:

    <class name="Customer">
       <composite-id class="CustomerId" mapped="true">
          <key-many-to-one name="user">
             <column name="userfirstname_fk"/>
             <column name="userlastname_fk"/>
          </key-many-to-one>
          <key-property name="customerNumber"/>
       </composite-id>

       <property name="preferredCustomer"/>
    </class>

    <class name="User">
       <composite-id name="id" class="UserId">
          <key-property name="firstName"/>
          <key-property name="lastName"/>
       </composite-id>

       <property name="age"/>
    </class>

#### 5.1.2.2. Identifier generator

Hibernate can generate and populate identifier values for you automatically. This is the recommended approach over "business" or "natural" id (especially composite ids).

Hibernate offers various generation strategies, let's explore the most common ones first that happens to be standardized by JPA:

- IDENTITY: supports identity columns in DB2, MySQL, MS SQL Server, Sybase and HypersonicSQL. The returned identifier is of type `long`, `short` or `int`.

- SEQUENCE (called `seqhilo` in Hibernate): uses a hi/lo algorithm to efficiently generate identifiers of type `long`, `short` or `int`, given a named database sequence.

- TABLE (called `MultipleHiLoPerTableGenerator` in Hibernate) : uses a hi/lo algorithm to efficiently generate identifiers of type `long`, `short` or `int`, given a table and column as a source of hi values. The hi/lo algorithm generates identifiers that are unique only for a particular database.

- AUTO: selects `IDENTITY`, `SEQUENCE` or `TABLE` depending upon the capabilities of the underlying database.

## Important

We recommend all new projects to use the new enhanced identifier generators. They are deactivated by default for entities using annotations but can be activated using `hibernate.id.new_generator_mappings=true`. These new generators are more efficient and closer to the JPA 2 specification semantic.

However they are not backward compatible with existing Hibernate based application (if a sequence or a table is used for id generation). See XXXXXXX ??? for more information on how to activate them.

To mark an id property as generated, use the `@GeneratedValue` annotation. You can specify the strategy used (default to `AUTO`) by setting `strategy`.

    @Entity
    public class Customer {
       @Id @GeneratedValue
       Integer getId() { ... };
    }

    @Entity
    public class Invoice {
       @Id @GeneratedValue(strategy=GenerationType.IDENTITY)
       Integer getId() { ... };
    }

`SEQUENCE` and `TABLE` require additional configurations that you can set using `@SequenceGenerator` and `@TableGenerator`:

- `name`: name of the generator

- `table` / `sequenceName`: name of the table or the sequence (defaulting respectively to `hibernate_sequences` and `hibernate_sequence`)

- `catalog` / `schema`:

- `initialValue`: the value from which the id is to start generating

- `allocationSize`: the amount to increment by when allocating id numbers from the generator

In addition, the `TABLE` strategy also let you customize:

- `pkColumnName`: the column name containing the entity identifier

- `valueColumnName`: the column name containing the identifier value

- `pkColumnValue`: the entity identifier

- `uniqueConstraints`: any potential column constraint on the table containing the ids

To link a table or sequence generator definition with an actual generated property, use the same name in both the definition `name` and the generator value `generator` as shown below.

    @Id

    @GeneratedValue(

        strategy=GenerationType.SEQUENCE,

        generator="SEQ_GEN")

    @javax.persistence.SequenceGenerator(

        name="SEQ_GEN",

        sequenceName="my_sequence",

        allocationSize=20

    )

    public Integer getId() { ... }

The scope of a generator definition can be the application or the class. Class-defined generators are not visible outside the class and can override application level generators. Application level generators are defined in JPA's XML deployment descriptors (see XXXXXX ???):

    <table-generator name="EMP_GEN"

                table="GENERATOR_TABLE"

                pk-column-name="key"

                value-column-name="hi"

                pk-column-value="EMP"

                allocation-size="20"/>

    //and the annotation equivalent

    @javax.persistence.TableGenerator(

        name="EMP_GEN",

        table="GENERATOR_TABLE",

        pkColumnName = "key",

        valueColumnName = "hi"

        pkColumnValue="EMP",

        allocationSize=20

    )

    <sequence-generator name="SEQ_GEN"

        sequence-name="my_sequence"

        allocation-size="20"/>

    //and the annotation equivalent

    @javax.persistence.SequenceGenerator(

        name="SEQ_GEN",

        sequenceName="my_sequence",

        allocationSize=20

    )

If a JPA XML descriptor (like `META-INF/orm.xml`) is used to define the generators, `EMP_GEN` and `SEQ_GEN` are application level generators.

## Note

Package level definition is not supported by the JPA specification. However, you can use the `@GenericGenerator` at the package level (see ???).

These are the four standard JPA generators. Hibernate goes beyond that and provide additional generators or additional options as we will see below. You can also write your own custom identifier generator by implementing `org.hibernate.id.IdentifierGenerator`.

To define a custom generator, use the `@GenericGenerator` annotation (and its plural counter part `@GenericGenerators`) that describes the class of the identifier generator or its short cut name (as described below) and a list of key/value parameters. When using `@GenericGenerator` and assigning it via `@GeneratedValue.generator`, the `@GeneratedValue.strategy` is ignored: leave it blank.

    @Id @GeneratedValue(generator="system-uuid")

    @GenericGenerator(name="system-uuid", strategy = "uuid")

    public String getId() {

    @Id @GeneratedValue(generator="trigger-generated")

    @GenericGenerator(

        name="trigger-generated",

        strategy = "select",

        parameters = @Parameter(name="key", value = "socialSecurityNumber")

    )

    public String getId() {

The hbm.xml approach uses the optional `<generator>` child element inside `<id>`. If any parameters are required to configure or initialize the generator instance, they are passed using the `<param>` element.

    <id name="id" type="long" column="cat_id">
            <generator class="org.hibernate.id.TableHiLoGenerator">
                    <param name="table">uid_table</param>
                    <param name="column">next_hi_value_column</param>
            </generator>
    </id>

##### 5.1.2.2.1. Various additional generators

All generators implement the interface `org.hibernate.id.IdentifierGenerator`. This is a very simple interface. Some applications can choose to provide their own specialized implementations, however, Hibernate provides a range of built-in implementations. The shortcut names for the built-in generators are as follows:

`increment`

  generates identifiers of type `long`, `short` or `int` that are unique only when no other process is inserting data into the same table. Do not use in a cluster.

`identity`

  supports identity columns in DB2, MySQL, MS SQL Server, Sybase and HypersonicSQL. The returned identifier is of type `long`, `short` or `int`.

`sequence`

  uses a sequence in DB2, PostgreSQL, Oracle, SAP DB, McKoi or a generator in Interbase. The returned identifier is of type `long`, `short` or `int`

`hilo`

  uses a hi/lo algorithm to efficiently generate identifiers of type `long`, `short` or `int`, given a table and column (by default `hibernate_unique_key` and `next_hi` respectively) as a source of hi values. The hi/lo algorithm generates identifiers that are unique only for a particular database.

`seqhilo`

  uses a hi/lo algorithm to efficiently generate identifiers of type `long`, `short` or `int`, given a named database sequence.

`uuid`

  Generates a 128-bit UUID based on a custom algorithm. The value generated is represented as a string of 32 hexidecimal digits. Users can also configure it to use a separator (config parameter "separator") which separates the hexidecimal digits into 8{sep}8{sep}4{sep}8{sep}4. Note specifically that this is different than the IETF RFC 4122 representation of 8-4-4-4-12. If you need RFC 4122 compliant UUIDs, consider using "uuid2" generator discussed below.

`uuid2`

  Generates a IETF RFC 4122 compliant (variant 2) 128-bit UUID. The exact "version" (the RFC term) generated depends on the pluggable "generation strategy" used (see below). Capable of generating values as `java.util.UUID`, `java.lang.String` or as a byte array of length 16 (`byte[16]`). The "generation strategy" is defined by the interface `org.hibernate.id.UUIDGenerationStrategy`. The generator defines 2 configuration parameters for defining which generation strategy to use:

  `uuid_gen_strategy_class`

    Names the UUIDGenerationStrategy class to use

  `uuid_gen_strategy`

    Names the UUIDGenerationStrategy instance to use

  Out of the box, comes with the following strategies:

  - `org.hibernate.id.uuid.StandardRandomStrategy` (the default) - generates "version 3" (aka, "random") UUID values via the `randomUUID` method of `java.util.UUID`

  - `org.hibernate.id.uuid.CustomVersionOneStrategy` - generates "version 1" UUID values, using IP address since mac address not available. If you need mac address to be used, consider leveraging one of the existing third party UUID generators which sniff out mac address and integrating it via the `org.hibernate.id.UUIDGenerationStrategy` contract. Two such libraries known at time of this writing include http://johannburkard.de/software/uuid/ and http://commons.apache.org/sandbox/id/uuid.html

`guid`

  uses a database-generated GUID string on MS SQL Server and MySQL.

`native`

  selects `identity`, `sequence` or `hilo` depending upon the capabilities of the underlying database.

`assigned`

  lets the application assign an identifier to the object before `save()` is called. This is the default strategy if no `<generator>` element is specified.

`select`

  retrieves a primary key, assigned by a database trigger, by selecting the row by some unique key and retrieving the primary key value.

`foreign`

  uses the identifier of another associated object. It is usually used in conjunction with a `<one-to-one>` primary key association.

`sequence-identity`

  a specialized sequence generation strategy that utilizes a database sequence for the actual value generation, but combines this with JDBC3 getGeneratedKeys to return the generated identifier value as part of the insert statement execution. This strategy is only supported on Oracle 10g drivers targeted for JDK 1.4. Comments on these insert statements are disabled due to a bug in the Oracle drivers.

##### 5.1.2.2.2. Hi/lo algorithm

The `hilo` and `seqhilo` generators provide two alternate implementations of the hi/lo algorithm. The first implementation requires a "special" database table to hold the next available "hi" value. Where supported, the second uses an Oracle-style sequence.

    <id name="id" type="long" column="cat_id">
            <generator class="hilo">
                    <param name="table">hi_value</param>
                    <param name="column">next_value</param>
                    <param name="max_lo">100</param>
            </generator>
    </id>

    <id name="id" type="long" column="cat_id">
            <generator class="seqhilo">
                    <param name="sequence">hi_value</param>
                    <param name="max_lo">100</param>
            </generator>
    </id>

Unfortunately, you cannot use `hilo` when supplying your own `Connection` to Hibernate. When Hibernate uses an application server datasource to obtain connections enlisted with JTA, you must configure the `hibernate.transaction.manager_lookup_class`.

##### 5.1.2.2.3. UUID algorithm

The UUID contains: IP address, startup time of the JVM that is accurate to a quarter second, system time and a counter value that is unique within the JVM. It is not possible to obtain a MAC address or memory address from Java code, so this is the best option without using JNI.

##### 5.1.2.2.4. Identity columns and sequences

For databases that support identity columns (DB2, MySQL, Sybase, MS SQL), you can use `identity` key generation. For databases that support sequences (DB2, Oracle, PostgreSQL, Interbase, McKoi, SAP DB) you can use `sequence` style key generation. Both of these strategies require two SQL queries to insert a new object. For example:

    <id name="id" type="long" column="person_id">
            <generator class="sequence">
                    <param name="sequence">person_id_sequence</param>
            </generator>
    </id>

    <id name="id" type="long" column="person_id" unsaved-value="0">
            <generator class="identity"/>
    </id>

For cross-platform development, the `native` strategy will, depending on the capabilities of the underlying database, choose from the `identity`, `sequence` and `hilo` strategies.

##### 5.1.2.2.5. Assigned identifiers

If you want the application to assign identifiers, as opposed to having Hibernate generate them, you can use the `assigned` generator. This special generator uses the identifier value already assigned to the object's identifier property. The generator is used when the primary key is a natural key instead of a surrogate key. This is the default behavior if you do not specify `@GeneratedValue` nor `<generator>` elements.

The `assigned` generator makes Hibernate use `unsaved-value="undefined"`. This forces Hibernate to go to the database to determine if an instance is transient or detached, unless there is a version or timestamp property, or you define `Interceptor.isUnsaved()`.

##### 5.1.2.2.6. Primary keys assigned by triggers

Hibernate does not generate DDL with triggers. It is for legacy schemas only.

    <id name="id" type="long" column="person_id">
            <generator class="select">
                    <param name="key">socialSecurityNumber</param>
            </generator>
    </id>

In the above example, there is a unique valued property named `socialSecurityNumber`. It is defined by the class, as a natural key and a surrogate key named `person_id`, whose value is generated by a trigger.

##### 5.1.2.2.7. Identity copy (foreign generator)

Finally, you can ask Hibernate to copy the identifier from another associated entity. In the Hibernate jargon, it is known as a foreign generator but the JPA mapping reads better and is encouraged.

    @Entity

    class MedicalHistory implements Serializable {

      @Id @OneToOne

      @JoinColumn(name = "person_id")

      Person patient;

    }

    @Entity

    public class Person implements Serializable {

      @Id @GeneratedValue Integer id;

    }

Or alternatively

    @Entity

    class MedicalHistory implements Serializable {

      @Id Integer id;

      @MapsId @OneToOne

      @JoinColumn(name = "patient_id")

      Person patient;

    }

    @Entity

    class Person {

      @Id @GeneratedValue Integer id;

    }

In hbm.xml use the following approach:

    <class name="MedicalHistory">
       <id name="id">
          <generator class="foreign">
             <param name="property">patient</param>
          </generator>
       </id>
       <one-to-one name="patient" class="Person" constrained="true"/>
    </class>

#### 5.1.2.3. Enhanced identifier generators

Starting with release 3.2.3, there are 2 new generators which represent a re-thinking of 2 different aspects of identifier generation. The first aspect is database portability; the second is optimization Optimization means that you do not have to query the database for every request for a new identifier value. These two new generators are intended to take the place of some of the named generators described above, starting in 3.3.x. However, they are included in the current releases and can be referenced by FQN.

The first of these new generators is `org.hibernate.id.enhanced.SequenceStyleGenerator` which is intended, firstly, as a replacement for the `sequence` generator and, secondly, as a better portability generator than `native`. This is because `native` generally chooses between `identity` and `sequence` which have largely different semantics that can cause subtle issues in applications eyeing portability. `org.hibernate.id.enhanced.SequenceStyleGenerator`, however, achieves portability in a different manner. It chooses between a table or a sequence in the database to store its incrementing values, depending on the capabilities of the dialect being used. The difference between this and `native` is that table-based and sequence-based storage have the same exact semantic. In fact, sequences are exactly what Hibernate tries to emulate with its table-based generators. This generator has a number of configuration parameters:

- `sequence_name` (optional, defaults to `hibernate_sequence`): the name of the sequence or table to be used.

- `initial_value` (optional, defaults to `1`): the initial value to be retrieved from the sequence/table. In sequence creation terms, this is analogous to the clause typically named "STARTS WITH".

- `increment_size` (optional - defaults to `1`): the value by which subsequent calls to the sequence/table should differ. In sequence creation terms, this is analogous to the clause typically named "INCREMENT BY".

- `force_table_use` (optional - defaults to `false`): should we force the use of a table as the backing structure even though the dialect might support sequence?

- `value_column` (optional - defaults to `next_val`): only relevant for table structures, it is the name of the column on the table which is used to hold the value.

- `optimizer` (optional - defaults to `none`): See Section 5.1.2.3.1, “Identifier generator optimization”

The second of these new generators is `org.hibernate.id.enhanced.TableGenerator`, which is intended, firstly, as a replacement for the `table` generator, even though it actually functions much more like `org.hibernate.id.MultipleHiLoPerTableGenerator`, and secondly, as a re-implementation of `org.hibernate.id.MultipleHiLoPerTableGenerator` that utilizes the notion of pluggable optimizers. Essentially this generator defines a table capable of holding a number of different increment values simultaneously by using multiple distinctly keyed rows. This generator has a number of configuration parameters:

- `table_name` (optional - defaults to `hibernate_sequences`): the name of the table to be used.

- `value_column_name` (optional - defaults to `next_val`): the name of the column on the table that is used to hold the value.

- `segment_column_name` (optional - defaults to `sequence_name`): the name of the column on the table that is used to hold the "segment key". This is the value which identifies which increment value to use.

- `segment_value` (optional - defaults to `default`): The "segment key" value for the segment from which we want to pull increment values for this generator.

- `segment_value_length` (optional - defaults to `255`): Used for schema generation; the column size to create this segment key column.

- `initial_value` (optional - defaults to `1`): The initial value to be retrieved from the table.

- `increment_size` (optional - defaults to `1`): The value by which subsequent calls to the table should differ.

- `optimizer` (optional - defaults to `??`): See Section 5.1.2.3.1, “Identifier generator optimization”.

##### 5.1.2.3.1. Identifier generator optimization

For identifier generators that store values in the database, it is inefficient for them to hit the database on each and every call to generate a new identifier value. Instead, you can group a bunch of them in memory and only hit the database when you have exhausted your in-memory value group. This is the role of the pluggable optimizers. Currently only the two enhanced generators (Section 5.1.2.3, “Enhanced identifier generators” support this operation.

- `none` (generally this is the default if no optimizer was specified): this will not perform any optimizations and hit the database for each and every request.

- `hilo`: applies a hi/lo algorithm around the database retrieved values. The values from the database for this optimizer are expected to be sequential. The values retrieved from the database structure for this optimizer indicates the "group number". The `increment_size` is multiplied by that value in memory to define a group "hi value".

- `pooled`: as with the case of `hilo`, this optimizer attempts to minimize the number of hits to the database. Here, however, we simply store the starting value for the "next group" into the database structure rather than a sequential value in combination with an in-memory grouping algorithm. Here, `increment_size` refers to the values coming from the database.

#### 5.1.2.4. Partial identifier generation

Hibernate supports the automatic generation of some of the identifier properties. Simply use the `@GeneratedValue` annotation on one or several id properties.

## Warning

The Hibernate team has always felt such a construct as fundamentally wrong. Try hard to fix your data model before using this feature.

    @Entity

    public class CustomerInventory implements Serializable {

      @Id

      @TableGenerator(name = "inventory",

        table = "U_SEQUENCES",

        pkColumnName = "S_ID",

        valueColumnName = "S_NEXTNUM",

        pkColumnValue = "inventory",

        allocationSize = 1000)

      @GeneratedValue(strategy = GenerationType.TABLE, generator = "inventory")

      Integer id;

      @Id @ManyToOne(cascade = CascadeType.MERGE)

      Customer customer;

    }

    @Entity

    public class Customer implements Serializable {

       @Id

       private int id;

    }

You can also generate properties inside an `@EmbeddedId` class.

### 5.1.3. Optimistic locking properties (optional)

When using long transactions or conversations that span several database transactions, it is useful to store versioning data to ensure that if the same entity is updated by two conversations, the last to commit changes will be informed and not override the other conversation's work. It guarantees some isolation while still allowing for good scalability and works particularly well in read-often write-sometimes situations.

You can use two approaches: a dedicated version number or a timestamp.

A version or timestamp property should never be null for a detached instance. Hibernate will detect any instance with a null version or timestamp as transient, irrespective of what other `unsaved-value` strategies are specified. Declaring a nullable version or timestamp property is an easy way to avoid problems with transitive reattachment in Hibernate. It is especially useful for people using assigned identifiers or composite keys.

#### 5.1.3.1. Version number

You can add optimistic locking capability to an entity using the `@Version` annotation:

    @Entity

    public class Flight implements Serializable {

    ...

        @Version

        @Column(name="OPTLOCK")

        public Integer getVersion() { ... }

    }

The version property will be mapped to the `OPTLOCK` column, and the entity manager will use it to detect conflicting updates (preventing lost updates you might otherwise see with the last-commit-wins strategy).

The version column may be a numeric. Hibernate supports any kind of type provided that you define and implement the appropriate `UserVersionType`.

The application must not alter the version number set up by Hibernate in any way. To artificially increase the version number, check in Hibernate Entity Manager's reference documentation `LockModeType.OPTIMISTIC_FORCE_INCREMENT` or `LockModeType.PESSIMISTIC_FORCE_INCREMENT`.

If the version number is generated by the database (via a trigger for example), make sure to use `@org.hibernate.annotations.Generated(GenerationTime.ALWAYS).`

To declare a version property in hbm.xml, use:

    <version
            column="version_column"
            name="propertyName"
            type="typename"
            access="field|property|ClassName"
            unsaved-value="null|negative|undefined"
            generated="never|always"
            insert="true|false"
            node="element-name|@attribute-name|element/@attribute|."
    />

| `column` (optional - defaults to the property name): the name of the column holding the version number.
| `name`: the name of a property of the persistent class.
| `type` (optional - defaults to `integer`): the type of the version number.
| `access` (optional - defaults to `property`): the strategy Hibernate uses to access the property value.
| `unsaved-value` (optional - defaults to `undefined`): a version property value that indicates that an instance is newly instantiated (unsaved), distinguishing it from detached instances that were saved or loaded in a previous session. `Undefined` specifies that the identifier property value should be used.
| `generated` (optional - defaults to `never`): specifies that this version property value is generated by the database. See the discussion of generated properties for more information.
| `insert` (optional - defaults to `true`): specifies whether the version column should be included in SQL insert statements. It can be set to `false` if the database column is defined with a default value of `0`.

#### 5.1.3.2. Timestamp

Alternatively, you can use a timestamp. Timestamps are a less safe implementation of optimistic locking. However, sometimes an application might use the timestamps in other ways as well.

Simply mark a property of type `Date` or `Calendar` as `@Version`.

    @Entity

    public class Flight implements Serializable {

    ...

        @Version

        public Date getLastUpdate() { ... }

    }

When using timestamp versioning you can tell Hibernate where to retrieve the timestamp value from - database or JVM - by optionally adding the `@org.hibernate.annotations.Source` annotation to the property. Possible values for the value attribute of the annotation are `org.hibernate.annotations.SourceType.VM` and `org.hibernate.annotations.SourceType.DB`. The default is `SourceType.DB` which is also used in case there is no `@Source` annotation at all.

Like in the case of version numbers, the timestamp can also be generated by the database instead of Hibernate. To do that, use `@org.hibernate.annotations.Generated(GenerationTime.ALWAYS).`

In hbm.xml, use the `<timestamp>` element:

    <timestamp
            column="timestamp_column"
            name="propertyName"
            access="field|property|ClassName"
            unsaved-value="null|undefined"
            source="vm|db"
            generated="never|always"
            node="element-name|@attribute-name|element/@attribute|."
    />

| `column` (optional - defaults to the property name): the name of a column holding the timestamp.
| `name`: the name of a JavaBeans style property of Java type `Date` or `Timestamp` of the persistent class.
| `access` (optional - defaults to `property`): the strategy Hibernate uses for accessing the property value.
| `unsaved-value` (optional - defaults to `null`): a version property value that indicates that an instance is newly instantiated (unsaved), distinguishing it from detached instances that were saved or loaded in a previous session. `Undefined` specifies that the identifier property value should be used.
| `source` (optional - defaults to `vm`): Where should Hibernate retrieve the timestamp value from? From the database, or from the current JVM? Database-based timestamps incur an overhead because Hibernate must hit the database in order to determine the "next value". It is safer to use in clustered environments. Not all `Dialects` are known to support the retrieval of the database's current timestamp. Others may also be unsafe for usage in locking due to lack of precision (Oracle 8, for example).
| `generated` (optional - defaults to `never`): specifies that this timestamp property value is actually generated by the database. See the discussion of generated properties for more information.

## Note

`<Timestamp>` is equivalent to `<version type="timestamp">`. And `<timestamp source="db">` is equivalent to `<version type="dbtimestamp">`

### 5.1.4. Property

You need to decide which property needs to be made persistent in a given entity. This differs slightly between the annotation driven metadata and the hbm.xml files.

#### 5.1.4.1. Property mapping with annotations

In the annotations world, every non static non transient property (field or method depending on the access type) of an entity is considered persistent, unless you annotate it as `@Transient`. Not having an annotation for your property is equivalent to the appropriate `@Basic` annotation.

The `@Basic` annotation allows you to declare the fetching strategy for a property. If set to `LAZY`, specifies that this property should be fetched lazily when the instance variable is first accessed. It requires build-time bytecode instrumentation, if your classes are not instrumented, property level lazy loading is silently ignored. The default is `EAGER`. You can also mark a property as not optional thanks to the `@Basic.optional` attribute. This will ensure that the underlying column are not nullable (if possible). Note that a better approach is to use the `@NotNull` annotation of the Bean Validation specification.

Let's look at a few examples:

    public transient int counter; //transient property

    private String firstname; //persistent property

    @Transient

    String getLengthInMeter() { ... } //transient property

    String getName() {... } // persistent property

    @Basic

    int getLength() { ... } // persistent property

    @Basic(fetch = FetchType.LAZY)

    String getDetailedComment() { ... } // persistent property

    @Temporal(TemporalType.TIME)

    java.util.Date getDepartureTime() { ... } // persistent property

    @Enumerated(EnumType.STRING)

    Starred getNote() { ... } //enum persisted as String in database

`counter`, a transient field, and `lengthInMeter`, a method annotated as `@Transient`, and will be ignored by the Hibernate. `name`, `length`, and `firstname` properties are mapped persistent and eagerly fetched (the default for simple properties). The `detailedComment` property value will be lazily fetched from the database once a lazy property of the entity is accessed for the first time. Usually you don't need to lazy simple properties (not to be confused with lazy association fetching). The recommended alternative is to use the projection capability of JP-QL (Java Persistence Query Language) or Criteria queries.

JPA support property mapping of all basic types supported by Hibernate (all basic Java types , their respective wrappers and serializable classes). Hibernate Annotations supports out of the box enum type mapping either into a ordinal column (saving the enum ordinal) or a string based column (saving the enum string representation): the persistence representation, defaulted to ordinal, can be overridden through the `@Enumerated` annotation as shown in the `note` property example.

In plain Java APIs, the temporal precision of time is not defined. When dealing with temporal data you might want to describe the expected precision in database. Temporal data can have `DATE`, `TIME`, or `TIMESTAMP` precision (ie the actual date, only the time, or both). Use the `@Temporal` annotation to fine tune that.

`@Lob` indicates that the property should be persisted in a Blob or a Clob depending on the property type: `java.sql.Clob`, `Character[]`, `char[]` and java.lang.`String` will be persisted in a Clob. `java.sql.Blob`, `Byte[]`, `byte[]` and `Serializable` type will be persisted in a Blob.

    @Lob

    public String getFullText() {

        return fullText;

    }

    @Lob

    public byte[] getFullCode() {

        return fullCode;

    }

If the property type implements `java.io.Serializable` and is not a basic type, and if the property is not annotated with `@Lob`, then the Hibernate `serializable` type is used.

##### 5.1.4.1.1. Type

You can also manually specify a type using the `@org.hibernate.annotations.Type` and some parameters if needed. `@Type.type` could be:

1. The name of a Hibernate basic type: `integer, string, character, date, timestamp, float, binary, serializable, object, blob` etc.

2. The name of a Java class with a default basic type: `int, float, char, java.lang.String, java.util.Date, java.lang.Integer, java.sql.Clob` etc.

3. The name of a serializable Java class.

4. The class name of a custom type: `com.illflow.type.MyCustomType` etc.

If you do not specify a type, Hibernate will use reflection upon the named property and guess the correct Hibernate type. Hibernate will attempt to interpret the name of the return class of the property getter using, in order, rules 2, 3, and 4.

`@org.hibernate.annotations.TypeDef` and `@org.hibernate.annotations.TypeDefs` allows you to declare type definitions. These annotations can be placed at the class or package level. Note that these definitions are global for the session factory (even when defined at the class level). If the type is used on a single entity, you can place the definition on the entity itself. Otherwise, it is recommended to place the definition at the package level. In the example below, when Hibernate encounters a property of class `PhoneNumer`, it delegates the persistence strategy to the custom mapping type `PhoneNumberType`. However, properties belonging to other classes, too, can delegate their persistence strategy to `PhoneNumberType`, by explicitly using the `@Type` annotation.

## Note

Package level annotations are placed in a file named `package-info.java` in the appropriate package. Place your annotations before the package declaration.

    @TypeDef(

       name = "phoneNumber",

       defaultForType = PhoneNumber.class,

       typeClass = PhoneNumberType.class

    )

    @Entity

    public class ContactDetails {

       [...]

       private PhoneNumber localPhoneNumber;

       @Type(type="phoneNumber")

       private OverseasPhoneNumber overseasPhoneNumber;

       [...]

    }

The following example shows the usage of the `parameters` attribute to customize the TypeDef.

    //in org/hibernate/test/annotations/entity/package-info.java

    @TypeDefs(

        {

        @TypeDef(

            name="caster",

            typeClass = CasterStringType.class,

            parameters = {

                @Parameter(name="cast", value="lower")

            }

        )

        }

    )

    package org.hibernate.test.annotations.entity;

    //in org/hibernate/test/annotations/entity/Forest.java

    public class Forest {

        @Type(type="caster")

        public String getSmallText() {

        ...

    }

When using composite user type, you will have to express column definitions. The `@Columns` has been introduced for that purpose.

    @Type(type="org.hibernate.test.annotations.entity.MonetaryAmountUserType")

    @Columns(columns = {

        @Column(name="r_amount"),

        @Column(name="r_currency")

    })

    public MonetaryAmount getAmount() {

        return amount;

    }

    public class MonetaryAmount implements Serializable {

        private BigDecimal amount;

        private Currency currency;

        ...

    }

##### 5.1.4.1.2. Access type

By default the access type of a class hierarchy is defined by the position of the `@Id` or `@EmbeddedId` annotations. If these annotations are on a field, then only fields are considered for persistence and the state is accessed via the field. If there annotations are on a getter, then only the getters are considered for persistence and the state is accessed via the getter/setter. That works well in practice and is the recommended approach.

## Note

The placement of annotations within a class hierarchy has to be consistent (either field or on property) to be able to determine the default access type. It is recommended to stick to one single annotation placement strategy throughout your whole application.

However in some situations, you need to:

- force the access type of the entity hierarchy

- override the access type of a specific entity in the class hierarchy

- override the access type of an embeddable type

The best use case is an embeddable class used by several entities that might not use the same access type. In this case it is better to force the access type at the embeddable class level.

To force the access type on a given class, use the `@Access` annotation as showed below:

    @Entity

    public class Order {

       @Id private Long id;

       public Long getId() { return id; }

       public void setId(Long id) { this.id = id; }

       @Embedded private Address address;

       public Address getAddress() { return address; }

       public void setAddress() { this.address = address; }

    }

    @Entity

    public class User {

       private Long id;

       @Id public Long getId() { return id; }

       public void setId(Long id) { this.id = id; }

       private Address address;

       @Embedded public Address getAddress() { return address; }

       public void setAddress() { this.address = address; }

    }

    @Embeddable

    @Access(AcessType.PROPERTY)

    public class Address {

       private String street1;

       public String getStreet1() { return street1; }

       public void setStreet1() { this.street1 = street1; }

       private hashCode; //not persistent

    }

You can also override the access type of a single property while keeping the other properties standard.

    @Entity

    public class Order {

       @Id private Long id;

       public Long getId() { return id; }

       public void setId(Long id) { this.id = id; }

       @Transient private String userId;

       @Transient private String orderId;

       @Access(AccessType.PROPERTY)

       public String getOrderNumber() { return userId + ":" + orderId; }

       public void setOrderNumber() { this.userId = ...; this.orderId = ...; }

    }

In this example, the default access type is `FIELD` except for the `orderNumber` property. Note that the corresponding field, if any must be marked as `@Transient` or `transient`.

## @org.hibernate.annotations.AccessType

The annotation `@org.hibernate.annotations.AccessType` should be considered deprecated for FIELD and PROPERTY access. It is still useful however if you need to use a custom access type.

##### 5.1.4.1.3. Optimistic lock

It is sometimes useful to avoid increasing the version number even if a given property is dirty (particularly collections). You can do that by annotating the property (or collection) with `@OptimisticLock(excluded=true)`.

More formally, specifies that updates to this property do not require acquisition of the optimistic lock.

##### 5.1.4.1.4. Declaring column attributes

The column(s) used for a property mapping can be defined using the `@Column` annotation. Use it to override default values (see the JPA specification for more information on the defaults). You can use this annotation at the property level for properties that are:

- not annotated at all

- annotated with `@Basic`

- annotated with `@Version`

- annotated with `@Lob`

- annotated with `@Temporal`

    @Entity

    public class Flight implements Serializable {

    ...

    @Column(updatable = false, name = "flight_name", nullable = false, length=50)

    public String getName() { ... }

The `name` property is mapped to the `flight_name` column, which is not nullable, has a length of 50 and is not updatable (making the property immutable).

This annotation can be applied to regular properties as well as `@Id` or `@Version` properties.

    @Column(
        name="columnName";
        boolean unique() default false;
        boolean nullable() default true;
        boolean insertable() default true;
        boolean updatable() default true;
        String columnDefinition() default "";
        String table() default "";
        int length() default 255;
        int precision() default 0; // decimal precision
        int scale() default 0; // decimal scale

| `name` (optional): the column name (default to the property name)
| `unique` (optional): set a unique constraint on this column or not (default false)
| `nullable` (optional): set the column as nullable (default true).
| `insertable` (optional): whether or not the column will be part of the insert statement (default true)
| `updatable` (optional): whether or not the column will be part of the update statement (default true)
| `columnDefinition` (optional): override the sql DDL fragment for this particular column (non portable)
| `table` (optional): define the targeted table (default primary table)
| `length` (optional): column length (default 255)
| `precision` (optional): column decimal precision (default 0)
| `scale` (optional): column decimal scale if useful (default 0)

##### 5.1.4.1.5. Formula

Sometimes, you want the Database to do some computation for you rather than in the JVM, you might also create some kind of virtual column. You can use a SQL fragment (aka formula) instead of mapping a property into a column. This kind of property is read only (its value is calculated by your formula fragment).

    @Formula("obj_length * obj_height * obj_width")

    public long getObjectVolume()

The SQL fragment can be as complex as you want and even include subselects.

##### 5.1.4.1.6. Non-annotated property defaults

If a property is not annotated, the following rules apply:

- If the property is of a single type, it is mapped as @Basic

- Otherwise, if the type of the property is annotated as @Embeddable, it is mapped as @Embedded

- Otherwise, if the type of the property is `Serializable`, it is mapped as `@Basic` in a column holding the object in its serialized version

- Otherwise, if the type of the property is `java.sql.Clob` or `java.sql.Blob`, it is mapped as `@Lob` with the appropriate `LobType`

#### 5.1.4.2. Property mapping with hbm.xml

The `<property>` element declares a persistent JavaBean style property of the class.

    <property
            name="propertyName"
            column="column_name"
            type="typename"
            update="true|false"
            insert="true|false"
            formula="arbitrary SQL expression"
            access="field|property|ClassName"
            lazy="true|false"
            unique="true|false"
            not-null="true|false"
            optimistic-lock="true|false"
            generated="never|insert|always"
            node="element-name|@attribute-name|element/@attribute|."
            index="index_name"
            unique_key="unique_key_id"
            length="L"
            precision="P"
            scale="S"
    />

| `name`: the name of the property, with an initial lowercase letter.
| `column` (optional - defaults to the property name): the name of the mapped database table column. This can also be specified by nested `<column>` element(s).
| `type` (optional): a name that indicates the Hibernate type.
| `update, insert` (optional - defaults to `true`): specifies that the mapped columns should be included in SQL `UPDATE` and/or `INSERT` statements. Setting both to `false` allows a pure "derived" property whose value is initialized from some other property that maps to the same column(s), or by a trigger or other application.
| `formula` (optional): an SQL expression that defines the value for a computed property. Computed properties do not have a column mapping of their own.
| `access` (optional - defaults to `property`): the strategy Hibernate uses for accessing the property value.
| `lazy` (optional - defaults to `false`): specifies that this property should be fetched lazily when the instance variable is first accessed. It requires build-time bytecode instrumentation.
| `unique` (optional): enables the DDL generation of a unique constraint for the columns. Also, allow this to be the target of a `property-ref`.
| `not-null` (optional): enables the DDL generation of a nullability constraint for the columns.
| `optimistic-lock` (optional - defaults to `true`): specifies that updates to this property do or do not require acquisition of the optimistic lock. In other words, it determines if a version increment should occur when this property is dirty.
| `generated` (optional - defaults to `never`): specifies that this property value is actually generated by the database. See the discussion of generated properties for more information.

typename could be:

1. The name of a Hibernate basic type: `integer, string, character, date, timestamp, float, binary, serializable, object, blob` etc.

2. The name of a Java class with a default basic type: `int, float, char, java.lang.String, java.util.Date, java.lang.Integer, java.sql.Clob` etc.

3. The name of a serializable Java class.

4. The class name of a custom type: `com.illflow.type.MyCustomType` etc.

If you do not specify a type, Hibernate will use reflection upon the named property and guess the correct Hibernate type. Hibernate will attempt to interpret the name of the return class of the property getter using, in order, rules 2, 3, and 4. In certain cases you will need the `type` attribute. For example, to distinguish between `Hibernate.DATE` and `Hibernate.TIMESTAMP`, or to specify a custom type.

The `access` attribute allows you to control how Hibernate accesses the property at runtime. By default, Hibernate will call the property get/set pair. If you specify `access="field"`, Hibernate will bypass the get/set pair and access the field directly using reflection. You can specify your own strategy for property access by naming a class that implements the interface `org.hibernate.property.PropertyAccessor`.

A powerful feature is derived properties. These properties are by definition read-only. The property value is computed at load time. You declare the computation as an SQL expression. This then translates to a `SELECT` clause subquery in the SQL query that loads an instance:

    <property name="totalPrice"
        formula="( SELECT SUM (li.quantity*p.price) FROM LineItem li, Product p
                    WHERE li.productId = p.productId
                    AND li.customerId = customerId
                    AND li.orderNumber = orderNumber )"/>

You can reference the entity table by not declaring an alias on a particular column. This would be `customerId` in the given example. You can also use the nested `<formula>` mapping element if you do not want to use the attribute.

### 5.1.5. Embedded objects (aka components)

Embeddable objects (or components) are objects whose properties are mapped to the same table as the owning entity's table. Components can, in turn, declare their own properties, components or collections

It is possible to declare an embedded component inside an entity and even override its column mapping. Component classes have to be annotated at the class level with the `@Embeddable` annotation. It is possible to override the column mapping of an embedded object for a particular entity using the `@Embedded` and `@AttributeOverride` annotation in the associated property:

    @Entity

    public class Person implements Serializable {

        // Persistent component using defaults

        Address homeAddress;

        @Embedded

        @AttributeOverrides( {

                @AttributeOverride(name="iso2", column = @Column(name="bornIso2") ),

                @AttributeOverride(name="name", column = @Column(name="bornCountryName") )

        } )

        Country bornIn;

        ...

    }

    @Embeddable

    public class Address implements Serializable {

        String city;

        Country nationality; //no overriding here

    }

    @Embeddable

    public class Country implements Serializable {

        private String iso2;

        @Column(name="countryName") private String name;

        public String getIso2() { return iso2; }

        public void setIso2(String iso2) { this.iso2 = iso2; }

        public String getName() { return name; }

        public void setName(String name) { this.name = name; }

        ...

    }

An embeddable object inherits the access type of its owning entity (note that you can override that using the `@Access` annotation).

The `Person` entity has two component properties, `homeAddress` and `bornIn`. `homeAddress` property has not been annotated, but Hibernate will guess that it is a persistent component by looking for the `@Embeddable` annotation in the Address class. We also override the mapping of a column name (to `bornCountryName`) with the `@Embedded` and `@AttributeOverride` annotations for each mapped attribute of `Country`. As you can see, `Country` is also a nested component of `Address`, again using auto-detection by Hibernate and JPA defaults. Overriding columns of embedded objects of embedded objects is through dotted expressions.

        @Embedded

        @AttributeOverrides( {

                @AttributeOverride(name="city", column = @Column(name="fld_city") ),

                @AttributeOverride(name="nationality.iso2", column = @Column(name="nat_Iso2") ),

                @AttributeOverride(name="nationality.name", column = @Column(name="nat_CountryName") )

                //nationality columns in homeAddress are overridden

        } )

        Address homeAddress;

Hibernate Annotations supports something that is not explicitly supported by the JPA specification. You can annotate a embedded object with the `@MappedSuperclass` annotation to make the superclass properties persistent (see `@MappedSuperclass` for more informations).

You can also use association annotations in an embeddable object (ie `@OneToOne`, `@ManyToOne`, `@OneToMany` or `@ManyToMany`). To override the association columns you can use `@AssociationOverride`.

If you want to have the same embeddable object type twice in the same entity, the column name defaulting will not work as several embedded objects would share the same set of columns. In plain JPA, you need to override at least one set of columns. Hibernate, however, allows you to enhance the default naming mechanism through the `NamingStrategy` interface. You can write a strategy that prevent name clashing in such a situation. `DefaultComponentSafeNamingStrategy` is an example of this.

If a property of the embedded object points back to the owning entity, annotate it with the `@Parent` annotation. Hibernate will make sure this property is properly loaded with the entity reference.

In XML, use the `<component>` element.

    <component
            name="propertyName"
            class="className"
            insert="true|false"
            update="true|false"
            access="field|property|ClassName"
            lazy="true|false"
            optimistic-lock="true|false"
            unique="true|false"
            node="element-name|."
    >

            <property ...../>
            <many-to-one .... />
            ........
    </component>

| `name`: the name of the property.
| `class` (optional - defaults to the property type determined by reflection): the name of the component (child) class.
| `insert`: do the mapped columns appear in SQL `INSERTs`?
| `update`: do the mapped columns appear in SQL `UPDATEs`?
| `access` (optional - defaults to `property`): the strategy Hibernate uses for accessing the property value.
| `lazy` (optional - defaults to `false`): specifies that this component should be fetched lazily when the instance variable is first accessed. It requires build-time bytecode instrumentation.
| `optimistic-lock` (optional - defaults to `true`): specifies that updates to this component either do or do not require acquisition of the optimistic lock. It determines if a version increment should occur when this property is dirty.
| `unique` (optional - defaults to `false`): specifies that a unique constraint exists upon all mapped columns of the component.

The child `<property>` tags map properties of the child class to table columns.

The `<component>` element allows a `<parent>` subelement that maps a property of the component class as a reference back to the containing entity.

The `<dynamic-component>` element allows a `Map` to be mapped as a component, where the property names refer to keys of the map. See Section 9.5, “Dynamic components” for more information. This feature is not supported in annotations.

### 5.1.6. Inheritance strategy

Java is a language supporting polymorphism: a class can inherit from another. Several strategies are possible to persist a class hierarchy:

- Single table per class hierarchy strategy: a single table hosts all the instances of a class hierarchy

- Joined subclass strategy: one table per class and subclass is present and each table persist the properties specific to a given subclass. The state of the entity is then stored in its corresponding class table and all its superclasses

- Table per class strategy: one table per concrete class and subclass is present and each table persist the properties of the class and its superclasses. The state of the entity is then stored entirely in the dedicated table for its class.

#### 5.1.6.1. Single table per class hierarchy strategy

With this approach the properties of all the subclasses in a given mapped class hierarchy are stored in a single table.

Each subclass declares its own persistent properties and subclasses. Version and id properties are assumed to be inherited from the root class. Each subclass in a hierarchy must define a unique discriminator value. If this is not specified, the fully qualified Java class name is used.

    @Entity

    @Inheritance(strategy=InheritanceType.SINGLE_TABLE)

    @DiscriminatorColumn(

        name="planetype",

        discriminatorType=DiscriminatorType.STRING

    )

    @DiscriminatorValue("Plane")

    public class Plane { ... }

    @Entity

    @DiscriminatorValue("A320")

    public class A320 extends Plane { ... }

In hbm.xml, for the table-per-class-hierarchy mapping strategy, the `<subclass>` declaration is used. For example:

    <subclass
            name="ClassName"
            discriminator-value="discriminator_value"
            proxy="ProxyInterface"
            lazy="true|false"
            dynamic-update="true|false"
            dynamic-insert="true|false"
            entity-name="EntityName"
            node="element-name"
            extends="SuperclassName">

            <property .... />
            .....
    </subclass>

| `name`: the fully qualified class name of the subclass.
| `discriminator-value` (optional - defaults to the class name): a value that distinguishes individual subclasses.
| `proxy` (optional): specifies a class or interface used for lazy initializing proxies.
| `lazy` (optional - defaults to `true`): setting `lazy="false"` disables the use of lazy fetching.

For information about inheritance mappings see Chapter 10, Inheritance mapping.

##### 5.1.6.1.1. Discriminator

Discriminators are required for polymorphic persistence using the table-per-class-hierarchy mapping strategy. It declares a discriminator column of the table. The discriminator column contains marker values that tell the persistence layer what subclass to instantiate for a particular row. Hibernate Core supports the follwoing restricted set of types as discriminator column: `string`, `character`, `integer`, `byte`, `short`, `boolean`, `yes_no`, `true_false`.

Use the `@DiscriminatorColumn` to define the discriminator column as well as the discriminator type.

## Note

`The enum DiscriminatorType` used in `javax.persitence.DiscriminatorColumn` only contains the values `STRING`, `CHAR` and `INTEGER` which means that not all Hibernate supported types are available via the `@DiscriminatorColumn` annotation.

You can also use `@DiscriminatorFormula` to express in SQL a virtual discriminator column. This is particularly useful when the discriminator value can be extracted from one or more columns of the table. Both `@DiscriminatorColumn` and `@DiscriminatorFormula` are to be set on the root entity (once per persisted hierarchy).

`@org.hibernate.annotations.DiscriminatorOptions` allows to optionally specify Hibernate specific discriminator options which are not standardized in JPA. The available options are `force` and `insert`. The `force` attribute is useful if the table contains rows with "extra" discriminator values that are not mapped to a persistent class. This could for example occur when working with a legacy database. If `force` is set to `true` Hibernate will specify the allowed discriminator values in the `SELECT` query, even when retrieving all instances of the root class. The second option - `insert` - tells Hibernate whether or not to include the discriminator column in SQL `INSERTs`. Usually the column should be part of the `INSERT` statement, but if your discriminator column is also part of a mapped composite identifier you have to set this option to `false`.

## Tip

There is also a `@org.hibernate.annotations.ForceDiscriminator` annotation which is deprecated since version 3.6. Use `@DiscriminatorOptions` instead.

Finally, use `@DiscriminatorValue` on each class of the hierarchy to specify the value stored in the discriminator column for a given entity. If you do not set `@DiscriminatorValue` on a class, the fully qualified class name is used.

    @Entity

    @Inheritance(strategy=InheritanceType.SINGLE_TABLE)

    @DiscriminatorColumn(

        name="planetype",

        discriminatorType=DiscriminatorType.STRING

    )

    @DiscriminatorValue("Plane")

    public class Plane { ... }

    @Entity

    @DiscriminatorValue("A320")

    public class A320 extends Plane { ... }

In hbm.xml, the `<discriminator>` element is used to define the discriminator column or formula:

    <discriminator
            column="discriminator_column"
            type="discriminator_type"
            force="true|false"
            insert="true|false"
            formula="arbitrary sql expression"
    />

| `column` (optional - defaults to `class`): the name of the discriminator column.
| `type` (optional - defaults to `string`): a name that indicates the Hibernate type
| `force` (optional - defaults to `false`): "forces" Hibernate to specify the allowed discriminator values, even when retrieving all instances of the root class.
| `insert` (optional - defaults to `true`): set this to `false` if your discriminator column is also part of a mapped composite identifier. It tells Hibernate not to include the column in SQL `INSERTs`.
| `formula` (optional): an arbitrary SQL expression that is executed when a type has to be evaluated. It allows content-based discrimination.

Actual values of the discriminator column are specified by the `discriminator-value` attribute of the `<class>` and `<subclass>` elements.

The `formula` attribute allows you to declare an arbitrary SQL expression that will be used to evaluate the type of a row. For example:

    <discriminator
        formula="case when CLASS_TYPE in ('a', 'b', 'c') then 0 else 1 end"
        type="integer"/>

#### 5.1.6.2. Joined subclass strategy

Each subclass can also be mapped to its own table. This is called the table-per-subclass mapping strategy. An inherited state is retrieved by joining with the table of the superclass. A discriminator column is not required for this mapping strategy. Each subclass must, however, declare a table column holding the object identifier. The primary key of this table is also a foreign key to the superclass table and described by the `@PrimaryKeyJoinColumn`s or the `<key>` element.

    @Entity @Table(name="CATS")

    @Inheritance(strategy=InheritanceType.JOINED)

    public class Cat implements Serializable {

        @Id @GeneratedValue(generator="cat-uuid")

        @GenericGenerator(name="cat-uuid", strategy="uuid")

        String getId() { return id; }

        ...

    }

    @Entity @Table(name="DOMESTIC_CATS")

    @PrimaryKeyJoinColumn(name="CAT")

    public class DomesticCat extends Cat {

        public String getName() { return name; }

    }

## Note

The table name still defaults to the non qualified class name. Also if `@PrimaryKeyJoinColumn` is not set, the primary key / foreign key columns are assumed to have the same names as the primary key columns of the primary table of the superclass.

In hbm.xml, use the `<joined-subclass>` element. For example:

    <joined-subclass
            name="ClassName"
            table="tablename"
            proxy="ProxyInterface"
            lazy="true|false"
            dynamic-update="true|false"
            dynamic-insert="true|false"
            schema="schema"
            catalog="catalog"
            extends="SuperclassName"
            persister="ClassName"
            subselect="SQL expression"
            entity-name="EntityName"
            node="element-name">

            <key .... >

            <property .... />
            .....
    </joined-subclass>

| `name`: the fully qualified class name of the subclass.
| `table`: the name of the subclass table.
| `proxy` (optional): specifies a class or interface to use for lazy initializing proxies.
| `lazy` (optional, defaults to `true`): setting `lazy="false"` disables the use of lazy fetching.

Use the `<key>` element to declare the primary key / foreign key column. The mapping at the start of the chapter would then be re-written as:

    <?xml version="1.0"?>
    <!DOCTYPE hibernate-mapping PUBLIC
            "-//Hibernate/Hibernate Mapping DTD//EN"
            "http://www.hibernate.org/dtd/hibernate-mapping-3.0.dtd">

    <hibernate-mapping package="eg">

            <class name="Cat" table="CATS">
                    <id name="id" column="uid" type="long">
                            <generator class="hilo"/>
                    </id>
                    <property name="birthdate" type="date"/>
                    <property name="color" not-null="true"/>
                    <property name="sex" not-null="true"/>
                    <property name="weight"/>
                    <many-to-one name="mate"/>
                    <set name="kittens">
                            <key column="MOTHER"/>
                            <one-to-many class="Cat"/>
                    </set>
                    <joined-subclass name="DomesticCat" table="DOMESTIC_CATS">
                        <key column="CAT"/>
                        <property name="name" type="string"/>
                    </joined-subclass>
            </class>

            <class name="eg.Dog">
                    <!-- mapping for Dog could go here -->
            </class>

    </hibernate-mapping>

For information about inheritance mappings see Chapter 10, Inheritance mapping.

#### 5.1.6.3. Table per class strategy

A third option is to map only the concrete classes of an inheritance hierarchy to tables. This is called the table-per-concrete-class strategy. Each table defines all persistent states of the class, including the inherited state. In Hibernate, it is not necessary to explicitly map such inheritance hierarchies. You can map each class as a separate entity root. However, if you wish use polymorphic associations (e.g. an association to the superclass of your hierarchy), you need to use the union subclass mapping.

    @Entity

    @Inheritance(strategy = InheritanceType.TABLE_PER_CLASS)

    public class Flight implements Serializable { ... }

Or in hbm.xml:

    <union-subclass
            name="ClassName"
            table="tablename"
            proxy="ProxyInterface"
            lazy="true|false"
            dynamic-update="true|false"
            dynamic-insert="true|false"
            schema="schema"
            catalog="catalog"
            extends="SuperclassName"
            abstract="true|false"
            persister="ClassName"
            subselect="SQL expression"
            entity-name="EntityName"
            node="element-name">

            <property .... />
            .....
    </union-subclass>

| `name`: the fully qualified class name of the subclass.
| `table`: the name of the subclass table.
| `proxy` (optional): specifies a class or interface to use for lazy initializing proxies.
| `lazy` (optional, defaults to `true`): setting `lazy="false"` disables the use of lazy fetching.

No discriminator column or key column is required for this mapping strategy.

For information about inheritance mappings see Chapter 10, Inheritance mapping.

#### 5.1.6.4. Inherit properties from superclasses

This is sometimes useful to share common properties through a technical or a business superclass without including it as a regular mapped entity (ie no specific table for this entity). For that purpose you can map them as `@MappedSuperclass`.

    @MappedSuperclass

    public class BaseEntity {

        @Basic

        @Temporal(TemporalType.TIMESTAMP)

        public Date getLastUpdate() { ... }

        public String getLastUpdater() { ... }

        ...

    }

    @Entity class Order extends BaseEntity {

        @Id public Integer getId() { ... }

        ...

    }

In database, this hierarchy will be represented as an `Order` table having the `id`, `lastUpdate` and `lastUpdater` columns. The embedded superclass property mappings are copied into their entity subclasses. Remember that the embeddable superclass is not the root of the hierarchy though.

## Note

Properties from superclasses not mapped as `@MappedSuperclass` are ignored.

## Note

The default access type (field or methods) is used, unless you use the `@Access` annotation.

## Note

The same notion can be applied to `@Embeddable` objects to persist properties from their superclasses. You also need to use `@MappedSuperclass` to do that (this should not be considered as a standard EJB3 feature though)

## Note

It is allowed to mark a class as `@MappedSuperclass` in the middle of the mapped inheritance hierarchy.

## Note

Any class in the hierarchy non annotated with `@MappedSuperclass` nor `@Entity` will be ignored.

You can override columns defined in entity superclasses at the root entity level using the `@AttributeOverride` annotation.

    @MappedSuperclass

    public class FlyingObject implements Serializable {

        public int getAltitude() {

            return altitude;

        }

        @Transient

        public int getMetricAltitude() {

            return metricAltitude;

        }

        @ManyToOne

        public PropulsionType getPropulsion() {

            return metricAltitude;

        }

        ...

    }

    @Entity

    @AttributeOverride( name="altitude", column = @Column(name="fld_altitude") )

    @AssociationOverride(

       name="propulsion",

       joinColumns = @JoinColumn(name="fld_propulsion_fk")

    )

    public class Plane extends FlyingObject {

        ...

    }

The `altitude` property will be persisted in an `fld_altitude` column of table `Plane` and the propulsion association will be materialized in a `fld_propulsion_fk` foreign key column.

You can define `@AttributeOverride`(s) and `@AssociationOverride`(s) on `@Entity` classes, `@MappedSuperclass` classes and properties pointing to an `@Embeddable` object.

In hbm.xml, simply map the properties of the superclass in the `<class>` element of the entity that needs to inherit them.

#### 5.1.6.5. Mapping one entity to several tables

While not recommended for a fresh schema, some legacy databases force your to map a single entity on several tables.

Using the `@SecondaryTable` or `@SecondaryTables` class level annotations. To express that a column is in a particular table, use the `table` parameter of `@Column` or `@JoinColumn`.

    @Entity

    @Table(name="MainCat")

    @SecondaryTables({

        @SecondaryTable(name="Cat1", pkJoinColumns={

            @PrimaryKeyJoinColumn(name="cat_id", referencedColumnName="id")

        ),

        @SecondaryTable(name="Cat2", uniqueConstraints={@UniqueConstraint(columnNames={"storyPart2"})})

    })

    public class Cat implements Serializable {

        private Integer id;

        private String name;

        private String storyPart1;

        private String storyPart2;

        @Id @GeneratedValue

        public Integer getId() {

            return id;

        }

        public String getName() {

            return name;

        }

        @Column(table="Cat1")

        public String getStoryPart1() {

            return storyPart1;

        }

        @Column(table="Cat2")

        public String getStoryPart2() {

            return storyPart2;

        }

    }

In this example, `name` will be in `MainCat`. `storyPart1` will be in `Cat1` and `storyPart2` will be in `Cat2`. `Cat1` will be joined to `MainCat` using the `cat_id` as a foreign key, and `Cat2` using `id` (ie the same column name, the `MainCat` id column has). Plus a unique constraint on `storyPart2` has been set.

There is also additional tuning accessible via the `@org.hibernate.annotations.Table` annotation:

- `fetch`: If set to JOIN, the default, Hibernate will use an inner join to retrieve a secondary table defined by a class or its superclasses and an outer join for a secondary table defined by a subclass. If set to `SELECT` then Hibernate will use a sequential select for a secondary table defined on a subclass, which will be issued only if a row turns out to represent an instance of the subclass. Inner joins will still be used to retrieve a secondary defined by the class and its superclasses.

- `inverse`: If true, Hibernate will not try to insert or update the properties defined by this join. Default to false.

- `optional`: If enabled (the default), Hibernate will insert a row only if the properties defined by this join are non-null and will always use an outer join to retrieve the properties.

- `foreignKey`: defines the Foreign Key name of a secondary table pointing back to the primary table.

Make sure to use the secondary table name in the `appliesto` property

    @Entity

    @Table(name="MainCat")

    @SecondaryTable(name="Cat1")

    @org.hibernate.annotations.Table(

       appliesTo="Cat1",

       fetch=FetchMode.SELECT,

       optional=true)

    public class Cat implements Serializable {

        private Integer id;

        private String name;

        private String storyPart1;

        private String storyPart2;

        @Id @GeneratedValue

        public Integer getId() {

            return id;

        }

        public String getName() {

            return name;

        }

        @Column(table="Cat1")

        public String getStoryPart1() {

            return storyPart1;

        }

        @Column(table="Cat2")

        public String getStoryPart2() {

            return storyPart2;

        }

    }

In hbm.xml, use the `<join>` element.

    <join
            table="tablename"
            schema="owner"
            catalog="catalog"
            fetch="join|select"
            inverse="true|false"
            optional="true|false">

            <key ... />

            <property ... />
            ...
    </join>

| `table`: the name of the joined table.
| `schema` (optional): overrides the schema name specified by the root `<hibernate-mapping>` element.
| `catalog` (optional): overrides the catalog name specified by the root `<hibernate-mapping>` element.
| `fetch` (optional - defaults to `join`): if set to `join`, the default, Hibernate will use an inner join to retrieve a `<join>` defined by a class or its superclasses. It will use an outer join for a `<join>` defined by a subclass. If set to `select` then Hibernate will use a sequential select for a `<join>` defined on a subclass. This will be issued only if a row represents an instance of the subclass. Inner joins will still be used to retrieve a `<join>` defined by the class and its superclasses.
| `inverse` (optional - defaults to `false`): if enabled, Hibernate will not insert or update the properties defined by this join.
| `optional` (optional - defaults to `false`): if enabled, Hibernate will insert a row only if the properties defined by this join are non-null. It will always use an outer join to retrieve the properties.

For example, address information for a person can be mapped to a separate table while preserving value type semantics for all properties:

    <class name="Person"
        table="PERSON">

        <id name="id" column="PERSON_ID">...</id>

        <join table="ADDRESS">
            <key column="ADDRESS_ID"/>
            <property name="address"/>
            <property name="zip"/>
            <property name="country"/>
        </join>
        ...

This feature is often only useful for legacy data models. We recommend fewer tables than classes and a fine-grained domain model. However, it is useful for switching between inheritance mapping strategies in a single hierarchy, as explained later.

### 5.1.7. Mapping one to one and one to many associations

To link one entity to an other, you need to map the association property as a to one association. In the relational model, you can either use a foreign key or an association table, or (a bit less common) share the same primary key value between the two entities.

To mark an association, use either `@ManyToOne` or `@OnetoOne`.

`@ManyToOne` and `@OneToOne` have a parameter named `targetEntity` which describes the target entity name. You usually don't need this parameter since the default value (the type of the property that stores the association) is good in almost all cases. However this is useful when you want to use interfaces as the return type instead of the regular entity.

Setting a value of the `cascade` attribute to any meaningful value other than nothing will propagate certain operations to the associated object. The meaningful values are divided into three categories.

1. basic operations, which include: `persist, merge, delete, save-update, evict, replicate, lock and refresh`;

2. special values: `delete-orphan` or `all` ;

3. comma-separated combinations of operation names: `cascade="persist,merge,evict"` or `cascade="all,delete-orphan"`. See Section 11.11, “Transitive persistence” for a full explanation. Note that single valued many-to-one associations do not support orphan delete.

By default, single point associations are eagerly fetched in JPA 2. You can mark it as lazily fetched by using `@ManyToOne(fetch=FetchType.LAZY)` in which case Hibernate will proxy the association and load it when the state of the associated entity is reached. You can force Hibernate not to use a proxy by using `@LazyToOne(NO_PROXY)`. In this case, the property is fetched lazily when the instance variable is first accessed. This requires build-time bytecode instrumentation. lazy="false" specifies that the association will always be eagerly fetched.

With the default JPA options, single-ended associations are loaded with a subsequent select if set to `LAZY`, or a SQL JOIN is used for `EAGER` associations. You can however adjust the fetching strategy, ie how data is fetched by using `@Fetch`. `FetchMode` can be `SELECT` (a select is triggered when the association needs to be loaded) or `JOIN` (use a SQL JOIN to load the association while loading the owner entity). `JOIN` overrides any lazy attribute (an association loaded through a `JOIN` strategy cannot be lazy).

#### 5.1.7.1. Using a foreign key or an association table

An ordinary association to another persistent class is declared using a

- `@ManyToOne` if several entities can point to the the target entity

- `@OneToOne` if only a single entity can point to the the target entity

and a foreign key in one table is referencing the primary key column(s) of the target table.

    @Entity

    public class Flight implements Serializable {

        @ManyToOne( cascade = {CascadeType.PERSIST, CascadeType.MERGE} )

        @JoinColumn(name="COMP_ID")

        public Company getCompany() {

            return company;

        }

        ...

    }

The `@JoinColumn` attribute is optional, the default value(s) is the concatenation of the name of the relationship in the owner side, _ (underscore), and the name of the primary key column in the owned side. In this example `company_id` because the property name is `company` and the column id of Company is `id`.

    @Entity

    public class Flight implements Serializable {

        @ManyToOne( cascade = {CascadeType.PERSIST, CascadeType.MERGE}, targetEntity=CompanyImpl.class )

        @JoinColumn(name="COMP_ID")

        public Company getCompany() {

            return company;

        }

        ...

    }

    public interface Company {

        ...

    }

You can also map a to one association through an association table. This association table described by the `@JoinTable` annotation will contains a foreign key referencing back the entity table (through `@JoinTable.joinColumns`) and a a foreign key referencing the target entity table (through `@JoinTable.inverseJoinColumns`).

    @Entity

    public class Flight implements Serializable {

        @ManyToOne( cascade = {CascadeType.PERSIST, CascadeType.MERGE} )

        @JoinTable(name="Flight_Company",

            joinColumns = @JoinColumn(name="FLIGHT_ID"),

            inverseJoinColumns = @JoinColumn(name="COMP_ID")

        )

        public Company getCompany() {

            return company;

        }

        ...

    }

## Note

You can use a SQL fragment to simulate a physical join column using the `@JoinColumnOrFormula` / `@JoinColumnOrformulas` annotations (just like you can use a SQL fragment to simulate a property column via the `@Formula` annotation).

    @Entity

    public class Ticket implements Serializable {

        @ManyToOne

        @JoinColumnOrFormula(formula="(firstname + ' ' + lastname)")

        public Person getOwner() {

            return person;

        }

        ...

    }

You can mark an association as mandatory by using the `optional=false` attribute. We recommend to use Bean Validation's `@NotNull` annotation as a better alternative however. As a consequence, the foreign key column(s) will be marked as not nullable (if possible).

When Hibernate cannot resolve the association because the expected associated element is not in database (wrong id on the association column), an exception is raised. This might be inconvenient for legacy and badly maintained schemas. You can ask Hibernate to ignore such elements instead of raising an exception using the `@NotFound` annotation.

Example 5.1. @NotFound annotation

    @Entity

    public class Child {

        ...

        @ManyToOne

        @NotFound(action=NotFoundAction.IGNORE)

        public Parent getParent() { ... }

        ...

    }

Sometimes you want to delegate to your database the deletion of cascade when a given entity is deleted. In this case Hibernate generates a cascade delete constraint at the database level.

Example 5.2. @OnDelete annotation

    @Entity

    public class Child {

        ...

        @ManyToOne

        @OnDelete(action=OnDeleteAction.CASCADE)

        public Parent getParent() { ... }

        ...

    }

Foreign key constraints, while generated by Hibernate, have a fairly unreadable name. You can override the constraint name using `@ForeignKey`.

Example 5.3. @ForeignKey annotation

    @Entity

    public class Child {

        ...

        @ManyToOne

        @ForeignKey(name="FK_PARENT")

        public Parent getParent() { ... }

        ...

    }

    alter table Child add constraint FK_PARENT foreign key (parent_id) references Parent

Sometimes, you want to link one entity to an other not by the target entity primary key but by a different unique key. You can achieve that by referencing the unique key column(s) in `@JoinColumn.referenceColumnName`.

    @Entity
    class Person {
       @Id Integer personNumber;
       String firstName;
       @Column(name="I")
       String initial;
       String lastName;
    }

    @Entity
    class Home {
       @ManyToOne
       @JoinColumns({
          @JoinColumn(name="first_name", referencedColumnName="firstName"),
          @JoinColumn(name="init", referencedColumnName="I"),
          @JoinColumn(name="last_name", referencedColumnName="lastName"),
       })
       Person owner
    }

This is not encouraged however and should be reserved to legacy mappings.

In hbm.xml, mapping an association is similar. The main difference is that a `@OneToOne` is mapped as `<many-to-one unique="true"/>`, let's dive into the subject.

    <many-to-one
            name="propertyName"
            column="column_name"
            class="ClassName"
            cascade="cascade_style"
            fetch="join|select"
            update="true|false"
            insert="true|false"
            property-ref="propertyNameFromAssociatedClass"
            access="field|property|ClassName"
            unique="true|false"
            not-null="true|false"
            optimistic-lock="true|false"
            lazy="proxy|no-proxy|false"
            not-found="ignore|exception"
            entity-name="EntityName"
            formula="arbitrary SQL expression"
            node="element-name|@attribute-name|element/@attribute|."
            embed-xml="true|false"
            index="index_name"
            unique_key="unique_key_id"
            foreign-key="foreign_key_name"
    />

| `name`: the name of the property.
| `column` (optional): the name of the foreign key column. This can also be specified by nested `<column>` element(s).
| `class` (optional - defaults to the property type determined by reflection): the name of the associated class.
| `cascade` (optional): specifies which operations should be cascaded from the parent object to the associated object.
| `fetch` (optional - defaults to `select`): chooses between outer-join fetching or sequential select fetching.
| `update, insert` (optional - defaults to `true`): specifies that the mapped columns should be included in SQL `UPDATE` and/or `INSERT` statements. Setting both to `false` allows a pure "derived" association whose value is initialized from another property that maps to the same column(s), or by a trigger or other application.
| `property-ref` (optional): the name of a property of the associated class that is joined to this foreign key. If not specified, the primary key of the associated class is used.
| `access` (optional - defaults to `property`): the strategy Hibernate uses for accessing the property value.
| `unique` (optional): enables the DDL generation of a unique constraint for the foreign-key column. By allowing this to be the target of a `property-ref`, you can make the association multiplicity one-to-one.
| `not-null` (optional): enables the DDL generation of a nullability constraint for the foreign key columns.
| `optimistic-lock` (optional - defaults to `true`): specifies that updates to this property do or do not require acquisition of the optimistic lock. In other words, it determines if a version increment should occur when this property is dirty.
| `lazy` (optional - defaults to `proxy`): by default, single point associations are proxied. `lazy="no-proxy"` specifies that the property should be fetched lazily when the instance variable is first accessed. This requires build-time bytecode instrumentation. `lazy="false"` specifies that the association will always be eagerly fetched.
| `not-found` (optional - defaults to `exception`): specifies how foreign keys that reference missing rows will be handled. `ignore` will treat a missing row as a null association.
| `entity-name` (optional): the entity name of the associated class.
| `formula` (optional): an SQL expression that defines the value for a computed foreign key.

Setting a value of the `cascade` attribute to any meaningful value other than `none` will propagate certain operations to the associated object. The meaningful values are divided into three categories. First, basic operations, which include: `persist, merge, delete, save-update, evict, replicate, lock and refresh`; second, special values: `delete-orphan`; and third,`all` comma-separated combinations of operation names: `cascade="persist,merge,evict"` or `cascade="all,delete-orphan"`. See Section 11.11, “Transitive persistence” for a full explanation. Note that single valued, many-to-one and one-to-one, associations do not support orphan delete.

Here is an example of a typical `many-to-one` declaration:

    <many-to-one name="product" class="Product" column="PRODUCT_ID"/>

The `property-ref` attribute should only be used for mapping legacy data where a foreign key refers to a unique key of the associated table other than the primary key. This is a complicated and confusing relational model. For example, if the `Product` class had a unique serial number that is not the primary key. The `unique` attribute controls Hibernate's DDL generation with the SchemaExport tool.

    <property name="serialNumber" unique="true" type="string" column="SERIAL_NUMBER"/>

Then the mapping for `OrderItem` might use:

    <many-to-one name="product" property-ref="serialNumber" column="PRODUCT_SERIAL_NUMBER"/>

This is not encouraged, however.

If the referenced unique key comprises multiple properties of the associated entity, you should map the referenced properties inside a named `<properties>` element.

If the referenced unique key is the property of a component, you can specify a property path:

    <many-to-one name="owner" property-ref="identity.ssn" column="OWNER_SSN"/>

#### 5.1.7.2. Sharing the primary key with the associated entity

The second approach is to ensure an entity and its associated entity share the same primary key. In this case the primary key column is also a foreign key and there is no extra column. These associations are always one to one.

Example 5.4. One to One association

    @Entity

    public class Body {

        @Id

        public Long getId() { return id; }

        @OneToOne(cascade = CascadeType.ALL)

        @MapsId

        public Heart getHeart() {

            return heart;

        }

        ...

    }

    @Entity

    public class Heart {

        @Id

        public Long getId() { ...}

    }

## Note

Many people got confused by these primary key based one to one associations. They can only be lazily loaded if Hibernate knows that the other side of the association is always present. To indicate to Hibernate that it is the case, use `@OneToOne(optional=false)`.

In hbm.xml, use the following mapping.

    <one-to-one
            name="propertyName"
            class="ClassName"
            cascade="cascade_style"
            constrained="true|false"
            fetch="join|select"
            property-ref="propertyNameFromAssociatedClass"
            access="field|property|ClassName"
            formula="any SQL expression"
            lazy="proxy|no-proxy|false"
            entity-name="EntityName"
            node="element-name|@attribute-name|element/@attribute|."
            embed-xml="true|false"
            foreign-key="foreign_key_name"
    />

| `name`: the name of the property.
| `class` (optional - defaults to the property type determined by reflection): the name of the associated class.
| `cascade` (optional): specifies which operations should be cascaded from the parent object to the associated object.
| `constrained` (optional): specifies that a foreign key constraint on the primary key of the mapped table and references the table of the associated class. This option affects the order in which `save()` and `delete()` are cascaded, and determines whether the association can be proxied. It is also used by the schema export tool.
| `fetch` (optional - defaults to `select`): chooses between outer-join fetching or sequential select fetching.
| `property-ref` (optional): the name of a property of the associated class that is joined to the primary key of this class. If not specified, the primary key of the associated class is used.
| `access` (optional - defaults to `property`): the strategy Hibernate uses for accessing the property value.
| `formula` (optional): almost all one-to-one associations map to the primary key of the owning entity. If this is not the case, you can specify another column, columns or expression to join on using an SQL formula. See `org.hibernate.test.onetooneformula` for an example.
| `lazy` (optional - defaults to `proxy`): by default, single point associations are proxied. `lazy="no-proxy"` specifies that the property should be fetched lazily when the instance variable is first accessed. It requires build-time bytecode instrumentation. `lazy="false"` specifies that the association will always be eagerly fetched. Note that if `constrained="false"`, proxying is impossible and Hibernate will eagerly fetch the association.
| `entity-name` (optional): the entity name of the associated class.

Primary key associations do not need an extra table column. If two rows are related by the association, then the two table rows share the same primary key value. To relate two objects by a primary key association, ensure that they are assigned the same identifier value.

For a primary key association, add the following mappings to `Employee` and `Person` respectively:

    <one-to-one name="person" class="Person"/>

    <one-to-one name="employee" class="Employee" constrained="true"/>

Ensure that the primary keys of the related rows in the PERSON and EMPLOYEE tables are equal. You use a special Hibernate identifier generation strategy called `foreign`:

    <class name="person" table="PERSON">
        <id name="id" column="PERSON_ID">
            <generator class="foreign">
                <param name="property">employee</param>
            </generator>
        </id>
        ...
        <one-to-one name="employee"
            class="Employee"
            constrained="true"/>
    </class>

A newly saved instance of `Person` is assigned the same primary key value as the `Employee` instance referred with the `employee` property of that `Person`.

### 5.1.8. Natural-id

Although we recommend the use of surrogate keys as primary keys, you should try to identify natural keys for all entities. A natural key is a property or combination of properties that is unique and non-null. It is also immutable. Map the properties of the natural key as `@NaturalId` or map them inside the `<natural-id>` element. Hibernate will generate the necessary unique key and nullability constraints and, as a result, your mapping will be more self-documenting.

    @Entity

    public class Citizen {

        @Id

        @GeneratedValue

        private Integer id;

        private String firstname;

        private String lastname;

        @NaturalId

        @ManyToOne

        private State state;

        @NaturalId

        private String ssn;

        ...

    }

    //and later on query

    List results = s.createCriteria( Citizen.class )

                    .add( Restrictions.naturalId().set( "ssn", "1234" ).set( "state", ste ) )

                    .list();

Or in XML,

    <natural-id mutable="true|false"/>
            <property ... />
            <many-to-one ... />
            ......
    </natural-id>

It is recommended that you implement `equals()` and `hashCode()` to compare the natural key properties of the entity.

This mapping is not intended for use with entities that have natural primary keys.

- `mutable` (optional - defaults to `false`): by default, natural identifier properties are assumed to be immutable (constant).

### 5.1.9. Any

There is one more type of property mapping. The `@Any` mapping defines a polymorphic association to classes from multiple tables. This type of mapping requires more than one column. The first column contains the type of the associated entity. The remaining columns contain the identifier. It is impossible to specify a foreign key constraint for this kind of association. This is not the usual way of mapping polymorphic associations and you should use this only in special cases. For example, for audit logs, user session data, etc.

The `@Any` annotation describes the column holding the metadata information. To link the value of the metadata information and an actual entity type, The `@AnyDef` and `@AnyDefs` annotations are used. The `metaType` attribute allows the application to specify a custom type that maps database column values to persistent classes that have identifier properties of the type specified by `idType`. You must specify the mapping from values of the `metaType` to class names.

    @Any( metaColumn = @Column( name = "property_type" ), fetch=FetchType.EAGER )

    @AnyMetaDef(

        idType = "integer",

        metaType = "string",

        metaValues = {

            @MetaValue( value = "S", targetEntity = StringProperty.class ),

            @MetaValue( value = "I", targetEntity = IntegerProperty.class )

        } )

    @JoinColumn( name = "property_id" )

    public Property getMainProperty() {

        return mainProperty;

    }

Note that `@AnyDef` can be mutualized and reused. It is recommended to place it as a package metadata in this case.

    //on a package

    @AnyMetaDef( name="property"

        idType = "integer",

        metaType = "string",

        metaValues = {

            @MetaValue( value = "S", targetEntity = StringProperty.class ),

            @MetaValue( value = "I", targetEntity = IntegerProperty.class )

        } )

    package org.hibernate.test.annotations.any;

    //in a class

        @Any( metaDef="property", metaColumn = @Column( name = "property_type" ), fetch=FetchType.EAGER )

        @JoinColumn( name = "property_id" )

        public Property getMainProperty() {

            return mainProperty;

        }

The hbm.xml equivalent is:

    <any name="being" id-type="long" meta-type="string">
        <meta-value value="TBL_ANIMAL" class="Animal"/>
        <meta-value value="TBL_HUMAN" class="Human"/>
        <meta-value value="TBL_ALIEN" class="Alien"/>
        <column name="table_name"/>
        <column name="id"/>
    </any>

## Note

You cannot mutualize the metadata in hbm.xml as you can in annotations.

    <any
            name="propertyName"
            id-type="idtypename"
            meta-type="metatypename"
            cascade="cascade_style"
            access="field|property|ClassName"
            optimistic-lock="true|false"
    >
            <meta-value ... />
            <meta-value ... />
            .....
            <column .... />
            <column .... />
            .....
    </any>

| `name`: the property name.
| `id-type`: the identifier type.
| `meta-type` (optional - defaults to `string`): any type that is allowed for a discriminator mapping.
| `cascade` (optional- defaults to `none`): the cascade style.
| `access` (optional - defaults to `property`): the strategy Hibernate uses for accessing the property value.
| `optimistic-lock` (optional - defaults to `true`): specifies that updates to this property either do or do not require acquisition of the optimistic lock. It defines whether a version increment should occur if this property is dirty.

### 5.1.10. Properties

The `<properties>` element allows the definition of a named, logical grouping of the properties of a class. The most important use of the construct is that it allows a combination of properties to be the target of a `property-ref`. It is also a convenient way to define a multi-column unique constraint. For example:

    <properties
            name="logicalName"
            insert="true|false"
            update="true|false"
            optimistic-lock="true|false"
            unique="true|false"
    >

            <property ...../>
            <many-to-one .... />
            ........
    </properties>

| `name`: the logical name of the grouping. It is not an actual property name.
| `insert`: do the mapped columns appear in SQL `INSERTs`?
| `update`: do the mapped columns appear in SQL `UPDATEs`?
| `optimistic-lock` (optional - defaults to `true`): specifies that updates to these properties either do or do not require acquisition of the optimistic lock. It determines if a version increment should occur when these properties are dirty.
| `unique` (optional - defaults to `false`): specifies that a unique constraint exists upon all mapped columns of the component.

For example, if we have the following `<properties>` mapping:

    <class name="Person">
        <id name="personNumber"/>

        ...
        <properties name="name"
                unique="true" update="false">
            <property name="firstName"/>
            <property name="initial"/>
            <property name="lastName"/>
        </properties>
    </class>

You might have some legacy data association that refers to this unique key of the `Person` table, instead of to the primary key:

    <many-to-one name="owner"
             class="Person" property-ref="name">
        <column name="firstName"/>
        <column name="initial"/>
        <column name="lastName"/>
    </many-to-one>

## Note

When using annotations as a mapping strategy, such construct is not necessary as the binding between a column and its related column on the associated table is done directly

    @Entity
    class Person {
       @Id Integer personNumber;
       String firstName;
       @Column(name="I")
       String initial;
       String lastName;
    }

    @Entity
    class Home {
       @ManyToOne
       @JoinColumns({
          @JoinColumn(name="first_name", referencedColumnName="firstName"),
          @JoinColumn(name="init", referencedColumnName="I"),
          @JoinColumn(name="last_name", referencedColumnName="lastName"),
       })
       Person owner
    }

The use of this outside the context of mapping legacy data is not recommended.

### 5.1.11. Some hbm.xml specificities

The hbm.xml structure has some specificities naturally not present when using annotations, let's describe them briefly.

#### 5.1.11.1. Doctype

All XML mappings should declare the doctype shown. The actual DTD can be found at the URL above, in the directory `hibernate-x.x.x/src/org/hibernate` , or in `hibernate3.jar`. Hibernate will always look for the DTD in its classpath first. If you experience lookups of the DTD using an Internet connection, check the DTD declaration against the contents of your classpath.

##### 5.1.11.1.1. EntityResolver

Hibernate will first attempt to resolve DTDs in its classpath. It does this is by registering a custom `org.xml.sax.EntityResolver` implementation with the SAXReader it uses to read in the xml files. This custom `EntityResolver` recognizes two different systemId namespaces:

- a `hibernate namespace` is recognized whenever the resolver encounters a systemId starting with `http://www.hibernate.org/dtd/`. The resolver attempts to resolve these entities via the classloader which loaded the Hibernate classes.

- a `user namespace` is recognized whenever the resolver encounters a systemId using a `classpath://` URL protocol. The resolver will attempt to resolve these entities via (1) the current thread context classloader and (2) the classloader which loaded the Hibernate classes.

The following is an example of utilizing user namespacing:

    <?xml version="1.0"?>

    <!DOCTYPE hibernate-mapping PUBLIC

            "-//Hibernate/Hibernate Mapping DTD 3.0//EN"

            "http://hibernate.sourceforge.net/hibernate-mapping-3.0.dtd" [

        <!ENTITY types SYSTEM "classpath://your/domain/types.xml">

    ]>

    <hibernate-mapping package="your.domain">

        <class name="MyEntity">

            <id name="id" type="my-custom-id-type">

                ...

            </id>

        <class>

        &types;

    </hibernate-mapping>

Where `types.xml` is a resource in the `your.domain` package and contains a custom typedef.

#### 5.1.11.2. Hibernate-mapping

This element has several optional attributes. The `schema` and `catalog` attributes specify that tables referred to in this mapping belong to the named schema and/or catalog. If they are specified, tablenames will be qualified by the given schema and catalog names. If they are missing, tablenames will be unqualified. The `default-cascade` attribute specifies what cascade style should be assumed for properties and collections that do not specify a `cascade` attribute. By default, the `auto-import` attribute allows you to use unqualified class names in the query language.

    <hibernate-mapping
             schema="schemaName"
             catalog="catalogName"
             default-cascade="cascade_style"
             default-access="field|property|ClassName"
             default-lazy="true|false"
             auto-import="true|false"
             package="package.name"
     />

| `schema` (optional): the name of a database schema.
| `catalog` (optional): the name of a database catalog.
| `default-cascade` (optional - defaults to `none`): a default cascade style.
| `default-access` (optional - defaults to `property`): the strategy Hibernate should use for accessing all properties. It can be a custom implementation of `PropertyAccessor`.
| `default-lazy` (optional - defaults to `true`): the default value for unspecified `lazy` attributes of class and collection mappings.
| `auto-import` (optional - defaults to `true`): specifies whether we can use unqualified class names of classes in this mapping in the query language.
| `package` (optional): specifies a package prefix to use for unqualified class names in the mapping document.

If you have two persistent classes with the same unqualified name, you should set `auto-import="false"`. An exception will result if you attempt to assign two classes to the same "imported" name.

The `hibernate-mapping` element allows you to nest several persistent `<class>` mappings, as shown above. It is, however, good practice (and expected by some tools) to map only a single persistent class, or a single class hierarchy, in one mapping file and name it after the persistent superclass. For example, `Cat.hbm.xml`, `Dog.hbm.xml`, or if using inheritance, `Animal.hbm.xml`.

#### 5.1.11.3. Key

The `<key>` element is featured a few times within this guide. It appears anywhere the parent mapping element defines a join to a new table that references the primary key of the original table. It also defines the foreign key in the joined table:

    <key
            column="columnname"
            on-delete="noaction|cascade"
            property-ref="propertyName"
            not-null="true|false"
            update="true|false"
            unique="true|false"
    />

| `column` (optional): the name of the foreign key column. This can also be specified by nested `<column>` element(s).
| `on-delete` (optional - defaults to `noaction`): specifies whether the foreign key constraint has database-level cascade delete enabled.
| `property-ref` (optional): specifies that the foreign key refers to columns that are not the primary key of the original table. It is provided for legacy data.
| `not-null` (optional): specifies that the foreign key columns are not nullable. This is implied whenever the foreign key is also part of the primary key.
| `update` (optional): specifies that the foreign key should never be updated. This is implied whenever the foreign key is also part of the primary key.
| `unique` (optional): specifies that the foreign key should have a unique constraint. This is implied whenever the foreign key is also the primary key.

For systems where delete performance is important, we recommend that all keys should be defined `on-delete="cascade"`. Hibernate uses a database-level `ON CASCADE DELETE` constraint, instead of many individual `DELETE` statements. Be aware that this feature bypasses Hibernate's usual optimistic locking strategy for versioned data.

The `not-null` and `update` attributes are useful when mapping a unidirectional one-to-many association. If you map a unidirectional one-to-many association to a non-nullable foreign key, you must declare the key column using `<key not-null="true">`.

#### 5.1.11.4. Import

If your application has two persistent classes with the same name, and you do not want to specify the fully qualified package name in Hibernate queries, classes can be "imported" explicitly, rather than relying upon `auto-import="true"`. You can also import classes and interfaces that are not explicitly mapped:

    <import class="java.lang.Object" rename="Universe"/>

    <import
            class="ClassName"
            rename="ShortName"
    />

| `class`: the fully qualified class name of any Java class.
| `rename` (optional - defaults to the unqualified class name): a name that can be used in the query language.

## Note

This feature is unique to hbm.xml and is not supported in annotations.

#### 5.1.11.5. Column and formula elements

Mapping elements which accept a `column` attribute will alternatively accept a `<column>` subelement. Likewise, `<formula>` is an alternative to the `formula` attribute. For example:

    <column
            name="column_name"
            length="N"
            precision="N"
            scale="N"
            not-null="true|false"
            unique="true|false"
            unique-key="multicolumn_unique_key_name"
            index="index_name"
            sql-type="sql_type_name"
            check="SQL expression"
            default="SQL expression"
            read="SQL expression"
            write="SQL expression"/>

    <formula>SQL expression</formula>

Most of the attributes on `column` provide a means of tailoring the DDL during automatic schema generation. The `read` and `write` attributes allow you to specify custom SQL that Hibernate will use to access the column's value. For more on this, see the discussion of column read and write expressions.

The `column` and `formula` elements can even be combined within the same property or association mapping to express, for example, exotic join conditions.

    <many-to-one name="homeAddress" class="Address"
            insert="false" update="false">
        <column name="person_id" not-null="true" length="10"/>
        <formula>'MAILING'</formula>
    </many-to-one>

## 5.2. Hibernate types

### 5.2.1. Entities and values

In relation to the persistence service, Java language-level objects are classified into two groups:

An entity exists independently of any other objects holding references to the entity. Contrast this with the usual Java model, where an unreferenced object is garbage collected. Entities must be explicitly saved and deleted. Saves and deletions, however, can be cascaded from a parent entity to its children. This is different from the ODMG model of object persistence by reachability and corresponds more closely to how application objects are usually used in large systems. Entities support circular and shared references. They can also be versioned.

An entity's persistent state consists of references to other entities and instances of value types. Values are primitives: collections (not what is inside a collection), components and certain immutable objects. Unlike entities, values in particular collections and components, are persisted and deleted by reachability. Since value objects and primitives are persisted and deleted along with their containing entity, they cannot be independently versioned. Values have no independent identity, so they cannot be shared by two entities or collections.

Until now, we have been using the term "persistent class" to refer to entities. We will continue to do that. Not all user-defined classes with a persistent state, however, are entities. A component is a user-defined class with value semantics. A Java property of type `java.lang.String` also has value semantics. Given this definition, all types (classes) provided by the JDK have value type semantics in Java, while user-defined types can be mapped with entity or value type semantics. This decision is up to the application developer. An entity class in a domain model will normally have shared references to a single instance of that class, while composition or aggregation usually translates to a value type.

We will revisit both concepts throughout this reference guide.

The challenge is to map the Java type system, and the developers' definition of entities and value types, to the SQL/database type system. The bridge between both systems is provided by Hibernate. For entities, `<class>`, `<subclass>` and so on are used. For value types we use `<property>`, `<component>`etc., that usually have a `type` attribute. The value of this attribute is the name of a Hibernate mapping type. Hibernate provides a range of mappings for standard JDK value types out of the box. You can write your own mapping types and implement your own custom conversion strategies.

With the exception of collections, all built-in Hibernate types support null semantics.

### 5.2.2. Basic value types

The built-in basic mapping types can be roughly categorized into the following:

`integer, long, short, float, double, character, byte, boolean, yes_no, true_false`

  Type mappings from Java primitives or wrapper classes to appropriate (vendor-specific) SQL column types. `boolean, yes_no` and `true_false` are all alternative encodings for a Java `boolean` or `java.lang.Boolean`.

`string`

  A type mapping from `java.lang.String` to `VARCHAR` (or Oracle `VARCHAR2`).

`date, time, timestamp`

  Type mappings from `java.util.Date` and its subclasses to SQL types `DATE`, `TIME` and `TIMESTAMP` (or equivalent).

`calendar, calendar_date`

  Type mappings from `java.util.Calendar` to SQL types `TIMESTAMP` and `DATE` (or equivalent).

`big_decimal, big_integer`

  Type mappings from `java.math.BigDecimal` and `java.math.BigInteger` to `NUMERIC` (or Oracle `NUMBER`).

`locale, timezone, currency`

  Type mappings from `java.util.Locale`, `java.util.TimeZone` and `java.util.Currency` to `VARCHAR` (or Oracle `VARCHAR2`). Instances of `Locale` and `Currency` are mapped to their ISO codes. Instances of `TimeZone` are mapped to their `ID`.

`class`

  A type mapping from `java.lang.Class` to `VARCHAR` (or Oracle `VARCHAR2`). A `Class` is mapped to its fully qualified name.

`binary`

  Maps byte arrays to an appropriate SQL binary type.

`text`

  Maps long Java strings to a SQL `LONGVARCHAR` or `TEXT` type.

`image`

  Maps long byte arrays to a SQL `LONGVARBINARY`.

`serializable`

  Maps serializable Java types to an appropriate SQL binary type. You can also indicate the Hibernate type `serializable` with the name of a serializable Java class or interface that does not default to a basic type.

`clob, blob`

  Type mappings for the JDBC classes `java.sql.Clob` and `java.sql.Blob`. These types can be inconvenient for some applications, since the blob or clob object cannot be reused outside of a transaction. Driver support is patchy and inconsistent.

`materialized_clob`

  Maps long Java strings to a SQL `CLOB` type. When read, the `CLOB` value is immediately materialized into a Java string. Some drivers require the `CLOB` value to be read within a transaction. Once materialized, the Java string is available outside of the transaction.

`materialized_blob`

  Maps long Java byte arrays to a SQL `BLOB` type. When read, the `BLOB` value is immediately materialized into a byte array. Some drivers require the `BLOB` value to be read within a transaction. Once materialized, the byte array is available outside of the transaction.

`imm_date, imm_time, imm_timestamp, imm_calendar, imm_calendar_date, imm_serializable, imm_binary`

  Type mappings for what are considered mutable Java types. This is where Hibernate makes certain optimizations appropriate only for immutable Java types, and the application treats the object as immutable. For example, you should not call `Date.setTime()` for an instance mapped as `imm_timestamp`. To change the value of the property, and have that change made persistent, the application must assign a new, nonidentical, object to the property.

Unique identifiers of entities and collections can be of any basic type except `binary`, `blob` and `clob`. Composite identifiers are also allowed. See below for more information.

The basic value types have corresponding `Type` constants defined on `org.hibernate.Hibernate`. For example, `Hibernate.STRING` represents the `string` type.

### 5.2.3. Custom value types

It is relatively easy for developers to create their own value types. For example, you might want to persist properties of type `java.lang.BigInteger` to `VARCHAR` columns. Hibernate does not provide a built-in type for this. Custom types are not limited to mapping a property, or collection element, to a single table column. So, for example, you might have a Java property `getName()`/`setName()` of type `java.lang.String` that is persisted to the columns `FIRST_NAME`, `INITIAL`, `SURNAME`.

To implement a custom type, implement either `org.hibernate.UserType` or `org.hibernate.CompositeUserType` and declare properties using the fully qualified classname of the type. View `org.hibernate.test.DoubleStringType` to see the kind of things that are possible.

    <property name="twoStrings" type="org.hibernate.test.DoubleStringType">
        <column name="first_string"/>
        <column name="second_string"/>
    </property>

Notice the use of `<column>` tags to map a property to multiple columns.

The `CompositeUserType`, `EnhancedUserType`, `UserCollectionType`, and `UserVersionType` interfaces provide support for more specialized uses.

You can even supply parameters to a `UserType` in the mapping file. To do this, your `UserType` must implement the `org.hibernate.usertype.ParameterizedType` interface. To supply parameters to your custom type, you can use the `<type>` element in your mapping files.

    <property name="priority">
        <type name="com.mycompany.usertypes.DefaultValueIntegerType">
            <param name="default">0</param>
        </type>
    </property>

The `UserType` can now retrieve the value for the parameter named `default` from the `Properties` object passed to it.

If you regularly use a certain `UserType`, it is useful to define a shorter name for it. You can do this using the `<typedef>` element. Typedefs assign a name to a custom type, and can also contain a list of default parameter values if the type is parameterized.

    <typedef class="com.mycompany.usertypes.DefaultValueIntegerType" name="default_zero">
        <param name="default">0</param>
    </typedef>

    <property name="priority" type="default_zero"/>

It is also possible to override the parameters supplied in a typedef on a case-by-case basis by using type parameters on the property mapping.

Even though Hibernate's rich range of built-in types and support for components means you will rarely need to use a custom type, it is considered good practice to use custom types for non-entity classes that occur frequently in your application. For example, a `MonetaryAmount` class is a good candidate for a `CompositeUserType`, even though it could be mapped as a component. One reason for this is abstraction. With a custom type, your mapping documents would be protected against changes to the way monetary values are represented.

## 5.3. Mapping a class more than once

It is possible to provide more than one mapping for a particular persistent class. In this case, you must specify an entity name to disambiguate between instances of the two mapped entities. By default, the entity name is the same as the class name. Hibernate lets you specify the entity name when working with persistent objects, when writing queries, or when mapping associations to the named entity.

    <class name="Contract" table="Contracts"
            entity-name="CurrentContract">
        ...
        <set name="history" inverse="true"
                order-by="effectiveEndDate desc">
            <key column="currentContractId"/>
            <one-to-many entity-name="HistoricalContract"/>
        </set>
    </class>

    <class name="Contract" table="ContractHistory"
            entity-name="HistoricalContract">
        ...
        <many-to-one name="currentContract"
                column="currentContractId"
                entity-name="CurrentContract"/>
    </class>

Associations are now specified using `entity-name` instead of `class`.

## Note

This feature is not supported in Annotations

## 5.4. SQL quoted identifiers

You can force Hibernate to quote an identifier in the generated SQL by enclosing the table or column name in backticks in the mapping document. Hibernate will use the correct quotation style for the SQL `Dialect`. This is usually double quotes, but the SQL Server uses brackets and MySQL uses backticks.

    @Entity @Table(name="`Line Item`")
    class LineItem {
       @id @Column(name="`Item Id`") Integer id;
       @Column(name="`Item #`") int itemNumber
    }

    <class name="LineItem" table="`Line Item`">
        <id name="id" column="`Item Id`"/><generator class="assigned"/></id>
        <property name="itemNumber" column="`Item #`"/>
        ...
    </class>

## 5.5. Generated properties

Generated properties are properties that have their values generated by the database. Typically, Hibernate applications needed to `refresh` objects that contain any properties for which the database was generating values. Marking properties as generated, however, lets the application delegate this responsibility to Hibernate. When Hibernate issues an SQL INSERT or UPDATE for an entity that has defined generated properties, it immediately issues a select afterwards to retrieve the generated values.

Properties marked as generated must additionally be non-insertable and non-updateable. Only versions, timestamps, and simple properties, can be marked as generated.

`never` (the default): the given property value is not generated within the database.

`insert`: the given property value is generated on insert, but is not regenerated on subsequent updates. Properties like created-date fall into this category. Even though version and timestamp properties can be marked as generated, this option is not available.

`always`: the property value is generated both on insert and on update.

To mark a property as generated, use `@Generated`.

## 5.6. Column transformers: read and write expressions

Hibernate allows you to customize the SQL it uses to read and write the values of columns mapped to simple properties. For example, if your database provides a set of data encryption functions, you can invoke them for individual columns like this:

    @Entity
    class CreditCard {
       @Column(name="credit_card_num")
       @ColumnTransformer(
          read="decrypt(credit_card_num)",
          write="encrypt(?)")
       public String getCreditCardNumber() { return creditCardNumber; }
       public void setCreditCardNumber(String number) { this.creditCardNumber = number; }
       private String creditCardNumber;
    }

or in XML

    <property name="creditCardNumber">
            <column
              name="credit_card_num"
              read="decrypt(credit_card_num)"
              write="encrypt(?)"/>
    </property>

## Note

You can use the plural form `@ColumnTransformers` if more than one columns need to define either of these rules.

If a property uses more that one column, you must use the `forColumn` attribute to specify which column, the expressions are targeting.

    @Entity
    class User {
       @Type(type="com.acme.type.CreditCardType")
       @Columns( {
          @Column(name="credit_card_num"),
          @Column(name="exp_date") } )
       @ColumnTransformer(
          forColumn="credit_card_num",
          read="decrypt(credit_card_num)",
          write="encrypt(?)")
       public CreditCard getCreditCard() { return creditCard; }
       public void setCreditCard(CreditCard card) { this.creditCard = card; }
       private CreditCard creditCard;
    }

Hibernate applies the custom expressions automatically whenever the property is referenced in a query. This functionality is similar to a derived-property `formula` with two differences:

- The property is backed by one or more columns that are exported as part of automatic schema generation.

- The property is read-write, not read-only.

The `write` expression, if specified, must contain exactly one '?' placeholder for the value.

## 5.7. Auxiliary database objects

Auxiliary database objects allow for the CREATE and DROP of arbitrary database objects. In conjunction with Hibernate's schema evolution tools, they have the ability to fully define a user schema within the Hibernate mapping files. Although designed specifically for creating and dropping things like triggers or stored procedures, any SQL command that can be run via a `java.sql.Statement.execute()` method is valid (for example, ALTERs, INSERTS, etc.). There are essentially two modes for defining auxiliary database objects:

The first mode is to explicitly list the CREATE and DROP commands in the mapping file:

    <hibernate-mapping>
        ...
        <database-object>
            <create>CREATE TRIGGER my_trigger ...</create>
            <drop>DROP TRIGGER my_trigger</drop>
        </database-object>
    </hibernate-mapping>

The second mode is to supply a custom class that constructs the CREATE and DROP commands. This custom class must implement the `org.hibernate.mapping.AuxiliaryDatabaseObject` interface.

    <hibernate-mapping>
        ...
        <database-object>
            <definition class="MyTriggerDefinition"/>
        </database-object>
    </hibernate-mapping>

Additionally, these database objects can be optionally scoped so that they only apply when certain dialects are used.

    <hibernate-mapping>
        ...
        <database-object>
            <definition class="MyTriggerDefinition"/>
            <dialect-scope name="org.hibernate.dialect.Oracle9iDialect"/>
            <dialect-scope name="org.hibernate.dialect.Oracle10gDialect"/>
        </database-object>
    </hibernate-mapping>

## Note

This feature is not supported in Annotations

## Chapter 6. Types

As an Object/Relational Mapping solution, Hibernate deals with both the Java and JDBC representations of application data. An online catalog application, for example, most likely has `Product` object with a number of attributes such as a `sku`, `name`, etc. For these individual attributes, Hibernate must be able to read the values out of the database and write them back. This 'marshalling' is the function of a Hibernate type, which is an implementation of the `org.hibernate.type.Type` interface. In addition, a Hibernate type describes various aspects of behavior of the Java type such as "how is equality checked?" or "how are values cloned?".

## Important

A Hibernate type is neither a Java type nor a SQL datatype; it provides a information about both.

When you encounter the term type in regards to Hibernate be aware that usage might refer to the Java type, the SQL/JDBC type or the Hibernate type.

Hibernate categorizes types into two high-level groups: value types (see Section 6.1, “Value types”) and entity types (see Section 6.2, “Entity types”).

## 6.1. Value types

The main distinguishing characteristic of a value type is the fact that they do not define their own lifecycle. We say that they are "owned" by something else (specifically an entity, as we will see later) which defines their lifecycle. Value types are further classified into 3 sub-categories: basic types (see Section 6.1.1, “Basic value types”), composite types (see Section 6.1.2, “Composite types”) amd collection types (see Section 6.1.3, “Collection types”).

### 6.1.1. Basic value types

The norm for basic value types is that they map a single database value (column) to a single, non-aggregated Java type. Hibernate provides a number of built-in basic types, which we will present in the following sections by the Java type. Mainly these follow the natural mappings recommended in the JDBC specification. We will later cover how to override these mapping and how to provide and use alternative type mappings.

#### 6.1.1.1. java.lang.String

`org.hibernate.type.StringType`

  Maps a string to the JDBC VARCHAR type. This is the standard mapping for a string if no Hibernate type is specified.

  Registered under `string` and `java.lang.String` in the type registry (see Section 6.5, “Type registry”).

`org.hibernate.type.MaterializedClob`

  Maps a string to a JDBC CLOB type

  Registered under `materialized_clob` in the type registry (see Section 6.5, “Type registry”).

`org.hibernate.type.TextType`

  Maps a string to a JDBC LONGVARCHAR type

  Registered under `text` in the type registry (see Section 6.5, “Type registry”).

#### 6.1.1.2. `java.lang.Character` (or char primitive)

`org.hibernate.type.CharacterType`

  Maps a char or `java.lang.Character` to a JDBC CHAR

  Registered under `char` and `java.lang.Character` in the type registry (see Section 6.5, “Type registry”).

#### 6.1.1.3. `java.lang.Boolean` (or boolean primitive)

`org.hibernate.type.BooleanType`

  Maps a boolean to a JDBC BIT type

  Registered under `boolean` and `java.lang.Boolean` in the type registry (see Section 6.5, “Type registry”).

`org.hibernate.type.NumericBooleanType`

  Maps a boolean to a JDBC INTEGER type as 0 = false, 1 = true

  Registered under `numeric_boolean` in the type registry (see Section 6.5, “Type registry”).

`org.hibernate.type.YesNoType`

  Maps a boolean to a JDBC CHAR type as ('N' | 'n') = false, ( 'Y' | 'y' ) = true

  Registered under `yes_no` in the type registry (see Section 6.5, “Type registry”).

`org.hibernate.type.TrueFalseType`

  Maps a boolean to a JDBC CHAR type as ('F' | 'f') = false, ( 'T' | 't' ) = true

  Registered under `true_false` in the type registry (see Section 6.5, “Type registry”).

#### 6.1.1.4. `java.lang.Byte` (or byte primitive)

`org.hibernate.type.ByteType`

  Maps a byte or `java.lang.Byte` to a JDBC TINYINT

  Registered under `byte` and `java.lang.Byte` in the type registry (see Section 6.5, “Type registry”).

#### 6.1.1.5. `java.lang.Short` (or short primitive)

`org.hibernate.type.ShortType`

  Maps a short or `java.lang.Short` to a JDBC SMALLINT

  Registered under `short` and `java.lang.Short` in the type registry (see Section 6.5, “Type registry”).

#### 6.1.1.6. `java.lang.Integer` (or int primitive)

`org.hibernate.type.IntegerTypes`

  Maps an int or `java.lang.Integer` to a JDBC INTEGER

  Registered under `int` and `java.lang.Integer`in the type registry (see Section 6.5, “Type registry”).

#### 6.1.1.7. `java.lang.Long` (or long primitive)

`org.hibernate.type.LongType`

  Maps a long or `java.lang.Long` to a JDBC BIGINT

  Registered under `long` and `java.lang.Long` in the type registry (see Section 6.5, “Type registry”).

#### 6.1.1.8. `java.lang.Float` (or float primitive)

`org.hibernate.type.FloatType`

  Maps a float or `java.lang.Float` to a JDBC FLOAT

  Registered under `float` and `java.lang.Float` in the type registry (see Section 6.5, “Type registry”).

#### 6.1.1.9. `java.lang.Double` (or double primitive)

`org.hibernate.type.DoubleType`

  Maps a double or `java.lang.Double` to a JDBC DOUBLE

  Registered under `double` and `java.lang.Double` in the type registry (see Section 6.5, “Type registry”).

#### 6.1.1.10. `java.math.BigInteger`

`org.hibernate.type.BigIntegerType`

  Maps a `java.math.BigInteger` to a JDBC NUMERIC

  Registered under `big_integer` and `java.math.BigInteger` in the type registry (see Section 6.5, “Type registry”).

#### 6.1.1.11. `java.math.BigDecimal`

`org.hibernate.type.BigDecimalType`

  Maps a `java.math.BigDecimal` to a JDBC NUMERIC

  Registered under `big_decimal` and `java.math.BigDecimal` in the type registry (see Section 6.5, “Type registry”).

#### 6.1.1.12. `java.util.Date` or `java.sql.Timestamp`

`org.hibernate.type.TimestampType`

  Maps a `java.sql.Timestamp` to a JDBC TIMESTAMP

  Registered under `timestamp`, `java.sql.Timestamp` and `java.util.Date` in the type registry (see Section 6.5, “Type registry”).

#### 6.1.1.13. `java.sql.Time`

`org.hibernate.type.TimeType`

  Maps a `java.sql.Time` to a JDBC TIME

  Registered under `time` and `java.sql.Time` in the type registry (see Section 6.5, “Type registry”).

#### 6.1.1.14. `java.sql.Date`

`org.hibernate.type.DateType`

  Maps a `java.sql.Date` to a JDBC DATE

  Registered under `date` and `java.sql.Date` in the type registry (see Section 6.5, “Type registry”).

#### 6.1.1.15. `java.util.Calendar`

`org.hibernate.type.CalendarType`

  Maps a `java.util.Calendar` to a JDBC TIMESTAMP

  Registered under `calendar`, `java.util.Calendar` and `java.util.GregorianCalendar` in the type registry (see Section 6.5, “Type registry”).

`org.hibernate.type.CalendarDateType`

  Maps a `java.util.Calendar` to a JDBC DATE

  Registered under `calendar_date` in the type registry (see Section 6.5, “Type registry”).

#### 6.1.1.16. `java.util.Currency`

`org.hibernate.type.CurrencyType`

  Maps a `java.util.Currency` to a JDBC VARCHAR (using the Currency code)

  Registered under `currency` and `java.util.Currency` in the type registry (see Section 6.5, “Type registry”).

#### 6.1.1.17. `java.util.Locale`

`org.hibernate.type.LocaleType`

  Maps a `java.util.Locale` to a JDBC VARCHAR (using the Locale code)

  Registered under `locale` and `java.util.Locale` in the type registry (see Section 6.5, “Type registry”).

#### 6.1.1.18. `java.util.TimeZone`

`org.hibernate.type.TimeZoneType`

  Maps a `java.util.TimeZone` to a JDBC VARCHAR (using the TimeZone ID)

  Registered under `timezone` and `java.util.TimeZone` in the type registry (see Section 6.5, “Type registry”).

#### 6.1.1.19. `java.net.URL`

`org.hibernate.type.UrlType`

  Maps a `java.net.URL` to a JDBC VARCHAR (using the external form)

  Registered under `url` and `java.net.URL` in the type registry (see Section 6.5, “Type registry”).

#### 6.1.1.20. `java.lang.Class`

`org.hibernate.type.ClassType`

  Maps a `java.lang.Class` to a JDBC VARCHAR (using the Class name)

  Registered under `class` and `java.lang.Class` in the type registry (see Section 6.5, “Type registry”).

#### 6.1.1.21. `java.sql.Blob`

`org.hibernate.type.BlobType`

  Maps a `java.sql.Blob` to a JDBC BLOB

  Registered under `blob` and `java.sql.Blob` in the type registry (see Section 6.5, “Type registry”).

#### 6.1.1.22. `java.sql.Clob`

`org.hibernate.type.ClobType`

  Maps a `java.sql.Clob` to a JDBC CLOB

  Registered under `clob` and `java.sql.Clob` in the type registry (see Section 6.5, “Type registry”).

#### 6.1.1.23. byte[]

`org.hibernate.type.BinaryType`

  Maps a primitive byte[] to a JDBC VARBINARY

  Registered under `binary` and `byte[]` in the type registry (see Section 6.5, “Type registry”).

`org.hibernate.type.MaterializedBlobType`

  Maps a primitive byte[] to a JDBC BLOB

  Registered under `materialized_blob` in the type registry (see Section 6.5, “Type registry”).

`org.hibernate.type.ImageType`

  Maps a primitive byte[] to a JDBC LONGVARBINARY

  Registered under `image` in the type registry (see Section 6.5, “Type registry”).

#### 6.1.1.24. Byte[]

`org.hibernate.type.BinaryType`

  Maps a java.lang.Byte[] to a JDBC VARBINARY

  Registered under `wrapper-binary`, `Byte[]` and `java.lang.Byte[]` in the type registry (see Section 6.5, “Type registry”).

#### 6.1.1.25. char[]

`org.hibernate.type.CharArrayType`

  Maps a char[] to a JDBC VARCHAR

  Registered under `characters` and `char[]` in the type registry (see Section 6.5, “Type registry”).

#### 6.1.1.26. Character[]

`org.hibernate.type.CharacterArrayType`

  Maps a java.lang.Character[] to a JDBC VARCHAR

  Registered under `wrapper-characters`, `Character[]` and `java.lang.Character[]` in the type registry (see Section 6.5, “Type registry”).

#### 6.1.1.27. `java.util.UUID`

`org.hibernate.type.UUIDBinaryType`

  Maps a java.util.UUID to a JDBC BINARY

  Registered under `uuid-binary` and `java.util.UUID` in the type registry (see Section 6.5, “Type registry”).

`org.hibernate.type.UUIDCharType`

  Maps a java.util.UUID to a JDBC CHAR (though VARCHAR is fine too for existing schemas)

  Registered under `uuid-char` in the type registry (see Section 6.5, “Type registry”).

`org.hibernate.type.PostgresUUIDType`

  Maps a java.util.UUID to the PostgreSQL UUID data type (through `Types#OTHER` which is how the PostgreSQL JDBC driver defines it).

  Registered under `pg-uuid` in the type registry (see Section 6.5, “Type registry”).

#### 6.1.1.28. `java.io.Serializable`

`org.hibernate.type.SerializableType`

  Maps implementors of java.lang.Serializable to a JDBC VARBINARY

  Unlike the other value types, there are multiple instances of this type. It gets registered once under `java.io.Serializable`. Additionally it gets registered under the specific `java.io.Serializable` implementation class names.

### 6.1.2. Composite types

## Note

The Java Persistence API calls these embedded types, while Hibernate traditionally called them components. Just be aware that both terms are used and mean the same thing in the scope of discussing Hibernate.

Components represent aggregations of values into a single Java type. For example, you might have an Address class that aggregates street, city, state, etc information or a Name class that aggregates the parts of a person's Name. In many ways a component looks exactly like an entity. They are both (generally speaking) classes written specifically for the application. They both might have references to other application-specific classes, as well as to collections and simple JDK types. As discussed before, the only distinguishing factory is the fact that a component does not own its own lifecycle nor does it define an identifier.

### 6.1.3. Collection types

## Important

It is critical understand that we mean the collection itself, not its contents. The contents of the collection can in turn be basic, component or entity types (though not collections), but the collection itself is owned.

Collections are covered in Chapter 7, Collection mapping.

## 6.2. Entity types

The definition of entities is covered in detail in Chapter 4, Persistent Classes. For the purpose of this discussion, it is enough to say that entities are (generally application-specific) classes which correlate to rows in a table. Specifically they correlate to the row by means of a unique identifier. Because of this unique identifier, entities exist independently and define their own lifecycle. As an example, when we delete a `Membership`, both the `User` and `Group` entities remain.

## Note

This notion of entity independence can be modified by the application developer using the concept of cascades. Cascades allow certain operations to continue (or "cascade") across an association from one entity to another. Cascades are covered in detail in Chapter 8, Association Mappings.

## 6.3. Significance of type categories

Why do we spend so much time categorizing the various types of types? What is the significance of the distinction?

The main categorization was between entity types and value types. To review we said that entities, by nature of their unique identifier, exist independently of other objects whereas values do not. An application cannot "delete" a Product sku; instead, the sku is removed when the Product itself is deleted (obviously you can update the sku of that Product to null to make it "go away", but even there the access is done through the Product).

Nor can you define an association to that Product sku. You can define an association to Product based on its sku, assuming sku is unique, but that is totally different.

TBC...

## 6.4. Custom types

Hibernate makes it relatively easy for developers to create their own value types. For example, you might want to persist properties of type `java.lang.BigInteger` to `VARCHAR` columns. Custom types are not limited to mapping values to a single table column. So, for example, you might want to concatenate together `FIRST_NAME`, `INITIAL` and `SURNAME` columns into a `java.lang.String`.

There are 3 approaches to developing a custom Hibernate type. As a means of illustrating the different approaches, lets consider a use case where we need to compose a `java.math.BigDecimal` and `java.util.Currency` together into a custom `Money` class.

### 6.4.1. Custom types using `org.hibernate.type.Type`

The first approach is to directly implement the `org.hibernate.type.Type` interface (or one of its derivatives). Probably, you will be more interested in the more specific `org.hibernate.type.BasicType` contract which would allow registration of the type (see Section 6.5, “Type registry”). The benefit of this registration is that whenever the metadata for a particular property does not specify the Hibernate type to use, Hibernate will consult the registry for the exposed property type. In our example, the property type would be `Money`, which is the key we would use to register our type in the registry:

Example 6.1. Defining and registering the custom Type

    public class MoneyType implements BasicType {
        public String[] getRegistrationKeys() {
            return new String[] { Money.class.getName() };
        }

    	public int[] sqlTypes(Mapping mapping) {
    	    // We will simply use delegation to the standard basic types for BigDecimal and Currency for many of the
    	    // Type methods...
    	    return new int[] {
    	             BigDecimalType.INSTANCE.sqlType(),
    	             CurrencyType.INSTANCE.sqlType(),
    	    };
    	    // we could also have honored any registry overrides via...
    	    //return new int[] {
    	    //         mappings.getTypeResolver().basic( BigDecimal.class.getName() ).sqlTypes( mappings )[0],
    	    //         mappings.getTypeResolver().basic( Currency.class.getName() ).sqlTypes( mappings )[0]
    	    //};
    	}

        public Class getReturnedClass() {
            return Money.class;
        }

        public Object nullSafeGet(ResultSet rs, String[] names, SessionImplementor session, Object owner) throws SQLException {
            assert names.length == 2;
            BigDecimal amount = BigDecimalType.INSTANCE.get( names[0] ); // already handles null check
            Currency currency = CurrencyType.INSTANCE.get( names[1] ); // already handles null check
            return amount == null && currency == null
                    ? null
                    : new Money( amount, currency );
        }

        public void nullSafeSet(PreparedStatement st, Object value, int index, boolean[] settable, SessionImplementor session)
                throws SQLException {
            if ( value == null ) {
                BigDecimalType.INSTANCE.set( st, null, index );
                CurrencyType.INSTANCE.set( st, null, index+1 );
            }
            else {
                final Money money = (Money) value;
                BigDecimalType.INSTANCE.set( st, money.getAmount(), index );
                CurrencyType.INSTANCE.set( st, money.getCurrency(), index+1 );
            }
        }

        ...
    }

    Configuration cfg = new Configuration();
    cfg.registerTypeOverride( new MoneyType() );
    cfg...;

## Important

It is important that we registered the type before adding mappings.

### 6.4.2. Custom types using `org.hibernate.usertype.UserType`

## Note

Both `org.hibernate.usertype.UserType` and `org.hibernate.usertype.CompositeUserType` were originally added to isolate user code from internal changes to the `org.hibernate.type.Type` interfaces.

The second approach is the use the `org.hibernate.usertype.UserType` interface, which presents a somewhat simplified view of the `org.hibernate.type.Type` interface. Using a `org.hibernate.usertype.UserType`, our `Money` custom type would look as follows:

Example 6.2. Defining the custom UserType

    public class MoneyType implements UserType {
        public int[] sqlTypes() {
            return new int[] {
                    BigDecimalType.INSTANCE.sqlType(),
                    CurrencyType.INSTANCE.sqlType(),
            };
        }

        public Class getReturnedClass() {
            return Money.class;
        }

        public Object nullSafeGet(ResultSet rs, String[] names, Object owner) throws SQLException {
            assert names.length == 2;
            BigDecimal amount = BigDecimalType.INSTANCE.get( names[0] ); // already handles null check
            Currency currency = CurrencyType.INSTANCE.get( names[1] ); // already handles null check
            return amount == null && currency == null
                    ? null
                    : new Money( amount, currency );
        }

        public void nullSafeSet(PreparedStatement st, Object value, int index) throws SQLException {
            if ( value == null ) {
                BigDecimalType.INSTANCE.set( st, null, index );
                CurrencyType.INSTANCE.set( st, null, index+1 );
            }
            else {
                final Money money = (Money) value;
                BigDecimalType.INSTANCE.set( st, money.getAmount(), index );
                CurrencyType.INSTANCE.set( st, money.getCurrency(), index+1 );
            }
        }

        ...
    }

There is not much difference between the `org.hibernate.type.Type` example and the `org.hibernate.usertype.UserType` example, but that is only because of the snippets shown. If you choose the `org.hibernate.type.Type` approach there are quite a few more methods you would need to implement as compared to the `org.hibernate.usertype.UserType`.

### 6.4.3. Custom types using `org.hibernate.usertype.CompositeUserType`

The third and final approach is the use the `org.hibernate.usertype.CompositeUserType` interface, which differs from `org.hibernate.usertype.UserType` in that it gives us the ability to provide Hibernate the information to handle the composition within the `Money` class (specifically the 2 attributes). This would give us the capability, for example, to reference the `amount` attribute in an HQL query. Using a `org.hibernate.usertype.CompositeUserType`, our `Money` custom type would look as follows:

Example 6.3. Defining the custom CompositeUserType

    public class MoneyType implements CompositeUserType {
        public String[] getPropertyNames() {
            // ORDER IS IMPORTANT!  it must match the order the columns are defined in the property mapping
            return new String[] { "amount", "currency" };
        }

        public Type[] getPropertyTypes() {
            return new Type[] { BigDecimalType.INSTANCE, CurrencyType.INSTANCE };
        }

        public Class getReturnedClass() {
            return Money.class;
        }

        public Object getPropertyValue(Object component, int propertyIndex) {
            if ( component == null ) {
                return null;
            }

            final Money money = (Money) component;
            switch ( propertyIndex ) {
                case 0: {
                    return money.getAmount();
                }
                case 1: {
                    return money.getCurrency();
                }
                default: {
                    throw new HibernateException( "Invalid property index [" + propertyIndex + "]" );
                }
            }
        }

    	public void setPropertyValue(Object component, int propertyIndex, Object value) throws HibernateException {
            if ( component == null ) {
                return;
            }

            final Money money = (Money) component;
            switch ( propertyIndex ) {
                case 0: {
                    money.setAmount( (BigDecimal) value );
                    break;
                }
                case 1: {
                    money.setCurrency( (Currency) value );
                    break;
                }
                default: {
                    throw new HibernateException( "Invalid property index [" + propertyIndex + "]" );
                }
            }
    	}

        public Object nullSafeGet(ResultSet rs, String[] names, SessionImplementor session, Object owner) throws SQLException {
            assert names.length == 2;
            BigDecimal amount = BigDecimalType.INSTANCE.get( names[0] ); // already handles null check
            Currency currency = CurrencyType.INSTANCE.get( names[1] ); // already handles null check
            return amount == null && currency == null
                    ? null
                    : new Money( amount, currency );
        }

        public void nullSafeSet(PreparedStatement st, Object value, int index, SessionImplementor session) throws SQLException {
            if ( value == null ) {
                BigDecimalType.INSTANCE.set( st, null, index );
                CurrencyType.INSTANCE.set( st, null, index+1 );
            }
            else {
                final Money money = (Money) value;
                BigDecimalType.INSTANCE.set( st, money.getAmount(), index );
                CurrencyType.INSTANCE.set( st, money.getCurrency(), index+1 );
            }
        }

        ...
    }

## 6.5. Type registry

Internally Hibernate uses a registry of basic types (see Section 6.1.1, “Basic value types”) when it needs to resolve the specific `org.hibernate.type.Type` to use in certain situations. It also provides a way for applications to add extra basic type registrations as well as override the standard basic type registrations.

To register a new type or to override an existing type registration, applications would make use of the `registerTypeOverride` method of the `org.hibernate.cfg.Configuration` class when bootstrapping Hibernate. For example, lets say you want Hibernate to use your custom `SuperDuperStringType`; during bootstrap you would call:

Example 6.4. Overriding the standard `StringType`

    Configuration cfg = ...;
    cfg.registerTypeOverride( new SuperDuperStringType() );

The argument to `registerTypeOverride` is a `org.hibernate.type.BasicType` which is a specialization of the `org.hibernate.type.Type` we saw before. It adds a single method:

Example 6.5. Snippet from BasicType.java

        /**
    	 * Get the names under which this type should be registered in the type registry.
    	 *
    	 * @return The keys under which to register this type.
    	 */
    	public String[] getRegistrationKeys();

One approach is to use inheritance (`SuperDuperStringType` extends `org.hibernate.type.StringType`); another is to use delegation.

## Chapter 7. Collection mapping

## 7.1. Persistent collections

Naturally Hibernate also allows to persist collections. These persistent collections can contain almost any other Hibernate type, including: basic types, custom types, components and references to other entities. The distinction between value and reference semantics is in this context very important. An object in a collection might be handled with "value" semantics (its life cycle fully depends on the collection owner), or it might be a reference to another entity with its own life cycle. In the latter case, only the "link" between the two objects is considered to be a state held by the collection.

As a requirement persistent collection-valued fields must be declared as an interface type (see Example 7.2, “Collection mapping using @OneToMany and @JoinColumn”). The actual interface might be `java.util.Set`, `java.util.Collection`, `java.util.List`, `java.util.Map`, `java.util.SortedSet`, `java.util.SortedMap` or anything you like ("anything you like" means you will have to write an implementation of `org.hibernate.usertype.UserCollectionType`).

Notice how in Example 7.2, “Collection mapping using @OneToMany and @JoinColumn” the instance variable `parts` was initialized with an instance of `HashSet`. This is the best way to initialize collection valued properties of newly instantiated (non-persistent) instances. When you make the instance persistent, by calling `persist()`, Hibernate will actually replace the `HashSet` with an instance of Hibernate's own implementation of `Set`. Be aware of the following error:

Example 7.1. Hibernate uses its own collection implementations

    Cat cat = new DomesticCat();
    Cat kitten = new DomesticCat();
    ....
    Set kittens = new HashSet();
    kittens.add(kitten);
    cat.setKittens(kittens);
    session.persist(cat);

    kittens = cat.getKittens(); // Okay, kittens collection is a Set
    (HashSet) cat.getKittens(); // Error!

The persistent collections injected by Hibernate behave like `HashMap`, `HashSet`, `TreeMap`, `TreeSet` or `ArrayList`, depending on the interface type.

Collections instances have the usual behavior of value types. They are automatically persisted when referenced by a persistent object and are automatically deleted when unreferenced. If a collection is passed from one persistent object to another, its elements might be moved from one table to another. Two entities cannot share a reference to the same collection instance. Due to the underlying relational model, collection-valued properties do not support null value semantics. Hibernate does not distinguish between a null collection reference and an empty collection.

## Note

Use persistent collections the same way you use ordinary Java collections. However, ensure you understand the semantics of bidirectional associations (see Section 7.3.2, “Bidirectional associations”).

## 7.2. How to map collections

Using annotations you can map `Collection`s, `List`s, `Map`s and `Set`s of associated entities using @OneToMany and @ManyToMany. For collections of a basic or embeddable type use @ElementCollection. In the simplest case a collection mapping looks like this:

Example 7.2. Collection mapping using @OneToMany and @JoinColumn

    @Entity
    public class Product {

        private String serialNumber;
        private Set<Part> parts = new HashSet<Part>();

        @Id
        public String getSerialNumber() { return serialNumber; }
        void setSerialNumber(String sn) { serialNumber = sn; }

        @OneToMany
        @JoinColumn(name="PART_ID")
        public Set<Part> getParts() { return parts; }
        void setParts(Set parts) { this.parts = parts; }
    }

    @Entity
    public class Part {
       ...
    }

Product describes a unidirectional relationship with Part using the join column PART_ID. In this unidirectional one to many scenario you can also use a join table as seen in Example 7.3, “Collection mapping using @OneToMany and @JoinTable”.

Example 7.3. Collection mapping using @OneToMany and @JoinTable

    @Entity
    public class Product {

        private String serialNumber;
        private Set<Part> parts = new HashSet<Part>();

        @Id
        public String getSerialNumber() { return serialNumber; }
        void setSerialNumber(String sn) { serialNumber = sn; }

        @OneToMany
        @JoinTable(
                name="PRODUCT_PARTS",
                joinColumns = @JoinColumn( name="PRODUCT_ID"),
                inverseJoinColumns = @JoinColumn( name="PART_ID")
        )
        public Set<Part> getParts() { return parts; }
        void setParts(Set parts) { this.parts = parts; }
    }

    @Entity
    public class Part {
       ...
    }

Without describing any physical mapping (no `@JoinColumn` or `@JoinTable`), a unidirectional one to many with join table is used. The table name is the concatenation of the owner table name, _, and the other side table name. The foreign key name(s) referencing the owner table is the concatenation of the owner table, _, and the owner primary key column(s) name. The foreign key name(s) referencing the other side is the concatenation of the owner property name, _, and the other side primary key column(s) name. A unique constraint is added to the foreign key referencing the other side table to reflect the one to many.

Lets have a look now how collections are mapped using Hibernate mapping files. In this case the first step is to chose the right mapping element. It depends on the type of interface. For example, a `<set>` element is used for mapping properties of type `Set`.

Example 7.4. Mapping a Set using <set>

    <class name="Product">
        <id name="serialNumber" column="productSerialNumber"/>
        <set name="parts">
            <key column="productSerialNumber" not-null="true"/>
            <one-to-many class="Part"/>
        </set>
    </class>

In Example 7.4, “Mapping a Set using <set>” a one-to-many association links the `Product` and `Part` entities. This association requires the existence of a foreign key column and possibly an index column to the `Part` table. This mapping loses certain semantics of normal Java collections:

- An instance of the contained entity class cannot belong to more than one instance of the collection.

- An instance of the contained entity class cannot appear at more than one value of the collection index.

Looking closer at the used `<one-to-many>` tag we see that it has the following options.

Example 7.5. options of <one-to-many> element

    <one-to-many
            class="ClassName"
            not-found="ignore|exception"
            entity-name="EntityName"
            node="element-name"
            embed-xml="true|false"
        />

| `class` (required): the name of the associated class.
| `not-found` (optional - defaults to `exception`): specifies how cached identifiers that reference missing rows will be handled. `ignore` will treat a missing row as a null association.
| `entity-name` (optional): the entity name of the associated class, as an alternative to `class`.

The `<one-to-many>` element does not need to declare any columns. Nor is it necessary to specify the `table` name anywhere.

## Warning

If the foreign key column of a `<one-to-many>` association is declared `NOT NULL`, you must declare the `<key>` mapping `not-null="true"` or use a bidirectional association with the collection mapping marked `inverse="true"`. See Section 7.3.2, “Bidirectional associations”.

Apart from the `<set>` tag as shown in Example 7.4, “Mapping a Set using <set>”, there is also `<list>`, `<map>`, `<bag>`, `<array>` and `<primitive-array>` mapping elements. The `<map>` element is representative:

Example 7.6. Elements of the <map> mapping

    <map
        name="propertyName"
        table="table_name"
        schema="schema_name"
        lazy="true|extra|false"
        inverse="true|false"
        cascade="all|none|save-update|delete|all-delete-orphan|delete-orphan"
        sort="unsorted|natural|comparatorClass"
        order-by="column_name asc|desc"
        where="arbitrary sql where condition"
        fetch="join|select|subselect"
        batch-size="N"
        access="field|property|ClassName"
        optimistic-lock="true|false"
        mutable="true|false"
        node="element-name|."
        embed-xml="true|false"
    >

        <key .... />
        <map-key .... />
        <element .... />
    </map>

| `name`: the collection property name
| `table` (optional - defaults to property name): the name of the collection table. It is not used for one-to-many associations.
| `schema` (optional): the name of a table schema to override the schema declared on the root element
| `lazy` (optional - defaults to `true`): disables lazy fetching and specifies that the association is always eagerly fetched. It can also be used to enable "extra-lazy" fetching where most operations do not initialize the collection. This is suitable for large collections.
| `inverse` (optional - defaults to `false`): marks this collection as the "inverse" end of a bidirectional association.
| `cascade` (optional - defaults to `none`): enables operations to cascade to child entities.
| `sort` (optional): specifies a sorted collection with `natural` sort order or a given comparator class.
| `order-by` (optional): specifies a table column or columns that define the iteration order of the `Map`, `Set` or bag, together with an optional `asc` or `desc`.
| `where` (optional): specifies an arbitrary SQL `WHERE` condition that is used when retrieving or removing the collection. This is useful if the collection needs to contain only a subset of the available data.
| `fetch` (optional, defaults to `select`): chooses between outer-join fetching, fetching by sequential select, and fetching by sequential subselect.
| `batch-size` (optional, defaults to `1`): specifies a "batch size" for lazily fetching instances of this collection.
| `access` (optional - defaults to `property`): the strategy Hibernate uses for accessing the collection property value.
| `optimistic-lock` (optional - defaults to `true`): specifies that changes to the state of the collection results in increments of the owning entity's version. For one-to-many associations you may want to disable this setting.
| `mutable` (optional - defaults to `true`): a value of `false` specifies that the elements of the collection never change. This allows for minor performance optimization in some cases.

After exploring the basic mapping of collections in the preceding paragraphs we will now focus details like physical mapping considerations, indexed collections and collections of value types.

### 7.2.1. Collection foreign keys

On the database level collection instances are distinguished by the foreign key of the entity that owns the collection. This foreign key is referred to as the collection key column, or columns, of the collection table. The collection key column is mapped by the `@JoinColumn` annotation respectively the `<key>` XML element.

There can be a nullability constraint on the foreign key column. For most collections, this is implied. For unidirectional one-to-many associations, the foreign key column is nullable by default, so you may need to specify

    @JoinColumn(nullable=false)

or

    <key column="productSerialNumber" not-null="true"/>

The foreign key constraint can use `ON DELETE CASCADE`. In XML this can be expressed via:

    <key column="productSerialNumber" on-delete="cascade"/>

In annotations the Hibernate specific annotation @OnDelete has to be used.

    @OnDelete(action=OnDeleteAction.CASCADE)

See Section 5.1.11.3, “Key” for more information about the `<key>` element.

### 7.2.2. Indexed collections

In the following paragraphs we have a closer at the indexed collections `List` and `Map` how the their index can be mapped in Hibernate.

#### 7.2.2.1. Lists

Lists can be mapped in two different ways:

- as ordered lists, where the order is not materialized in the database

- as indexed lists, where the order is materialized in the database

To order lists in memory, add `@javax.persistence.OrderBy` to your property. This annotation takes as parameter a list of comma separated properties (of the target entity) and orders the collection accordingly (eg `firstname asc, age desc`), if the string is empty, the collection will be ordered by the primary key of the target entity.

Example 7.7. Ordered lists using `@OrderBy`

    @Entity

    public class Customer {

       @Id @GeneratedValue public Integer getId() { return id; }

       public void setId(Integer id) { this.id = id; }

       private Integer id;

       @OneToMany(mappedBy="customer")

       @OrderBy("number")

       public List<Order> getOrders() { return orders; }

       public void setOrders(List<Order> orders) { this.orders = orders; }

       private List<Order> orders;

    }

    @Entity

    public class Order {

       @Id @GeneratedValue public Integer getId() { return id; }

       public void setId(Integer id) { this.id = id; }

       private Integer id;

       public String getNumber() { return number; }

       public void setNumber(String number) { this.number = number; }

       private String number;

       @ManyToOne

       public Customer getCustomer() { return customer; }

       public void setCustomer(Customer customer) { this.customer = customer; }

       private Customer number;

    }

    -- Table schema

    |-------------| |----------|

    | Order       | | Customer |

    |-------------| |----------|

    | id          | | id       |

    | number      | |----------|

    | customer_id |

    |-------------|

To store the index value in a dedicated column, use the `@javax.persistence.OrderColumn` annotation on your property. This annotations describes the column name and attributes of the column keeping the index value. This column is hosted on the table containing the association foreign key. If the column name is not specified, the default is the name of the referencing property, followed by underscore, followed by `ORDER` (in the following example, it would be `orders_ORDER`).

Example 7.8. Explicit index column using `@OrderColumn`

    @Entity

    public class Customer {

       @Id @GeneratedValue public Integer getId() { return id; }

       public void setId(Integer id) { this.id = id; }

       private Integer id;

       @OneToMany(mappedBy="customer")

       @OrderColumn(name="orders_index")

       public List<Order> getOrders() { return orders; }

       public void setOrders(List<Order> orders) { this.orders = orders; }

       private List<Order> orders;

    }

    @Entity

    public class Order {

       @Id @GeneratedValue public Integer getId() { return id; }

       public void setId(Integer id) { this.id = id; }

       private Integer id;

       public String getNumber() { return number; }

       public void setNumber(String number) { this.number = number; }

       private String number;

       @ManyToOne

       public Customer getCustomer() { return customer; }

       public void setCustomer(Customer customer) { this.customer = customer; }

       private Customer number;

    }

    -- Table schema

    |--------------| |----------|

    | Order        | | Customer |

    |--------------| |----------|

    | id           | | id       |

    | number       | |----------|

    | customer_id  |

    | orders_order |

    |--------------|

## Note

We recommend you to convert `the legacy @org.hibernate.annotations.IndexColumn` usages to `@OrderColumn` unless you are making use of the base property. The `base` property lets you define the index value of the first element (aka as base index). The usual value is `0` or `1`. The default is 0 like in Java.

Looking again at the Hibernate mapping file equivalent, the index of an array or list is always of type `integer` and is mapped using the `<list-index>` element. The mapped column contains sequential integers that are numbered from zero by default.

Example 7.9. index-list element for indexed collections in xml mapping

    <list-index
            column="column_name"
            base="0|1|..."/>

| `column_name` (required): the name of the column holding the collection index values.
| `base` (optional - defaults to `0`): the value of the index column that corresponds to the first element of the list or array.

If your table does not have an index column, and you still wish to use `List` as the property type, you can map the property as a Hibernate <bag>. A bag does not retain its order when it is retrieved from the database, but it can be optionally sorted or ordered.

#### 7.2.2.2. Maps

The question with `Map`s is where the key value is stored. There are everal options. Maps can borrow their keys from one of the associated entity properties or have dedicated columns to store an explicit key.

To use one of the target entity property as a key of the map, use `@MapKey(name="myProperty")`, where `myProperty` is a property name in the target entity. When using `@MapKey` without the name attribuate, the target entity primary key is used. The map key uses the same column as the property pointed out. There is no additional column defined to hold the map key, because the map key represent a target property. Be aware that once loaded, the key is no longer kept in sync with the property. In other words, if you change the property value, the key will not change automatically in your Java model.

Example 7.10. Use of target entity property as map key via `@MapKey`

    @Entity

    public class Customer {

       @Id @GeneratedValue public Integer getId() { return id; }

       public void setId(Integer id) { this.id = id; }

       private Integer id;

       @OneToMany(mappedBy="customer")

       @MapKey(name="number")

       public Map<String,Order> getOrders() { return orders; }

       public void setOrders(Map<String,Order> order) { this.orders = orders; }

       private Map<String,Order> orders;

    }

    @Entity

    public class Order {

       @Id @GeneratedValue public Integer getId() { return id; }

       public void setId(Integer id) { this.id = id; }

       private Integer id;

       public String getNumber() { return number; }

       public void setNumber(String number) { this.number = number; }

       private String number;

       @ManyToOne

       public Customer getCustomer() { return customer; }

       public void setCustomer(Customer customer) { this.customer = customer; }

       private Customer number;

    }

    -- Table schema

    |-------------| |----------|

    | Order       | | Customer |

    |-------------| |----------|

    | id          | | id       |

    | number      | |----------|

    | customer_id |

    |-------------|

Alternatively the map key is mapped to a dedicated column or columns. In order to customize the mapping use one of the following annotations:

- @`MapKeyColumn` if the map key is a basic type. If you don't specify the column name, the name of the property followed by underscore followed by `KEY` is used (for example `orders_KEY`).

- `@MapKeyEnumerated` / `@MapKeyTemporal` if the map key type is respectively an enum or a `Date`.

- `@MapKeyJoinColumn`/`@MapKeyJoinColumns` if the map key type is another entity.

- `@AttributeOverride`/`@AttributeOverrides` when the map key is a embeddable object. Use `key.` as a prefix for your embeddable object property names.

You can also use `@MapKeyClass` to define the type of the key if you don't use generics.

Example 7.11. Map key as basic type using `@MapKeyColumn`

    @Entity

    public class Customer {

       @Id @GeneratedValue public Integer getId() { return id; }

       public void setId(Integer id) { this.id = id; }

       private Integer id;

       @OneToMany @JoinTable(name="Cust_Order")

       @MapKeyColumn(name="orders_number")

       public Map<String,Order> getOrders() { return orders; }

       public void setOrders(Map<String,Order> orders) { this.orders = orders; }

       private Map<String,Order> orders;

    }

    @Entity

    public class Order {

       @Id @GeneratedValue public Integer getId() { return id; }

       public void setId(Integer id) { this.id = id; }

       private Integer id;

       public String getNumber() { return number; }

       public void setNumber(String number) { this.number = number; }

       private String number;

       @ManyToOne

       public Customer getCustomer() { return customer; }

       public void setCustomer(Customer customer) { this.customer = customer; }

       private Customer number;

    }

    -- Table schema

    |-------------| |----------| |---------------|

    | Order       | | Customer | | Cust_Order    |

    |-------------| |----------| |---------------|

    | id          | | id       | | customer_id   |

    | number      | |----------| | order_id      |

    | customer_id |              | orders_number |

    |-------------|              |---------------|

## Note

We recommend you to migrate from `@org.hibernate.annotations.MapKey` / `@org.hibernate.annotation.MapKeyManyToMany` to the new standard approach described above

Using Hibernate mapping files there exists equivalent concepts to the descibed annotations. You have to use `<map-key>`, `<map-key-many-to-many>` and `<composite-map-key>`. `<map-key>` is used for any basic type, `<map-key-many-to-many>` for an entity reference and `<composite-map-key>` for a composite type.

Example 7.12. map-key xml mapping element

    <map-key
            column="column_name"
            formula="any SQL expression"
            type="type_name"
            node="@attribute-name"
            length="N"/>

| `column` (optional): the name of the column holding the collection index values.
| `formula` (optional): a SQL formula used to evaluate the key of the map.
| `type` (required): the type of the map keys.

Example 7.13. map-key-many-to-many

    <map-key-many-to-many
            column="column_name"
            formula="any SQL expression"
            class="ClassName"
    />

| `column` (optional): the name of the foreign key column for the collection index values.
| `formula` (optional): a SQ formula used to evaluate the foreign key of the map key.
| `class` (required): the entity class used as the map key.

### 7.2.3. Collections of basic types and embeddable objects

In some situations you don't need to associate two entities but simply create a collection of basic types or embeddable objects. Use the `@ElementCollection` for this case.

Example 7.14. Collection of basic types mapped via `@ElementCollection`

    @Entity

    public class User {

       [...]

       public String getLastname() { ...}

       @ElementCollection

       @CollectionTable(name="Nicknames", joinColumns=@JoinColumn(name="user_id"))

       @Column(name="nickname")

       public Set<String> getNicknames() { ... }

    }

The collection table holding the collection data is set using the `@CollectionTable` annotation. If omitted the collection table name defaults to the concatenation of the name of the containing entity and the name of the collection attribute, separated by an underscore. In our example, it would be `User_nicknames`.

The column holding the basic type is set using the `@Column` annotation. If omitted, the column name defaults to the property name: in our example, it would be `nicknames`.

But you are not limited to basic types, the collection type can be any embeddable object. To override the columns of the embeddable object in the collection table, use the `@AttributeOverride` annotation.

Example 7.15. @ElementCollection for embeddable objects

    @Entity

    public class User {

       [...]

       public String getLastname() { ...}

       @ElementCollection

       @CollectionTable(name="Addresses", joinColumns=@JoinColumn(name="user_id"))

       @AttributeOverrides({

          @AttributeOverride(name="street1", column=@Column(name="fld_street"))

       })

       public Set<Address> getAddresses() { ... }

    }

    @Embeddable

    public class Address {

       public String getStreet1() {...}

       [...]

    }

Such an embeddable object cannot contains a collection itself.

## Note

in `@AttributeOverride`, you must use the `value.` prefix to override properties of the embeddable object used in the map value and the `key.` prefix to override properties of the embeddable object used in the map key.

    @Entity

    public class User {

       @ElementCollection

       @AttributeOverrides({

          @AttributeOverride(name="key.street1", column=@Column(name="fld_street")),

          @AttributeOverride(name="value.stars", column=@Column(name="fld_note"))

       })

       public Map<Address,Rating> getFavHomes() { ... }

## Note

We recommend you to migrate from `@org.hibernate.annotations.CollectionOfElements` to the new `@ElementCollection` annotation.

Using the mapping file approach a collection of values is mapped using the `<element>` tag. For example:

Example 7.16. <element> tag for collection values using mapping files

    <element
            column="column_name"
            formula="any SQL expression"
            type="typename"
            length="L"
            precision="P"
            scale="S"
            not-null="true|false"
            unique="true|false"
            node="element-name"
    />

| `column` (optional): the name of the column holding the collection element values.
| `formula` (optional): an SQL formula used to evaluate the element.
| `type` (required): the type of the collection element.

## 7.3. Advanced collection mappings

### 7.3.1. Sorted collections

Hibernate supports collections implementing `java.util.SortedMap` and `java.util.SortedSet`. With annotations you declare a sort comparator using `@Sort`. You chose between the comparator types unsorted, natural or custom. If you want to use your own comparator implementation, you'll also have to specify the implementation class using the `comparator` attribute. Note that you need to use either a `SortedSet` or a `SortedMap` interface.

Example 7.17. Sorted collection with @Sort

    @OneToMany(cascade=CascadeType.ALL, fetch=FetchType.EAGER)

    @JoinColumn(name="CUST_ID")

    @Sort(type = SortType.COMPARATOR, comparator = TicketComparator.class)

    public SortedSet<Ticket> getTickets() {

        return tickets;

    }

Using Hibernate mapping files you specify a comparator in the mapping file with `<sort>`:

Example 7.18. Sorted collection using xml mapping

    <set name="aliases"
                table="person_aliases"
                sort="natural">
        <key column="person"/>
        <element column="name" type="string"/>
    </set>

    <map name="holidays" sort="my.custom.HolidayComparator">
        <key column="year_id"/>
        <map-key column="hol_name" type="string"/>
        <element column="hol_date" type="date"/>
    </map>

Allowed values of the `sort` attribute are `unsorted`, `natural` and the name of a class implementing `java.util.Comparator`.

## Tip

Sorted collections actually behave like `java.util.TreeSet` or `java.util.TreeMap`.

If you want the database itself to order the collection elements, use the `order-by` attribute of `set`, `bag` or `map` mappings. This solution is implemented using `LinkedHashSet` or `LinkedHashMap` and performs the ordering in the SQL query and not in the memory.

Example 7.19. Sorting in database using order-by

    <set name="aliases" table="person_aliases" order-by="lower(name) asc">
        <key column="person"/>
        <element column="name" type="string"/>
    </set>

    <map name="holidays" order-by="hol_date, hol_name">
        <key column="year_id"/>
        <map-key column="hol_name" type="string"/>
        <element column="hol_date type="date"/>
    </map>

## Note

The value of the `order-by` attribute is an SQL ordering, not an HQL ordering.

Associations can even be sorted by arbitrary criteria at runtime using a collection `filter()`:

Example 7.20. Sorting via a query filter

    sortedUsers = s.createFilter( group.getUsers(), "order by this.name" ).list();

### 7.3.2. Bidirectional associations

A bidirectional association allows navigation from both "ends" of the association. Two kinds of bidirectional association are supported:

one-to-many

  set or bag valued at one end and single-valued at the other

many-to-many

  set or bag valued at both ends

Often there exists a many to one association which is the owner side of a bidirectional relationship. The corresponding one to many association is in this case annotated by `@OneToMany(mappedBy=...)`

Example 7.21. Bidirectional one to many with many to one side as association owner

    @Entity

    public class Troop {

        @OneToMany(mappedBy="troop")

        public Set<Soldier> getSoldiers() {

        ...

    }

    @Entity

    public class Soldier {

        @ManyToOne

        @JoinColumn(name="troop_fk")

        public Troop getTroop() {

        ...

    }

`Troop` has a bidirectional one to many relationship with `Soldier` through the `troop` property. You don't have to (must not) define any physical mapping in the `mappedBy` side.

To map a bidirectional one to many, with the one-to-many side as the owning side, you have to remove the `mappedBy` element and set the many to one `@JoinColumn` as insertable and updatable to false. This solution is not optimized and will produce additional UPDATE statements.

Example 7.22. Bidirectional associtaion with one to many side as owner

    @Entity

    public class Troop {

        @OneToMany

        @JoinColumn(name="troop_fk") //we need to duplicate the physical information

        public Set<Soldier> getSoldiers() {

        ...

    }

    @Entity

    public class Soldier {

        @ManyToOne

        @JoinColumn(name="troop_fk", insertable=false, updatable=false)

        public Troop getTroop() {

        ...

    }

How does the mappping of a bidirectional mapping look like in Hibernate mapping xml? There you define a bidirectional one-to-many association by mapping a one-to-many association to the same table column(s) as a many-to-one association and declaring the many-valued end `inverse="true"`.

Example 7.23. Bidirectional one to many via Hibernate mapping files

    <class name="Parent">
        <id name="id" column="parent_id"/>
        ....
        <set name="children" inverse="true">
            <key column="parent_id"/>
            <one-to-many class="Child"/>
        </set>
    </class>

    <class name="Child">
        <id name="id" column="child_id"/>
        ....
        <many-to-one name="parent"
            class="Parent"
            column="parent_id"
            not-null="true"/>
    </class>

Mapping one end of an association with `inverse="true"` does not affect the operation of cascades as these are orthogonal concepts.

A many-to-many association is defined logically using the `@ManyToMany` annotation. You also have to describe the association table and the join conditions using the `@JoinTable` annotation. If the association is bidirectional, one side has to be the owner and one side has to be the inverse end (ie. it will be ignored when updating the relationship values in the association table):

Example 7.24. Many to many association via @ManyToMany

    @Entity

    public class Employer implements Serializable {

        @ManyToMany(

            targetEntity=org.hibernate.test.metadata.manytomany.Employee.class,

            cascade={CascadeType.PERSIST, CascadeType.MERGE}

        )

        @JoinTable(

            name="EMPLOYER_EMPLOYEE",

            joinColumns=@JoinColumn(name="EMPER_ID"),

            inverseJoinColumns=@JoinColumn(name="EMPEE_ID")

        )

        public Collection getEmployees() {

            return employees;

        }

        ...

    }

    @Entity

    public class Employee implements Serializable {

        @ManyToMany(

            cascade = {CascadeType.PERSIST, CascadeType.MERGE},

            mappedBy = "employees",

            targetEntity = Employer.class

        )

        public Collection getEmployers() {

            return employers;

        }

    }

In this example `@JoinTable` defines a `name`, an array of join columns, and an array of inverse join columns. The latter ones are the columns of the association table which refer to the `Employee` primary key (the "other side"). As seen previously, the other side don't have to (must not) describe the physical mapping: a simple `mappedBy` argument containing the owner side property name bind the two.

As any other annotations, most values are guessed in a many to many relationship. Without describing any physical mapping in a unidirectional many to many the following rules applied. The table name is the concatenation of the owner table name, _ and the other side table name. The foreign key name(s) referencing the owner table is the concatenation of the owner table name, _ and the owner primary key column(s). The foreign key name(s) referencing the other side is the concatenation of the owner property name, _, and the other side primary key column(s). These are the same rules used for a unidirectional one to many relationship.

Example 7.25. Default values for `@ManyToMany` (uni-directional)

    @Entity

    public class Store {

        @ManyToMany(cascade = CascadeType.PERSIST)

        public Set<City> getImplantedIn() {

            ...

        }

    }

    @Entity

    public class City {

        ... //no bidirectional relationship

    }

A `Store_City` is used as the join table. The `Store_id` column is a foreign key to the `Store` table. The `implantedIn_id` column is a foreign key to the `City` table.

Without describing any physical mapping in a bidirectional many to many the following rules applied. The table name is the concatenation of the owner table name, _ and the other side table name. The foreign key name(s) referencing the owner table is the concatenation of the other side property name, _, and the owner primary key column(s). The foreign key name(s) referencing the other side is the concatenation of the owner property name, _, and the other side primary key column(s). These are the same rules used for a unidirectional one to many relationship.

Example 7.26. Default values for `@ManyToMany` (bi-directional)

    @Entity

    public class Store {

        @ManyToMany(cascade = {CascadeType.PERSIST, CascadeType.MERGE})

        public Set<Customer> getCustomers() {

            ...

        }

    }

    @Entity

    public class Customer {

        @ManyToMany(mappedBy="customers")

        public Set<Store> getStores() {

            ...

        }

    }

A `Store_Customer` is used as the join table. The `stores_id` column is a foreign key to the `Store` table. The `customers_id` column is a foreign key to the `Customer` table.

Using Hibernate mapping files you can map a bidirectional many-to-many association by mapping two many-to-many associations to the same database table and declaring one end as inverse.

## Note

You cannot select an indexed collection.

Example 7.27, “Many to many association using Hibernate mapping files” shows a bidirectional many-to-many association that illustrates how each category can have many items and each item can be in many categories:

Example 7.27. Many to many association using Hibernate mapping files

    <class name="Category">
        <id name="id" column="CATEGORY_ID"/>
        ...
        <bag name="items" table="CATEGORY_ITEM">
            <key column="CATEGORY_ID"/>
            <many-to-many class="Item" column="ITEM_ID"/>
        </bag>
    </class>

    <class name="Item">
        <id name="id" column="ITEM_ID"/>
        ...

        <!-- inverse end -->
        <bag name="categories" table="CATEGORY_ITEM" inverse="true">
            <key column="ITEM_ID"/>
            <many-to-many class="Category" column="CATEGORY_ID"/>
        </bag>
    </class>

Changes made only to the inverse end of the association are not persisted. This means that Hibernate has two representations in memory for every bidirectional association: one link from A to B and another link from B to A. This is easier to understand if you think about the Java object model and how a many-to-many relationship in Javais created:

Example 7.28. Effect of inverse vs. non-inverse side of many to many associations

    category.getItems().add(item);           // The category now "knows" about the relationship
    item.getCategories().add(category);      // The item now "knows" about the relationship

    session.persist(item);                   // The relationship won't be saved!
    session.persist(category);               // The relationship will be saved

The non-inverse side is used to save the in-memory representation to the database.

### 7.3.3. Bidirectional associations with indexed collections

There are some additional considerations for bidirectional mappings with indexed collections (where one end is represented as a `<list>` or `<map>`) when using Hibernate mapping files. If there is a property of the child class that maps to the index column you can use `inverse="true"` on the collection mapping:

Example 7.29. Bidirectional association with indexed collection

    <class name="Parent">
        <id name="id" column="parent_id"/>
        ....
        <map name="children" inverse="true">
            <key column="parent_id"/>
            <map-key column="name"
                type="string"/>
            <one-to-many class="Child"/>
        </map>
    </class>

    <class name="Child">
        <id name="id" column="child_id"/>
        ....
        <property name="name"
            not-null="true"/>
        <many-to-one name="parent"
            class="Parent"
            column="parent_id"
            not-null="true"/>
    </class>

If there is no such property on the child class, the association cannot be considered truly bidirectional. That is, there is information available at one end of the association that is not available at the other end. In this case, you cannot map the collection `inverse="true"`. Instead, you could use the following mapping:

Example 7.30. Bidirectional association with indexed collection, but no index column

    <class name="Parent">
        <id name="id" column="parent_id"/>
        ....
        <map name="children">
            <key column="parent_id"
                not-null="true"/>
            <map-key column="name"
                type="string"/>
            <one-to-many class="Child"/>
        </map>
    </class>

    <class name="Child">
        <id name="id" column="child_id"/>
        ....
        <many-to-one name="parent"
            class="Parent"
            column="parent_id"
            insert="false"
            update="false"
            not-null="true"/>
    </class>

Note that in this mapping, the collection-valued end of the association is responsible for updates to the foreign key.

### 7.3.4. Ternary associations

There are three possible approaches to mapping a ternary association. One approach is to use a `Map` with an association as its index:

Example 7.31. Ternary association mapping

    @Entity
    public class Company {
       @Id
       int id;
       ...
       @OneToMany // unidirectional
       @MapKeyJoinColumn(name="employee_id")
       Map<Employee, Contract> contracts;
    }

    // or

    <map name="contracts">
        <key column="employer_id" not-null="true"/>
        <map-key-many-to-many column="employee_id" class="Employee"/>
        <one-to-many class="Contract"/>
    </map>

A second approach is to remodel the association as an entity class. This is the most common approach. A final alternative is to use composite elements, which will be discussed later.

### 7.3.5. `Using an <idbag>`

The majority of the many-to-many associations and collections of values shown previously all map to tables with composite keys, even though it has been suggested that entities should have synthetic identifiers (surrogate keys). A pure association table does not seem to benefit much from a surrogate key, although a collection of composite values might. For this reason Hibernate provides a feature that allows you to map many-to-many associations and collections of values to a table with a surrogate key.

The `<idbag>` element lets you map a `List` (or `Collection`) with bag semantics. For example:

    <idbag name="lovers" table="LOVERS">
        <collection-id column="ID" type="long">
            <generator class="sequence"/>
        </collection-id>
        <key column="PERSON1"/>
        <many-to-many column="PERSON2" class="Person" fetch="join"/>
    </idbag>

An `<idbag>` has a synthetic id generator, just like an entity class. A different surrogate key is assigned to each collection row. Hibernate does not, however, provide any mechanism for discovering the surrogate key value of a particular row.

The update performance of an `<idbag>` supersedes a regular `<bag>`. Hibernate can locate individual rows efficiently and update or delete them individually, similar to a list, map or set.

In the current implementation, the `native` identifier generation strategy is not supported for `<idbag>` collection identifiers.

## 7.4. Collection examples

This section covers collection examples.

The following class has a collection of `Child` instances:

Example 7.32. Example classes `Parent` and `Child`

    public class Parent {
        private long id;
        private Set<Child> children;

        // getter/setter
        ...
    }

    public class Child {
       private long id;
       private String name

       // getter/setter
       ...
    }

If each child has, at most, one parent, the most natural mapping is a one-to-many association:

Example 7.33. One to many unidirectional `Parent-Child` relationship using annotations

    public class Parent {
        @Id
        @GeneratedValue
        private long id;

        @OneToMany
        private Set<Child> children;

        // getter/setter
        ...
    }

    public class Child {
       @Id
       @GeneratedValue
       private long id;
       private String name;

       // getter/setter
       ...
    }

Example 7.34. One to many unidirectional `Parent-Child` relationship using mapping files

    <hibernate-mapping>

        <class name="Parent">
            <id name="id">
                <generator class="sequence"/>
            </id>
            <set name="children">
                <key column="parent_id"/>
                <one-to-many class="Child"/>
            </set>
        </class>

        <class name="Child">
            <id name="id">
                <generator class="sequence"/>
            </id>
            <property name="name"/>
        </class>

    </hibernate-mapping>

This maps to the following table definitions:

Example 7.35. Table definitions for unidirectional `Parent`-`Child` relationship

    create table parent ( id bigint not null primary key )
    create table child ( id bigint not null primary key, name varchar(255), parent_id bigint )
    alter table child add constraint childfk0 (parent_id) references parent

If the parent is required, use a bidirectional one-to-many association:

Example 7.36. One to many bidirectional `Parent-Child` relationship using annotations

    public class Parent {
        @Id
        @GeneratedValue
        private long id;

        @OneToMany(mappedBy="parent")
        private Set<Child> children;

        // getter/setter
        ...
    }

    public class Child {
       @Id
       @GeneratedValue
       private long id;

       private String name;

       @ManyToOne
       private Parent parent;

       // getter/setter
       ...
    }

Example 7.37. One to many bidirectional `Parent-Child` relationship using mapping files

    <hibernate-mapping>

        <class name="Parent">
            <id name="id">
                <generator class="sequence"/>
            </id>
            <set name="children" inverse="true">
                <key column="parent_id"/>
                <one-to-many class="Child"/>
            </set>
        </class>

        <class name="Child">
            <id name="id">
                <generator class="sequence"/>
            </id>
            <property name="name"/>
            <many-to-one name="parent" class="Parent" column="parent_id" not-null="true"/>
        </class>

    </hibernate-mapping>

Notice the `NOT NULL` constraint:

Example 7.38. Table definitions for bidirectional `Parent`-`Child` relationship

    create table parent ( id bigint not null primary key )
    create table child ( id bigint not null
                         primary key,
                         name varchar(255),
                         parent_id bigint not null )
    alter table child add constraint childfk0 (parent_id) references parent

Alternatively, if this association must be unidirectional you can enforce the `NOT NULL` constraint.

Example 7.39. Enforcing NOT NULL constraint in unidirectional relation using annotations

    public class Parent {
        @Id
        @GeneratedValue
        private long id;

        @OneToMany(optional=false)
        private Set<Child> children;

        // getter/setter
        ...
    }

    public class Child {
       @Id
       @GeneratedValue
       private long id;
       private String name;

       // getter/setter
       ...
    }

Example 7.40. Enforcing NOT NULL constraint in unidirectional relation using mapping files

    <hibernate-mapping>

        <class name="Parent">
            <id name="id">
                <generator class="sequence"/>
            </id>
            <set name="children">
                <key column="parent_id" not-null="true"/>
                <one-to-many class="Child"/>
            </set>
        </class>

        <class name="Child">
            <id name="id">
                <generator class="sequence"/>
            </id>
            <property name="name"/>
        </class>

    </hibernate-mapping>

On the other hand, if a child has multiple parents, a many-to-many association is appropriate.

Example 7.41. Many to many `Parent-Child` relationship using annotations

    public class Parent {
        @Id
        @GeneratedValue
        private long id;

        @ManyToMany
        private Set<Child> children;

        // getter/setter
        ...
    }

    public class Child {
       @Id
       @GeneratedValue
       private long id;

       private String name;

       // getter/setter
       ...
    }

Example 7.42. Many to many `Parent-Child` relationship using mapping files

    <hibernate-mapping>

        <class name="Parent">
            <id name="id">
                <generator class="sequence"/>
            </id>
            <set name="children" table="childset">
                <key column="parent_id"/>
                <many-to-many class="Child" column="child_id"/>
            </set>
        </class>

        <class name="Child">
            <id name="id">
                <generator class="sequence"/>
            </id>
            <property name="name"/>
        </class>

    </hibernate-mapping>

Table definitions:

Example 7.43. Table definitions for many to many releationship

    create table parent ( id bigint not null primary key )
    create table child ( id bigint not null primary key, name varchar(255) )
    create table childset ( parent_id bigint not null,
                            child_id bigint not null,
                            primary key ( parent_id, child_id ) )
    alter table childset add constraint childsetfk0 (parent_id) references parent
    alter table childset add constraint childsetfk1 (child_id) references child

For more examples and a complete explanation of a parent/child relationship mapping, see Chapter 24, Example: Parent/Child for more information. Even more complex association mappings are covered in the next chapter.

## Chapter 8. Association Mappings

## 8.1. Introduction

Association mappings are often the most difficult thing to implement correctly. In this section we examine some canonical cases one by one, starting with unidirectional mappings and then bidirectional cases. We will use `Person` and `Address` in all the examples.

Associations will be classified by multiplicity and whether or not they map to an intervening join table.

Nullable foreign keys are not considered to be good practice in traditional data modelling, so our examples do not use nullable foreign keys. This is not a requirement of Hibernate, and the mappings will work if you drop the nullability constraints.

## 8.2. Unidirectional associations

### 8.2.1. Many-to-one

A unidirectional many-to-one association is the most common kind of unidirectional association.

    <class name="Person">
        <id name="id" column="personId">
            <generator class="native"/>
        </id>
        <many-to-one name="address"
            column="addressId"
            not-null="true"/>
    </class>

    <class name="Address">
        <id name="id" column="addressId">
            <generator class="native"/>
        </id>
    </class>

    create table Person ( personId bigint not null primary key, addressId bigint not null )
    create table Address ( addressId bigint not null primary key )

### 8.2.2. One-to-one

A unidirectional one-to-one association on a foreign key is almost identical. The only difference is the column unique constraint.

    <class name="Person">
        <id name="id" column="personId">
            <generator class="native"/>
        </id>
        <many-to-one name="address"
            column="addressId"
            unique="true"
            not-null="true"/>
    </class>

    <class name="Address">
        <id name="id" column="addressId">
            <generator class="native"/>
        </id>
    </class>

    create table Person ( personId bigint not null primary key, addressId bigint not null unique )
    create table Address ( addressId bigint not null primary key )

A unidirectional one-to-one association on a primary key usually uses a special id generator In this example, however, we have reversed the direction of the association:

    <class name="Person">
        <id name="id" column="personId">
            <generator class="native"/>
        </id>
    </class>

    <class name="Address">
        <id name="id" column="personId">
            <generator class="foreign">
                <param name="property">person</param>
            </generator>
        </id>
        <one-to-one name="person" constrained="true"/>
    </class>

    create table Person ( personId bigint not null primary key )
    create table Address ( personId bigint not null primary key )

### 8.2.3. One-to-many

A unidirectional one-to-many association on a foreign key is an unusual case, and is not recommended.

    <class name="Person">
        <id name="id" column="personId">
            <generator class="native"/>
        </id>
        <set name="addresses">
            <key column="personId"
                not-null="true"/>
            <one-to-many class="Address"/>
        </set>
    </class>

    <class name="Address">
        <id name="id" column="addressId">
            <generator class="native"/>
        </id>
    </class>

    create table Person ( personId bigint not null primary key )
    create table Address ( addressId bigint not null primary key, personId bigint not null )

You should instead use a join table for this kind of association.

## 8.3. Unidirectional associations with join tables

### 8.3.1. One-to-many

A unidirectional one-to-many association on a join table is the preferred option. Specifying `unique="true"`, changes the multiplicity from many-to-many to one-to-many.

    <class name="Person">
        <id name="id" column="personId">
            <generator class="native"/>
        </id>
        <set name="addresses" table="PersonAddress">
            <key column="personId"/>
            <many-to-many column="addressId"
                unique="true"
                class="Address"/>
        </set>
    </class>

    <class name="Address">
        <id name="id" column="addressId">
            <generator class="native"/>
        </id>
    </class>

    create table Person ( personId bigint not null primary key )
    create table PersonAddress ( personId not null, addressId bigint not null primary key )
    create table Address ( addressId bigint not null primary key )

### 8.3.2. Many-to-one

A unidirectional many-to-one association on a join table is common when the association is optional. For example:

    <class name="Person">
        <id name="id" column="personId">
            <generator class="native"/>
        </id>
        <join table="PersonAddress"
            optional="true">
            <key column="personId" unique="true"/>
            <many-to-one name="address"
                column="addressId"
                not-null="true"/>
        </join>
    </class>

    <class name="Address">
        <id name="id" column="addressId">
            <generator class="native"/>
        </id>
    </class>

    create table Person ( personId bigint not null primary key )
    create table PersonAddress ( personId bigint not null primary key, addressId bigint not null )
    create table Address ( addressId bigint not null primary key )

### 8.3.3. One-to-one

A unidirectional one-to-one association on a join table is possible, but extremely unusual.

    <class name="Person">
        <id name="id" column="personId">
            <generator class="native"/>
        </id>
        <join table="PersonAddress"
            optional="true">
            <key column="personId"
                unique="true"/>
            <many-to-one name="address"
                column="addressId"
                not-null="true"
                unique="true"/>
        </join>
    </class>

    <class name="Address">
        <id name="id" column="addressId">
            <generator class="native"/>
        </id>
    </class>

    create table Person ( personId bigint not null primary key )
    create table PersonAddress ( personId bigint not null primary key, addressId bigint not null unique )
    create table Address ( addressId bigint not null primary key )

### 8.3.4. Many-to-many

Finally, here is an example of a unidirectional many-to-many association.

    <class name="Person">
        <id name="id" column="personId">
            <generator class="native"/>
        </id>
        <set name="addresses" table="PersonAddress">
            <key column="personId"/>
            <many-to-many column="addressId"
                class="Address"/>
        </set>
    </class>

    <class name="Address">
        <id name="id" column="addressId">
            <generator class="native"/>
        </id>
    </class>

    create table Person ( personId bigint not null primary key )
    create table PersonAddress ( personId bigint not null, addressId bigint not null, primary key (personId, addressId) )
    create table Address ( addressId bigint not null primary key )

## 8.4. Bidirectional associations

### 8.4.1. one-to-many / many-to-one

A bidirectional many-to-one association is the most common kind of association. The following example illustrates the standard parent/child relationship.

    <class name="Person">
        <id name="id" column="personId">
            <generator class="native"/>
        </id>
        <many-to-one name="address"
            column="addressId"
            not-null="true"/>
    </class>

    <class name="Address">
        <id name="id" column="addressId">
            <generator class="native"/>
        </id>
        <set name="people" inverse="true">
            <key column="addressId"/>
            <one-to-many class="Person"/>
        </set>
    </class>

    create table Person ( personId bigint not null primary key, addressId bigint not null )
    create table Address ( addressId bigint not null primary key )

If you use a `List`, or other indexed collection, set the `key` column of the foreign key to `not null`. Hibernate will manage the association from the collections side to maintain the index of each element, making the other side virtually inverse by setting `update="false"` and `insert="false"`:

    <class name="Person">
       <id name="id"/>
       ...
       <many-to-one name="address"
          column="addressId"
          not-null="true"
          insert="false"
          update="false"/>
    </class>

    <class name="Address">
       <id name="id"/>
       ...
       <list name="people">
          <key column="addressId" not-null="true"/>
          <list-index column="peopleIdx"/>
          <one-to-many class="Person"/>
       </list>
    </class>

If the underlying foreign key column is `NOT NULL`, it is important that you define `not-null="true"` on the `<key>` element of the collection mapping. Do not only declare `not-null="true"` on a possible nested `<column>` element, but on the `<key>` element.

### 8.4.2. One-to-one

A bidirectional one-to-one association on a foreign key is common:

    <class name="Person">
        <id name="id" column="personId">
            <generator class="native"/>
        </id>
        <many-to-one name="address"
            column="addressId"
            unique="true"
            not-null="true"/>
    </class>

    <class name="Address">
        <id name="id" column="addressId">
            <generator class="native"/>
        </id>
       <one-to-one name="person"
            property-ref="address"/>
    </class>

    create table Person ( personId bigint not null primary key, addressId bigint not null unique )
    create table Address ( addressId bigint not null primary key )

A bidirectional one-to-one association on a primary key uses the special id generator:

    <class name="Person">
        <id name="id" column="personId">
            <generator class="native"/>
        </id>
        <one-to-one name="address"/>
    </class>

    <class name="Address">
        <id name="id" column="personId">
            <generator class="foreign">
                <param name="property">person</param>
            </generator>
        </id>
        <one-to-one name="person"
            constrained="true"/>
    </class>

    create table Person ( personId bigint not null primary key )
    create table Address ( personId bigint not null primary key )

## 8.5. Bidirectional associations with join tables

### 8.5.1. one-to-many / many-to-one

The following is an example of a bidirectional one-to-many association on a join table. The `inverse="true"` can go on either end of the association, on the collection, or on the join.

    <class name="Person">
        <id name="id" column="personId">
            <generator class="native"/>
        </id>
        <set name="addresses"
            table="PersonAddress">
            <key column="personId"/>
            <many-to-many column="addressId"
                unique="true"
                class="Address"/>
        </set>
    </class>

    <class name="Address">
        <id name="id" column="addressId">
            <generator class="native"/>
        </id>
        <join table="PersonAddress"
            inverse="true"
            optional="true">
            <key column="addressId"/>
            <many-to-one name="person"
                column="personId"
                not-null="true"/>
        </join>
    </class>

    create table Person ( personId bigint not null primary key )
    create table PersonAddress ( personId bigint not null, addressId bigint not null primary key )
    create table Address ( addressId bigint not null primary key )

### 8.5.2. one to one

A bidirectional one-to-one association on a join table is possible, but extremely unusual.

    <class name="Person">
        <id name="id" column="personId">
            <generator class="native"/>
        </id>
        <join table="PersonAddress"
            optional="true">
            <key column="personId"
                unique="true"/>
            <many-to-one name="address"
                column="addressId"
                not-null="true"
                unique="true"/>
        </join>
    </class>

    <class name="Address">
        <id name="id" column="addressId">
            <generator class="native"/>
        </id>
        <join table="PersonAddress"
            optional="true"
            inverse="true">
            <key column="addressId"
                unique="true"/>
            <many-to-one name="person"
                column="personId"
                not-null="true"
                unique="true"/>
        </join>
    </class>

    create table Person ( personId bigint not null primary key )
    create table PersonAddress ( personId bigint not null primary key, addressId bigint not null unique )
    create table Address ( addressId bigint not null primary key )

### 8.5.3. Many-to-many

Here is an example of a bidirectional many-to-many association.

    <class name="Person">
        <id name="id" column="personId">
            <generator class="native"/>
        </id>
        <set name="addresses" table="PersonAddress">
            <key column="personId"/>
            <many-to-many column="addressId"
                class="Address"/>
        </set>
    </class>

    <class name="Address">
        <id name="id" column="addressId">
            <generator class="native"/>
        </id>
        <set name="people" inverse="true" table="PersonAddress">
            <key column="addressId"/>
            <many-to-many column="personId"
                class="Person"/>
        </set>
    </class>

    create table Person ( personId bigint not null primary key )
    create table PersonAddress ( personId bigint not null, addressId bigint not null, primary key (personId, addressId) )
    create table Address ( addressId bigint not null primary key )

## 8.6. More complex association mappings

More complex association joins are extremely rare. Hibernate handles more complex situations by using SQL fragments embedded in the mapping document. For example, if a table with historical account information data defines `accountNumber`, `effectiveEndDate` and `effectiveStartDate`columns, it would be mapped as follows:

    <properties name="currentAccountKey">
        <property name="accountNumber" type="string" not-null="true"/>
        <property name="currentAccount" type="boolean">
            <formula>case when effectiveEndDate is null then 1 else 0 end</formula>
        </property>
    </properties>
    <property name="effectiveEndDate" type="date"/>
    <property name="effectiveStateDate" type="date" not-null="true"/>

You can then map an association to the current instance, the one with null `effectiveEndDate`, by using:

    <many-to-one name="currentAccountInfo"
            property-ref="currentAccountKey"
            class="AccountInfo">
        <column name="accountNumber"/>
        <formula>'1'</formula>
    </many-to-one>

In a more complex example, imagine that the association between `Employee` and `Organization` is maintained in an `Employment` table full of historical employment data. An association to the employee's most recent employer, the one with the most recent `startDate`, could be mapped in the following way:

    <join>
        <key column="employeeId"/>
        <subselect>
            select employeeId, orgId
            from Employments
            group by orgId
            having startDate = max(startDate)
        </subselect>
        <many-to-one name="mostRecentEmployer"
                class="Organization"
                column="orgId"/>
    </join>

This functionality allows a degree of creativity and flexibility, but it is more practical to handle these kinds of cases using HQL or a criteria query.

## Chapter 9. Component Mapping

The notion of a component is re-used in several different contexts and purposes throughout Hibernate.

## 9.1. Dependent objects

A component is a contained object that is persisted as a value type and not an entity reference. The term "component" refers to the object-oriented notion of composition and not to architecture-level components. For example, you can model a person like this:

    public class Person {
        private java.util.Date birthday;
        private Name name;
        private String key;
        public String getKey() {
            return key;
        }
        private void setKey(String key) {
            this.key=key;
        }
        public java.util.Date getBirthday() {
            return birthday;
        }
        public void setBirthday(java.util.Date birthday) {
            this.birthday = birthday;
        }
        public Name getName() {
            return name;
        }
        public void setName(Name name) {
            this.name = name;
        }
        ......
        ......
    }

    public class Name {
        char initial;
        String first;
        String last;
        public String getFirst() {
            return first;
        }
        void setFirst(String first) {
            this.first = first;
        }
        public String getLast() {
            return last;
        }
        void setLast(String last) {
            this.last = last;
        }
        public char getInitial() {
            return initial;
        }
        void setInitial(char initial) {
            this.initial = initial;
        }
    }

Now `Name` can be persisted as a component of `Person`. `Name` defines getter and setter methods for its persistent properties, but it does not need to declare any interfaces or identifier properties.

Our Hibernate mapping would look like this:

    <class name="eg.Person" table="person">
        <id name="Key" column="pid" type="string">
            <generator class="uuid"/>
        </id>
        <property name="birthday" type="date"/>
        <component name="Name" class="eg.Name"> <!-- class attribute optional -->
            <property name="initial"/>
            <property name="first"/>
            <property name="last"/>
        </component>
    </class>

The person table would have the columns `pid`, `birthday`, `initial`, `first` and `last`.

Like value types, components do not support shared references. In other words, two persons could have the same name, but the two person objects would contain two independent name objects that were only "the same" by value. The null value semantics of a component are ad hoc. When reloading the containing object, Hibernate will assume that if all component columns are null, then the entire component is null. This is suitable for most purposes.

The properties of a component can be of any Hibernate type (collections, many-to-one associations, other components, etc). Nested components should not be considered an exotic usage. Hibernate is intended to support a fine-grained object model.

The `<component>` element allows a `<parent>` subelement that maps a property of the component class as a reference back to the containing entity.

    <class name="eg.Person" table="person">
        <id name="Key" column="pid" type="string">
            <generator class="uuid"/>
        </id>
        <property name="birthday" type="date"/>
        <component name="Name" class="eg.Name" unique="true">
            <parent name="namedPerson"/> <!-- reference back to the Person -->
            <property name="initial"/>
            <property name="first"/>
            <property name="last"/>
        </component>
    </class>

## 9.2. Collections of dependent objects

Collections of components are supported (e.g. an array of type `Name`). Declare your component collection by replacing the `<element>` tag with a `<composite-element>` tag:

    <set name="someNames" table="some_names" lazy="true">
        <key column="id"/>
        <composite-element class="eg.Name"> <!-- class attribute required -->
            <property name="initial"/>
            <property name="first"/>
            <property name="last"/>
        </composite-element>
    </set>

## Important

If you define a `Set` of composite elements, it is important to implement `equals()` and `hashCode()` correctly.

Composite elements can contain components but not collections. If your composite element contains components, use the `<nested-composite-element>` tag. This case is a collection of components which themselves have components. You may want to consider if a one-to-many association is more appropriate. Remodel the composite element as an entity, but be aware that even though the Java model is the same, the relational model and persistence semantics are still slightly different.

A composite element mapping does not support null-able properties if you are using a `<set>`. There is no separate primary key column in the composite element table. Hibernate uses each column's value to identify a record when deleting objects, which is not possible with null values. You have to either use only not-null properties in a composite-element or choose a `<list>`, `<map>`, `<bag>` or `<idbag>`.

A special case of a composite element is a composite element with a nested `<many-to-one>` element. This mapping allows you to map extra columns of a many-to-many association table to the composite element class. The following is a many-to-many association from `Order` to `Item`, where `purchaseDate`, `price` and `quantity` are properties of the association:

    <class name="eg.Order" .... >
        ....
        <set name="purchasedItems" table="purchase_items" lazy="true">
            <key column="order_id">
            <composite-element class="eg.Purchase">
                <property name="purchaseDate"/>
                <property name="price"/>
                <property name="quantity"/>
                <many-to-one name="item" class="eg.Item"/> <!-- class attribute is optional -->
            </composite-element>
        </set>
    </class>

There cannot be a reference to the purchase on the other side for bidirectional association navigation. Components are value types and do not allow shared references. A single `Purchase` can be in the set of an `Order`, but it cannot be referenced by the `Item` at the same time.

Even ternary (or quaternary, etc) associations are possible:

    <class name="eg.Order" .... >
        ....
        <set name="purchasedItems" table="purchase_items" lazy="true">
            <key column="order_id">
            <composite-element class="eg.OrderLine">
                <many-to-one name="purchaseDetails class="eg.Purchase"/>
                <many-to-one name="item" class="eg.Item"/>
            </composite-element>
        </set>
    </class>

Composite elements can appear in queries using the same syntax as associations to other entities.

## 9.3. Components as Map indices

The `<composite-map-key>` element allows you to map a component class as the key of a `Map`. Ensure that you override `hashCode()` and `equals()` correctly on the component class.

## 9.4. Components as composite identifiers

You can use a component as an identifier of an entity class. Your component class must satisfy certain requirements:

- It must implement `java.io.Serializable`.

- It must re-implement `equals()` and `hashCode()` consistently with the database's notion of composite key equality.

## Note

In Hibernate3, although the second requirement is not an absolutely hard requirement of Hibernate, it is recommended.

You cannot use an `IdentifierGenerator` to generate composite keys. Instead the application must assign its own identifiers.

Use the `<composite-id>` tag, with nested `<key-property>` elements, in place of the usual `<id>` declaration. For example, the `OrderLine` class has a primary key that depends upon the (composite) primary key of `Order`.

    <class name="OrderLine">

        <composite-id name="id" class="OrderLineId">
            <key-property name="lineId"/>
            <key-property name="orderId"/>
            <key-property name="customerId"/>
        </composite-id>

        <property name="name"/>

        <many-to-one name="order" class="Order"
                insert="false" update="false">
            <column name="orderId"/>
            <column name="customerId"/>
        </many-to-one>
        ....

    </class>

Any foreign keys referencing the `OrderLine` table are now composite. Declare this in your mappings for other classes. An association to `OrderLine` is mapped like this:

    <many-to-one name="orderLine" class="OrderLine">
    <!-- the "class" attribute is optional, as usual -->
        <column name="lineId"/>
        <column name="orderId"/>
        <column name="customerId"/>
    </many-to-one>

## Tip

The `column` element is an alternative to the `column` attribute everywhere. Using the `column` element just gives more declaration options, which are mostly useful when utilizing `hbm2ddl`

A `many-to-many` association to `OrderLine` also uses the composite foreign key:

    <set name="undeliveredOrderLines">
        <key column name="warehouseId"/>
        <many-to-many class="OrderLine">
            <column name="lineId"/>
            <column name="orderId"/>
            <column name="customerId"/>
        </many-to-many>
    </set>

The collection of `OrderLine`s in `Order` would use:

    <set name="orderLines" inverse="true">
        <key>
            <column name="orderId"/>
            <column name="customerId"/>
        </key>
        <one-to-many class="OrderLine"/>
    </set>

The `<one-to-many>` element declares no columns.

If `OrderLine` itself owns a collection, it also has a composite foreign key.

    <class name="OrderLine">
        ....
        ....
        <list name="deliveryAttempts">
            <key>   <!-- a collection inherits the composite key type -->
                <column name="lineId"/>
                <column name="orderId"/>
                <column name="customerId"/>
            </key>
            <list-index column="attemptId" base="1"/>
            <composite-element class="DeliveryAttempt">
                ...
            </composite-element>
        </set>
    </class>

## 9.5. Dynamic components

You can also map a property of type `Map`:

    <dynamic-component name="userAttributes">
        <property name="foo" column="FOO" type="string"/>
        <property name="bar" column="BAR" type="integer"/>
        <many-to-one name="baz" class="Baz" column="BAZ_ID"/>
    </dynamic-component>

The semantics of a `<dynamic-component>` mapping are identical to `<component>`. The advantage of this kind of mapping is the ability to determine the actual properties of the bean at deployment time just by editing the mapping document. Runtime manipulation of the mapping document is also possible, using a DOM parser. You can also access, and change, Hibernate's configuration-time metamodel via the `Configuration` object.

## Chapter 10. Inheritance mapping

## 10.1. The three strategies

Hibernate supports the three basic inheritance mapping strategies:

- table per class hierarchy

- table per subclass

- table per concrete class

In addition, Hibernate supports a fourth, slightly different kind of polymorphism:

- implicit polymorphism

It is possible to use different mapping strategies for different branches of the same inheritance hierarchy. You can then make use of implicit polymorphism to achieve polymorphism across the whole hierarchy. However, Hibernate does not support mixing `<subclass>`, `<joined-subclass>` and `<union-subclass>` mappings under the same root `<class>` element. It is possible to mix together the table per hierarchy and table per subclass strategies under the the same `<class>` element, by combining the `<subclass>` and `<join>` elements (see below for an example).

It is possible to define `subclass`, `union-subclass`, and `joined-subclass` mappings in separate mapping documents directly beneath `hibernate-mapping`. This allows you to extend a class hierarchy by adding a new mapping file. You must specify an `extends` attribute in the subclass mapping, naming a previously mapped superclass. Previously this feature made the ordering of the mapping documents important. Since Hibernate3, the ordering of mapping files is irrelevant when using the extends keyword. The ordering inside a single mapping file still needs to be defined as superclasses before subclasses.

     <hibernate-mapping>
         <subclass name="DomesticCat" extends="Cat" discriminator-value="D">
              <property name="name" type="string"/>
         </subclass>
     </hibernate-mapping>

### 10.1.1. Table per class hierarchy

Suppose we have an interface `Payment` with the implementors `CreditCardPayment`, `CashPayment`, and `ChequePayment`. The table per hierarchy mapping would display in the following way:

    <class name="Payment" table="PAYMENT">
        <id name="id" type="long" column="PAYMENT_ID">
            <generator class="native"/>
        </id>
        <discriminator column="PAYMENT_TYPE" type="string"/>
        <property name="amount" column="AMOUNT"/>
        ...
        <subclass name="CreditCardPayment" discriminator-value="CREDIT">
            <property name="creditCardType" column="CCTYPE"/>
            ...
        </subclass>
        <subclass name="CashPayment" discriminator-value="CASH">
            ...
        </subclass>
        <subclass name="ChequePayment" discriminator-value="CHEQUE">
            ...
        </subclass>
    </class>

Exactly one table is required. There is a limitation of this mapping strategy: columns declared by the subclasses, such as `CCTYPE`, cannot have `NOT NULL` constraints.

### 10.1.2. Table per subclass

A table per subclass mapping looks like this:

    <class name="Payment" table="PAYMENT">
        <id name="id" type="long" column="PAYMENT_ID">
            <generator class="native"/>
        </id>
        <property name="amount" column="AMOUNT"/>
        ...
        <joined-subclass name="CreditCardPayment" table="CREDIT_PAYMENT">
            <key column="PAYMENT_ID"/>
            <property name="creditCardType" column="CCTYPE"/>
            ...
        </joined-subclass>
        <joined-subclass name="CashPayment" table="CASH_PAYMENT">
            <key column="PAYMENT_ID"/>
            ...
        </joined-subclass>
        <joined-subclass name="ChequePayment" table="CHEQUE_PAYMENT">
            <key column="PAYMENT_ID"/>
            ...
        </joined-subclass>
    </class>

Four tables are required. The three subclass tables have primary key associations to the superclass table so the relational model is actually a one-to-one association.

### 10.1.3. Table per subclass: using a discriminator

Hibernate's implementation of table per subclass does not require a discriminator column. Other object/relational mappers use a different implementation of table per subclass that requires a type discriminator column in the superclass table. The approach taken by Hibernate is much more difficult to implement, but arguably more correct from a relational point of view. If you want to use a discriminator column with the table per subclass strategy, you can combine the use of `<subclass>` and `<join>`, as follows:

    <class name="Payment" table="PAYMENT">
        <id name="id" type="long" column="PAYMENT_ID">
            <generator class="native"/>
        </id>
        <discriminator column="PAYMENT_TYPE" type="string"/>
        <property name="amount" column="AMOUNT"/>
        ...
        <subclass name="CreditCardPayment" discriminator-value="CREDIT">
            <join table="CREDIT_PAYMENT">
                <key column="PAYMENT_ID"/>
                <property name="creditCardType" column="CCTYPE"/>
                ...
            </join>
        </subclass>
        <subclass name="CashPayment" discriminator-value="CASH">
            <join table="CASH_PAYMENT">
                <key column="PAYMENT_ID"/>
                ...
            </join>
        </subclass>
        <subclass name="ChequePayment" discriminator-value="CHEQUE">
            <join table="CHEQUE_PAYMENT" fetch="select">
                <key column="PAYMENT_ID"/>
                ...
            </join>
        </subclass>
    </class>

The optional `fetch="select"` declaration tells Hibernate not to fetch the `ChequePayment` subclass data using an outer join when querying the superclass.

### 10.1.4. Mixing table per class hierarchy with table per subclass

You can even mix the table per hierarchy and table per subclass strategies using the following approach:

    <class name="Payment" table="PAYMENT">
        <id name="id" type="long" column="PAYMENT_ID">
            <generator class="native"/>
        </id>
        <discriminator column="PAYMENT_TYPE" type="string"/>
        <property name="amount" column="AMOUNT"/>
        ...
        <subclass name="CreditCardPayment" discriminator-value="CREDIT">
            <join table="CREDIT_PAYMENT">
                <property name="creditCardType" column="CCTYPE"/>
                ...
            </join>
        </subclass>
        <subclass name="CashPayment" discriminator-value="CASH">
            ...
        </subclass>
        <subclass name="ChequePayment" discriminator-value="CHEQUE">
            ...
        </subclass>
    </class>

For any of these mapping strategies, a polymorphic association to the root `Payment` class is mapped using `<many-to-one>`.

    <many-to-one name="payment" column="PAYMENT_ID" class="Payment"/>

### 10.1.5. Table per concrete class

There are two ways we can map the table per concrete class strategy. First, you can use `<union-subclass>`.

    <class name="Payment">
        <id name="id" type="long" column="PAYMENT_ID">
            <generator class="sequence"/>
        </id>
        <property name="amount" column="AMOUNT"/>
        ...
        <union-subclass name="CreditCardPayment" table="CREDIT_PAYMENT">
            <property name="creditCardType" column="CCTYPE"/>
            ...
        </union-subclass>
        <union-subclass name="CashPayment" table="CASH_PAYMENT">
            ...
        </union-subclass>
        <union-subclass name="ChequePayment" table="CHEQUE_PAYMENT">
            ...
        </union-subclass>
    </class>

Three tables are involved for the subclasses. Each table defines columns for all properties of the class, including inherited properties.

The limitation of this approach is that if a property is mapped on the superclass, the column name must be the same on all subclass tables. The identity generator strategy is not allowed in union subclass inheritance. The primary key seed has to be shared across all unioned subclasses of a hierarchy.

If your superclass is abstract, map it with `abstract="true"`. If it is not abstract, an additional table (it defaults to `PAYMENT` in the example above), is needed to hold instances of the superclass.

### 10.1.6. Table per concrete class using implicit polymorphism

An alternative approach is to make use of implicit polymorphism:

    <class name="CreditCardPayment" table="CREDIT_PAYMENT">
        <id name="id" type="long" column="CREDIT_PAYMENT_ID">
            <generator class="native"/>
        </id>
        <property name="amount" column="CREDIT_AMOUNT"/>
        ...
    </class>

    <class name="CashPayment" table="CASH_PAYMENT">
        <id name="id" type="long" column="CASH_PAYMENT_ID">
            <generator class="native"/>
        </id>
        <property name="amount" column="CASH_AMOUNT"/>
        ...
    </class>

    <class name="ChequePayment" table="CHEQUE_PAYMENT">
        <id name="id" type="long" column="CHEQUE_PAYMENT_ID">
            <generator class="native"/>
        </id>
        <property name="amount" column="CHEQUE_AMOUNT"/>
        ...
    </class>

Notice that the `Payment` interface is not mentioned explicitly. Also notice that properties of `Payment` are mapped in each of the subclasses. If you want to avoid duplication, consider using XML entities (for example, `[ <!ENTITY allproperties SYSTEM "allproperties.xml"> ]` in the `DOCTYPE` declaration and `%allproperties;` in the mapping).

The disadvantage of this approach is that Hibernate does not generate SQL `UNION`s when performing polymorphic queries.

For this mapping strategy, a polymorphic association to `Payment` is usually mapped using `<any>`.

    <any name="payment" meta-type="string" id-type="long">
        <meta-value value="CREDIT" class="CreditCardPayment"/>
        <meta-value value="CASH" class="CashPayment"/>
        <meta-value value="CHEQUE" class="ChequePayment"/>
        <column name="PAYMENT_CLASS"/>
        <column name="PAYMENT_ID"/>
    </any>

### 10.1.7. Mixing implicit polymorphism with other inheritance mappings

Since the subclasses are each mapped in their own `<class>` element, and since `Payment` is just an interface), each of the subclasses could easily be part of another inheritance hierarchy. You can still use polymorphic queries against the `Payment` interface.

    <class name="CreditCardPayment" table="CREDIT_PAYMENT">
        <id name="id" type="long" column="CREDIT_PAYMENT_ID">
            <generator class="native"/>
        </id>
        <discriminator column="CREDIT_CARD" type="string"/>
        <property name="amount" column="CREDIT_AMOUNT"/>
        ...
        <subclass name="MasterCardPayment" discriminator-value="MDC"/>
        <subclass name="VisaPayment" discriminator-value="VISA"/>
    </class>

    <class name="NonelectronicTransaction" table="NONELECTRONIC_TXN">
        <id name="id" type="long" column="TXN_ID">
            <generator class="native"/>
        </id>
        ...
        <joined-subclass name="CashPayment" table="CASH_PAYMENT">
            <key column="PAYMENT_ID"/>
            <property name="amount" column="CASH_AMOUNT"/>
            ...
        </joined-subclass>
        <joined-subclass name="ChequePayment" table="CHEQUE_PAYMENT">
            <key column="PAYMENT_ID"/>
            <property name="amount" column="CHEQUE_AMOUNT"/>
            ...
        </joined-subclass>
    </class>

Once again, `Payment` is not mentioned explicitly. If we execute a query against the `Payment` interface, for example `from Payment`, Hibernate automatically returns instances of `CreditCardPayment` (and its subclasses, since they also implement `Payment`), `CashPayment` and `ChequePayment`, but not instances of `NonelectronicTransaction`.

## 10.2. Limitations

There are limitations to the "implicit polymorphism" approach to the table per concrete-class mapping strategy. There are somewhat less restrictive limitations to `<union-subclass>` mappings.

The following table shows the limitations of table per concrete-class mappings, and of implicit polymorphism, in Hibernate.

Table 10.1. Features of inheritance mappings

Inheritance strategy | Polymorphic many-to-one | Polymorphic one-to-one | Polymorphic one-to-many | Polymorphic many-to-many | Polymorphic `load()/get()` | Polymorphic queries | Polymorphic joins | Outer join fetching
--- | --- | --- | --- | --- | --- | --- | --- | ---
table per class-hierarchy | `<many-to-one>` | `<one-to-one>` | `<one-to-many>` | `<many-to-many>` | `s.get(Payment.class, id)` | `from Payment p` | `from Order o join o.payment p` | supported
table per subclass | `<many-to-one>` | `<one-to-one>` | `<one-to-many>` | `<many-to-many>` | `s.get(Payment.class, id)` | `from Payment p` | `from Order o join o.payment p` | supported
table per concrete-class (union-subclass) | `<many-to-one>` | `<one-to-one>` | `<one-to-many>` (for `inverse="true"` only) | `<many-to-many>` | `s.get(Payment.class, id)` | `from Payment p` | `from Order o join o.payment p` | supported
table per concrete class (implicit polymorphism) | `<any>` | not supported | not supported | `<many-to-any>` | `s.createCriteria(Payment.class).add( Restrictions.idEq(id) ).uniqueResult()` | `from Payment p` | not supported | not supported

## Chapter 11. Working with objects

Hibernate is a full object/relational mapping solution that not only shields the developer from the details of the underlying database management system, but also offers state management of objects. This is, contrary to the management of SQL `statements` in common JDBC/SQL persistence layers, a natural object-oriented view of persistence in Java applications.

In other words, Hibernate application developers should always think about the state of their objects, and not necessarily about the execution of SQL statements. This part is taken care of by Hibernate and is only relevant for the application developer when tuning the performance of the system.

## 11.1. Hibernate object states

Hibernate defines and supports the following object states:

- Transient - an object is transient if it has just been instantiated using the `new` operator, and it is not associated with a Hibernate `Session`. It has no persistent representation in the database and no identifier value has been assigned. Transient instances will be destroyed by the garbage collector if the application does not hold a reference anymore. Use the Hibernate `Session` to make an object persistent (and let Hibernate take care of the SQL statements that need to be executed for this transition).

- Persistent - a persistent instance has a representation in the database and an identifier value. It might just have been saved or loaded, however, it is by definition in the scope of a `Session`. Hibernate will detect any changes made to an object in persistent state and synchronize the state with the database when the unit of work completes. Developers do not execute manual `UPDATE` statements, or `DELETE` statements when an object should be made transient.

- Detached - a detached instance is an object that has been persistent, but its `Session` has been closed. The reference to the object is still valid, of course, and the detached instance might even be modified in this state. A detached instance can be reattached to a new `Session` at a later point in time, making it (and all the modifications) persistent again. This feature enables a programming model for long running units of work that require user think-time. We call them application transactions, i.e., a unit of work from the point of view of the user.

We will now discuss the states and state transitions (and the Hibernate methods that trigger a transition) in more detail.

## 11.2. Making objects persistent

Newly instantiated instances of a persistent class are considered transient by Hibernate. We can make a transient instance persistent by associating it with a session:

    DomesticCat fritz = new DomesticCat();
    fritz.setColor(Color.GINGER);
    fritz.setSex('M');
    fritz.setName("Fritz");
    Long generatedId = (Long) sess.save(fritz);

If `Cat` has a generated identifier, the identifier is generated and assigned to the `cat` when `save()` is called. If `Cat` has an `assigned` identifier, or a composite key, the identifier should be assigned to the `cat` instance before calling `save()`. You can also use `persist()` instead of `save()`, with the semantics defined in the EJB3 early draft.

- `persist()` makes a transient instance persistent. However, it does not guarantee that the identifier value will be assigned to the persistent instance immediately, the assignment might happen at flush time. `persist()` also guarantees that it will not execute an `INSERT` statement if it is called outside of transaction boundaries. This is useful in long-running conversations with an extended Session/persistence context.

- `save()` does guarantee to return an identifier. If an INSERT has to be executed to get the identifier ( e.g. "identity" generator, not "sequence"), this INSERT happens immediately, no matter if you are inside or outside of a transaction. This is problematic in a long-running conversation with an extended Session/persistence context.

Alternatively, you can assign the identifier using an overloaded version of `save()`.

    DomesticCat pk = new DomesticCat();
    pk.setColor(Color.TABBY);
    pk.setSex('F');
    pk.setName("PK");
    pk.setKittens( new HashSet() );
    pk.addKitten(fritz);
    sess.save( pk, new Long(1234) );

If the object you make persistent has associated objects (e.g. the `kittens` collection in the previous example), these objects can be made persistent in any order you like unless you have a `NOT NULL` constraint upon a foreign key column. There is never a risk of violating foreign key constraints. However, you might violate a `NOT NULL` constraint if you `save()` the objects in the wrong order.

Usually you do not bother with this detail, as you will normally use Hibernate's transitive persistence feature to save the associated objects automatically. Then, even `NOT NULL` constraint violations do not occur - Hibernate will take care of everything. Transitive persistence is discussed later in this chapter.

## 11.3. Loading an object

The `load()` methods of `Session` provide a way of retrieving a persistent instance if you know its identifier. `load()` takes a class object and loads the state into a newly instantiated instance of that class in a persistent state.

    Cat fritz = (Cat) sess.load(Cat.class, generatedId);

    // you need to wrap primitive identifiers
    long id = 1234;
    DomesticCat pk = (DomesticCat) sess.load( DomesticCat.class, new Long(id) );

Alternatively, you can load state into a given instance:

    Cat cat = new DomesticCat();
    // load pk's state into cat
    sess.load( cat, new Long(pkId) );
    Set kittens = cat.getKittens();

Be aware that `load()` will throw an unrecoverable exception if there is no matching database row. If the class is mapped with a proxy, `load()` just returns an uninitialized proxy and does not actually hit the database until you invoke a method of the proxy. This is useful if you wish to create an association to an object without actually loading it from the database. It also allows multiple instances to be loaded as a batch if `batch-size` is defined for the class mapping.

If you are not certain that a matching row exists, you should use the `get()` method which hits the database immediately and returns null if there is no matching row.

    Cat cat = (Cat) sess.get(Cat.class, id);
    if (cat==null) {
        cat = new Cat();
        sess.save(cat, id);
    }
    return cat;

You can even load an object using an SQL `SELECT ... FOR UPDATE`, using a `LockMode`. See the API documentation for more information.

    Cat cat = (Cat) sess.get(Cat.class, id, LockMode.UPGRADE);

Any associated instances or contained collections will not be selected `FOR UPDATE`, unless you decide to specify `lock` or `all` as a cascade style for the association.

It is possible to re-load an object and all its collections at any time, using the `refresh()` method. This is useful when database triggers are used to initialize some of the properties of the object.

    sess.save(cat);
    sess.flush(); //force the SQL INSERT
    sess.refresh(cat); //re-read the state (after the trigger executes)

How much does Hibernate load from the database and how many SQL `SELECT`s will it use? This depends on the fetching strategy. This is explained in Section 21.1, “Fetching strategies”.

## 11.4. Querying

If you do not know the identifiers of the objects you are looking for, you need a query. Hibernate supports an easy-to-use but powerful object oriented query language (HQL). For programmatic query creation, Hibernate supports a sophisticated Criteria and Example query feature (QBC and QBE). You can also express your query in the native SQL of your database, with optional support from Hibernate for result set conversion into objects.

### 11.4.1. Executing queries

HQL and native SQL queries are represented with an instance of `org.hibernate.Query`. This interface offers methods for parameter binding, result set handling, and for the execution of the actual query. You always obtain a `Query` using the current `Session`:

    List cats = session.createQuery(
        "from Cat as cat where cat.birthdate < ?")
        .setDate(0, date)
        .list();

    List mothers = session.createQuery(
        "select mother from Cat as cat join cat.mother as mother where cat.name = ?")
        .setString(0, name)
        .list();

    List kittens = session.createQuery(
        "from Cat as cat where cat.mother = ?")
        .setEntity(0, pk)
        .list();

    Cat mother = (Cat) session.createQuery(
        "select cat.mother from Cat as cat where cat = ?")
        .setEntity(0, izi)
        .uniqueResult();]]

    Query mothersWithKittens = (Cat) session.createQuery(
        "select mother from Cat as mother left join fetch mother.kittens");
    Set uniqueMothers = new HashSet(mothersWithKittens.list());

A query is usually executed by invoking `list()`. The result of the query will be loaded completely into a collection in memory. Entity instances retrieved by a query are in a persistent state. The `uniqueResult()` method offers a shortcut if you know your query will only return a single object. Queries that make use of eager fetching of collections usually return duplicates of the root objects, but with their collections initialized. You can filter these duplicates through a `Set`.

#### 11.4.1.1. Iterating results

Occasionally, you might be able to achieve better performance by executing the query using the `iterate()` method. This will usually be the case if you expect that the actual entity instances returned by the query will already be in the session or second-level cache. If they are not already cached, `iterate()` will be slower than `list()` and might require many database hits for a simple query, usually 1 for the initial select which only returns identifiers, and n additional selects to initialize the actual instances.

    // fetch ids
    Iterator iter = sess.createQuery("from eg.Qux q order by q.likeliness").iterate();
    while ( iter.hasNext() ) {
        Qux qux = (Qux) iter.next();  // fetch the object
        // something we couldnt express in the query
        if ( qux.calculateComplicatedAlgorithm() ) {
            // delete the current instance
            iter.remove();
            // dont need to process the rest
            break;
        }
    }

#### 11.4.1.2. Queries that return tuples

Hibernate queries sometimes return tuples of objects. Each tuple is returned as an array:

    Iterator kittensAndMothers = sess.createQuery(
                "select kitten, mother from Cat kitten join kitten.mother mother")
                .list()
                .iterator();

    while ( kittensAndMothers.hasNext() ) {
        Object[] tuple = (Object[]) kittensAndMothers.next();
        Cat kitten = (Cat) tuple[0];
        Cat mother = (Cat) tuple[1];
        ....
    }

#### 11.4.1.3. Scalar results

Queries can specify a property of a class in the `select` clause. They can even call SQL aggregate functions. Properties or aggregates are considered "scalar" results and not entities in persistent state.

    Iterator results = sess.createQuery(
            "select cat.color, min(cat.birthdate), count(cat) from Cat cat " +
            "group by cat.color")
            .list()
            .iterator();

    while ( results.hasNext() ) {
        Object[] row = (Object[]) results.next();
        Color type = (Color) row[0];
        Date oldest = (Date) row[1];
        Integer count = (Integer) row[2];
        .....
    }

#### 11.4.1.4. Bind parameters

Methods on `Query` are provided for binding values to named parameters or JDBC-style `?` parameters. Contrary to JDBC, Hibernate numbers parameters from zero. Named parameters are identifiers of the form `:name` in the query string. The advantages of named parameters are as follows:

- named parameters are insensitive to the order they occur in the query string

- they can occur multiple times in the same query

- they are self-documenting

    //named parameter (preferred)
    Query q = sess.createQuery("from DomesticCat cat where cat.name = :name");
    q.setString("name", "Fritz");
    Iterator cats = q.iterate();

    //positional parameter
    Query q = sess.createQuery("from DomesticCat cat where cat.name = ?");
    q.setString(0, "Izi");
    Iterator cats = q.iterate();

    //named parameter list
    List names = new ArrayList();
    names.add("Izi");
    names.add("Fritz");
    Query q = sess.createQuery("from DomesticCat cat where cat.name in (:namesList)");
    q.setParameterList("namesList", names);
    List cats = q.list();

#### 11.4.1.5. Pagination

If you need to specify bounds upon your result set, that is, the maximum number of rows you want to retrieve and/or the first row you want to retrieve, you can use methods of the `Query` interface:

    Query q = sess.createQuery("from DomesticCat cat");
    q.setFirstResult(20);
    q.setMaxResults(10);
    List cats = q.list();

Hibernate knows how to translate this limit query into the native SQL of your DBMS.

#### 11.4.1.6. Scrollable iteration

If your JDBC driver supports scrollable `ResultSet`s, the `Query` interface can be used to obtain a `ScrollableResults` object that allows flexible navigation of the query results.

    Query q = sess.createQuery("select cat.name, cat from DomesticCat cat " +
                                "order by cat.name");
    ScrollableResults cats = q.scroll();
    if ( cats.first() ) {

        // find the first name on each page of an alphabetical list of cats by name
        firstNamesOfPages = new ArrayList();
        do {
            String name = cats.getString(0);
            firstNamesOfPages.add(name);
        }
        while ( cats.scroll(PAGE_SIZE) );

        // Now get the first page of cats
        pageOfCats = new ArrayList();
        cats.beforeFirst();
        int i=0;
        while( ( PAGE_SIZE > i++ ) && cats.next() ) pageOfCats.add( cats.get(1) );

    }
    cats.close()

Note that an open database connection and cursor is required for this functionality. Use `setMaxResult()`/`setFirstResult()` if you need offline pagination functionality.

#### 11.4.1.7. Externalizing named queries

Queries can also be configured as so called named queries using annotations or Hibernate mapping documents. `@NamedQuery` and `@NamedQueries` can be defined at the class level as seen in Example 11.1, “Defining a named query using `@NamedQuery`” . However their definitions are global to the session factory/entity manager factory scope. A named query is defined by its name and the actual query string.

Example 11.1. Defining a named query using `@NamedQuery`

    @Entity

    @NamedQuery(name="night.moreRecentThan", query="select n from Night n where n.date >= :date")

    public class Night {

        ...

    }

    public class MyDao {

        doStuff() {

            Query q = s.getNamedQuery("night.moreRecentThan");

            q.setDate( "date", aMonthAgo );

            List results = q.list();

            ...

        }

        ...

    }

Using a mapping document can be configured using the `<query>` node. Remember to use a `CDATA` section if your query contains characters that could be interpreted as markup.

Example 11.2. Defining a named query using `<query>`

    <query name="ByNameAndMaximumWeight"><![CDATA[
        from eg.DomesticCat as cat
            where cat.name = ?
            and cat.weight > ?
    ] ]></query>

Parameter binding and executing is done programatically as seen in Example 11.3, “Parameter binding of a named query”.

Example 11.3. Parameter binding of a named query

    Query q = sess.getNamedQuery("ByNameAndMaximumWeight");
    q.setString(0, name);
    q.setInt(1, minWeight);
    List cats = q.list();

The actual program code is independent of the query language that is used. You can also define native SQL queries in metadata, or migrate existing queries to Hibernate by placing them in mapping files.

Also note that a query declaration inside a `<hibernate-mapping>` element requires a global unique name for the query, while a query declaration inside a `<class>` element is made unique automatically by prepending the fully qualified name of the class. For example `eg.Cat.ByNameAndMaximumWeight`.

### 11.4.2. Filtering collections

A collection filter is a special type of query that can be applied to a persistent collection or array. The query string can refer to `this`, meaning the current collection element.

    Collection blackKittens = session.createFilter(
        pk.getKittens(),
        "where this.color = ?")
        .setParameter( Color.BLACK, Hibernate.custom(ColorUserType.class) )
        .list()
    );

The returned collection is considered a bag that is a copy of the given collection. The original collection is not modified. This is contrary to the implication of the name "filter", but consistent with expected behavior.

Observe that filters do not require a `from` clause, although they can have one if required. Filters are not limited to returning the collection elements themselves.

    Collection blackKittenMates = session.createFilter(
        pk.getKittens(),
        "select this.mate where this.color = eg.Color.BLACK.intValue")
        .list();

Even an empty filter query is useful, e.g. to load a subset of elements in a large collection:

    Collection tenKittens = session.createFilter(
        mother.getKittens(), "")
        .setFirstResult(0).setMaxResults(10)
        .list();

### 11.4.3. Criteria queries

HQL is extremely powerful, but some developers prefer to build queries dynamically using an object-oriented API, rather than building query strings. Hibernate provides an intuitive `Criteria` query API for these cases:

    Criteria crit = session.createCriteria(Cat.class);
    crit.add( Restrictions.eq( "color", eg.Color.BLACK ) );
    crit.setMaxResults(10);
    List cats = crit.list();

The `Criteria` and the associated `Example` API are discussed in more detail in Chapter 17, Criteria Queries.

### 11.4.4. Queries in native SQL

You can express a query in SQL, using `createSQLQuery()` and let Hibernate manage the mapping from result sets to objects. You can at any time call `session.connection()` and use the JDBC `Connection` directly. If you choose to use the Hibernate API, you must enclose SQL aliases in braces:

    List cats = session.createSQLQuery("SELECT {cat.*} FROM CAT {cat} WHERE ROWNUM<10")
        .addEntity("cat", Cat.class)
    .list();

    List cats = session.createSQLQuery(
        "SELECT {cat}.ID AS {cat.id}, {cat}.SEX AS {cat.sex}, " +
               "{cat}.MATE AS {cat.mate}, {cat}.SUBCLASS AS {cat.class}, ... " +
        "FROM CAT {cat} WHERE ROWNUM<10")
        .addEntity("cat", Cat.class)
    .list()

SQL queries can contain named and positional parameters, just like Hibernate queries. More information about native SQL queries in Hibernate can be found in Chapter 18, Native SQL.

## 11.5. Modifying persistent objects

Transactional persistent instances (i.e. objects loaded, saved, created or queried by the `Session`) can be manipulated by the application, and any changes to persistent state will be persisted when the `Session` is flushed. This is discussed later in this chapter. There is no need to call a particular method (like `update()`, which has a different purpose) to make your modifications persistent. The most straightforward way to update the state of an object is to `load()` it and then manipulate it directly while the `Session` is open:

    DomesticCat cat = (DomesticCat) sess.load( Cat.class, new Long(69) );
    cat.setName("PK");
    sess.flush();  // changes to cat are automatically detected and persisted

Sometimes this programming model is inefficient, as it requires in the same session both an SQL `SELECT` to load an object and an SQL `UPDATE` to persist its updated state. Hibernate offers an alternate approach by using detached instances.

## 11.6. Modifying detached objects

Many applications need to retrieve an object in one transaction, send it to the UI layer for manipulation, then save the changes in a new transaction. Applications that use this kind of approach in a high-concurrency environment usually use versioned data to ensure isolation for the "long" unit of work.

Hibernate supports this model by providing for reattachment of detached instances using the `Session.update()` or `Session.merge()` methods:

    // in the first session
    Cat cat = (Cat) firstSession.load(Cat.class, catId);
    Cat potentialMate = new Cat();
    firstSession.save(potentialMate);

    // in a higher layer of the application
    cat.setMate(potentialMate);

    // later, in a new session
    secondSession.update(cat);  // update cat
    secondSession.update(mate); // update mate

If the `Cat` with identifier `catId` had already been loaded by `secondSession` when the application tried to reattach it, an exception would have been thrown.

Use `update()` if you are certain that the session does not contain an already persistent instance with the same identifier. Use `merge()` if you want to merge your modifications at any time without consideration of the state of the session. In other words, `update()` is usually the first method you would call in a fresh session, ensuring that the reattachment of your detached instances is the first operation that is executed.

The application should individually `update()` detached instances that are reachable from the given detached instance only if it wants their state to be updated. This can be automated using transitive persistence. See Section 11.11, “Transitive persistence” for more information.

The `lock()` method also allows an application to reassociate an object with a new session. However, the detached instance has to be unmodified.

    //just reassociate:
    sess.lock(fritz, LockMode.NONE);
    //do a version check, then reassociate:
    sess.lock(izi, LockMode.READ);
    //do a version check, using SELECT ... FOR UPDATE, then reassociate:
    sess.lock(pk, LockMode.UPGRADE);

Note that `lock()` can be used with various `LockMode`s. See the API documentation and the chapter on transaction handling for more information. Reattachment is not the only usecase for `lock()`.

Other models for long units of work are discussed in Section 13.3, “Optimistic concurrency control”.

## 11.7. Automatic state detection

Hibernate users have requested a general purpose method that either saves a transient instance by generating a new identifier or updates/reattaches the detached instances associated with its current identifier. The `saveOrUpdate()` method implements this functionality.

    // in the first session
    Cat cat = (Cat) firstSession.load(Cat.class, catID);

    // in a higher tier of the application
    Cat mate = new Cat();
    cat.setMate(mate);

    // later, in a new session
    secondSession.saveOrUpdate(cat);   // update existing state (cat has a non-null id)
    secondSession.saveOrUpdate(mate);  // save the new instance (mate has a null id)

The usage and semantics of `saveOrUpdate()` seems to be confusing for new users. Firstly, so long as you are not trying to use instances from one session in another new session, you should not need to use `update()`, `saveOrUpdate()`, or `merge()`. Some whole applications will never use either of these methods.

Usually `update()` or `saveOrUpdate()` are used in the following scenario:

- the application loads an object in the first session

- the object is passed up to the UI tier

- some modifications are made to the object

- the object is passed back down to the business logic tier

- the application persists these modifications by calling `update()` in a second session

`saveOrUpdate()` does the following:

- if the object is already persistent in this session, do nothing

- if another object associated with the session has the same identifier, throw an exception

- if the object has no identifier property, `save()` it

- if the object's identifier has the value assigned to a newly instantiated object, `save()` it

- if the object is versioned by a `<version>` or `<timestamp>`, and the version property value is the same value assigned to a newly instantiated object, `save()` it

- otherwise `update()` the object

and `merge()` is very different:

- if there is a persistent instance with the same identifier currently associated with the session, copy the state of the given object onto the persistent instance

- if there is no persistent instance currently associated with the session, try to load it from the database, or create a new persistent instance

- the persistent instance is returned

- the given instance does not become associated with the session, it remains detached

## 11.8. Deleting persistent objects

`Session.delete()` will remove an object's state from the database. Your application, however, can still hold a reference to a deleted object. It is best to think of `delete()` as making a persistent instance, transient.

    sess.delete(cat);

You can delete objects in any order, without risk of foreign key constraint violations. It is still possible to violate a `NOT NULL` constraint on a foreign key column by deleting objects in the wrong order, e.g. if you delete the parent, but forget to delete the children.

## 11.9. Replicating object between two different datastores

It is sometimes useful to be able to take a graph of persistent instances and make them persistent in a different datastore, without regenerating identifier values.

    //retrieve a cat from one database
    Session session1 = factory1.openSession();
    Transaction tx1 = session1.beginTransaction();
    Cat cat = session1.get(Cat.class, catId);
    tx1.commit();
    session1.close();

    //reconcile with a second database
    Session session2 = factory2.openSession();
    Transaction tx2 = session2.beginTransaction();
    session2.replicate(cat, ReplicationMode.LATEST_VERSION);
    tx2.commit();
    session2.close();

The `ReplicationMode` determines how `replicate()` will deal with conflicts with existing rows in the database:

- `ReplicationMode.IGNORE`: ignores the object when there is an existing database row with the same identifier

- `ReplicationMode.OVERWRITE`: overwrites any existing database row with the same identifier

- `ReplicationMode.EXCEPTION`: throws an exception if there is an existing database row with the same identifier

- `ReplicationMode.LATEST_VERSION`: overwrites the row if its version number is earlier than the version number of the object, or ignore the object otherwise

Usecases for this feature include reconciling data entered into different database instances, upgrading system configuration information during product upgrades, rolling back changes made during non-ACID transactions and more.

## 11.10. Flushing the Session

Sometimes the `Session` will execute the SQL statements needed to synchronize the JDBC connection's state with the state of objects held in memory. This process, called flush, occurs by default at the following points:

- before some query executions

- from `org.hibernate.Transaction.commit()`

- from `Session.flush()`

The SQL statements are issued in the following order:

1. all entity insertions in the same order the corresponding objects were saved using `Session.save()`

2. all entity updates

3. all collection deletions

4. all collection element deletions, updates and insertions

5. all collection insertions

6. all entity deletions in the same order the corresponding objects were deleted using `Session.delete()`

An exception is that objects using `native` ID generation are inserted when they are saved.

Except when you explicitly `flush()`, there are absolutely no guarantees about when the `Session` executes the JDBC calls, only the order in which they are executed. However, Hibernate does guarantee that the `Query.list(..)` will never return stale or incorrect data.

It is possible to change the default behavior so that flush occurs less frequently. The `FlushMode` class defines three different modes: only flush at commit time when the Hibernate `Transaction` API is used, flush automatically using the explained routine, or never flush unless `flush()` is called explicitly. The last mode is useful for long running units of work, where a `Session` is kept open and disconnected for a long time (see Section 13.3.2, “Extended session and automatic versioning”).

    sess = sf.openSession();
    Transaction tx = sess.beginTransaction();
    sess.setFlushMode(FlushMode.COMMIT); // allow queries to return stale state

    Cat izi = (Cat) sess.load(Cat.class, id);
    izi.setName(iznizi);

    // might return stale data
    sess.find("from Cat as cat left outer join cat.kittens kitten");

    // change to izi is not flushed!
    ...
    tx.commit(); // flush occurs
    sess.close();

During flush, an exception might occur (e.g. if a DML operation violates a constraint). Since handling exceptions involves some understanding of Hibernate's transactional behavior, we discuss it in Chapter 13, Transactions and Concurrency.

## 11.11. Transitive persistence

It is quite cumbersome to save, delete, or reattach individual objects, especially if you deal with a graph of associated objects. A common case is a parent/child relationship. Consider the following example:

If the children in a parent/child relationship would be value typed (e.g. a collection of addresses or strings), their life cycle would depend on the parent and no further action would be required for convenient "cascading" of state changes. When the parent is saved, the value-typed child objects are saved and when the parent is deleted, the children will be deleted, etc. This works for operations such as the removal of a child from the collection. Since value-typed objects cannot have shared references, Hibernate will detect this and delete the child from the database.

Now consider the same scenario with parent and child objects being entities, not value-types (e.g. categories and items, or parent and child cats). Entities have their own life cycle and support shared references. Removing an entity from the collection does not mean it can be deleted), and there is by default no cascading of state from one entity to any other associated entities. Hibernate does not implement persistence by reachability by default.

For each basic operation of the Hibernate session - including `persist(), merge(), saveOrUpdate(), delete(), lock(), refresh(), evict(), replicate()` - there is a corresponding cascade style. Respectively, the cascade styles are named `create, merge, save-update, delete, lock, refresh, evict, replicate`. If you want an operation to be cascaded along an association, you must indicate that in the mapping document. For example:

    <one-to-one name="person" cascade="persist"/>

Cascade styles my be combined:

    <one-to-one name="person" cascade="persist,delete,lock"/>

You can even use `cascade="all"` to specify that all operations should be cascaded along the association. The default `cascade="none"` specifies that no operations are to be cascaded.

In case you are using annotatons you probably have noticed the `cascade` attribute taking an array of `CascadeType` as a value. The cascade concept in JPA is very is similar to the transitive persistence and cascading of operations as described above, but with slightly different semantics and cascading types:

- `CascadeType.PERSIST`: cascades the persist (create) operation to associated entities persist() is called or if the entity is managed

- `CascadeType.MERGE`: cascades the merge operation to associated entities if merge() is called or if the entity is managed

- `CascadeType.REMOVE`: cascades the remove operation to associated entities if delete() is called

- `CascadeType.REFRESH:` cascades the refresh operation to associated entities if refresh() is called

- `CascadeType.DETACH:` cascades the detach operation to associated entities if detach() is called

- `CascadeType.ALL`: all of the above

## Note

CascadeType.ALL also covers Hibernate specific operations like save-update, lock etc...

A special cascade style, `delete-orphan`, applies only to one-to-many associations, and indicates that the `delete()` operation should be applied to any child object that is removed from the association. Using annotations there is no `CascadeType.DELETE-ORPHAN` equivalent. Instead you can use the attribute `orphanRemoval as seen in` Example 11.4, “`@OneToMany` with `orphanRemoval`”. If an entity is removed from a `@OneToMany` collection or an associated entity is dereferenced from a `@OneToOne` association, this associated entity can be marked for deletion if `orphanRemoval` is set to true.

Example 11.4. `@OneToMany` with `orphanRemoval`

    @Entity

    public class Customer {

       private Set<Order> orders;

       @OneToMany(cascade=CascadeType.ALL, orphanRemoval=true)

       public Set<Order> getOrders() { return orders; }

       public void setOrders(Set<Order> orders) { this.orders = orders; }

       [...]

    }

    @Entity

    public class Order { ... }

    Customer customer = em.find(Customer.class, 1l);

    Order order = em.find(Order.class, 1l);

    customer.getOrders().remove(order); //order will be deleted by cascade

Recommendations:

- It does not usually make sense to enable cascade on a many-to-one or many-to-many association. In fact the `@ManyToOne` and `@ManyToMany` don't even offer a `orphanRemoval` attribute. Cascading is often useful for one-to-one and one-to-many associations.

- If the child object's lifespan is bounded by the lifespan of the parent object, make it a life cycle object by specifying `cascade="all,delete-orphan"` (`@OneToMany(cascade=CascadeType.ALL, orphanRemoval=true)`).

- Otherwise, you might not need cascade at all. But if you think that you will often be working with the parent and children together in the same transaction, and you want to save yourself some typing, consider using `cascade="persist,merge,save-update"`.

Mapping an association (either a single valued association, or a collection) with `cascade="all"` marks the association as a parent/child style relationship where save/update/delete of the parent results in save/update/delete of the child or children.

Furthermore, a mere reference to a child from a persistent parent will result in save/update of the child. This metaphor is incomplete, however. A child which becomes unreferenced by its parent is not automatically deleted, except in the case of a one-to-many association mapped with `cascade="delete-orphan"`. The precise semantics of cascading operations for a parent/child relationship are as follows:

- If a parent is passed to `persist()`, all children are passed to `persist()`

- If a parent is passed to `merge()`, all children are passed to `merge()`

- If a parent is passed to `save()`, `update()` or `saveOrUpdate()`, all children are passed to `saveOrUpdate()`

- If a transient or detached child becomes referenced by a persistent parent, it is passed to `saveOrUpdate()`

- If a parent is deleted, all children are passed to `delete()`

- If a child is dereferenced by a persistent parent, nothing special happens - the application should explicitly delete the child if necessary - unless `cascade="delete-orphan"`, in which case the "orphaned" child is deleted.

Finally, note that cascading of operations can be applied to an object graph at call time or at flush time. All operations, if enabled, are cascaded to associated entities reachable when the operation is executed. However, `save-update` and `delete-orphan` are transitive for all associated entities reachable during flush of the `Session`.

## 11.12. Using metadata

Hibernate requires a rich meta-level model of all entity and value types. This model can be useful to the application itself. For example, the application might use Hibernate's metadata to implement a "smart" deep-copy algorithm that understands which objects should be copied (eg. mutable value types) and which objects that should not (e.g. immutable value types and, possibly, associated entities).

Hibernate exposes metadata via the `ClassMetadata` and `CollectionMetadata` interfaces and the `Type` hierarchy. Instances of the metadata interfaces can be obtained from the `SessionFactory`.

    Cat fritz = ......;
    ClassMetadata catMeta = sessionfactory.getClassMetadata(Cat.class);

    Object[] propertyValues = catMeta.getPropertyValues(fritz);
    String[] propertyNames = catMeta.getPropertyNames();
    Type[] propertyTypes = catMeta.getPropertyTypes();

    // get a Map of all properties which are not collections or associations
    Map namedValues = new HashMap();
    for ( int i=0; i<propertyNames.length; i++ ) {
        if ( !propertyTypes[i].isEntityType() && !propertyTypes[i].isCollectionType() ) {
            namedValues.put( propertyNames[i], propertyValues[i] );
        }
    }

## Chapter 12. Read-only entities

## Important

Hibernate's treatment of read-only entities may differ from what you may have encountered elsewhere. Incorrect usage may cause unexpected results.

When an entity is read-only:

- Hibernate does not dirty-check the entity's simple properties or single-ended associations;

- Hibernate will not update simple properties or updatable single-ended associations;

- Hibernate will not update the version of the read-only entity if only simple properties or single-ended updatable associations are changed;

In some ways, Hibernate treats read-only entities the same as entities that are not read-only:

- Hibernate cascades operations to associations as defined in the entity mapping.

- Hibernate updates the version if the entity has a collection with changes that dirties the entity;

- A read-only entity can be deleted.

Even if an entity is not read-only, its collection association can be affected if it contains a read-only entity.

For details about the affect of read-only entities on different property and association types, see Section 12.2, “Read-only affect on property type”.

For details about how to make entities read-only, see Section 12.1, “Making persistent entities read-only”

Hibernate does some optimizing for read-only entities:

- It saves execution time by not dirty-checking simple properties or single-ended associations.

- It saves memory by deleting database snapshots.

## 12.1. Making persistent entities read-only

Only persistent entities can be made read-only. Transient and detached entities must be put in persistent state before they can be made read-only.

Hibernate provides the following ways to make persistent entities read-only:

- you can map an entity class as immutable; when an entity of an immutable class is made persistent, Hibernate automatically makes it read-only. see Section 12.1.1, “Entities of immutable classes” for details

- you can change a default so that entities loaded into the session by Hibernate are automatically made read-only; see Section 12.1.2, “Loading persistent entities as read-only” for details

- you can make an HQL query or criteria read-only so that entities loaded when the query or criteria executes, scrolls, or iterates, are automatically made read-only; see Section 12.1.3, “Loading read-only entities from an HQL query/criteria” for details

- you can make a persistent entity that is already in the in the session read-only; see Section 12.1.4, “Making a persistent entity read-only” for details

### 12.1.1. Entities of immutable classes

When an entity instance of an immutable class is made persistent, Hibernate automatically makes it read-only.

An entity of an immutable class can created and deleted the same as an entity of a mutable class.

Hibernate treats a persistent entity of an immutable class the same way as a read-only persistent entity of a mutable class. The only exception is that Hibernate will not allow an entity of an immutable class to be changed so it is not read-only.

### 12.1.2. Loading persistent entities as read-only

## Note

Entities of immutable classes are automatically loaded as read-only.

To change the default behavior so Hibernate loads entity instances of mutable classes into the session and automatically makes them read-only, call:

    Session.setDefaultReadOnly( true );

To change the default back so entities loaded by Hibernate are not made read-only, call:

    Session.setDefaultReadOnly( false );

You can determine the current setting by calling:

    Session.isDefaultReadOnly();

If Session.isDefaultReadOnly() returns true, entities loaded by the following are automatically made read-only:

- Session.load()

- Session.get()

- Session.merge()

- executing, scrolling, or iterating HQL queries and criteria; to override this setting for a particular HQL query or criteria see Section 12.1.3, “Loading read-only entities from an HQL query/criteria”

Changing this default has no effect on:

- persistent entities already in the session when the default was changed

- persistent entities that are refreshed via Session.refresh(); a refreshed persistent entity will only be read-only if it was read-only before refreshing

- persistent entities added by the application via Session.persist(), Session.save(), and Session.update() Session.saveOrUpdate()

### 12.1.3. Loading read-only entities from an HQL query/criteria

## Note

Entities of immutable classes are automatically loaded as read-only.

If Session.isDefaultReadOnly() returns false (the default) when an HQL query or criteria executes, then entities and proxies of mutable classes loaded by the query will not be read-only.

You can override this behavior so that entities and proxies loaded by an HQL query or criteria are automatically made read-only.

For an HQL query, call:

    Query.setReadOnly( true );

`Query.setReadOnly( true )` must be called before `Query.list()`, `Query.uniqueResult()`, `Query.scroll()`, or `Query.iterate()`

For an HQL criteria, call:

    Criteria.setReadOnly( true );

`Criteria.setReadOnly( true )` must be called before `Criteria.list()`, `Criteria.uniqueResult()`, or `Criteria.scroll()`

Entities and proxies that exist in the session before being returned by an HQL query or criteria are not affected.

Uninitialized persistent collections returned by the query are not affected. Later, when the collection is initialized, entities loaded into the session will be read-only if Session.isDefaultReadOnly() returns true.

Using `Query.setReadOnly( true )` or `Criteria.setReadOnly( true )` works well when a single HQL query or criteria loads all the entities and intializes all the proxies and collections that the application needs to be read-only.

When it is not possible to load and initialize all necessary entities in a single query or criteria, you can temporarily change the session default to load entities as read-only before the query is executed. Then you can explicitly initialize proxies and collections before restoring the session default.

    Session session = factory.openSession();
    Transaction tx = session.beginTransaction();

    setDefaultReadOnly( true );
    Contract contract =
       ( Contract ) session.createQuery(
               "from Contract where customerName = 'Sherman'" )
               .uniqueResult();
    Hibernate.initialize( contract.getPlan() );
    Hibernate.initialize( contract.getVariations() );
    Hibernate.initialize( contract.getNotes() );
    setDefaultReadOnly( false );
    ...
    tx.commit();
    session.close();

If Session.isDefaultReadOnly() returns true, then you can use Query.setReadOnly( false ) and Criteria.setReadOnly( false ) to override this session setting and load entities that are not read-only.

### 12.1.4. Making a persistent entity read-only

## Note

Persistent entities of immutable classes are automatically made read-only.

To make a persistent entity or proxy read-only, call:

    Session.setReadOnly(entityOrProxy, true)

To change a read-only entity or proxy of a mutable class so it is no longer read-only, call:

    Session.setReadOnly(entityOrProxy, false)

## Important

When a read-only entity or proxy is changed so it is no longer read-only, Hibernate assumes that the current state of the read-only entity is consistent with its database representation. If this is not true, then any non-flushed changes made before or while the entity was read-only, will be ignored.

To throw away non-flushed changes and make the persistent entity consistent with its database representation, call:

    session.refresh( entity );

To flush changes made before or while the entity was read-only and make the database representation consistent with the current state of the persistent entity:

    // evict the read-only entity so it is detached
    session.evict( entity );

    // make the detached entity (with the non-flushed changes) persistent
    session.update( entity );

    // now entity is no longer read-only and its changes can be flushed
    s.flush();

## 12.2. Read-only affect on property type

The following table summarizes how different property types are affected by making an entity read-only.

Table 12.1. Affect of read-only entity on property types

Property/Association Type | Changes flushed to DB?
--- | ---
Simple (Section 12.2.1, “Simple properties”) | no*
Unidirectional one-to-one Unidirectional many-to-one (Section 12.2.2.1, “Unidirectional one-to-one and many-to-one”) | no* no*
Unidirectional one-to-many Unidirectional many-to-many (Section 12.2.2.2, “Unidirectional one-to-many and many-to-many”) | yes yes
Bidirectional one-to-one (Section 12.2.3.1, “Bidirectional one-to-one”) | only if the owning entity is not read-only*
Bidirectional one-to-many/many-to-one inverse collection non-inverse collection (Section 12.2.3.2, “Bidirectional one-to-many/many-to-one”) | only added/removed entities that are not read-only* yes
Bidirectional many-to-many (Section 12.2.3.3, “Bidirectional many-to-many”) | yes

* Behavior is different when the entity having the property/association is read-only, compared to when it is not read-only.

### 12.2.1. Simple properties

When a persistent object is read-only, Hibernate does not dirty-check simple properties.

Hibernate will not synchronize simple property state changes to the database. If you have automatic versioning, Hibernate will not increment the version if any simple properties change.

    Session session = factory.openSession();
    Transaction tx = session.beginTransaction();

    // get a contract and make it read-only
    Contract contract = ( Contract ) session.get( Contract.class, contractId );
    session.setReadOnly( contract, true );

    // contract.getCustomerName() is "Sherman"
    contract.setCustomerName( "Yogi" );
    tx.commit();

    tx = session.beginTransaction();

    contract = ( Contract ) session.get( Contract.class, contractId );
    // contract.getCustomerName() is still "Sherman"
    ...
    tx.commit();
    session.close();

### 12.2.2. Unidirectional associations

#### 12.2.2.1. Unidirectional one-to-one and many-to-one

Hibernate treats unidirectional one-to-one and many-to-one associations in the same way when the owning entity is read-only.

We use the term unidirectional single-ended association when referring to functionality that is common to unidirectional one-to-one and many-to-one associations.

Hibernate does not dirty-check unidirectional single-ended associations when the owning entity is read-only.

If you change a read-only entity's reference to a unidirectional single-ended association to null, or to refer to a different entity, that change will not be flushed to the database.

## Note

If an entity is of an immutable class, then its references to unidirectional single-ended associations must be assigned when that entity is first created. Because the entity is automatically made read-only, these references can not be updated.

If automatic versioning is used, Hibernate will not increment the version due to local changes to unidirectional single-ended associations.

In the following examples, Contract has a unidirectional many-to-one association with Plan. Contract cascades save and update operations to the association.

The following shows that changing a read-only entity's many-to-one association reference to null has no effect on the entity's database representation.

    // get a contract with an existing plan;
    // make the contract read-only and set its plan to null
    tx = session.beginTransaction();
    Contract contract = ( Contract ) session.get( Contract.class, contractId );
    session.setReadOnly( contract, true );
    contract.setPlan( null );
    tx.commit();

    // get the same contract
    tx = session.beginTransaction();
    contract = ( Contract ) session.get( Contract.class, contractId );

    // contract.getPlan() still refers to the original plan;

    tx.commit();
    session.close();

The following shows that, even though an update to a read-only entity's many-to-one association has no affect on the entity's database representation, flush still cascades the save-update operation to the locally changed association.

    // get a contract with an existing plan;
    // make the contract read-only and change to a new plan
    tx = session.beginTransaction();
    Contract contract = ( Contract ) session.get( Contract.class, contractId );
    session.setReadOnly( contract, true );
    Plan newPlan = new Plan( "new plan"
    contract.setPlan( newPlan);
    tx.commit();

    // get the same contract
    tx = session.beginTransaction();
    contract = ( Contract ) session.get( Contract.class, contractId );
    newPlan = ( Contract ) session.get( Plan.class, newPlan.getId() );

    // contract.getPlan() still refers to the original plan;
    // newPlan is non-null because it was persisted when
    // the previous transaction was committed;

    tx.commit();
    session.close();

#### 12.2.2.2. Unidirectional one-to-many and many-to-many

Hibernate treats unidirectional one-to-many and many-to-many associations owned by a read-only entity the same as when owned by an entity that is not read-only.

Hibernate dirty-checks unidirectional one-to-many and many-to-many associations;

The collection can contain entities that are read-only, as well as entities that are not read-only.

Entities can be added and removed from the collection; changes are flushed to the database.

If automatic versioning is used, Hibernate will update the version due to changes in the collection if they dirty the owning entity.

### 12.2.3. Bidirectional associations

#### 12.2.3.1. Bidirectional one-to-one

If a read-only entity owns a bidirectional one-to-one association:

- Hibernate does not dirty-check the association.

- updates that change the association reference to null or to refer to a different entity will not be flushed to the database.

- If automatic versioning is used, Hibernate will not increment the version due to local changes to the association.

## Note

If an entity is of an immutable class, and it owns a bidirectional one-to-one association, then its reference must be assigned when that entity is first created. Because the entity is automatically made read-only, these references cannot be updated.

When the owner is not read-only, Hibernate treats an association with a read-only entity the same as when the association is with an entity that is not read-only.

#### 12.2.3.2. Bidirectional one-to-many/many-to-one

A read-only entity has no impact on a bidirectional one-to-many/many-to-one association if:

- the read-only entity is on the one-to-many side using an inverse collection;

- the read-only entity is on the one-to-many side using a non-inverse collection;

- the one-to-many side uses a non-inverse collection that contains the read-only entity

When the one-to-many side uses an inverse collection:

- a read-only entity can only be added to the collection when it is created;

- a read-only entity can only be removed from the collection by an orphan delete or by explicitly deleting the entity.

#### 12.2.3.3. Bidirectional many-to-many

Hibernate treats bidirectional many-to-many associations owned by a read-only entity the same as when owned by an entity that is not read-only.

Hibernate dirty-checks bidirectional many-to-many associations.

The collection on either side of the association can contain entities that are read-only, as well as entities that are not read-only.

Entities are added and removed from both sides of the collection; changes are flushed to the database.

If automatic versioning is used, Hibernate will update the version due to changes in both sides of the collection if they dirty the entity owning the respective collections.

## Chapter 13. Transactions and Concurrency

The most important point about Hibernate and concurrency control is that it is easy to understand. Hibernate directly uses JDBC connections and JTA resources without adding any additional locking behavior. It is recommended that you spend some time with the JDBC, ANSI, and transaction isolation specification of your database management system.

Hibernate does not lock objects in memory. Your application can expect the behavior as defined by the isolation level of your database transactions. Through `Session`, which is also a transaction-scoped cache, Hibernate provides repeatable reads for lookup by identifier and entity queries and not reporting queries that return scalar values.

In addition to versioning for automatic optimistic concurrency control, Hibernate also offers, using the `SELECT FOR UPDATE` syntax, a (minor) API for pessimistic locking of rows. Optimistic concurrency control and this API are discussed later in this chapter.

The discussion of concurrency control in Hibernate begins with the granularity of `Configuration`, `SessionFactory`, and `Session`, as well as database transactions and long conversations.

## 13.1. Session and transaction scopes

A `SessionFactory` is an expensive-to-create, threadsafe object, intended to be shared by all application threads. It is created once, usually on application startup, from a `Configuration` instance.

A `Session` is an inexpensive, non-threadsafe object that should be used once and then discarded for: a single request, a conversation or a single unit of work. A `Session` will not obtain a JDBC `Connection`, or a `Datasource`, unless it is needed. It will not consume any resources until used.

In order to reduce lock contention in the database, a database transaction has to be as short as possible. Long database transactions will prevent your application from scaling to a highly concurrent load. It is not recommended that you hold a database transaction open during user think time until the unit of work is complete.

What is the scope of a unit of work? Can a single Hibernate `Session` span several database transactions, or is this a one-to-one relationship of scopes? When should you open and close a `Session` and how do you demarcate the database transaction boundaries? These questions are addressed in the following sections.

### 13.1.1. Unit of work

First, let's define a unit of work. A unit of work is a design pattern described by Martin Fowler as “ [maintaining] a list of objects affected by a business transaction and coordinates the writing out of changes and the resolution of concurrency problems. ”[PoEAA] In other words, its a series of operations we wish to carry out against the database together. Basically, it is a transaction, though fulfilling a unit of work will often span multiple physical database transactions (see Section 13.1.2, “Long conversations”). So really we are talking about a more abstract notion of a transaction. The term "business transaction" is also sometimes used in lieu of unit of work.

Do not use the session-per-operation antipattern: do not open and close a `Session` for every simple database call in a single thread. The same is true for database transactions. Database calls in an application are made using a planned sequence; they are grouped into atomic units of work. This also means that auto-commit after every single SQL statement is useless in an application as this mode is intended for ad-hoc SQL console work. Hibernate disables, or expects the application server to disable, auto-commit mode immediately. Database transactions are never optional. All communication with a database has to occur inside a transaction. Auto-commit behavior for reading data should be avoided, as many small transactions are unlikely to perform better than one clearly defined unit of work. The latter is also more maintainable and extensible.

The most common pattern in a multi-user client/server application is session-per-request. In this model, a request from the client is sent to the server, where the Hibernate persistence layer runs. A new Hibernate `Session` is opened, and all database operations are executed in this unit of work. On completion of the work, and once the response for the client has been prepared, the session is flushed and closed. Use a single database transaction to serve the clients request, starting and committing it when you open and close the `Session`. The relationship between the two is one-to-one and this model is a perfect fit for many applications.

The challenge lies in the implementation. Hibernate provides built-in management of the "current session" to simplify this pattern. Start a transaction when a server request has to be processed, and end the transaction before the response is sent to the client. Common solutions are `ServletFilter`, AOP interceptor with a pointcut on the service methods, or a proxy/interception container. An EJB container is a standardized way to implement cross-cutting aspects such as transaction demarcation on EJB session beans, declaratively with CMT. If you use programmatic transaction demarcation, for ease of use and code portability use the Hibernate `Transaction` API shown later in this chapter.

Your application code can access a "current session" to process the request by calling `sessionFactory.getCurrentSession()`. You will always get a `Session` scoped to the current database transaction. This has to be configured for either resource-local or JTA environments, see Section 2.3, “Contextual sessions”.

You can extend the scope of a `Session` and database transaction until the "view has been rendered". This is especially useful in servlet applications that utilize a separate rendering phase after the request has been processed. Extending the database transaction until view rendering, is achieved by implementing your own interceptor. However, this will be difficult if you rely on EJBs with container-managed transactions. A transaction will be completed when an EJB method returns, before rendering of any view can start. See the Hibernate website and forum for tips and examples relating to this Open Session in View pattern.

### 13.1.2. Long conversations

The session-per-request pattern is not the only way of designing units of work. Many business processes require a whole series of interactions with the user that are interleaved with database accesses. In web and enterprise applications, it is not acceptable for a database transaction to span a user interaction. Consider the following example:

- The first screen of a dialog opens. The data seen by the user has been loaded in a particular `Session` and database transaction. The user is free to modify the objects.

- The user clicks "Save" after 5 minutes and expects their modifications to be made persistent. The user also expects that they were the only person editing this information and that no conflicting modification has occurred.

From the point of view of the user, we call this unit of work a long-running conversation or application transaction. There are many ways to implement this in your application.

A first naive implementation might keep the `Session` and database transaction open during user think time, with locks held in the database to prevent concurrent modification and to guarantee isolation and atomicity. This is an anti-pattern, since lock contention would not allow the application to scale with the number of concurrent users.

You have to use several database transactions to implement the conversation. In this case, maintaining isolation of business processes becomes the partial responsibility of the application tier. A single conversation usually spans several database transactions. It will be atomic if only one of these database transactions (the last one) stores the updated data. All others simply read data (for example, in a wizard-style dialog spanning several request/response cycles). This is easier to implement than it might sound, especially if you utilize some of Hibernate's features:

- Automatic Versioning: Hibernate can perform automatic optimistic concurrency control for you. It can automatically detect if a concurrent modification occurred during user think time. Check for this at the end of the conversation.

- Detached Objects: if you decide to use the session-per-request pattern, all loaded instances will be in the detached state during user think time. Hibernate allows you to reattach the objects and persist the modifications. The pattern is called session-per-request-with-detached-objects. Automatic versioning is used to isolate concurrent modifications.

- Extended (or Long) Session: the Hibernate `Session` can be disconnected from the underlying JDBC connection after the database transaction has been committed and reconnected when a new client request occurs. This pattern is known as session-per-conversation and makes even reattachment unnecessary. Automatic versioning is used to isolate concurrent modifications and the `Session` will not be allowed to be flushed automatically, but explicitly.

Both session-per-request-with-detached-objects and session-per-conversation have advantages and disadvantages. These disadvantages are discussed later in this chapter in the context of optimistic concurrency control.

### 13.1.3. Considering object identity

An application can concurrently access the same persistent state in two different `Session`s. However, an instance of a persistent class is never shared between two `Session` instances. It is for this reason that there are two different notions of identity:

Database Identity

  `foo.getId().equals( bar.getId() )`

JVM Identity

  `foo==bar`

For objects attached to a particular `Session` (i.e., in the scope of a `Session`), the two notions are equivalent and JVM identity for database identity is guaranteed by Hibernate. While the application might concurrently access the "same" (persistent identity) business object in two different sessions, the two instances will actually be "different" (JVM identity). Conflicts are resolved using an optimistic approach and automatic versioning at flush/commit time.

This approach leaves Hibernate and the database to worry about concurrency. It also provides the best scalability, since guaranteeing identity in single-threaded units of work means that it does not need expensive locking or other means of synchronization. The application does not need to synchronize on any business object, as long as it maintains a single thread per `Session`. Within a `Session` the application can safely use `==` to compare objects.

However, an application that uses `==` outside of a `Session` might produce unexpected results. This might occur even in some unexpected places. For example, if you put two detached instances into the same `Set`, both might have the same database identity (i.e., they represent the same row). JVM identity, however, is by definition not guaranteed for instances in a detached state. The developer has to override the `equals()` and `hashCode()` methods in persistent classes and implement their own notion of object equality. There is one caveat: never use the database identifier to implement equality. Use a business key that is a combination of unique, usually immutable, attributes. The database identifier will change if a transient object is made persistent. If the transient instance (usually together with detached instances) is held in a `Set`, changing the hashcode breaks the contract of the `Set`. Attributes for business keys do not have to be as stable as database primary keys; you only have to guarantee stability as long as the objects are in the same `Set`. See the Hibernate website for a more thorough discussion of this issue. Please note that this is not a Hibernate issue, but simply how Java object identity and equality has to be implemented.

### 13.1.4. Common issues

Do not use the anti-patterns session-per-user-session or session-per-application (there are, however, rare exceptions to this rule). Some of the following issues might also arise within the recommended patterns, so ensure that you understand the implications before making a design decision:

- A `Session` is not thread-safe. Things that work concurrently, like HTTP requests, session beans, or Swing workers, will cause race conditions if a `Session` instance is shared. If you keep your Hibernate `Session` in your `HttpSession` (this is discussed later in the chapter), you should consider synchronizing access to your Http session. Otherwise, a user that clicks reload fast enough can use the same `Session` in two concurrently running threads.

- An exception thrown by Hibernate means you have to rollback your database transaction and close the `Session` immediately (this is discussed in more detail later in the chapter). If your `Session` is bound to the application, you have to stop the application. Rolling back the database transaction does not put your business objects back into the state they were at the start of the transaction. This means that the database state and the business objects will be out of sync. Usually this is not a problem, because exceptions are not recoverable and you will have to start over after rollback anyway.

- The `Session` caches every object that is in a persistent state (watched and checked for dirty state by Hibernate). If you keep it open for a long time or simply load too much data, it will grow endlessly until you get an OutOfMemoryException. One solution is to call `clear()` and `evict()` to manage the `Session` cache, but you should consider a Stored Procedure if you need mass data operations. Some solutions are shown in Chapter 15, Batch processing. Keeping a `Session` open for the duration of a user session also means a higher probability of stale data.

## 13.2. Database transaction demarcation

Database, or system, transaction boundaries are always necessary. No communication with the database can occur outside of a database transaction (this seems to confuse many developers who are used to the auto-commit mode). Always use clear transaction boundaries, even for read-only operations. Depending on your isolation level and database capabilities this might not be required, but there is no downside if you always demarcate transactions explicitly. Certainly, a single database transaction is going to perform better than many small transactions, even for reading data.

A Hibernate application can run in non-managed (i.e., standalone, simple Web- or Swing applications) and managed J2EE environments. In a non-managed environment, Hibernate is usually responsible for its own database connection pool. The application developer has to manually set transaction boundaries (begin, commit, or rollback database transactions) themselves. A managed environment usually provides container-managed transactions (CMT), with the transaction assembly defined declaratively (in deployment descriptors of EJB session beans, for example). Programmatic transaction demarcation is then no longer necessary.

However, it is often desirable to keep your persistence layer portable between non-managed resource-local environments, and systems that can rely on JTA but use BMT instead of CMT. In both cases use programmatic transaction demarcation. Hibernate offers a wrapper API called `Transaction` that translates into the native transaction system of your deployment environment. This API is actually optional, but we strongly encourage its use unless you are in a CMT session bean.

Ending a `Session` usually involves four distinct phases:

- flush the session

- commit the transaction

- close the session

- handle exceptions

We discussed Flushing the session earlier, so we will now have a closer look at transaction demarcation and exception handling in both managed and non-managed environments.

### 13.2.1. Non-managed environment

If a Hibernate persistence layer runs in a non-managed environment, database connections are usually handled by simple (i.e., non-DataSource) connection pools from which Hibernate obtains connections as needed. The session/transaction handling idiom looks like this:

    // Non-managed environment idiom
    Session sess = factory.openSession();
    Transaction tx = null;
    try {
        tx = sess.beginTransaction();

        // do some work
        ...

        tx.commit();
    }
    catch (RuntimeException e) {
        if (tx != null) tx.rollback();
        throw e; // or display error message
    }
    finally {
        sess.close();
    }

You do not have to `flush()` the `Session` explicitly: the call to `commit()` automatically triggers the synchronization depending on the Section 11.10, “Flushing the Session” for the session. A call to `close()` marks the end of a session. The main implication of `close()` is that the JDBC connection will be relinquished by the session. This Java code is portable and runs in both non-managed and JTA environments.

As outlined earlier, a much more flexible solution is Hibernate's built-in "current session" context management:

    // Non-managed environment idiom with getCurrentSession()
    try {
        factory.getCurrentSession().beginTransaction();

        // do some work
        ...

        factory.getCurrentSession().getTransaction().commit();
    }
    catch (RuntimeException e) {
        factory.getCurrentSession().getTransaction().rollback();
        throw e; // or display error message
    }

You will not see these code snippets in a regular application; fatal (system) exceptions should always be caught at the "top". In other words, the code that executes Hibernate calls in the persistence layer, and the code that handles `RuntimeException` (and usually can only clean up and exit), are in different layers. The current context management by Hibernate can significantly simplify this design by accessing a `SessionFactory`. Exception handling is discussed later in this chapter.

You should select `org.hibernate.transaction.JDBCTransactionFactory`, which is the default, and for the second example select `"thread"` as your `hibernate.current_session_context_class`.

### 13.2.2. Using JTA

If your persistence layer runs in an application server (for example, behind EJB session beans), every datasource connection obtained by Hibernate will automatically be part of the global JTA transaction. You can also install a standalone JTA implementation and use it without EJB. Hibernate offers two strategies for JTA integration.

If you use bean-managed transactions (BMT), Hibernate will tell the application server to start and end a BMT transaction if you use the `Transaction` API. The transaction management code is identical to the non-managed environment.

    // BMT idiom
    Session sess = factory.openSession();
    Transaction tx = null;
    try {
        tx = sess.beginTransaction();

        // do some work
        ...

        tx.commit();
    }
    catch (RuntimeException e) {
        if (tx != null) tx.rollback();
        throw e; // or display error message
    }
    finally {
        sess.close();
    }

If you want to use a transaction-bound `Session`, that is, the `getCurrentSession()` functionality for easy context propagation, use the JTA `UserTransaction` API directly:

    // BMT idiom with getCurrentSession()
    try {
        UserTransaction tx = (UserTransaction)new InitialContext()
                                .lookup("java:comp/UserTransaction");

        tx.begin();

        // Do some work on Session bound to transaction
        factory.getCurrentSession().load(...);
        factory.getCurrentSession().persist(...);

        tx.commit();
    }
    catch (RuntimeException e) {
        tx.rollback();
        throw e; // or display error message
    }

With CMT, transaction demarcation is completed in session bean deployment descriptors, not programmatically. The code is reduced to:

    // CMT idiom
     Session sess = factory.getCurrentSession();

     // do some work
     ...

In a CMT/EJB, even rollback happens automatically. An unhandled `RuntimeException` thrown by a session bean method tells the container to set the global transaction to rollback. You do not need to use the Hibernate `Transaction` API at all with BMT or CMT, and you get automatic propagation of the "current" Session bound to the transaction.

When configuring Hibernate's transaction factory, choose `org.hibernate.transaction.JTATransactionFactory` if you use JTA directly (BMT), and `org.hibernate.transaction.CMTTransactionFactory` in a CMT session bean. Remember to also set `hibernate.transaction.manager_lookup_class`. Ensure that your `hibernate.current_session_context_class` is either unset (backwards compatibility), or is set to `"jta"`.

The `getCurrentSession()` operation has one downside in a JTA environment. There is one caveat to the use of `after_statement` connection release mode, which is then used by default. Due to a limitation of the JTA spec, it is not possible for Hibernate to automatically clean up any unclosed `ScrollableResults` or `Iterator` instances returned by `scroll()` or `iterate()`. You must release the underlying database cursor by calling `ScrollableResults.close()` or `Hibernate.close(Iterator)` explicitly from a `finally` block. Most applications can easily avoid using `scroll()` or `iterate()` from the JTA or CMT code.)

### 13.2.3. Exception handling

If the `Session` throws an exception, including any `SQLException`, immediately rollback the database transaction, call `Session.close()` and discard the `Session` instance. Certain methods of `Session` will not leave the session in a consistent state. No exception thrown by Hibernate can be treated as recoverable. Ensure that the `Session` will be closed by calling `close()` in a `finally` block.

The `HibernateException`, which wraps most of the errors that can occur in a Hibernate persistence layer, is an unchecked exception. It was not in older versions of Hibernate. In our opinion, we should not force the application developer to catch an unrecoverable exception at a low layer. In most systems, unchecked and fatal exceptions are handled in one of the first frames of the method call stack (i.e., in higher layers) and either an error message is presented to the application user or some other appropriate action is taken. Note that Hibernate might also throw other unchecked exceptions that are not a `HibernateException`. These are not recoverable and appropriate action should be taken.

Hibernate wraps `SQLException`s thrown while interacting with the database in a `JDBCException`. In fact, Hibernate will attempt to convert the exception into a more meaningful subclass of `JDBCException`. The underlying `SQLException` is always available via `JDBCException.getCause()`. Hibernate converts the `SQLException` into an appropriate `JDBCException` subclass using the `SQLExceptionConverter` attached to the `SessionFactory`. By default, the `SQLExceptionConverter` is defined by the configured dialect. However, it is also possible to plug in a custom implementation. See the javadocs for the `SQLExceptionConverterFactory` class for details. The standard `JDBCException` subtypes are:

- `JDBCConnectionException`: indicates an error with the underlying JDBC communication.

- `SQLGrammarException`: indicates a grammar or syntax problem with the issued SQL.

- `ConstraintViolationException`: indicates some form of integrity constraint violation.

- `LockAcquisitionException`: indicates an error acquiring a lock level necessary to perform the requested operation.

- `GenericJDBCException`: a generic exception which did not fall into any of the other categories.

### 13.2.4. Transaction timeout

An important feature provided by a managed environment like EJB, that is never provided for non-managed code, is transaction timeout. Transaction timeouts ensure that no misbehaving transaction can indefinitely tie up resources while returning no response to the user. Outside a managed (JTA) environment, Hibernate cannot fully provide this functionality. However, Hibernate can at least control data access operations, ensuring that database level deadlocks and queries with huge result sets are limited by a defined timeout. In a managed environment, Hibernate can delegate transaction timeout to JTA. This functionality is abstracted by the Hibernate `Transaction` object.

    Session sess = factory.openSession();
    try {
        //set transaction timeout to 3 seconds
        sess.getTransaction().setTimeout(3);
        sess.getTransaction().begin();

        // do some work
        ...

        sess.getTransaction().commit()
    }
    catch (RuntimeException e) {
        sess.getTransaction().rollback();
        throw e; // or display error message
    }
    finally {
        sess.close();
    }

`setTimeout()` cannot be called in a CMT bean, where transaction timeouts must be defined declaratively.

## 13.3. Optimistic concurrency control

The only approach that is consistent with high concurrency and high scalability, is optimistic concurrency control with versioning. Version checking uses version numbers, or timestamps, to detect conflicting updates and to prevent lost updates. Hibernate provides three possible approaches to writing application code that uses optimistic concurrency. The use cases we discuss are in the context of long conversations, but version checking also has the benefit of preventing lost updates in single database transactions.

### 13.3.1. Application version checking

In an implementation without much help from Hibernate, each interaction with the database occurs in a new `Session` and the developer is responsible for reloading all persistent instances from the database before manipulating them. The application is forced to carry out its own version checking to ensure conversation transaction isolation. This approach is the least efficient in terms of database access. It is the approach most similar to entity EJBs.

    // foo is an instance loaded by a previous Session
    session = factory.openSession();
    Transaction t = session.beginTransaction();

    int oldVersion = foo.getVersion();
    session.load( foo, foo.getKey() ); // load the current state
    if ( oldVersion != foo.getVersion() ) throw new StaleObjectStateException();
    foo.setProperty("bar");

    t.commit();
    session.close();

The `version` property is mapped using `<version>`, and Hibernate will automatically increment it during flush if the entity is dirty.

If you are operating in a low-data-concurrency environment, and do not require version checking, you can use this approach and skip the version check. In this case, last commit wins is the default strategy for long conversations. Be aware that this might confuse the users of the application, as they might experience lost updates without error messages or a chance to merge conflicting changes.

Manual version checking is only feasible in trivial circumstances and not practical for most applications. Often not only single instances, but complete graphs of modified objects, have to be checked. Hibernate offers automatic version checking with either an extended `Session` or detached instances as the design paradigm.

### 13.3.2. Extended session and automatic versioning

A single `Session` instance and its persistent instances that are used for the whole conversation are known as session-per-conversation. Hibernate checks instance versions at flush time, throwing an exception if concurrent modification is detected. It is up to the developer to catch and handle this exception. Common options are the opportunity for the user to merge changes or to restart the business conversation with non-stale data.

The `Session` is disconnected from any underlying JDBC connection when waiting for user interaction. This approach is the most efficient in terms of database access. The application does not version check or reattach detached instances, nor does it have to reload instances in every database transaction.

    // foo is an instance loaded earlier by the old session
    Transaction t = session.beginTransaction(); // Obtain a new JDBC connection, start transaction

    foo.setProperty("bar");

    session.flush();    // Only for last transaction in conversation
    t.commit();         // Also return JDBC connection
    session.close();    // Only for last transaction in conversation

The `foo` object knows which `Session` it was loaded in. Beginning a new database transaction on an old session obtains a new connection and resumes the session. Committing a database transaction disconnects a session from the JDBC connection and returns the connection to the pool. After reconnection, to force a version check on data you are not updating, you can call `Session.lock()` with `LockMode.READ` on any objects that might have been updated by another transaction. You do not need to lock any data that you are updating. Usually you would set `FlushMode.MANUAL` on an extended `Session`, so that only the last database transaction cycle is allowed to actually persist all modifications made in this conversation. Only this last database transaction will include the `flush()` operation, and then `close()` the session to end the conversation.

This pattern is problematic if the `Session` is too big to be stored during user think time (for example, an `HttpSession` should be kept as small as possible). As the `Session` is also the first-level cache and contains all loaded objects, we can probably use this strategy only for a few request/response cycles. Use a `Session` only for a single conversation as it will soon have stale data.

## Note

Earlier versions of Hibernate required explicit disconnection and reconnection of a `Session`. These methods are deprecated, as beginning and ending a transaction has the same effect.

Keep the disconnected `Session` close to the persistence layer. Use an EJB stateful session bean to hold the `Session` in a three-tier environment. Do not transfer it to the web layer, or even serialize it to a separate tier, to store it in the `HttpSession`.

The extended session pattern, or session-per-conversation, is more difficult to implement with automatic current session context management. You need to supply your own implementation of the `CurrentSessionContext` for this. See the Hibernate Wiki for examples.

### 13.3.3. Detached objects and automatic versioning

Each interaction with the persistent store occurs in a new `Session`. However, the same persistent instances are reused for each interaction with the database. The application manipulates the state of detached instances originally loaded in another `Session` and then reattaches them using `Session.update()`, `Session.saveOrUpdate()`, or `Session.merge()`.

    // foo is an instance loaded by a previous Session
    foo.setProperty("bar");
    session = factory.openSession();
    Transaction t = session.beginTransaction();
    session.saveOrUpdate(foo); // Use merge() if "foo" might have been loaded already
    t.commit();
    session.close();

Again, Hibernate will check instance versions during flush, throwing an exception if conflicting updates occurred.

You can also call `lock()` instead of `update()`, and use `LockMode.READ` (performing a version check and bypassing all caches) if you are sure that the object has not been modified.

### 13.3.4. Customizing automatic versioning

You can disable Hibernate's automatic version increment for particular properties and collections by setting the `optimistic-lock` mapping attribute to `false`. Hibernate will then no longer increment versions if the property is dirty.

Legacy database schemas are often static and cannot be modified. Or, other applications might access the same database and will not know how to handle version numbers or even timestamps. In both cases, versioning cannot rely on a particular column in a table. To force a version check with a comparison of the state of all fields in a row but without a version or timestamp property mapping, turn on `optimistic-lock="all"` in the `<class>` mapping. This conceptually only works if Hibernate can compare the old and the new state (i.e., if you use a single long `Session` and not session-per-request-with-detached-objects).

Concurrent modification can be permitted in instances where the changes that have been made do not overlap. If you set `optimistic-lock="dirty"` when mapping the `<class>`, Hibernate will only compare dirty fields during flush.

In both cases, with dedicated version/timestamp columns or with a full/dirty field comparison, Hibernate uses a single `UPDATE` statement, with an appropriate `WHERE` clause, per entity to execute the version check and update the information. If you use transitive persistence to cascade reattachment to associated entities, Hibernate may execute unnecessary updates. This is usually not a problem, but on update triggers in the database might be executed even when no changes have been made to detached instances. You can customize this behavior by setting `select-before-update="true"` in the `<class>` mapping, forcing Hibernate to `SELECT` the instance to ensure that changes did occur before updating the row.

## 13.4. Pessimistic locking

It is not intended that users spend much time worrying about locking strategies. It is usually enough to specify an isolation level for the JDBC connections and then simply let the database do all the work. However, advanced users may wish to obtain exclusive pessimistic locks or re-obtain locks at the start of a new transaction.

Hibernate will always use the locking mechanism of the database; it never lock objects in memory.

The `LockMode` class defines the different lock levels that can be acquired by Hibernate. A lock is obtained by the following mechanisms:

- `LockMode.WRITE` is acquired automatically when Hibernate updates or inserts a row.

- `LockMode.UPGRADE` can be acquired upon explicit user request using `SELECT ... FOR UPDATE` on databases which support that syntax.

- `LockMode.UPGRADE_NOWAIT` can be acquired upon explicit user request using a `SELECT ... FOR UPDATE NOWAIT` under Oracle.

- `LockMode.READ` is acquired automatically when Hibernate reads data under Repeatable Read or Serializable isolation level. It can be re-acquired by explicit user request.

- `LockMode.NONE` represents the absence of a lock. All objects switch to this lock mode at the end of a `Transaction`. Objects associated with the session via a call to `update()` or `saveOrUpdate()` also start out in this lock mode.

The "explicit user request" is expressed in one of the following ways:

- A call to `Session.load()`, specifying a `LockMode`.

- A call to `Session.lock()`.

- A call to `Query.setLockMode()`.

If `Session.load()` is called with `UPGRADE` or `UPGRADE_NOWAIT`, and the requested object was not yet loaded by the session, the object is loaded using `SELECT ... FOR UPDATE`. If `load()` is called for an object that is already loaded with a less restrictive lock than the one requested, Hibernate calls `lock()` for that object.

`Session.lock()` performs a version number check if the specified lock mode is `READ`, `UPGRADE` or `UPGRADE_NOWAIT`. In the case of `UPGRADE` or `UPGRADE_NOWAIT`, `SELECT ... FOR UPDATE` is used.

If the requested lock mode is not supported by the database, Hibernate uses an appropriate alternate mode instead of throwing an exception. This ensures that applications are portable.

## 13.5. Connection release modes

One of the legacies of Hibernate 2.x JDBC connection management meant that a `Session` would obtain a connection when it was first required and then maintain that connection until the session was closed. Hibernate 3.x introduced the notion of connection release modes that would instruct a session how to handle its JDBC connections. The following discussion is pertinent only to connections provided through a configured `ConnectionProvider`. User-supplied connections are outside the breadth of this discussion. The different release modes are identified by the enumerated values of `org.hibernate.ConnectionReleaseMode`:

- `ON_CLOSE`: is the legacy behavior described above. The Hibernate session obtains a connection when it first needs to perform some JDBC access and maintains that connection until the session is closed.

- `AFTER_TRANSACTION`: releases connections after a `org.hibernate.Transaction` has been completed.

- `AFTER_STATEMENT` (also referred to as aggressive release): releases connections after every statement execution. This aggressive releasing is skipped if that statement leaves open resources associated with the given session. Currently the only situation where this occurs is through the use of `org.hibernate.ScrollableResults`.

The configuration parameter `hibernate.connection.release_mode` is used to specify which release mode to use. The possible values are as follows:

- `auto` (the default): this choice delegates to the release mode returned by the `org.hibernate.transaction.TransactionFactory.getDefaultReleaseMode()` method. For JTATransactionFactory, this returns ConnectionReleaseMode.AFTER_STATEMENT; for JDBCTransactionFactory, this returns ConnectionReleaseMode.AFTER_TRANSACTION. Do not change this default behavior as failures due to the value of this setting tend to indicate bugs and/or invalid assumptions in user code.

- `on_close`: uses ConnectionReleaseMode.ON_CLOSE. This setting is left for backwards compatibility, but its use is discouraged.

- `after_transaction`: uses ConnectionReleaseMode.AFTER_TRANSACTION. This setting should not be used in JTA environments. Also note that with ConnectionReleaseMode.AFTER_TRANSACTION, if a session is considered to be in auto-commit mode, connections will be released as if the release mode were AFTER_STATEMENT.

- `after_statement`: uses ConnectionReleaseMode.AFTER_STATEMENT. Additionally, the configured `ConnectionProvider` is consulted to see if it supports this setting (`supportsAggressiveRelease()`). If not, the release mode is reset to ConnectionReleaseMode.AFTER_TRANSACTION. This setting is only safe in environments where we can either re-acquire the same underlying JDBC connection each time you make a call into `ConnectionProvider.getConnection()` or in auto-commit environments where it does not matter if we re-establish the same connection.

## Chapter 14. Interceptors and events

It is useful for the application to react to certain events that occur inside Hibernate. This allows for the implementation of generic functionality and the extension of Hibernate functionality.

## 14.1. Interceptors

The `Interceptor` interface provides callbacks from the session to the application, allowing the application to inspect and/or manipulate properties of a persistent object before it is saved, updated, deleted or loaded. One possible use for this is to track auditing information. For example, the following `Interceptor` automatically sets the `createTimestamp` when an `Auditable` is created and updates the `lastUpdateTimestamp` property when an `Auditable` is updated.

You can either implement `Interceptor` directly or extend `EmptyInterceptor`.

    package org.hibernate.test;

    import java.io.Serializable;
    import java.util.Date;
    import java.util.Iterator;

    import org.hibernate.EmptyInterceptor;
    import org.hibernate.Transaction;
    import org.hibernate.type.Type;

    public class AuditInterceptor extends EmptyInterceptor {

        private int updates;
        private int creates;
        private int loads;

        public void onDelete(Object entity,
                             Serializable id,
                             Object[] state,
                             String[] propertyNames,
                             Type[] types) {
            // do nothing
        }

        public boolean onFlushDirty(Object entity,
                                    Serializable id,
                                    Object[] currentState,
                                    Object[] previousState,
                                    String[] propertyNames,
                                    Type[] types) {

            if ( entity instanceof Auditable ) {
                updates++;
                for ( int i=0; i < propertyNames.length; i++ ) {
                    if ( "lastUpdateTimestamp".equals( propertyNames[i] ) ) {
                        currentState[i] = new Date();
                        return true;
                    }
                }
            }
            return false;
        }

        public boolean onLoad(Object entity,
                              Serializable id,
                              Object[] state,
                              String[] propertyNames,
                              Type[] types) {
            if ( entity instanceof Auditable ) {
                loads++;
            }
            return false;
        }

        public boolean onSave(Object entity,
                              Serializable id,
                              Object[] state,
                              String[] propertyNames,
                              Type[] types) {

            if ( entity instanceof Auditable ) {
                creates++;
                for ( int i=0; i<propertyNames.length; i++ ) {
                    if ( "createTimestamp".equals( propertyNames[i] ) ) {
                        state[i] = new Date();
                        return true;
                    }
                }
            }
            return false;
        }

        public void afterTransactionCompletion(Transaction tx) {
            if ( tx.wasCommitted() ) {
                System.out.println("Creations: " + creates + ", Updates: " + updates, "Loads: " + loads);
            }
            updates=0;
            creates=0;
            loads=0;
        }

    }

There are two kinds of inteceptors: `Session`-scoped and `SessionFactory`-scoped.

A `Session`-scoped interceptor is specified when a session is opened.

    Session session = sf.withOptions( new AuditInterceptor() ).openSession();

A `SessionFactory`-scoped interceptor is registered with the `Configuration` object prior to building the `SessionFactory`. Unless a session is opened explicitly specifying the interceptor to use, the supplied interceptor will be applied to all sessions opened from that `SessionFactory`. `SessionFactory`-scoped interceptors must be thread safe. Ensure that you do not store session-specific states, since multiple sessions will use this interceptor potentially concurrently.

    new Configuration().setInterceptor( new AuditInterceptor() );

## 14.2. Event system

If you have to react to particular events in your persistence layer, you can also use the Hibernate event architecture. The event system can be used in addition, or as a replacement, for interceptors.

Many methods of the `Session` interface correlate to an event type. The full range of defined event types is declared as enum values on `org.hibernate.event.spi.EventType`. When a request is made of one of these methods, the Hibernate `Session` generates an appropriate event and passes it to the configured event listeners for that type. Out-of-the-box, these listeners implement the same processing in which those methods always resulted. However, you are free to implement a customization of one of the listener interfaces (i.e., the `LoadEvent` is processed by the registered implementation of the `LoadEventListener` interface), in which case their implementation would be responsible for processing any `load()` requests made of the `Session`.

## Note

See the Hibernate Developer Guide for information on registering custom event listeners.

The listeners should be considered stateless; they are shared between requests, and should not save any state as instance variables.

A custom listener implements the appropriate interface for the event it wants to process and/or extend one of the convenience base classes (or even the default event listeners used by Hibernate out-of-the-box as these are declared non-final for this purpose). Here is an example of a custom load event listener:

    public class MyLoadListener implements LoadEventListener {
        // this is the single method defined by the LoadEventListener interface
        public void onLoad(LoadEvent event, LoadEventListener.LoadType loadType)
                throws HibernateException {
            if ( !MySecurity.isAuthorized( event.getEntityClassName(), event.getEntityId() ) ) {
                throw MySecurityException("Unauthorized access");
            }
        }
    }

## 14.3. Hibernate declarative security

Usually, declarative security in Hibernate applications is managed in a session facade layer. Hibernate allows certain actions to be permissioned via JACC, and authorized via JAAS. This is an optional functionality that is built on top of the event architecture.

First, you must configure the appropriate event listeners, to enable the use of JACC authorization. Again, see Hibernate Developer Guide for the details. Below is an example of an appropriate `org.hibernate.integrator.spi.Integrator` implementation for this purpose.

    import org.hibernate.event.service.spi.DuplicationStrategy;
    import org.hibernate.event.service.spi.EventListenerRegistry;
    import org.hibernate.integrator.spi.Integrator;
    import org.hibernate.secure.internal.JACCPreDeleteEventListener;
    import org.hibernate.secure.internal.JACCPreInsertEventListener;
    import org.hibernate.secure.internal.JACCPreLoadEventListener;
    import org.hibernate.secure.internal.JACCPreUpdateEventListener;
    import org.hibernate.secure.internal.JACCSecurityListener;

    public class JaccEventListenerIntegrator implements Integrator {

    	private static final DuplicationStrategy JACC_DUPLICATION_STRATEGY = new DuplicationStrategy() {
    		@Override
    		public boolean areMatch(Object listener, Object original) {
    			return listener.getClass().equals( original.getClass() ) &&
    					JACCSecurityListener.class.isInstance( original );
    		}

    		@Override
    		public Action getAction() {
    			return Action.KEEP_ORIGINAL;
    		}
    	};

    	@Override
    	@SuppressWarnings( {"unchecked"})
    	public void integrate(
    			Configuration configuration,
    			SessionFactoryImplementor sessionFactory,
    			SessionFactoryServiceRegistry serviceRegistry) {
    		boolean isSecurityEnabled = configuration.getProperties().containsKey( AvailableSettings.JACC_ENABLED );
    		if ( !isSecurityEnabled ) {
    			return;
    		}

    		final EventListenerRegistry eventListenerRegistry = serviceRegistry.getService( EventListenerRegistry.class );
    		eventListenerRegistry.addDuplicationStrategy( JACC_DUPLICATION_STRATEGY );

    		final String jaccContextId = configuration.getProperty( Environment.JACC_CONTEXTID );
    		eventListenerRegistry.prependListeners( EventType.PRE_DELETE, new JACCPreDeleteEventListener(jaccContextId) );
    		eventListenerRegistry.prependListeners( EventType.PRE_INSERT, new JACCPreInsertEventListener(jaccContextId) );
    		eventListenerRegistry.prependListeners( EventType.PRE_UPDATE, new JACCPreUpdateEventListener(jaccContextId) );
    		eventListenerRegistry.prependListeners( EventType.PRE_LOAD, new JACCPreLoadEventListener(jaccContextId) );
    	}
    }

You must also decide how to configure your JACC provider. One option is to tell Hibernate what permissions to bind to what roles and have it configure the JACC provider. This would be done in the `hibernate.cfg.xml` file.

    <grant role="admin" entity-name="User" actions="insert,update,read"/>
    <grant role="su" entity-name="User" actions="*"/>

## Chapter 15. Batch processing

A naive approach to inserting 100,000 rows in the database using Hibernate might look like this:

    Session session = sessionFactory.openSession();
    Transaction tx = session.beginTransaction();
    for ( int i=0; i<100000; i++ ) {
        Customer customer = new Customer(.....);
        session.save(customer);
    }
    tx.commit();
    session.close();

This would fall over with an `OutOfMemoryException` somewhere around the 50,000th row. That is because Hibernate caches all the newly inserted `Customer` instances in the session-level cache. In this chapter we will show you how to avoid this problem.

If you are undertaking batch processing you will need to enable the use of JDBC batching. This is absolutely essential if you want to achieve optimal performance. Set the JDBC batch size to a reasonable number (10-50, for example):

    hibernate.jdbc.batch_size 20

Hibernate disables insert batching at the JDBC level transparently if you use an `identity` identifier generator.

You can also do this kind of work in a process where interaction with the second-level cache is completely disabled:

    hibernate.cache.use_second_level_cache false

However, this is not absolutely necessary, since we can explicitly set the `CacheMode` to disable interaction with the second-level cache.

## 15.1. Batch inserts

When making new objects persistent `flush()` and then `clear()` the session regularly in order to control the size of the first-level cache.

    Session session = sessionFactory.openSession();
    Transaction tx = session.beginTransaction();

    for ( int i=0; i<100000; i++ ) {
        Customer customer = new Customer(.....);
        session.save(customer);
        if ( i % 20 == 0 ) { //20, same as the JDBC batch size
            //flush a batch of inserts and release memory:
            session.flush();
            session.clear();
        }
    }

    tx.commit();
    session.close();

## 15.2. Batch updates

For retrieving and updating data, the same ideas apply. In addition, you need to use `scroll()` to take advantage of server-side cursors for queries that return many rows of data.

    Session session = sessionFactory.openSession();
    Transaction tx = session.beginTransaction();

    ScrollableResults customers = session.getNamedQuery("GetCustomers")
        .setCacheMode(CacheMode.IGNORE)
        .scroll(ScrollMode.FORWARD_ONLY);
    int count=0;
    while ( customers.next() ) {
        Customer customer = (Customer) customers.get(0);
        customer.updateStuff(...);
        if ( ++count % 20 == 0 ) {
            //flush a batch of updates and release memory:
            session.flush();
            session.clear();
        }
    }

    tx.commit();
    session.close();

## 15.3. The StatelessSession interface

Alternatively, Hibernate provides a command-oriented API that can be used for streaming data to and from the database in the form of detached objects. A `StatelessSession` has no persistence context associated with it and does not provide many of the higher-level life cycle semantics. In particular, a stateless session does not implement a first-level cache nor interact with any second-level or query cache. It does not implement transactional write-behind or automatic dirty checking. Operations performed using a stateless session never cascade to associated instances. Collections are ignored by a stateless session. Operations performed via a stateless session bypass Hibernate's event model and interceptors. Due to the lack of a first-level cache, Stateless sessions are vulnerable to data aliasing effects. A stateless session is a lower-level abstraction that is much closer to the underlying JDBC.

    StatelessSession session = sessionFactory.openStatelessSession();
    Transaction tx = session.beginTransaction();

    ScrollableResults customers = session.getNamedQuery("GetCustomers")
        .scroll(ScrollMode.FORWARD_ONLY);
    while ( customers.next() ) {
        Customer customer = (Customer) customers.get(0);
        customer.updateStuff(...);
        session.update(customer);
    }

    tx.commit();
    session.close();

In this code example, the `Customer` instances returned by the query are immediately detached. They are never associated with any persistence context.

The `insert(), update()` and `delete()` operations defined by the `StatelessSession` interface are considered to be direct database row-level operations. They result in the immediate execution of a SQL `INSERT, UPDATE` or `DELETE` respectively. They have different semantics to the `save(), saveOrUpdate()` and `delete()` operations defined by the `Session` interface.

## 15.4. DML-style operations

As already discussed, automatic and transparent object/relational mapping is concerned with the management of the object state. The object state is available in memory. This means that manipulating data directly in the database (using the SQL `Data Manipulation Language` (DML) the statements: `INSERT`, `UPDATE`, `DELETE`) will not affect in-memory state. However, Hibernate provides methods for bulk SQL-style DML statement execution that is performed through the Hibernate Query Language (Chapter 16, HQL: The Hibernate Query Language).

The pseudo-syntax for `UPDATE` and `DELETE` statements is: `( UPDATE | DELETE ) FROM? EntityName (WHERE where_conditions)?`.

Some points to note:

- In the from-clause, the FROM keyword is optional

- There can only be a single entity named in the from-clause. It can, however, be aliased. If the entity name is aliased, then any property references must be qualified using that alias. If the entity name is not aliased, then it is illegal for any property references to be qualified.

- No Section 16.4, “Forms of join syntax”, either implicit or explicit, can be specified in a bulk HQL query. Sub-queries can be used in the where-clause, where the subqueries themselves may contain joins.

- The where-clause is also optional.

As an example, to execute an HQL `UPDATE`, use the `Query.executeUpdate()` method. The method is named for those familiar with JDBC's `PreparedStatement.executeUpdate()`:

    Session session = sessionFactory.openSession();
    Transaction tx = session.beginTransaction();

    String hqlUpdate = "update Customer c set c.name = :newName where c.name = :oldName";
    // or String hqlUpdate = "update Customer set name = :newName where name = :oldName";
    int updatedEntities = s.createQuery( hqlUpdate )
            .setString( "newName", newName )
            .setString( "oldName", oldName )
            .executeUpdate();
    tx.commit();
    session.close();

In keeping with the EJB3 specification, HQL `UPDATE` statements, by default, do not effect the ??? or the Section 5.1.3.2, “Timestamp” property values for the affected entities. However, you can force Hibernate to reset the `version` or `timestamp` property values through the use of a `versioned update`. This is achieved by adding the `VERSIONED` keyword after the `UPDATE` keyword.

    Session session = sessionFactory.openSession();
    Transaction tx = session.beginTransaction();
    String hqlVersionedUpdate = "update versioned Customer set name = :newName where name = :oldName";
    int updatedEntities = s.createQuery( hqlUpdate )
            .setString( "newName", newName )
            .setString( "oldName", oldName )
            .executeUpdate();
    tx.commit();
    session.close();

Custom version types, `org.hibernate.usertype.UserVersionType`, are not allowed in conjunction with a `update versioned` statement.

To execute an HQL `DELETE`, use the same `Query.executeUpdate()` method:

    Session session = sessionFactory.openSession();
    Transaction tx = session.beginTransaction();

    String hqlDelete = "delete Customer c where c.name = :oldName";
    // or String hqlDelete = "delete Customer where name = :oldName";
    int deletedEntities = s.createQuery( hqlDelete )
            .setString( "oldName", oldName )
            .executeUpdate();
    tx.commit();
    session.close();

The `int` value returned by the `Query.executeUpdate()` method indicates the number of entities effected by the operation. This may or may not correlate to the number of rows effected in the database. An HQL bulk operation might result in multiple actual SQL statements being executed (for joined-subclass, for example). The returned number indicates the number of actual entities affected by the statement. Going back to the example of joined-subclass, a delete against one of the subclasses may actually result in deletes against not just the table to which that subclass is mapped, but also the "root" table and potentially joined-subclass tables further down the inheritance hierarchy.

The pseudo-syntax for `INSERT` statements is: `INSERT INTO EntityName properties_list select_statement`. Some points to note:

- Only the INSERT INTO ... SELECT ... form is supported; not the INSERT INTO ... VALUES ... form.

  The properties_list is analogous to the `column specification` in the SQL `INSERT` statement. For entities involved in mapped inheritance, only properties directly defined on that given class-level can be used in the properties_list. Superclass properties are not allowed and subclass properties do not make sense. In other words, `INSERT` statements are inherently non-polymorphic.

- select_statement can be any valid HQL select query, with the caveat that the return types must match the types expected by the insert. Currently, this is checked during query compilation rather than allowing the check to relegate to the database. This might, however, cause problems between Hibernate `Type`s which are equivalent as opposed to equal. This might cause issues with mismatches between a property defined as a `org.hibernate.type.DateType` and a property defined as a `org.hibernate.type.TimestampType`, even though the database might not make a distinction or might be able to handle the conversion.

- For the id property, the insert statement gives you two options. You can either explicitly specify the id property in the properties_list, in which case its value is taken from the corresponding select expression, or omit it from the properties_list, in which case a generated value is used. This latter option is only available when using id generators that operate in the database; attempting to use this option with any "in memory" type generators will cause an exception during parsing. For the purposes of this discussion, in-database generators are considered to be `org.hibernate.id.SequenceGenerator` (and its subclasses) and any implementers of `org.hibernate.id.PostInsertIdentifierGenerator`. The most notable exception here is `org.hibernate.id.TableHiLoGenerator`, which cannot be used because it does not expose a selectable way to get its values.

- For properties mapped as either `version` or `timestamp`, the insert statement gives you two options. You can either specify the property in the properties_list, in which case its value is taken from the corresponding select expressions, or omit it from the properties_list, in which case the `seed value` defined by the `org.hibernate.type.VersionType` is used.

The following is an example of an HQL `INSERT` statement execution:

    Session session = sessionFactory.openSession();
    Transaction tx = session.beginTransaction();

    String hqlInsert = "insert into DelinquentAccount (id, name) select c.id, c.name from Customer c where ...";
    int createdEntities = s.createQuery( hqlInsert )
            .executeUpdate();
    tx.commit();
    session.close();

## Chapter 16. HQL: The Hibernate Query Language

Hibernate uses a powerful query language (HQL) that is similar in appearance to SQL. Compared with SQL, however, HQL is fully object-oriented and understands notions like inheritance, polymorphism and association.

## 16.1. Case Sensitivity

With the exception of names of Java classes and properties, queries are case-insensitive. So `SeLeCT` is the same as `sELEct` is the same as `SELECT`, but `org.hibernate.eg.FOO` is not `org.hibernate.eg.Foo`, and `foo.barSet` is not `foo.BARSET`.

This manual uses lowercase HQL keywords. Some users find queries with uppercase keywords more readable, but this convention is unsuitable for queries embedded in Java code.

## 16.2. The from clause

The simplest possible Hibernate query is of the form:

    from eg.Cat

This returns all instances of the class `eg.Cat`. You do not usually need to qualify the class name, since `auto-import` is the default. For example:

    from Cat

In order to refer to the `Cat` in other parts of the query, you will need to assign an alias. For example:

    from Cat as cat

This query assigns the alias `cat` to `Cat` instances, so you can use that alias later in the query. The `as` keyword is optional. You could also write:

    from Cat cat

Multiple classes can appear, resulting in a cartesian product or "cross" join.

    from Formula, Parameter

    from Formula as form, Parameter as param

It is good practice to name query aliases using an initial lowercase as this is consistent with Java naming standards for local variables (e.g. `domesticCat`).

## 16.3. Associations and joins

You can also assign aliases to associated entities or to elements of a collection of values using a `join`. For example:

    from Cat as cat
        inner join cat.mate as mate
        left outer join cat.kittens as kitten

    from Cat as cat left join cat.mate.kittens as kittens

    from Formula form full join form.parameter param

The supported join types are borrowed from ANSI SQL:

- `inner join`

- `left outer join`

- `right outer join`

- `full join` (not usually useful)

The `inner join`, `left outer join` and `right outer join` constructs may be abbreviated.

    from Cat as cat
        join cat.mate as mate
        left join cat.kittens as kitten

You may supply extra join conditions using the HQL `with` keyword.

    from Cat as cat
        left join cat.kittens as kitten
            with kitten.bodyWeight > 10.0

A "fetch" join allows associations or collections of values to be initialized along with their parent objects using a single select. This is particularly useful in the case of a collection. It effectively overrides the outer join and lazy declarations of the mapping file for associations and collections. See Section 21.1, “Fetching strategies” for more information.

    from Cat as cat
        inner join fetch cat.mate
        left join fetch cat.kittens

A fetch join does not usually need to assign an alias, because the associated objects should not be used in the `where` clause (or any other clause). The associated objects are also not returned directly in the query results. Instead, they may be accessed via the parent object. The only reason you might need an alias is if you are recursively join fetching a further collection:

    from Cat as cat
        inner join fetch cat.mate
        left join fetch cat.kittens child
        left join fetch child.kittens

The `fetch` construct cannot be used in queries called using `iterate()` (though `scroll()` can be used). `Fetch` should be used together with `setMaxResults()` or `setFirstResult()`, as these operations are based on the result rows which usually contain duplicates for eager collection fetching, hence, the number of rows is not what you would expect. `Fetch` should also not be used together with impromptu `with` condition. It is possible to create a cartesian product by join fetching more than one collection in a query, so take care in this case. Join fetching multiple collection roles can produce unexpected results for bag mappings, so user discretion is advised when formulating queries in this case. Finally, note that `full join fetch` and `right join fetch` are not meaningful.

If you are using property-level lazy fetching (with bytecode instrumentation), it is possible to force Hibernate to fetch the lazy properties in the first query immediately using `fetch all properties`.

    from Document fetch all properties order by name

    from Document doc fetch all properties where lower(doc.name) like '%cats%'

## 16.4. Forms of join syntax

HQL supports two forms of association joining: `implicit` and `explicit`.

The queries shown in the previous section all use the `explicit` form, that is, where the join keyword is explicitly used in the from clause. This is the recommended form.

The `implicit` form does not use the join keyword. Instead, the associations are "dereferenced" using dot-notation. `implicit` joins can appear in any of the HQL clauses. `implicit` join result in inner joins in the resulting SQL statement.

    from Cat as cat where cat.mate.name like '%s%'

## 16.5. Referring to identifier property

There are 2 ways to refer to an entity's identifier property:

- The special property (lowercase) `id` may be used to reference the identifier property of an entity provided that the entity does not define a non-identifier property named id.

- If the entity defines a named identifier property, you can use that property name.

References to composite identifier properties follow the same naming rules. If the entity has a non-identifier property named id, the composite identifier property can only be referenced by its defined named. Otherwise, the special `id` property can be used to reference the identifier property.

## Important

Please note that, starting in version 3.2.2, this has changed significantly. In previous versions, `id` always referred to the identifier property regardless of its actual name. A ramification of that decision was that non-identifier properties named `id` could never be referenced in Hibernate queries.

## 16.6. The select clause

The `select` clause picks which objects and properties to return in the query result set. Consider the following:

    select mate
    from Cat as cat
        inner join cat.mate as mate

The query will select `mate`s of other `Cat`s. You can express this query more compactly as:

    select cat.mate from Cat cat

Queries can return properties of any value type including properties of component type:

    select cat.name from DomesticCat cat
    where cat.name like 'fri%'

    select cust.name.firstName from Customer as cust

Queries can return multiple objects and/or properties as an array of type `Object[]`:

    select mother, offspr, mate.name
    from DomesticCat as mother
        inner join mother.mate as mate
        left outer join mother.kittens as offspr

Or as a `List`:

    select new list(mother, offspr, mate.name)
    from DomesticCat as mother
        inner join mother.mate as mate
        left outer join mother.kittens as offspr

Or - assuming that the class `Family` has an appropriate constructor - as an actual typesafe Java object:

    select new Family(mother, mate, offspr)
    from DomesticCat as mother
        join mother.mate as mate
        left join mother.kittens as offspr

You can assign aliases to selected expressions using `as`:

    select max(bodyWeight) as max, min(bodyWeight) as min, count(*) as n
    from Cat cat

This is most useful when used together with `select new map`:

    select new map( max(bodyWeight) as max, min(bodyWeight) as min, count(*) as n )
    from Cat cat

This query returns a `Map` from aliases to selected values.

## 16.7. Aggregate functions

HQL queries can even return the results of aggregate functions on properties:

    select avg(cat.weight), sum(cat.weight), max(cat.weight), count(cat)
    from Cat cat

The supported aggregate functions are:

- `avg(...), sum(...), min(...), max(...)`

- `count(*)`

- `count(...), count(distinct ...), count(all...)`

You can use arithmetic operators, concatenation, and recognized SQL functions in the select clause:

    select cat.weight + sum(kitten.weight)
    from Cat cat
        join cat.kittens kitten
    group by cat.id, cat.weight

    select firstName||' '||initial||' '||upper(lastName) from Person

The `distinct` and `all` keywords can be used and have the same semantics as in SQL.

    select distinct cat.name from Cat cat

    select count(distinct cat.name), count(cat) from Cat cat

## 16.8. Polymorphic queries

A query like:

    from Cat as cat

returns instances not only of `Cat`, but also of subclasses like `DomesticCat`. Hibernate queries can name any Java class or interface in the `from` clause. The query will return instances of all persistent classes that extend that class or implement the interface. The following query would return all persistent objects:

    from java.lang.Object o

The interface `Named` might be implemented by various persistent classes:

    from Named n, Named m where n.name = m.name

These last two queries will require more than one SQL `SELECT`. This means that the `order by` clause does not correctly order the whole result set. It also means you cannot call these queries using `Query.scroll()`.

## 16.9. The where clause

The `where` clause allows you to refine the list of instances returned. If no alias exists, you can refer to properties by name:

    from Cat where name='Fritz'

If there is an alias, use a qualified property name:

    from Cat as cat where cat.name='Fritz'

This returns instances of `Cat` named 'Fritz'.

The following query:

    select foo
    from Foo foo, Bar bar
    where foo.startDate = bar.date

returns all instances of `Foo` with an instance of `bar` with a `date` property equal to the `startDate` property of the `Foo`. Compound path expressions make the `where` clause extremely powerful. Consider the following:

    from Cat cat where cat.mate.name is not null

This query translates to an SQL query with a table (inner) join. For example:

    from Foo foo
    where foo.bar.baz.customer.address.city is not null

would result in a query that would require four table joins in SQL.

The `=` operator can be used to compare not only properties, but also instances:

    from Cat cat, Cat rival where cat.mate = rival.mate

    select cat, mate
    from Cat cat, Cat mate
    where cat.mate = mate

The special property (lowercase) `id` can be used to reference the unique identifier of an object. See Section 16.5, “Referring to identifier property” for more information.

    from Cat as cat where cat.id = 123

    from Cat as cat where cat.mate.id = 69

The second query is efficient and does not require a table join.

Properties of composite identifiers can also be used. Consider the following example where `Person` has composite identifiers consisting of `country` and `medicareNumber`:

    from bank.Person person
    where person.id.country = 'AU'
        and person.id.medicareNumber = 123456

    from bank.Account account
    where account.owner.id.country = 'AU'
        and account.owner.id.medicareNumber = 123456

Once again, the second query does not require a table join.

See Section 16.5, “Referring to identifier property” for more information regarding referencing identifier properties)

The special property `class` accesses the discriminator value of an instance in the case of polymorphic persistence. A Java class name embedded in the where clause will be translated to its discriminator value.

    from Cat cat where cat.class = DomesticCat

You can also use components or composite user types, or properties of said component types. See Section 16.17, “Components” for more information.

An "any" type has the special properties `id` and `class` that allows you to express a join in the following way (where `AuditLog.item` is a property mapped with `<any>`):

    from AuditLog log, Payment payment
    where log.item.class = 'Payment' and log.item.id = payment.id

The `log.item.class` and `payment.class` would refer to the values of completely different database columns in the above query.

## 16.10. Expressions

Expressions used in the `where` clause include the following:

- mathematical operators: `+, -, *, /`

- binary comparison operators: `=, >=, <=, <>, !=, like`

- logical operations `and, or, not`

- Parentheses `( )` that indicates grouping

- `in`, `not in`, `between`, `is null`, `is not null`, `is empty`, `is not empty`, `member of` and `not member of`

- "Simple" case, `case ... when ... then ... else ... end`, and "searched" case, `case when ... then ... else ... end`

- string concatenation `...||...` or `concat(...,...)`

- `current_date()`, `current_time()`, and `current_timestamp()`

- `second(...)`, `minute(...)`, `hour(...)`, `day(...)`, `month(...)`, and `year(...)`

- Any function or operator defined by EJB-QL 3.0: `substring(), trim(), lower(), upper(), length(), locate(), abs(), sqrt(), bit_length(), mod()`

- `coalesce()` and `nullif()`

- `str()` for converting numeric or temporal values to a readable string

- `cast(... as ...)`, where the second argument is the name of a Hibernate type, and `extract(... from ...)` if ANSI `cast()` and `extract()` is supported by the underlying database

- the HQL `index()` function, that applies to aliases of a joined indexed collection

- HQL functions that take collection-valued path expressions: `size(), minelement(), maxelement(), minindex(), maxindex()`, along with the special `elements()` and `indices` functions that can be quantified using `some, all, exists, any, in`.

- Any database-supported SQL scalar function like `sign()`, `trunc()`, `rtrim()`, and `sin()`

- JDBC-style positional parameters `?`

- named parameters `:name`, `:start_date`, and `:x1`

- SQL literals `'foo'`, `69`, `6.66E+2`, `'1970-01-01 10:00:01.0'`

- Java `public static final` constants `eg.Color.TABBY`

`in` and `between` can be used as follows:

    from DomesticCat cat where cat.name between 'A' and 'B'

    from DomesticCat cat where cat.name in ( 'Foo', 'Bar', 'Baz' )

The negated forms can be written as follows:

    from DomesticCat cat where cat.name not between 'A' and 'B'

    from DomesticCat cat where cat.name not in ( 'Foo', 'Bar', 'Baz' )

Similarly, `is null` and `is not null` can be used to test for null values.

Booleans can be easily used in expressions by declaring HQL query substitutions in Hibernate configuration:

    <property name="hibernate.query.substitutions">true 1, false 0</property>

This will replace the keywords `true` and `false` with the literals `1` and `0` in the translated SQL from this HQL:

    from Cat cat where cat.alive = true

You can test the size of a collection with the special property `size` or the special `size()` function.

    from Cat cat where cat.kittens.size > 0

    from Cat cat where size(cat.kittens) > 0

For indexed collections, you can refer to the minimum and maximum indices using `minindex` and `maxindex` functions. Similarly, you can refer to the minimum and maximum elements of a collection of basic type using the `minelement` and `maxelement` functions. For example:

    from Calendar cal where maxelement(cal.holidays) > current_date

    from Order order where maxindex(order.items) > 100

    from Order order where minelement(order.items) > 10000

The SQL functions `any, some, all, exists, in` are supported when passed the element or index set of a collection (`elements` and `indices` functions) or the result of a subquery (see below):

    select mother from Cat as mother, Cat as kit
    where kit in elements(foo.kittens)

    select p from NameList list, Person p
    where p.name = some elements(list.names)

    from Cat cat where exists elements(cat.kittens)

    from Player p where 3 > all elements(p.scores)

    from Show show where 'fizard' in indices(show.acts)

Note that these constructs - `size`, `elements`, `indices`, `minindex`, `maxindex`, `minelement`, `maxelement` - can only be used in the where clause in Hibernate3.

Elements of indexed collections (arrays, lists, and maps) can be referred to by index in a where clause only:

    from Order order where order.items[0].id = 1234

    select person from Person person, Calendar calendar
    where calendar.holidays['national day'] = person.birthDay
        and person.nationality.calendar = calendar

    select item from Item item, Order order
    where order.items[ order.deliveredItemIndices[0] ] = item and order.id = 11

    select item from Item item, Order order
    where order.items[ maxindex(order.items) ] = item and order.id = 11

The expression inside `[]` can even be an arithmetic expression:

    select item from Item item, Order order
    where order.items[ size(order.items) - 1 ] = item

HQL also provides the built-in `index()` function for elements of a one-to-many association or collection of values.

    select item, index(item) from Order order
        join order.items item
    where index(item) < 5

Scalar SQL functions supported by the underlying database can be used:

    from DomesticCat cat where upper(cat.name) like 'FRI%'

Consider how much longer and less readable the following query would be in SQL:

    select cust
    from Product prod,
        Store store
        inner join store.customers cust
    where prod.name = 'widget'
        and store.location.name in ( 'Melbourne', 'Sydney' )
        and prod = all elements(cust.currentOrder.lineItems)

Hint: something like

    SELECT cust.name, cust.address, cust.phone, cust.id, cust.current_order
    FROM customers cust,
        stores store,
        locations loc,
        store_customers sc,
        product prod
    WHERE prod.name = 'widget'
        AND store.loc_id = loc.id
        AND loc.name IN ( 'Melbourne', 'Sydney' )
        AND sc.store_id = store.id
        AND sc.cust_id = cust.id
        AND prod.id = ALL(
            SELECT item.prod_id
            FROM line_items item, orders o
            WHERE item.order_id = o.id
                AND cust.current_order = o.id
        )

## 16.11. The order by clause

The list returned by a query can be ordered by any property of a returned class or components:

    from DomesticCat cat
    order by cat.name asc, cat.weight desc, cat.birthdate

The optional `asc` or `desc` indicate ascending or descending order respectively.

## 16.12. The group by clause

A query that returns aggregate values can be grouped by any property of a returned class or components:

    select cat.color, sum(cat.weight), count(cat)
    from Cat cat
    group by cat.color

    select foo.id, avg(name), max(name)
    from Foo foo join foo.names name
    group by foo.id

A `having` clause is also allowed.

    select cat.color, sum(cat.weight), count(cat)
    from Cat cat
    group by cat.color
    having cat.color in (eg.Color.TABBY, eg.Color.BLACK)

SQL functions and aggregate functions are allowed in the `having` and `order by` clauses if they are supported by the underlying database (i.e., not in MySQL).

    select cat
    from Cat cat
        join cat.kittens kitten
    group by cat.id, cat.name, cat.other, cat.properties
    having avg(kitten.weight) > 100
    order by count(kitten) asc, sum(kitten.weight) desc

Neither the `group by` clause nor the `order by` clause can contain arithmetic expressions. Hibernate also does not currently expand a grouped entity, so you cannot write `group by cat` if all properties of `cat` are non-aggregated. You have to list all non-aggregated properties explicitly.

## 16.13. Subqueries

For databases that support subselects, Hibernate supports subqueries within queries. A subquery must be surrounded by parentheses (often by an SQL aggregate function call). Even correlated subqueries (subqueries that refer to an alias in the outer query) are allowed.

    from Cat as fatcat
    where fatcat.weight > (
        select avg(cat.weight) from DomesticCat cat
    )

    from DomesticCat as cat
    where cat.name = some (
        select name.nickName from Name as name
    )

    from Cat as cat
    where not exists (
        from Cat as mate where mate.mate = cat
    )

    from DomesticCat as cat
    where cat.name not in (
        select name.nickName from Name as name
    )

    select cat.id, (select max(kit.weight) from cat.kitten kit)
    from Cat as cat

Note that HQL subqueries can occur only in the select or where clauses.

Note that subqueries can also utilize `row value constructor` syntax. See Section 16.18, “Row value constructor syntax” for more information.

## 16.14. HQL examples

Hibernate queries can be quite powerful and complex. In fact, the power of the query language is one of Hibernate's main strengths. The following example queries are similar to queries that have been used on recent projects. Please note that most queries you will write will be much simpler than the following examples.

The following query returns the order id, number of items, the given minimum total value and the total value of the order for all unpaid orders for a particular customer. The results are ordered by total value. In determining the prices, it uses the current catalog. The resulting SQL query, against the `ORDER`, `ORDER_LINE`, `PRODUCT`, `CATALOG` and `PRICE` tables has four inner joins and an (uncorrelated) subselect.

    select order.id, sum(price.amount), count(item)
    from Order as order
        join order.lineItems as item
        join item.product as product,
        Catalog as catalog
        join catalog.prices as price
    where order.paid = false
        and order.customer = :customer
        and price.product = product
        and catalog.effectiveDate < sysdate
        and catalog.effectiveDate >= all (
            select cat.effectiveDate
            from Catalog as cat
            where cat.effectiveDate < sysdate
        )
    group by order
    having sum(price.amount) > :minAmount
    order by sum(price.amount) desc

What a monster! Actually, in real life, I'm not very keen on subqueries, so my query was really more like this:

    select order.id, sum(price.amount), count(item)
    from Order as order
        join order.lineItems as item
        join item.product as product,
        Catalog as catalog
        join catalog.prices as price
    where order.paid = false
        and order.customer = :customer
        and price.product = product
        and catalog = :currentCatalog
    group by order
    having sum(price.amount) > :minAmount
    order by sum(price.amount) desc

The next query counts the number of payments in each status, excluding all payments in the `AWAITING_APPROVAL` status where the most recent status change was made by the current user. It translates to an SQL query with two inner joins and a correlated subselect against the `PAYMENT`, `PAYMENT_STATUS` and `PAYMENT_STATUS_CHANGE` tables.

    select count(payment), status.name
    from Payment as payment
        join payment.currentStatus as status
        join payment.statusChanges as statusChange
    where payment.status.name <> PaymentStatus.AWAITING_APPROVAL
        or (
            statusChange.timeStamp = (
                select max(change.timeStamp)
                from PaymentStatusChange change
                where change.payment = payment
            )
            and statusChange.user <> :currentUser
        )
    group by status.name, status.sortOrder
    order by status.sortOrder

If the `statusChanges` collection was mapped as a list, instead of a set, the query would have been much simpler to write.

    select count(payment), status.name
    from Payment as payment
        join payment.currentStatus as status
    where payment.status.name <> PaymentStatus.AWAITING_APPROVAL
        or payment.statusChanges[ maxIndex(payment.statusChanges) ].user <> :currentUser
    group by status.name, status.sortOrder
    order by status.sortOrder

The next query uses the MS SQL Server `isNull()` function to return all the accounts and unpaid payments for the organization to which the current user belongs. It translates to an SQL query with three inner joins, an outer join and a subselect against the `ACCOUNT`, `PAYMENT`, `PAYMENT_STATUS`, `ACCOUNT_TYPE`, `ORGANIZATION` and `ORG_USER` tables.

    select account, payment
    from Account as account
        left outer join account.payments as payment
    where :currentUser in elements(account.holder.users)
        and PaymentStatus.UNPAID = isNull(payment.currentStatus.name, PaymentStatus.UNPAID)
    order by account.type.sortOrder, account.accountNumber, payment.dueDate

For some databases, we would need to do away with the (correlated) subselect.

    select account, payment
    from Account as account
        join account.holder.users as user
        left outer join account.payments as payment
    where :currentUser = user
        and PaymentStatus.UNPAID = isNull(payment.currentStatus.name, PaymentStatus.UNPAID)
    order by account.type.sortOrder, account.accountNumber, payment.dueDate

## 16.15. Bulk update and delete

HQL now supports `update`, `delete` and `insert ... select ...` statements. See Section 15.4, “DML-style operations” for more information.

## 16.16. Tips & Tricks

You can count the number of query results without returning them:

    ( (Integer) session.createQuery("select count(*) from ....").iterate().next() ).intValue()

To order a result by the size of a collection, use the following query:

    select usr.id, usr.name
    from User as usr
        left join usr.messages as msg
    group by usr.id, usr.name
    order by count(msg)

If your database supports subselects, you can place a condition upon selection size in the where clause of your query:

    from User usr where size(usr.messages) >= 1

If your database does not support subselects, use the following query:

    select usr.id, usr.name
    from User usr
        join usr.messages msg
    group by usr.id, usr.name
    having count(msg) >= 1

As this solution cannot return a `User` with zero messages because of the inner join, the following form is also useful:

    select usr.id, usr.name
    from User as usr
        left join usr.messages as msg
    group by usr.id, usr.name
    having count(msg) = 0

Properties of a JavaBean can be bound to named query parameters:

    Query q = s.createQuery("from foo Foo as foo where foo.name=:name and foo.size=:size");
    q.setProperties(fooBean); // fooBean has getName() and getSize()
    List foos = q.list();

Collections are pageable by using the `Query` interface with a filter:

    Query q = s.createFilter( collection, "" ); // the trivial filter
    q.setMaxResults(PAGE_SIZE);
    q.setFirstResult(PAGE_SIZE * pageNumber);
    List page = q.list();

Collection elements can be ordered or grouped using a query filter:

    Collection orderedCollection = s.filter( collection, "order by this.amount" );
    Collection counts = s.filter( collection, "select this.type, count(this) group by this.type" );

You can find the size of a collection without initializing it:

    ( (Integer) session.createQuery("select count(*) from ....").iterate().next() ).intValue();

## 16.17. Components

Components can be used similarly to the simple value types that are used in HQL queries. They can appear in the `select` clause as follows:

    select p.name from Person p

    select p.name.first from Person p

where the Person's name property is a component. Components can also be used in the `where` clause:

    from Person p where p.name = :name

    from Person p where p.name.first = :firstName

Components can also be used in the `order by` clause:

    from Person p order by p.name

    from Person p order by p.name.first

Another common use of components is in Section 16.18, “Row value constructor syntax”.

## 16.18. Row value constructor syntax

HQL supports the use of ANSI SQL `row value constructor` syntax, sometimes referred to AS `tuple` syntax, even though the underlying database may not support that notion. Here, we are generally referring to multi-valued comparisons, typically associated with components. Consider an entity Person which defines a name component:

    from Person p where p.name.first='John' and p.name.last='Jingleheimer-Schmidt'

That is valid syntax although it is a little verbose. You can make this more concise by using `row value constructor` syntax:

    from Person p where p.name=('John', 'Jingleheimer-Schmidt')

It can also be useful to specify this in the `select` clause:

    select p.name from Person p

Using `row value constructor` syntax can also be beneficial when using subqueries that need to compare against multiple values:

    from Cat as cat
    where not ( cat.name, cat.color ) in (
        select cat.name, cat.color from DomesticCat cat
    )

One thing to consider when deciding if you want to use this syntax, is that the query will be dependent upon the ordering of the component sub-properties in the metadata.

## Chapter 17. Criteria Queries

Hibernate features an intuitive, extensible criteria query API.

## 17.1. Creating a `Criteria` instance

The interface `org.hibernate.Criteria` represents a query against a particular persistent class. The `Session` is a factory for `Criteria` instances.

    Criteria crit = sess.createCriteria(Cat.class);
    crit.setMaxResults(50);
    List cats = crit.list();

## 17.2. Narrowing the result set

An individual query criterion is an instance of the interface `org.hibernate.criterion.Criterion`. The class `org.hibernate.criterion.Restrictions` defines factory methods for obtaining certain built-in `Criterion` types.

    List cats = sess.createCriteria(Cat.class)
        .add( Restrictions.like("name", "Fritz%") )
        .add( Restrictions.between("weight", minWeight, maxWeight) )
        .list();

Restrictions can be grouped logically.

    List cats = sess.createCriteria(Cat.class)
        .add( Restrictions.like("name", "Fritz%") )
        .add( Restrictions.or(
            Restrictions.eq( "age", new Integer(0) ),
            Restrictions.isNull("age")
        ) )
        .list();

    List cats = sess.createCriteria(Cat.class)
        .add( Restrictions.in( "name", new String[] { "Fritz", "Izi", "Pk" } ) )
        .add( Restrictions.disjunction()
            .add( Restrictions.isNull("age") )
            .add( Restrictions.eq("age", new Integer(0) ) )
            .add( Restrictions.eq("age", new Integer(1) ) )
            .add( Restrictions.eq("age", new Integer(2) ) )
        ) )
        .list();

There are a range of built-in criterion types (`Restrictions` subclasses). One of the most useful allows you to specify SQL directly.

    List cats = sess.createCriteria(Cat.class)
        .add( Restrictions.sqlRestriction("lower({alias}.name) like lower(?)", "Fritz%", Hibernate.STRING) )
        .list();

The `{alias}` placeholder will be replaced by the row alias of the queried entity.

You can also obtain a criterion from a `Property` instance. You can create a `Property` by calling `Property.forName()`:

    Property age = Property.forName("age");
    List cats = sess.createCriteria(Cat.class)
        .add( Restrictions.disjunction()
            .add( age.isNull() )
            .add( age.eq( new Integer(0) ) )
            .add( age.eq( new Integer(1) ) )
            .add( age.eq( new Integer(2) ) )
        ) )
        .add( Property.forName("name").in( new String[] { "Fritz", "Izi", "Pk" } ) )
        .list();

## 17.3. Ordering the results

You can order the results using `org.hibernate.criterion.Order`.

    List cats = sess.createCriteria(Cat.class)
        .add( Restrictions.like("name", "F%")
        .addOrder( Order.asc("name") )
        .addOrder( Order.desc("age") )
        .setMaxResults(50)
        .list();

    List cats = sess.createCriteria(Cat.class)
        .add( Property.forName("name").like("F%") )
        .addOrder( Property.forName("name").asc() )
        .addOrder( Property.forName("age").desc() )
        .setMaxResults(50)
        .list();

## 17.4. Associations

By navigating associations using `createCriteria()` you can specify constraints upon related entities:

    List cats = sess.createCriteria(Cat.class)
        .add( Restrictions.like("name", "F%") )
        .createCriteria("kittens")
            .add( Restrictions.like("name", "F%") )
        .list();

The second `createCriteria()` returns a new instance of `Criteria` that refers to the elements of the `kittens` collection.

There is also an alternate form that is useful in certain circumstances:

    List cats = sess.createCriteria(Cat.class)
        .createAlias("kittens", "kt")
        .createAlias("mate", "mt")
        .add( Restrictions.eqProperty("kt.name", "mt.name") )
        .list();

(`createAlias()` does not create a new instance of `Criteria`.)

The kittens collections held by the `Cat` instances returned by the previous two queries are not pre-filtered by the criteria. If you want to retrieve just the kittens that match the criteria, you must use a `ResultTransformer`.

    List cats = sess.createCriteria(Cat.class)
        .createCriteria("kittens", "kt")
            .add( Restrictions.eq("name", "F%") )
        .setResultTransformer(Criteria.ALIAS_TO_ENTITY_MAP)
        .list();
    Iterator iter = cats.iterator();
    while ( iter.hasNext() ) {
        Map map = (Map) iter.next();
        Cat cat = (Cat) map.get(Criteria.ROOT_ALIAS);
        Cat kitten = (Cat) map.get("kt");
    }

Additionally you may manipulate the result set using a left outer join:

    		List cats = session.createCriteria( Cat.class )
                           .createAlias("mate", "mt", Criteria.LEFT_JOIN, Restrictions.like("mt.name", "good%") )
                           .addOrder(Order.asc("mt.age"))
                           .list();

This will return all of the `Cat`s with a mate whose name starts with "good" ordered by their mate's age, and all cats who do not have a mate. This is useful when there is a need to order or limit in the database prior to returning complex/large result sets, and removes many instances where multiple queries would have to be performed and the results unioned by java in memory.

Without this feature, first all of the cats without a mate would need to be loaded in one query.

A second query would need to retreive the cats with mates who's name started with "good" sorted by the mates age.

Thirdly, in memory; the lists would need to be joined manually.

## 17.5. Dynamic association fetching

You can specify association fetching semantics at runtime using `setFetchMode()`.

    List cats = sess.createCriteria(Cat.class)
        .add( Restrictions.like("name", "Fritz%") )
        .setFetchMode("mate", FetchMode.EAGER)
        .setFetchMode("kittens", FetchMode.EAGER)
        .list();

This query will fetch both `mate` and `kittens` by outer join. See Section 21.1, “Fetching strategies” for more information.

## 17.6. Components

To add a restriction against a property of an embedded component, the component property name should be prepended to the property name when creating the `Restriction`. The criteria object should be created on the owning entity, and cannot be created on the component itself. For example, suppose the `Cat` has a component property `fullName` with sub-properties `firstName` and `lastName`:

    		List cats = session.createCriteria(Cat.class)
    			.add(Restrictions.eq("fullName.lastName", "Cattington"))
    			.list();

Note: this does not apply when querying collections of components, for that see below Section 17.7, “Collections”

## 17.7. Collections

When using criteria against collections, there are two distinct cases. One is if the collection contains entities (eg. `<one-to-many/>` or `<many-to-many/>`) or components (`<composite-element/>` ), and the second is if the collection contains scalar values (`<element/>`). In the first case, the syntax is as given above in the section Section 17.4, “Associations” where we restrict the `kittens` collection. Essentially we create a `Criteria` object against the collection property and restrict the entity or component properties using that instance.

For queryng a collection of basic values, we still create the `Criteria` object against the collection, but to reference the value, we use the special property "elements". For an indexed collection, we can also reference the index property using the special property "indices".

    		List cats = session.createCriteria(Cat.class)
    			.createCriteria("nickNames")
    				.add(Restrictions.eq("elements", "BadBoy"))
    			.list();

## 17.8. Example queries

The class `org.hibernate.criterion.Example` allows you to construct a query criterion from a given instance.

    Cat cat = new Cat();
    cat.setSex('F');
    cat.setColor(Color.BLACK);
    List results = session.createCriteria(Cat.class)
        .add( Example.create(cat) )
        .list();

Version properties, identifiers and associations are ignored. By default, null valued properties are excluded.

You can adjust how the `Example` is applied.

    Example example = Example.create(cat)
        .excludeZeroes()           //exclude zero valued properties
        .excludeProperty("color")  //exclude the property named "color"
        .ignoreCase()              //perform case insensitive string comparisons
        .enableLike();             //use like for string comparisons
    List results = session.createCriteria(Cat.class)
        .add(example)
        .list();

You can even use examples to place criteria upon associated objects.

    List results = session.createCriteria(Cat.class)
        .add( Example.create(cat) )
        .createCriteria("mate")
            .add( Example.create( cat.getMate() ) )
        .list();

## 17.9. Projections, aggregation and grouping

The class `org.hibernate.criterion.Projections` is a factory for `Projection` instances. You can apply a projection to a query by calling `setProjection()`.

    List results = session.createCriteria(Cat.class)
        .setProjection( Projections.rowCount() )
        .add( Restrictions.eq("color", Color.BLACK) )
        .list();

    List results = session.createCriteria(Cat.class)
        .setProjection( Projections.projectionList()
            .add( Projections.rowCount() )
            .add( Projections.avg("weight") )
            .add( Projections.max("weight") )
            .add( Projections.groupProperty("color") )
        )
        .list();

There is no explicit "group by" necessary in a criteria query. Certain projection types are defined to be grouping projections, which also appear in the SQL `group by` clause.

An alias can be assigned to a projection so that the projected value can be referred to in restrictions or orderings. Here are two different ways to do this:

    List results = session.createCriteria(Cat.class)
        .setProjection( Projections.alias( Projections.groupProperty("color"), "colr" ) )
        .addOrder( Order.asc("colr") )
        .list();

    List results = session.createCriteria(Cat.class)
        .setProjection( Projections.groupProperty("color").as("colr") )
        .addOrder( Order.asc("colr") )
        .list();

The `alias()` and `as()` methods simply wrap a projection instance in another, aliased, instance of `Projection`. As a shortcut, you can assign an alias when you add the projection to a projection list:

    List results = session.createCriteria(Cat.class)
        .setProjection( Projections.projectionList()
            .add( Projections.rowCount(), "catCountByColor" )
            .add( Projections.avg("weight"), "avgWeight" )
            .add( Projections.max("weight"), "maxWeight" )
            .add( Projections.groupProperty("color"), "color" )
        )
        .addOrder( Order.desc("catCountByColor") )
        .addOrder( Order.desc("avgWeight") )
        .list();

    List results = session.createCriteria(Domestic.class, "cat")
        .createAlias("kittens", "kit")
        .setProjection( Projections.projectionList()
            .add( Projections.property("cat.name"), "catName" )
            .add( Projections.property("kit.name"), "kitName" )
        )
        .addOrder( Order.asc("catName") )
        .addOrder( Order.asc("kitName") )
        .list();

You can also use `Property.forName()` to express projections:

    List results = session.createCriteria(Cat.class)
        .setProjection( Property.forName("name") )
        .add( Property.forName("color").eq(Color.BLACK) )
        .list();

    List results = session.createCriteria(Cat.class)
        .setProjection( Projections.projectionList()
            .add( Projections.rowCount().as("catCountByColor") )
            .add( Property.forName("weight").avg().as("avgWeight") )
            .add( Property.forName("weight").max().as("maxWeight") )
            .add( Property.forName("color").group().as("color" )
        )
        .addOrder( Order.desc("catCountByColor") )
        .addOrder( Order.desc("avgWeight") )
        .list();

## 17.10. Detached queries and subqueries

The `DetachedCriteria` class allows you to create a query outside the scope of a session and then execute it using an arbitrary `Session`.

    DetachedCriteria query = DetachedCriteria.forClass(Cat.class)
        .add( Property.forName("sex").eq('F') );

    Session session = ....;
    Transaction txn = session.beginTransaction();
    List results = query.getExecutableCriteria(session).setMaxResults(100).list();
    txn.commit();
    session.close();

A `DetachedCriteria` can also be used to express a subquery. Criterion instances involving subqueries can be obtained via `Subqueries` or `Property`.

    DetachedCriteria avgWeight = DetachedCriteria.forClass(Cat.class)
        .setProjection( Property.forName("weight").avg() );
    session.createCriteria(Cat.class)
        .add( Property.forName("weight").gt(avgWeight) )
        .list();

    DetachedCriteria weights = DetachedCriteria.forClass(Cat.class)
        .setProjection( Property.forName("weight") );
    session.createCriteria(Cat.class)
        .add( Subqueries.geAll("weight", weights) )
        .list();

Correlated subqueries are also possible:

    DetachedCriteria avgWeightForSex = DetachedCriteria.forClass(Cat.class, "cat2")
        .setProjection( Property.forName("weight").avg() )
        .add( Property.forName("cat2.sex").eqProperty("cat.sex") );
    session.createCriteria(Cat.class, "cat")
        .add( Property.forName("weight").gt(avgWeightForSex) )
        .list();

Example of multi-column restriction based on a subquery:

    DetachedCriteria sizeQuery = DetachedCriteria.forClass( Man.class )
        .setProjection( Projections.projectionList().add( Projections.property( "weight" ) )
                                                    .add( Projections.property( "height" ) ) )
        .add( Restrictions.eq( "name", "John" ) );
    session.createCriteria( Woman.class )
        .add( Subqueries.propertiesEq( new String[] { "weight", "height" }, sizeQuery ) )
        .list();

## 17.11. Queries by natural identifier

For most queries, including criteria queries, the query cache is not efficient because query cache invalidation occurs too frequently. However, there is a special kind of query where you can optimize the cache invalidation algorithm: lookups by a constant natural key. In some applications, this kind of query occurs frequently. The criteria API provides special provision for this use case.

First, map the natural key of your entity using `<natural-id>` and enable use of the second-level cache.

    <class name="User">
        <cache usage="read-write"/>
        <id name="id">
            <generator class="increment"/>
        </id>
        <natural-id>
            <property name="name"/>
            <property name="org"/>
        </natural-id>
        <property name="password"/>
    </class>

This functionality is not intended for use with entities with mutable natural keys.

Once you have enabled the Hibernate query cache, the `Restrictions.naturalId()` allows you to make use of the more efficient cache algorithm.

    session.createCriteria(User.class)
        .add( Restrictions.naturalId()
            .set("name", "gavin")
            .set("org", "hb")
        ).setCacheable(true)
        .uniqueResult();

## Chapter 18. Native SQL

You can also express queries in the native SQL dialect of your database. This is useful if you want to utilize database-specific features such as query hints or the `CONNECT` keyword in Oracle. It also provides a clean migration path from a direct SQL/JDBC based application to Hibernate.

Hibernate3 allows you to specify handwritten SQL, including stored procedures, for all create, update, delete, and load operations.

## 18.1. Using a `SQLQuery`

Execution of native SQL queries is controlled via the `SQLQuery` interface, which is obtained by calling `Session.createSQLQuery()`. The following sections describe how to use this API for querying.

### 18.1.1. Scalar queries

The most basic SQL query is to get a list of scalars (values).

    sess.createSQLQuery("SELECT * FROM CATS").list();
    sess.createSQLQuery("SELECT ID, NAME, BIRTHDATE FROM CATS").list();

These will return a List of Object arrays (Object[]) with scalar values for each column in the CATS table. Hibernate will use ResultSetMetadata to deduce the actual order and types of the returned scalar values.

To avoid the overhead of using `ResultSetMetadata`, or simply to be more explicit in what is returned, one can use `addScalar()`:

    sess.createSQLQuery("SELECT * FROM CATS")
     .addScalar("ID", Hibernate.LONG)
     .addScalar("NAME", Hibernate.STRING)
     .addScalar("BIRTHDATE", Hibernate.DATE)

This query specified:

- the SQL query string

- the columns and types to return

This will return Object arrays, but now it will not use `ResultSetMetadata` but will instead explicitly get the ID, NAME and BIRTHDATE column as respectively a Long, String and a Short from the underlying resultset. This also means that only these three columns will be returned, even though the query is using `*` and could return more than the three listed columns.

It is possible to leave out the type information for all or some of the scalars.

    sess.createSQLQuery("SELECT * FROM CATS")
     .addScalar("ID", Hibernate.LONG)
     .addScalar("NAME")
     .addScalar("BIRTHDATE")

This is essentially the same query as before, but now `ResultSetMetaData` is used to determine the type of NAME and BIRTHDATE, where as the type of ID is explicitly specified.

How the java.sql.Types returned from ResultSetMetaData is mapped to Hibernate types is controlled by the Dialect. If a specific type is not mapped, or does not result in the expected type, it is possible to customize it via calls to `registerHibernateType` in the Dialect.

### 18.1.2. Entity queries

The above queries were all about returning scalar values, basically returning the "raw" values from the resultset. The following shows how to get entity objects from a native sql query via `addEntity()`.

    sess.createSQLQuery("SELECT * FROM CATS").addEntity(Cat.class);
    sess.createSQLQuery("SELECT ID, NAME, BIRTHDATE FROM CATS").addEntity(Cat.class);

This query specified:

- the SQL query string

- the entity returned by the query

Assuming that Cat is mapped as a class with the columns ID, NAME and BIRTHDATE the above queries will both return a List where each element is a Cat entity.

If the entity is mapped with a `many-to-one` to another entity it is required to also return this when performing the native query, otherwise a database specific "column not found" error will occur. The additional columns will automatically be returned when using the * notation, but we prefer to be explicit as in the following example for a `many-to-one` to a `Dog`:

    sess.createSQLQuery("SELECT ID, NAME, BIRTHDATE, DOG_ID FROM CATS").addEntity(Cat.class);

This will allow cat.getDog() to function properly.

### 18.1.3. Handling associations and collections

It is possible to eagerly join in the `Dog` to avoid the possible extra roundtrip for initializing the proxy. This is done via the `addJoin()` method, which allows you to join in an association or collection.

    sess.createSQLQuery("SELECT c.ID, NAME, BIRTHDATE, DOG_ID, D_ID, D_NAME FROM CATS c, DOGS d WHERE c.DOG_ID = d.D_ID")
     .addEntity("cat", Cat.class)
     .addJoin("cat.dog");

In this example, the returned `Cat`'s will have their `dog` property fully initialized without any extra roundtrip to the database. Notice that you added an alias name ("cat") to be able to specify the target property path of the join. It is possible to do the same eager joining for collections, e.g. if the `Cat` had a one-to-many to `Dog` instead.

    sess.createSQLQuery("SELECT ID, NAME, BIRTHDATE, D_ID, D_NAME, CAT_ID FROM CATS c, DOGS d WHERE c.ID = d.CAT_ID")
     .addEntity("cat", Cat.class)
     .addJoin("cat.dogs");

At this stage you are reaching the limits of what is possible with native queries, without starting to enhance the sql queries to make them usable in Hibernate. Problems can arise when returning multiple entities of the same type or when the default alias/column names are not enough.

### 18.1.4. Returning multiple entities

Until now, the result set column names are assumed to be the same as the column names specified in the mapping document. This can be problematic for SQL queries that join multiple tables, since the same column names can appear in more than one table.

Column alias injection is needed in the following query (which most likely will fail):

    sess.createSQLQuery("SELECT c.*, m.*  FROM CATS c, CATS m WHERE c.MOTHER_ID = c.ID")
     .addEntity("cat", Cat.class)
     .addEntity("mother", Cat.class)

The query was intended to return two Cat instances per row: a cat and its mother. The query will, however, fail because there is a conflict of names; the instances are mapped to the same column names. Also, on some databases the returned column aliases will most likely be on the form "c.ID", "c.NAME", etc. which are not equal to the columns specified in the mappings ("ID" and "NAME").

The following form is not vulnerable to column name duplication:

    sess.createSQLQuery("SELECT {cat.*}, {m.*}  FROM CATS c, CATS m WHERE c.MOTHER_ID = m.ID")
     .addEntity("cat", Cat.class)
     .addEntity("mother", Cat.class)

This query specified:

- the SQL query string, with placeholders for Hibernate to inject column aliases

- the entities returned by the query

The {cat.*} and {mother.*} notation used above is a shorthand for "all properties". Alternatively, you can list the columns explicitly, but even in this case Hibernate injects the SQL column aliases for each property. The placeholder for a column alias is just the property name qualified by the table alias. In the following example, you retrieve Cats and their mothers from a different table (cat_log) to the one declared in the mapping metadata. You can even use the property aliases in the where clause.

    String sql = "SELECT ID as {c.id}, NAME as {c.name}, " +
             "BIRTHDATE as {c.birthDate}, MOTHER_ID as {c.mother}, {mother.*} " +
             "FROM CAT_LOG c, CAT_LOG m WHERE {c.mother} = c.ID";

    List loggedCats = sess.createSQLQuery(sql)
            .addEntity("cat", Cat.class)
            .addEntity("mother", Cat.class).list()

#### 18.1.4.1. Alias and property references

In most cases the above alias injection is needed. For queries relating to more complex mappings, like composite properties, inheritance discriminators, collections etc., you can use specific aliases that allow Hibernate to inject the proper aliases.

The following table shows the different ways you can use the alias injection. Please note that the alias names in the result are simply examples; each alias will have a unique and probably different name when used.

Table 18.1. Alias injection names

Description | Syntax | Example
--- | --- | ---
A simple property | `{[aliasname].[propertyname]` | `A_NAME as {item.name}`
A composite property | `{[aliasname].[componentname].[propertyname]}` | `CURRENCY as {item.amount.currency}, VALUE as {item.amount.value}`
Discriminator of an entity | `{[aliasname].class}` | `DISC as {item.class}`
All properties of an entity | `{[aliasname].*}` | `{item.*}`
A collection key | `{[aliasname].key}` | `ORGID as {coll.key}`
The id of an collection | `{[aliasname].id}` | `EMPID as {coll.id}`
The element of an collection | `{[aliasname].element}` | `XID as {coll.element}`
property of the element in the collection | `{[aliasname].element.[propertyname]}` | `NAME as {coll.element.name}`
All properties of the element in the collection | `{[aliasname].element.*}` | `{coll.element.*}`
All properties of the collection | `{[aliasname].*}` | `{coll.*}`

### 18.1.5. Returning non-managed entities

It is possible to apply a ResultTransformer to native SQL queries, allowing it to return non-managed entities.

    sess.createSQLQuery("SELECT NAME, BIRTHDATE FROM CATS")
            .setResultTransformer(Transformers.aliasToBean(CatDTO.class))

This query specified:

- the SQL query string

- a result transformer

The above query will return a list of `CatDTO` which has been instantiated and injected the values of NAME and BIRTHNAME into its corresponding properties or fields.

### 18.1.6. Handling inheritance

Native SQL queries which query for entities that are mapped as part of an inheritance must include all properties for the baseclass and all its subclasses.

### 18.1.7. Parameters

Native SQL queries support positional as well as named parameters:

    Query query = sess.createSQLQuery("SELECT * FROM CATS WHERE NAME like ?").addEntity(Cat.class);
    List pusList = query.setString(0, "Pus%").list();

    query = sess.createSQLQuery("SELECT * FROM CATS WHERE NAME like :name").addEntity(Cat.class);
    List pusList = query.setString("name", "Pus%").list();

## 18.2. Named SQL queries

Named SQL queries can also be defined in the mapping document and called in exactly the same way as a named HQL query (see Section 11.4.1.7, “Externalizing named queries”). In this case, you do not need to call `addEntity()`.

Example 18.1. Named sql query using the <sql-query> maping element

    <sql-query name="persons">
        <return alias="person" class="eg.Person"/>
        SELECT person.NAME AS {person.name},
               person.AGE AS {person.age},
               person.SEX AS {person.sex}
        FROM PERSON person
        WHERE person.NAME LIKE :namePattern
    </sql-query>

Example 18.2. Execution of a named query

    List people = sess.getNamedQuery("persons")
        .setString("namePattern", namePattern)
        .setMaxResults(50)
        .list();

The `<return-join>` element is use to join associations and the `<load-collection>` element is used to define queries which initialize collections,

Example 18.3. Named sql query with association

    <sql-query name="personsWith">
        <return alias="person" class="eg.Person"/>
        <return-join alias="address" property="person.mailingAddress"/>
        SELECT person.NAME AS {person.name},
               person.AGE AS {person.age},
               person.SEX AS {person.sex},
               address.STREET AS {address.street},
               address.CITY AS {address.city},
               address.STATE AS {address.state},
               address.ZIP AS {address.zip}
        FROM PERSON person
        JOIN ADDRESS address
            ON person.ID = address.PERSON_ID AND address.TYPE='MAILING'
        WHERE person.NAME LIKE :namePattern
    </sql-query>

A named SQL query may return a scalar value. You must declare the column alias and Hibernate type using the `<return-scalar>` element:

Example 18.4. Named query returning a scalar

    <sql-query name="mySqlQuery">
        <return-scalar column="name" type="string"/>
        <return-scalar column="age" type="long"/>
        SELECT p.NAME AS name,
               p.AGE AS age,
        FROM PERSON p WHERE p.NAME LIKE 'Hiber%'
    </sql-query>

You can externalize the resultset mapping information in a `<resultset>` element which will allow you to either reuse them across several named queries or through the `setResultSetMapping()` API.

Example 18.5. <resultset> mapping used to externalize mapping information

    <resultset name="personAddress">
        <return alias="person" class="eg.Person"/>
        <return-join alias="address" property="person.mailingAddress"/>
    </resultset>

    <sql-query name="personsWith" resultset-ref="personAddress">
        SELECT person.NAME AS {person.name},
               person.AGE AS {person.age},
               person.SEX AS {person.sex},
               address.STREET AS {address.street},
               address.CITY AS {address.city},
               address.STATE AS {address.state},
               address.ZIP AS {address.zip}
        FROM PERSON person
        JOIN ADDRESS address
            ON person.ID = address.PERSON_ID AND address.TYPE='MAILING'
        WHERE person.NAME LIKE :namePattern
    </sql-query>

You can, alternatively, use the resultset mapping information in your hbm files directly in java code.

Example 18.6. Programmatically specifying the result mapping information

    List cats = sess.createSQLQuery(
            "select {cat.*}, {kitten.*} from cats cat, cats kitten where kitten.mother = cat.id"
        )
        .setResultSetMapping("catAndKitten")
        .list();

So far we have only looked at externalizing SQL queries using Hibernate mapping files. The same concept is also available with anntations and is called named native queries. You can use `@NamedNativeQuery` (`@NamedNativeQueries`) in conjunction with `@SqlResultSetMapping` (`@SqlResultSetMappings`). Like `@NamedQuery`, `@NamedNativeQuery` and `@SqlResultSetMapping` can be defined at class level, but their scope is global to the application. Lets look at a view examples.

Example 18.7, “Named SQL query using `@NamedNativeQuery` together with `@SqlResultSetMapping`” shows how a `resultSetMapping` parameter is defined in `@NamedNativeQuery`. It represents the name of a defined `@SqlResultSetMapping`. The resultset mapping declares the entities retrieved by this native query. Each field of the entity is bound to an SQL alias (or column name). All fields of the entity including the ones of subclasses and the foreign key columns of related entities have to be present in the SQL query. Field definitions are optional provided that they map to the same column name as the one declared on the class property. In the example 2 entities, `Night` and `Area`, are returned and each property is declared and associated to a column name, actually the column name retrieved by the query.

In Example 18.8, “Implicit result set mapping” the result set mapping is implicit. We only describe the entity class of the result set mapping. The property / column mappings is done using the entity mapping values. In this case the model property is bound to the model_txt column.

Finally, if the association to a related entity involve a composite primary key, a `@FieldResult` element should be used for each foreign key column. The `@FieldResult` name is composed of the property name for the relationship, followed by a dot ("."), followed by the name or the field or property of the primary key. This can be seen in Example 18.9, “Using dot notation in @FieldResult for specifying associations ”.

Example 18.7. Named SQL query using `@NamedNativeQuery` together with `@SqlResultSetMapping`

    @NamedNativeQuery(name="night&area", query="select night.id nid, night.night_duration, "

        + " night.night_date, area.id aid, night.area_id, area.name "

        + "from Night night, Area area where night.area_id = area.id",

                      resultSetMapping="joinMapping")

    @SqlResultSetMapping(name="joinMapping", entities={

        @EntityResult(entityClass=Night.class, fields = {

            @FieldResult(name="id", column="nid"),

            @FieldResult(name="duration", column="night_duration"),

            @FieldResult(name="date", column="night_date"),

            @FieldResult(name="area", column="area_id"),

            discriminatorColumn="disc"

        }),

        @EntityResult(entityClass=org.hibernate.test.annotations.query.Area.class, fields = {

            @FieldResult(name="id", column="aid"),

            @FieldResult(name="name", column="name")

        })

        }

    )

Example 18.8. Implicit result set mapping

    @Entity

    @SqlResultSetMapping(name="implicit",

                         entities=@EntityResult(entityClass=SpaceShip.class))

    @NamedNativeQuery(name="implicitSample",

                      query="select * from SpaceShip",

                      resultSetMapping="implicit")

    public class SpaceShip {

        private String name;

        private String model;

        private double speed;

        @Id

        public String getName() {

            return name;

        }

        public void setName(String name) {

            this.name = name;

        }

        @Column(name="model_txt")

        public String getModel() {

            return model;

        }

        public void setModel(String model) {

            this.model = model;

        }

        public double getSpeed() {

            return speed;

        }

        public void setSpeed(double speed) {

            this.speed = speed;

        }

    }

Example 18.9. Using dot notation in @FieldResult for specifying associations

    @Entity

    @SqlResultSetMapping(name="compositekey",

            entities=@EntityResult(entityClass=SpaceShip.class,

                fields = {

                        @FieldResult(name="name", column = "name"),

                        @FieldResult(name="model", column = "model"),

                        @FieldResult(name="speed", column = "speed"),

                        @FieldResult(name="captain.firstname", column = "firstn"),

                        @FieldResult(name="captain.lastname", column = "lastn"),

                        @FieldResult(name="dimensions.length", column = "length"),

                        @FieldResult(name="dimensions.width", column = "width")

                        }),

            columns = { @ColumnResult(name = "surface"),

                        @ColumnResult(name = "volume") } )

    @NamedNativeQuery(name="compositekey",

        query="select name, model, speed, lname as lastn, fname as firstn, length, width, length * width as surface from SpaceShip",

        resultSetMapping="compositekey")

    } )

    public class SpaceShip {

        private String name;

        private String model;

        private double speed;

        private Captain captain;

        private Dimensions dimensions;

        @Id

        public String getName() {

            return name;

        }

        public void setName(String name) {

            this.name = name;

        }

        @ManyToOne(fetch= FetchType.LAZY)

        @JoinColumns( {

                @JoinColumn(name="fname", referencedColumnName = "firstname"),

                @JoinColumn(name="lname", referencedColumnName = "lastname")

                } )

        public Captain getCaptain() {

            return captain;

        }

        public void setCaptain(Captain captain) {

            this.captain = captain;

        }

        public String getModel() {

            return model;

        }

        public void setModel(String model) {

            this.model = model;

        }

        public double getSpeed() {

            return speed;

        }

        public void setSpeed(double speed) {

            this.speed = speed;

        }

        public Dimensions getDimensions() {

            return dimensions;

        }

        public void setDimensions(Dimensions dimensions) {

            this.dimensions = dimensions;

        }

    }

    @Entity

    @IdClass(Identity.class)

    public class Captain implements Serializable {

        private String firstname;

        private String lastname;

        @Id

        public String getFirstname() {

            return firstname;

        }

        public void setFirstname(String firstname) {

            this.firstname = firstname;

        }

        @Id

        public String getLastname() {

            return lastname;

        }

        public void setLastname(String lastname) {

            this.lastname = lastname;

        }

    }

## Tip

If you retrieve a single entity using the default mapping, you can specify the `resultClass` attribute instead of `resultSetMapping`:

    @NamedNativeQuery(name="implicitSample", query="select * from SpaceShip", resultClass=SpaceShip.class)

    public class SpaceShip {

In some of your native queries, you'll have to return scalar values, for example when building report queries. You can map them in the `@SqlResultsetMapping` through `@ColumnResult`. You actually can even mix, entities and scalar returns in the same native query (this is probably not that common though).

Example 18.10. Scalar values via `@ColumnResult`

    @SqlResultSetMapping(name="scalar", columns=@ColumnResult(name="dimension"))

    @NamedNativeQuery(name="scalar", query="select length*width as dimension from SpaceShip", resultSetMapping="scalar")

An other query hint specific to native queries has been introduced: `org.hibernate.callable` which can be true or false depending on whether the query is a stored procedure or not.

### 18.2.1. Using return-property to explicitly specify column/alias names

You can explicitly tell Hibernate what column aliases to use with `<return-property>`, instead of using the `{}`-syntax to let Hibernate inject its own aliases.For example:

    <sql-query name="mySqlQuery">
        <return alias="person" class="eg.Person">
            <return-property name="name" column="myName"/>
            <return-property name="age" column="myAge"/>
            <return-property name="sex" column="mySex"/>
        </return>
        SELECT person.NAME AS myName,
               person.AGE AS myAge,
               person.SEX AS mySex,
        FROM PERSON person WHERE person.NAME LIKE :name
    </sql-query>

`<return-property>` also works with multiple columns. This solves a limitation with the `{}`-syntax which cannot allow fine grained control of multi-column properties.

    <sql-query name="organizationCurrentEmployments">
        <return alias="emp" class="Employment">
            <return-property name="salary">
                <return-column name="VALUE"/>
                <return-column name="CURRENCY"/>
            </return-property>
            <return-property name="endDate" column="myEndDate"/>
        </return>
            SELECT EMPLOYEE AS {emp.employee}, EMPLOYER AS {emp.employer},
            STARTDATE AS {emp.startDate}, ENDDATE AS {emp.endDate},
            REGIONCODE as {emp.regionCode}, EID AS {emp.id}, VALUE, CURRENCY
            FROM EMPLOYMENT
            WHERE EMPLOYER = :id AND ENDDATE IS NULL
            ORDER BY STARTDATE ASC
    </sql-query>

In this example `<return-property>` was used in combination with the `{}`-syntax for injection. This allows users to choose how they want to refer column and properties.

If your mapping has a discriminator you must use `<return-discriminator>` to specify the discriminator column.

### 18.2.2. Using stored procedures for querying

Hibernate3 provides support for queries via stored procedures and functions. Most of the following documentation is equivalent for both. The stored procedure/function must return a resultset as the first out-parameter to be able to work with Hibernate. An example of such a stored function in Oracle 9 and higher is as follows:

    CREATE OR REPLACE FUNCTION selectAllEmployments
        RETURN SYS_REFCURSOR
    AS
        st_cursor SYS_REFCURSOR;
    BEGIN
        OPEN st_cursor FOR
     SELECT EMPLOYEE, EMPLOYER,
     STARTDATE, ENDDATE,
     REGIONCODE, EID, VALUE, CURRENCY
     FROM EMPLOYMENT;
          RETURN  st_cursor;
     END;

To use this query in Hibernate you need to map it via a named query.

    <sql-query name="selectAllEmployees_SP" callable="true">
        <return alias="emp" class="Employment">
            <return-property name="employee" column="EMPLOYEE"/>
            <return-property name="employer" column="EMPLOYER"/>
            <return-property name="startDate" column="STARTDATE"/>
            <return-property name="endDate" column="ENDDATE"/>
            <return-property name="regionCode" column="REGIONCODE"/>
            <return-property name="id" column="EID"/>
            <return-property name="salary">
                <return-column name="VALUE"/>
                <return-column name="CURRENCY"/>
            </return-property>
        </return>
        { ? = call selectAllEmployments() }
    </sql-query>

Stored procedures currently only return scalars and entities. `<return-join>` and `<load-collection>` are not supported.

#### 18.2.2.1. Rules/limitations for using stored procedures

You cannot use stored procedures with Hibernate unless you follow some procedure/function rules. If they do not follow those rules they are not usable with Hibernate. If you still want to use these procedures you have to execute them via `session.connection()`. The rules are different for each database, since database vendors have different stored procedure semantics/syntax.

Stored procedure queries cannot be paged with `setFirstResult()/setMaxResults()`.

The recommended call form is standard SQL92: `{ ? = call functionName(<parameters>) }` or `{ ? = call procedureName(<parameters>}`. Native call syntax is not supported.

For Oracle the following rules apply:

- A function must return a result set. The first parameter of a procedure must be an `OUT` that returns a result set. This is done by using a `SYS_REFCURSOR` type in Oracle 9 or 10. In Oracle you need to define a `REF CURSOR` type. See Oracle literature for further information.

For Sybase or MS SQL server the following rules apply:

- The procedure must return a result set. Note that since these servers can return multiple result sets and update counts, Hibernate will iterate the results and take the first result that is a result set as its return value. Everything else will be discarded.

- If you can enable `SET NOCOUNT ON` in your procedure it will probably be more efficient, but this is not a requirement.

## 18.3. Custom SQL for create, update and delete

Hibernate3 can use custom SQL for create, update, and delete operations. The SQL can be overridden at the statement level or inidividual column level. This section describes statement overrides. For columns, see Section 5.6, “Column transformers: read and write expressions”. Example 18.11, “Custom CRUD via annotations” shows how to define custom SQL operatons using annotations.

Example 18.11. Custom CRUD via annotations

    @Entity

    @Table(name="CHAOS")

    @SQLInsert( sql="INSERT INTO CHAOS(size, name, nickname, id) VALUES(?,upper(?),?,?)")

    @SQLUpdate( sql="UPDATE CHAOS SET size = ?, name = upper(?), nickname = ? WHERE id = ?")

    @SQLDelete( sql="DELETE CHAOS WHERE id = ?")

    @SQLDeleteAll( sql="DELETE CHAOS")

    @Loader(namedQuery = "chaos")

    @NamedNativeQuery(name="chaos", query="select id, size, name, lower( nickname ) as nickname from CHAOS where xml:id= ?", resultClass = Chaos.class)

    public class Chaos {

        @Id

        private Long id;

        private Long size;

        private String name;

        private String nickname;

`@SQLInsert`, `@SQLUpdate`, `@SQLDelete`, `@SQLDeleteAll` respectively override the INSERT, UPDATE, DELETE, and DELETE all statement. The same can be achieved using Hibernate mapping files and the `<sql-insert>`, `<sql-update>` and `<sql-delete>` nodes. This can be seen in Example 18.12, “Custom CRUD XML”.

Example 18.12. Custom CRUD XML

    <class name="Person">
        <id name="id">
            <generator class="increment"/>
        </id>
        <property name="name" not-null="true"/>
        <sql-insert>INSERT INTO PERSON (NAME, ID) VALUES ( UPPER(?), ? )</sql-insert>
        <sql-update>UPDATE PERSON SET NAME=UPPER(?) WHERE xml:id=?</sql-update>
        <sql-delete>DELETE FROM PERSON WHERE xml:id=?</sql-delete>
    </class>

If you expect to call a store procedure, be sure to set the `callable` attribute to `true`. In annotations as well as in xml.

To check that the execution happens correctly, Hibernate allows you to define one of those three strategies:

- none: no check is performed: the store procedure is expected to fail upon issues

- count: use of rowcount to check that the update is successful

- param: like COUNT but using an output parameter rather that the standard mechanism

To define the result check style, use the `check` parameter which is again available in annoations as well as in xml.

You can use the exact same set of annotations respectively xml nodes to override the collection related statements -see Example 18.13, “Overriding SQL statements for collections using annotations”.

Example 18.13. Overriding SQL statements for collections using annotations

    @OneToMany

    @JoinColumn(name="chaos_fk")

    @SQLInsert( sql="UPDATE CASIMIR_PARTICULE SET chaos_fk = ? where id = ?")

    @SQLDelete( sql="UPDATE CASIMIR_PARTICULE SET chaos_fk = null where id = ?")

    private Set<CasimirParticle> particles = new HashSet<CasimirParticle>();

## Tip

The parameter order is important and is defined by the order Hibernate handles properties. You can see the expected order by enabling debug logging for the `org.hibernate.persister.entity` level. With this level enabled Hibernate will print out the static SQL that is used to create, update, delete etc. entities. (To see the expected sequence, remember to not include your custom SQL through annotations or mapping files as that will override the Hibernate generated static sql)

Overriding SQL statements for secondary tables is also possible using `@org.hibernate.annotations.Table` and either (or all) attributes `sqlInsert`, `sqlUpdate`, `sqlDelete`:

Example 18.14. Overriding SQL statements for secondary tables

    @Entity

    @SecondaryTables({

        @SecondaryTable(name = "`Cat nbr1`"),

        @SecondaryTable(name = "Cat2"})

    @org.hibernate.annotations.Tables( {

        @Table(appliesTo = "Cat", comment = "My cat table" ),

        @Table(appliesTo = "Cat2", foreignKey = @ForeignKey(name="FK_CAT2_CAT"), fetch = FetchMode.SELECT,

            sqlInsert=@SQLInsert(sql="insert into Cat2(storyPart2, id) values(upper(?), ?)") )

    } )

    public class Cat implements Serializable {

The previous example also shows that you can give a comment to a given table (primary or secondary): This comment will be used for DDL generation.

## Tip

The SQL is directly executed in your database, so you can use any dialect you like. This will, however, reduce the portability of your mapping if you use database specific SQL.

Last but not least, stored procedures are in most cases required to return the number of rows inserted, updated and deleted. Hibernate always registers the first statement parameter as a numeric output parameter for the CUD operations:

Example 18.15. Stored procedures and their return value

    CREATE OR REPLACE FUNCTION updatePerson (uid IN NUMBER, uname IN VARCHAR2)
        RETURN NUMBER IS
    BEGIN

        update PERSON
        set
            NAME = uname,
        where
            ID = uid;

        return SQL%ROWCOUNT;

    END updatePerson;

## 18.4. Custom SQL for loading

You can also declare your own SQL (or HQL) queries for entity loading. As with inserts, updates, and deletes, this can be done at the individual column level as described in Section 5.6, “Column transformers: read and write expressions” or at the statement level. Here is an example of a statement level override:

    <sql-query name="person">
        <return alias="pers" class="Person" lock-mode="upgrade"/>
        SELECT NAME AS {pers.name}, ID AS {pers.id}
        FROM PERSON
        WHERE xml:id=?
        FOR UPDATE
    </sql-query>

This is just a named query declaration, as discussed earlier. You can reference this named query in a class mapping:

    <class name="Person">
        <id name="id">
            <generator class="increment"/>
        </id>
        <property name="name" not-null="true"/>
        <loader query-ref="person"/>
    </class>

This even works with stored procedures.

You can even define a query for collection loading:

    <set name="employments" inverse="true">
        <key/>
        <one-to-many class="Employment"/>
        <loader query-ref="employments"/>
    </set>

    <sql-query name="employments">
        <load-collection alias="emp" role="Person.employments"/>
        SELECT {emp.*}
        FROM EMPLOYMENT emp
        WHERE EMPLOYER = :id
        ORDER BY STARTDATE ASC, EMPLOYEE ASC
    </sql-query>

You can also define an entity loader that loads a collection by join fetching:

    <sql-query name="person">
        <return alias="pers" class="Person"/>
        <return-join alias="emp" property="pers.employments"/>
        SELECT NAME AS {pers.*}, {emp.*}
        FROM PERSON pers
        LEFT OUTER JOIN EMPLOYMENT emp
            ON pers.ID = emp.PERSON_ID
        WHERE xml:id=?
    </sql-query>

The annotation equivalent `<loader>` is the @Loader annotation as seen in Example 18.11, “Custom CRUD via annotations”.

## Chapter 19. Filtering data

Hibernate3 provides an innovative new approach to handling data with "visibility" rules. A Hibernate filter is a global, named, parameterized filter that can be enabled or disabled for a particular Hibernate session.

## 19.1. Hibernate filters

Hibernate3 has the ability to pre-define filter criteria and attach those filters at both a class level and a collection level. A filter criteria allows you to define a restriction clause similar to the existing "where" attribute available on the class and various collection elements. These filter conditions, however, can be parameterized. The application can then decide at runtime whether certain filters should be enabled and what their parameter values should be. Filters can be used like database views, but they are parameterized inside the application.

Using annotatons filters are defined via `@org.hibernate.annotations.FilterDef` or `@org.hibernate.annotations.FilterDefs`. A filter definition has a `name()` and an array of parameters(). A parameter will allow you to adjust the behavior of the filter at runtime. Each parameter is defined by a `@ParamDef` which has a name and a type. You can also define a `defaultCondition()` parameter for a given `@FilterDef` to set the default condition to use when none are defined in each individual `@Filter`. `@FilterDef`(s) can be defined at the class or package level.

We now need to define the SQL filter clause applied to either the entity load or the collection load. `@Filter` is used and placed either on the entity or the collection element. The connection between `@FilterName` and `@Filter` is a matching name.

Example 19.1. @FilterDef and @Filter annotations

    @Entity

    @FilterDef(name="minLength", parameters=@ParamDef( name="minLength", type="integer" ) )

    @Filters( {

        @Filter(name="betweenLength", condition=":minLength <= length and :maxLength >= length"),

        @Filter(name="minLength", condition=":minLength <= length")

    } )

    public class Forest { ... }

When the collection use an association table as a relational representation, you might want to apply the filter condition to the association table itself or to the target entity table. To apply the constraint on the target entity, use the regular `@Filter` annotation. However, if you want to target the association table, use the `@FilterJoinTable` annotation.

Example 19.2. Using `@FilterJoinTable` for filterting on the association table

        @OneToMany

        @JoinTable

        //filter on the target entity table

        @Filter(name="betweenLength", condition=":minLength <= length and :maxLength >= length")

        //filter on the association table

        @FilterJoinTable(name="security", condition=":userlevel >= requredLevel")

        public Set<Forest> getForests() { ... }

Using Hibernate mapping files for defining filters the situtation is very similar. The filters must first be defined and then attached to the appropriate mapping elements. To define a filter, use the `<filter-def/>` element within a `<hibernate-mapping/>` element:

Example 19.3. Defining a filter definition via `<filter-def>`

    <filter-def name="myFilter">
        <filter-param name="myFilterParam" type="string"/>
    </filter-def>

This filter can then be attached to a class or collection (or, to both or multiples of each at the same time):

Example 19.4. Attaching a filter to a class or collection using `<filter>`

    <class name="myClass" ...>
        ...
        <filter name="myFilter" condition=":myFilterParam = MY_FILTERED_COLUMN"/>

        <set ...>
            <filter name="myFilter" condition=":myFilterParam = MY_FILTERED_COLUMN"/>
        </set>
    </class>

The methods on `Session` are: `enableFilter(String filterName)`, `getEnabledFilter(String filterName)`, and `disableFilter(String filterName)`. By default, filters are not enabled for a given session. Filters must be enabled through use of the `Session.enableFilter()` method, which returns an instance of the `Filter` interface. If you used the simple filter defined above, it would look like this:

    session.enableFilter("myFilter").setParameter("myFilterParam", "some-value");

Methods on the org.hibernate.Filter interface do allow the method-chaining common to much of Hibernate.

The following is a full example, using temporal data with an effective record date pattern:

    <filter-def name="effectiveDate">
        <filter-param name="asOfDate" type="date"/>
    </filter-def>

    <class name="Employee" ...>
    ...
        <many-to-one name="department" column="dept_id" class="Department"/>
        <property name="effectiveStartDate" type="date" column="eff_start_dt"/>
        <property name="effectiveEndDate" type="date" column="eff_end_dt"/>
    ...
        <!--
            Note that this assumes non-terminal records have an eff_end_dt set to
            a max db date for simplicity-sake
        -->
        <filter name="effectiveDate"
                condition=":asOfDate BETWEEN eff_start_dt and eff_end_dt"/>
    </class>

    <class name="Department" ...>
    ...
        <set name="employees" lazy="true">
            <key column="dept_id"/>
            <one-to-many class="Employee"/>
            <filter name="effectiveDate"
                    condition=":asOfDate BETWEEN eff_start_dt and eff_end_dt"/>
        </set>
    </class>

In order to ensure that you are provided with currently effective records, enable the filter on the session prior to retrieving employee data:

    Session session = ...;
    session.enableFilter("effectiveDate").setParameter("asOfDate", new Date());
    List results = session.createQuery("from Employee as e where e.salary > :targetSalary")
             .setLong("targetSalary", new Long(1000000))
             .list();

Even though a salary constraint was mentioned explicitly on the results in the above HQL, because of the enabled filter, the query will return only currently active employees who have a salary greater than one million dollars.

If you want to use filters with outer joining, either through HQL or load fetching, be careful of the direction of the condition expression. It is safest to set this up for left outer joining. Place the parameter first followed by the column name(s) after the operator.

After being defined, a filter might be attached to multiple entities and/or collections each with its own condition. This can be problematic when the conditions are the same each time. Using `<filter-def/>` allows you to definine a default condition, either as an attribute or CDATA:

    <filter-def name="myFilter" condition="abc > xyz">...</filter-def>
    <filter-def name="myOtherFilter">abc=xyz</filter-def>

This default condition will be used whenever the filter is attached to something without specifying a condition. This means you can give a specific condition as part of the attachment of the filter that overrides the default condition in that particular case.

## Chapter 20. XML Mapping

XML Mapping is an experimental feature in Hibernate 3.0 and is currently under active development.

## 20.1. Working with XML data

Hibernate allows you to work with persistent XML data in much the same way you work with persistent POJOs. A parsed XML tree can be thought of as another way of representing the relational data at the object level, instead of POJOs.

Hibernate supports dom4j as API for manipulating XML trees. You can write queries that retrieve dom4j trees from the database and have any modification you make to the tree automatically synchronized to the database. You can even take an XML document, parse it using dom4j, and write it to the database with any of Hibernate's basic operations: `persist(), saveOrUpdate(), merge(), delete(), replicate()` (merging is not yet supported).

This feature has many applications including data import/export, externalization of entity data via JMS or SOAP and XSLT-based reporting.

A single mapping can be used to simultaneously map properties of a class and nodes of an XML document to the database, or, if there is no class to map, it can be used to map just the XML.

### 20.1.1. Specifying XML and class mapping together

Here is an example of mapping a POJO and XML simultaneously:

    <class name="Account"
            table="ACCOUNTS"
            node="account">

        <id name="accountId"
                column="ACCOUNT_ID"
                node="@id"/>

        <many-to-one name="customer"
                column="CUSTOMER_ID"
                node="customer/@id"
                embed-xml="false"/>

        <property name="balance"
                column="BALANCE"
                node="balance"/>

        ...

    </class>

### 20.1.2. Specifying only an XML mapping

Here is an example where there is no POJO class:

    <class entity-name="Account"
            table="ACCOUNTS"
            node="account">

        <id name="id"
                column="ACCOUNT_ID"
                node="@id"
                type="string"/>

        <many-to-one name="customerId"
                column="CUSTOMER_ID"
                node="customer/@id"
                embed-xml="false"
                entity-name="Customer"/>

        <property name="balance"
                column="BALANCE"
                node="balance"
                type="big_decimal"/>

        ...

    </class>

This mapping allows you to access the data as a dom4j tree, or as a graph of property name/value pairs or java `Map`s. The property names are purely logical constructs that can be referred to in HQL queries.

## 20.2. XML mapping metadata

A range of Hibernate mapping elements accept the `node` attribute. This lets you specify the name of an XML attribute or element that holds the property or entity data. The format of the `node` attribute must be one of the following:

- `"element-name"`: map to the named XML element

- `"@attribute-name"`: map to the named XML attribute

- `"."`: map to the parent element

- `"element-name/@attribute-name"`: map to the named attribute of the named element

For collections and single valued associations, there is an additional `embed-xml` attribute. If `embed-xml="true"`, the default, the XML tree for the associated entity (or collection of value type) will be embedded directly in the XML tree for the entity that owns the association. Otherwise, if `embed-xml="false"`, then only the referenced identifier value will appear in the XML for single point associations and collections will not appear at all.

Do not leave `embed-xml="true"` for too many associations, since XML does not deal well with circularity.

    <class name="Customer"
            table="CUSTOMER"
            node="customer">

        <id name="id"
                column="CUST_ID"
                node="@id"/>

        <map name="accounts"
                node="."
                embed-xml="true">
            <key column="CUSTOMER_ID"
                    not-null="true"/>
            <map-key column="SHORT_DESC"
                    node="@short-desc"
                    type="string"/>
            <one-to-many entity-name="Account"
                    embed-xml="false"
                    node="account"/>
        </map>

        <component name="name"
                node="name">
            <property name="firstName"
                    node="first-name"/>
            <property name="initial"
                    node="initial"/>
            <property name="lastName"
                    node="last-name"/>
        </component>

        ...

    </class>

In this case, the collection of account ids is embedded, but not the actual account data. The following HQL query:

    from Customer c left join fetch c.accounts where c.lastName like :lastName

would return datasets such as this:

    <customer xml:id="123456789">
        <account short-desc="Savings">987632567</account>
        <account short-desc="Credit Card">985612323</account>
        <name>
            <first-name>Gavin</first-name>
            <initial>A</initial>
            <last-name>King</last-name>
        </name>
        ...
    </customer>

If you set `embed-xml="true"` on the `<one-to-many>` mapping, the data might look more like this:

    <customer xml:id="123456789">
        <account xml:id="987632567" short-desc="Savings">
            <customer xml:id="123456789"/>
            <balance>100.29</balance>
        </account>
        <account xml:id="985612323" short-desc="Credit Card">
            <customer xml:id="123456789"/>
            <balance>-2370.34</balance>
        </account>
        <name>
            <first-name>Gavin</first-name>
            <initial>A</initial>
            <last-name>King</last-name>
        </name>
        ...
    </customer>

## 20.3. Manipulating XML data

You can also re-read and update XML documents in the application. You can do this by obtaining a dom4j session:

    Document doc = ....;

    Session session = factory.openSession();
    Session dom4jSession = session.getSession(EntityMode.DOM4J);
    Transaction tx = session.beginTransaction();

    List results = dom4jSession
        .createQuery("from Customer c left join fetch c.accounts where c.lastName like :lastName")
        .list();
    for ( int i=0; i<results.size(); i++ ) {
        //add the customer data to the XML document
        Element customer = (Element) results.get(i);
        doc.add(customer);
    }

    tx.commit();
    session.close();

    Session session = factory.openSession();
    Session dom4jSession = session.getSession(EntityMode.DOM4J);
    Transaction tx = session.beginTransaction();

    Element cust = (Element) dom4jSession.get("Customer", customerId);
    for ( int i=0; i<results.size(); i++ ) {
        Element customer = (Element) results.get(i);
        //change the customer name in the XML and database
        Element name = customer.element("name");
        name.element("first-name").setText(firstName);
        name.element("initial").setText(initial);
        name.element("last-name").setText(lastName);
    }

    tx.commit();
    session.close();

When implementing XML-based data import/export, it is useful to combine this feature with Hibernate's `replicate()` operation.

## Chapter 21. Improving performance

## 21.1. Fetching strategies

Hibernate uses a fetching strategy to retrieve associated objects if the application needs to navigate the association. Fetch strategies can be declared in the O/R mapping metadata, or over-ridden by a particular HQL or `Criteria` query.

Hibernate3 defines the following fetching strategies:

- Join fetching: Hibernate retrieves the associated instance or collection in the same `SELECT`, using an `OUTER JOIN`.

- Select fetching: a second `SELECT` is used to retrieve the associated entity or collection. Unless you explicitly disable lazy fetching by specifying `lazy="false"`, this second select will only be executed when you access the association.

- Subselect fetching: a second `SELECT` is used to retrieve the associated collections for all entities retrieved in a previous query or fetch. Unless you explicitly disable lazy fetching by specifying `lazy="false"`, this second select will only be executed when you access the association.

- Batch fetching: an optimization strategy for select fetching. Hibernate retrieves a batch of entity instances or collections in a single `SELECT` by specifying a list of primary or foreign keys.

Hibernate also distinguishes between:

- Immediate fetching: an association, collection or attribute is fetched immediately when the owner is loaded.

- Lazy collection fetching: a collection is fetched when the application invokes an operation upon that collection. This is the default for collections.

- "Extra-lazy" collection fetching: individual elements of the collection are accessed from the database as needed. Hibernate tries not to fetch the whole collection into memory unless absolutely needed. It is suitable for large collections.

- Proxy fetching: a single-valued association is fetched when a method other than the identifier getter is invoked upon the associated object.

- "No-proxy" fetching: a single-valued association is fetched when the instance variable is accessed. Compared to proxy fetching, this approach is less lazy; the association is fetched even when only the identifier is accessed. It is also more transparent, since no proxy is visible to the application. This approach requires buildtime bytecode instrumentation and is rarely necessary.

- Lazy attribute fetching: an attribute or single valued association is fetched when the instance variable is accessed. This approach requires buildtime bytecode instrumentation and is rarely necessary.

We have two orthogonal notions here: when is the association fetched and how is it fetched. It is important that you do not confuse them. We use `fetch` to tune performance. We can use `lazy` to define a contract for what data is always available in any detached instance of a particular class.

### 21.1.1. Working with lazy associations

By default, Hibernate3 uses lazy select fetching for collections and lazy proxy fetching for single-valued associations. These defaults make sense for most associations in the majority of applications.

If you set `hibernate.default_batch_fetch_size`, Hibernate will use the batch fetch optimization for lazy fetching. This optimization can also be enabled at a more granular level.

Please be aware that access to a lazy association outside of the context of an open Hibernate session will result in an exception. For example:

    s = sessions.openSession();
    Transaction tx = s.beginTransaction();

    User u = (User) s.createQuery("from User u where u.name=:userName")
        .setString("userName", userName).uniqueResult();
    Map permissions = u.getPermissions();

    tx.commit();
    s.close();

    Integer accessLevel = (Integer) permissions.get("accounts");  // Error!

Since the permissions collection was not initialized when the `Session` was closed, the collection will not be able to load its state. Hibernate does not support lazy initialization for detached objects. This can be fixed by moving the code that reads from the collection to just before the transaction is committed.

Alternatively, you can use a non-lazy collection or association, by specifying `lazy="false"` for the association mapping. However, it is intended that lazy initialization be used for almost all collections and associations. If you define too many non-lazy associations in your object model, Hibernate will fetch the entire database into memory in every transaction.

On the other hand, you can use join fetching, which is non-lazy by nature, instead of select fetching in a particular transaction. We will now explain how to customize the fetching strategy. In Hibernate3, the mechanisms for choosing a fetch strategy are identical for single-valued associations and collections.

### 21.1.2. Tuning fetch strategies

Select fetching (the default) is extremely vulnerable to N+1 selects problems, so we might want to enable join fetching in the mapping document:

    <set name="permissions"
                fetch="join">
        <key column="userId"/>
        <one-to-many class="Permission"/>
    </set

    <many-to-one name="mother" class="Cat" fetch="join"/>

The `fetch` strategy defined in the mapping document affects:

- retrieval via `get()` or `load()`

- retrieval that happens implicitly when an association is navigated

- `Criteria` queries

- HQL queries if `subselect` fetching is used

Irrespective of the fetching strategy you use, the defined non-lazy graph is guaranteed to be loaded into memory. This might, however, result in several immediate selects being used to execute a particular HQL query.

Usually, the mapping document is not used to customize fetching. Instead, we keep the default behavior, and override it for a particular transaction, using `left join fetch` in HQL. This tells Hibernate to fetch the association eagerly in the first select, using an outer join. In the `Criteria` query API, you would use `setFetchMode(FetchMode.JOIN)`.

If you want to change the fetching strategy used by `get()` or `load()`, you can use a `Criteria` query. For example:

    User user = (User) session.createCriteria(User.class)
                    .setFetchMode("permissions", FetchMode.JOIN)
                    .add( Restrictions.idEq(userId) )
                    .uniqueResult();

This is Hibernate's equivalent of what some ORM solutions call a "fetch plan".

A completely different approach to problems with N+1 selects is to use the second-level cache.

### 21.1.3. Single-ended association proxies

Lazy fetching for collections is implemented using Hibernate's own implementation of persistent collections. However, a different mechanism is needed for lazy behavior in single-ended associations. The target entity of the association must be proxied. Hibernate implements lazy initializing proxies for persistent objects using runtime bytecode enhancement which is accessed via the CGLIB library.

At startup, Hibernate3 generates proxies by default for all persistent classes and uses them to enable lazy fetching of `many-to-one` and `one-to-one` associations.

The mapping file may declare an interface to use as the proxy interface for that class, with the `proxy` attribute. By default, Hibernate uses a subclass of the class. The proxied class must implement a default constructor with at least package visibility. This constructor is recommended for all persistent classes.

There are potential problems to note when extending this approach to polymorphic classes.For example:

    <class name="Cat" proxy="Cat">
        ......
        <subclass name="DomesticCat">
            .....
        </subclass>
    </class>

Firstly, instances of `Cat` will never be castable to `DomesticCat`, even if the underlying instance is an instance of `DomesticCat`:

    Cat cat = (Cat) session.load(Cat.class, id);  // instantiate a proxy (does not hit the db)
    if ( cat.isDomesticCat() ) {                  // hit the db to initialize the proxy
        DomesticCat dc = (DomesticCat) cat;       // Error!
        ....
    }

Secondly, it is possible to break proxy `==`:

    Cat cat = (Cat) session.load(Cat.class, id);            // instantiate a Cat proxy
    DomesticCat dc =
            (DomesticCat) session.load(DomesticCat.class, id);  // acquire new DomesticCat proxy!
    System.out.println(cat==dc);                            // false

However, the situation is not quite as bad as it looks. Even though we now have two references to different proxy objects, the underlying instance will still be the same object:

    cat.setWeight(11.0);  // hit the db to initialize the proxy
    System.out.println( dc.getWeight() );  // 11.0

Third, you cannot use a CGLIB proxy for a `final` class or a class with any `final` methods.

Finally, if your persistent object acquires any resources upon instantiation (e.g. in initializers or default constructor), then those resources will also be acquired by the proxy. The proxy class is an actual subclass of the persistent class.

These problems are all due to fundamental limitations in Java's single inheritance model. To avoid these problems your persistent classes must each implement an interface that declares its business methods. You should specify these interfaces in the mapping file where `CatImpl` implements the interface `Cat` and `DomesticCatImpl` implements the interface `DomesticCat`. For example:

    <class name="CatImpl" proxy="Cat">
        ......
        <subclass name="DomesticCatImpl" proxy="DomesticCat">
            .....
        </subclass>
    </class>

Then proxies for instances of `Cat` and `DomesticCat` can be returned by `load()` or `iterate()`.

    Cat cat = (Cat) session.load(CatImpl.class, catid);
    Iterator iter = session.createQuery("from CatImpl as cat where cat.name='fritz'").iterate();
    Cat fritz = (Cat) iter.next();

## Note

`list()` does not usually return proxies.

Relationships are also lazily initialized. This means you must declare any properties to be of type `Cat`, not `CatImpl`.

Certain operations do not require proxy initialization:

- `equals()`: if the persistent class does not override `equals()`

- `hashCode()`: if the persistent class does not override `hashCode()`

- The identifier getter method

Hibernate will detect persistent classes that override `equals()` or `hashCode()`.

By choosing `lazy="no-proxy"` instead of the default `lazy="proxy"`, you can avoid problems associated with typecasting. However, buildtime bytecode instrumentation is required, and all operations will result in immediate proxy initialization.

### 21.1.4. Initializing collections and proxies

A `LazyInitializationException` will be thrown by Hibernate if an uninitialized collection or proxy is accessed outside of the scope of the `Session`, i.e., when the entity owning the collection or having the reference to the proxy is in the detached state.

Sometimes a proxy or collection needs to be initialized before closing the `Session`. You can force initialization by calling `cat.getSex()` or `cat.getKittens().size()`, for example. However, this can be confusing to readers of the code and it is not convenient for generic code.

The static methods `Hibernate.initialize()` and `Hibernate.isInitialized()`, provide the application with a convenient way of working with lazily initialized collections or proxies. `Hibernate.initialize(cat)` will force the initialization of a proxy, `cat`, as long as its `Session` is still open. `Hibernate.initialize( cat.getKittens() )` has a similar effect for the collection of kittens.

Another option is to keep the `Session` open until all required collections and proxies have been loaded. In some application architectures, particularly where the code that accesses data using Hibernate, and the code that uses it are in different application layers or different physical processes, it can be a problem to ensure that the `Session` is open when a collection is initialized. There are two basic ways to deal with this issue:

- In a web-based application, a servlet filter can be used to close the `Session` only at the end of a user request, once the rendering of the view is complete (the Open Session in View pattern). Of course, this places heavy demands on the correctness of the exception handling of your application infrastructure. It is vitally important that the `Session` is closed and the transaction ended before returning to the user, even when an exception occurs during rendering of the view. See the Hibernate Wiki for examples of this "Open Session in View" pattern.

- In an application with a separate business tier, the business logic must "prepare" all collections that the web tier needs before returning. This means that the business tier should load all the data and return all the data already initialized to the presentation/web tier that is required for a particular use case. Usually, the application calls `Hibernate.initialize()` for each collection that will be needed in the web tier (this call must occur before the session is closed) or retrieves the collection eagerly using a Hibernate query with a `FETCH` clause or a `FetchMode.JOIN` in `Criteria`. This is usually easier if you adopt the Command pattern instead of a Session Facade.

- You can also attach a previously loaded object to a new `Session` with `merge()` or `lock()` before accessing uninitialized collections or other proxies. Hibernate does not, and certainly should not, do this automatically since it would introduce impromptu transaction semantics.

Sometimes you do not want to initialize a large collection, but still need some information about it, like its size, for example, or a subset of the data.

You can use a collection filter to get the size of a collection without initializing it:

    ( (Integer) s.createFilter( collection, "select count(*)" ).list().get(0) ).intValue()

The `createFilter()` method is also used to efficiently retrieve subsets of a collection without needing to initialize the whole collection:

    s.createFilter( lazyCollection, "").setFirstResult(0).setMaxResults(10).list();

### 21.1.5. Using batch fetching

Using batch fetching, Hibernate can load several uninitialized proxies if one proxy is accessed. Batch fetching is an optimization of the lazy select fetching strategy. There are two ways you can configure batch fetching: on the class level and the collection level.

Batch fetching for classes/entities is easier to understand. Consider the following example: at runtime you have 25 `Cat` instances loaded in a `Session`, and each `Cat` has a reference to its `owner`, a `Person`. The `Person` class is mapped with a proxy, `lazy="true"`. If you now iterate through all cats and call `getOwner()` on each, Hibernate will, by default, execute 25 `SELECT` statements to retrieve the proxied owners. You can tune this behavior by specifying a `batch-size` in the mapping of `Person`:

    <class name="Person" batch-size="10">...</class>

Hibernate will now execute only three queries: the pattern is 10, 10, 5.

You can also enable batch fetching of collections. For example, if each `Person` has a lazy collection of `Cat`s, and 10 persons are currently loaded in the `Session`, iterating through all persons will generate 10 `SELECT`s, one for every call to `getCats()`. If you enable batch fetching for the `cats` collection in the mapping of `Person`, Hibernate can pre-fetch collections:

    <class name="Person">
        <set name="cats" batch-size="3">
            ...
        </set>
    </class>

With a `batch-size` of 3, Hibernate will load 3, 3, 3, 1 collections in four `SELECT`s. Again, the value of the attribute depends on the expected number of uninitialized collections in a particular `Session`.

Batch fetching of collections is particularly useful if you have a nested tree of items, i.e. the typical bill-of-materials pattern. However, a nested set or a materialized path might be a better option for read-mostly trees.

### 21.1.6. Using subselect fetching

If one lazy collection or single-valued proxy has to be fetched, Hibernate will load all of them, re-running the original query in a subselect. This works in the same way as batch-fetching but without the piecemeal loading.

### 21.1.7. Fetch profiles

Another way to affect the fetching strategy for loading associated objects is through something called a fetch profile, which is a named configuration associated with the `org.hibernate.SessionFactory` but enabled, by name, on the `org.hibernate.Session`. Once enabled on a `org.hibernate.Session`, the fetch profile will be in affect for that `org.hibernate.Session` until it is explicitly disabled.

So what does that mean? Well lets explain that by way of an example which show the different available approaches to configure a fetch profile:

Example 21.1. Specifying a fetch profile using `@FetchProfile`

    @Entity
    @FetchProfile(name = "customer-with-orders", fetchOverrides = {
       @FetchProfile.FetchOverride(entity = Customer.class, association = "orders", mode = FetchMode.JOIN)
    })
    public class Customer {
       @Id
       @GeneratedValue
       private long id;

       private String name;

       private long customerNumber;

       @OneToMany
       private Set<Order> orders;

       // standard getter/setter
       ...
    }

Example 21.2. Specifying a fetch profile using `<fetch-profile>` outside `<class>` node

    <hibernate-mapping>
        <class name="Customer">
            ...
            <set name="orders" inverse="true">
                <key column="cust_id"/>
                <one-to-many class="Order"/>
            </set>
        </class>
        <class name="Order">
            ...
        </class>
        <fetch-profile name="customer-with-orders">
            <fetch entity="Customer" association="orders" style="join"/>
        </fetch-profile>
    </hibernate-mapping>

Example 21.3. Specifying a fetch profile using `<fetch-profile>` inside `<class>` node

    <hibernate-mapping>
        <class name="Customer">
            ...
            <set name="orders" inverse="true">
                <key column="cust_id"/>
                <one-to-many class="Order"/>
            </set>
            <fetch-profile name="customer-with-orders">
                <fetch association="orders" style="join"/>
            </fetch-profile>
        </class>
        <class name="Order">
            ...
        </class>
    </hibernate-mapping>

Now normally when you get a reference to a particular customer, that customer's set of orders will be lazy meaning we will not yet have loaded those orders from the database. Normally this is a good thing. Now lets say that you have a certain use case where it is more efficient to load the customer and their orders together. One way certainly is to use "dynamic fetching" strategies via an HQL or criteria queries. But another option is to use a fetch profile to achieve that. The following code will load both the customer andtheir orders:

Example 21.4. Activating a fetch profile for a given `Session`

    Session session = ...;
    session.enableFetchProfile( "customer-with-orders" );  // name matches from mapping
    Customer customer = (Customer) session.get( Customer.class, customerId );

## Note

`@FetchProfile` definitions are global and it does not matter on which class you place them. You can place the `@FetchProfile` annotation either onto a class or package (package-info.java). In order to define multiple fetch profiles for the same class or package `@FetchProfiles` can be used.

Currently only join style fetch profiles are supported, but they plan is to support additional styles. See HHH-3414 for details.

### 21.1.8. Using lazy property fetching

Hibernate3 supports the lazy fetching of individual properties. This optimization technique is also known as fetch groups. Please note that this is mostly a marketing feature; optimizing row reads is much more important than optimization of column reads. However, only loading some properties of a class could be useful in extreme cases. For example, when legacy tables have hundreds of columns and the data model cannot be improved.

To enable lazy property loading, set the `lazy` attribute on your particular property mappings:

    <class name="Document">
           <id name="id">
            <generator class="native"/>
        </id>
        <property name="name" not-null="true" length="50"/>
        <property name="summary" not-null="true" length="200" lazy="true"/>
        <property name="text" not-null="true" length="2000" lazy="true"/>
    </class>

Lazy property loading requires buildtime bytecode instrumentation. If your persistent classes are not enhanced, Hibernate will ignore lazy property settings and return to immediate fetching.

For bytecode instrumentation, use the following Ant task:

    <target name="instrument" depends="compile">
        <taskdef name="instrument" classname="org.hibernate.tool.instrument.InstrumentTask">
            <classpath path="${jar.path}"/>
            <classpath path="${classes.dir}"/>
            <classpath refxml:id="lib.class.path"/>
        </taskdef>

        <instrument verbose="true">
            <fileset dir="${testclasses.dir}/org/hibernate/auction/model">
                <include name="*.class"/>
            </fileset>
        </instrument>
    </target>

A different way of avoiding unnecessary column reads, at least for read-only transactions, is to use the projection features of HQL or Criteria queries. This avoids the need for buildtime bytecode processing and is certainly a preferred solution.

You can force the usual eager fetching of properties using `fetch all properties` in HQL.

## 21.2. The Second Level Cache

A Hibernate `Session` is a transaction-level cache of persistent data. It is possible to configure a cluster or JVM-level (`SessionFactory`-level) cache on a class-by-class and collection-by-collection basis. You can even plug in a clustered cache. Be aware that caches are not aware of changes made to the persistent store by another application. They can, however, be configured to regularly expire cached data.

You have the option to tell Hibernate which caching implementation to use by specifying the name of a class that implements `org.hibernate.cache.spi.CacheProvider` using the property `hibernate.cache.provider_class`. Hibernate is bundled with a number of built-in integrations with the open-source cache providers that are listed in Table 21.1, “Cache Providers”. You can also implement your own and plug it in as outlined above. Note that versions prior to Hibernate 3.2 use EhCache as the default cache provider.

Table 21.1. Cache Providers

Cache | Provider class | Type | Cluster Safe | Query Cache Supported
--- | --- | --- | --- | ---
ConcurrentHashMap (only for testing purpose, in hibernate-testing module) | `org.hibernate.testing.cache.CachingRegionFactory` | memory |  | yes
EHCache | `org.hibernate.cache.ehcache.EhCacheRegionFactory` | memory, disk, transactional, clustered | yes | yes
Infinispan | `org.hibernate.cache.infinispan.InfinispanRegionFactory` | clustered (ip multicast), transactional | yes (replication or invalidation) | yes (clock sync req.)

### 21.2.1. Cache mappings

As we have done in previous chapters we are looking at the two different possibiltites to configure caching. First configuration via annotations and then via Hibernate mapping files.

By default, entities are not part of the second level cache and we recommend you to stick to this setting. However, you can override this by setting the `shared-cache-mode` element in your `persistence.xml` file or by using the `javax.persistence.sharedCache.mode` property in your configuration. The following values are possible:

- `ENABLE_SELECTIVE` (Default and recommended value): entities are not cached unless explicitly marked as cacheable.

- `DISABLE_SELECTIVE`: entities are cached unless explicitly marked as not cacheable.

- `ALL`: all entities are always cached even if marked as non cacheable.

- `NONE`: no entity are cached even if marked as cacheable. This option can make sense to disable second-level cache altogether.

The cache concurrency strategy used by default can be set globaly via the `hibernate.cache.default_cache_concurrency_strategy` configuration property. The values for this property are:

- `read-only`

- `read-write`

- `nonstrict-read-write`

- `transactional`

## Note

It is recommended to define the cache concurrency strategy per entity rather than using a global one. Use the `@org.hibernate.annotations.Cache` annotation for that.

Example 21.5. Definition of cache concurrency strategy via `@Cache`

    @Entity

    @Cacheable

    @Cache(usage = CacheConcurrencyStrategy.NONSTRICT_READ_WRITE)

    public class Forest { ... }

Hibernate also let's you cache the content of a collection or the identifiers if the collection contains other entities. Use the `@Cache` annotation on the collection property.

Example 21.6. Caching collections using annotations

    @OneToMany(cascade=CascadeType.ALL, fetch=FetchType.EAGER)

    @JoinColumn(name="CUST_ID")

    @Cache(usage = CacheConcurrencyStrategy.NONSTRICT_READ_WRITE)

    public SortedSet<Ticket> getTickets() {

        return tickets;

    }

Example 21.7, “`@Cache` annotation with attributes”shows the`@org.hibernate.annotations.Cache` annotations with its attributes. It allows you to define the caching strategy and region of a given second level cache.

Example 21.7. `@Cache` annotation with attributes

    @Cache(
        CacheConcurrencyStrategy usage();
        String region() default "";
        String include() default "all";
    )

| usage: the given cache concurrency strategy (NONE, READ_ONLY, NONSTRICT_READ_WRITE, READ_WRITE, TRANSACTIONAL)
| region (optional): the cache region (default to the fqcn of the class or the fq role name of the collection)
| `include` (optional): all to include all properties, non-lazy to only include non lazy properties (default all).

Let's now take a look at Hibernate mapping files. There the `<cache>` element of a class or collection mapping is used to configure the second level cache. Looking at Example 21.8, “The Hibernate `<cache>` mapping element” the parallels to anotations is obvious.

Example 21.8. The Hibernate `<cache>` mapping element

    <cache
        usage="transactional|read-write|nonstrict-read-write|read-only"
        region="RegionName"
        include="all|non-lazy"
    />

| `usage` (required) specifies the caching strategy: `transactional`, `read-write`, `nonstrict-read-write` or `read-only`
| `region` (optional: defaults to the class or collection role name): specifies the name of the second level cache region
| `include` (optional: defaults to `all`) `non-lazy`: specifies that properties of the entity mapped with `lazy="true"` cannot be cached when attribute-level lazy fetching is enabled

Alternatively to `<cache>`, you can use `<class-cache>` and `<collection-cache>` elements in `hibernate.cfg.xml`.

Let's now have a closer look at the different usage strategies

### 21.2.2. Strategy: read only

If your application needs to read, but not modify, instances of a persistent class, a `read-only` cache can be used. This is the simplest and optimal performing strategy. It is even safe for use in a cluster.

### 21.2.3. Strategy: read/write

If the application needs to update data, a `read-write` cache might be appropriate. This cache strategy should never be used if serializable transaction isolation level is required. If the cache is used in a JTA environment, you must specify the property `hibernate.transaction.manager_lookup_class` and naming a strategy for obtaining the JTA `TransactionManager`. In other environments, you should ensure that the transaction is completed when `Session.close()` or `Session.disconnect()` is called. If you want to use this strategy in a cluster, you should ensure that the underlying cache implementation supports locking. The built-in cache providers do not support locking.

### 21.2.4. Strategy: nonstrict read/write

If the application only occasionally needs to update data (i.e. if it is extremely unlikely that two transactions would try to update the same item simultaneously), and strict transaction isolation is not required, a `nonstrict-read-write` cache might be appropriate. If the cache is used in a JTA environment, you must specify `hibernate.transaction.manager_lookup_class`. In other environments, you should ensure that the transaction is completed when `Session.close()` or `Session.disconnect()` is called.

### 21.2.5. Strategy: transactional

The `transactional` cache strategy provides support for fully transactional cache providers such as JBoss TreeCache. Such a cache can only be used in a JTA environment and you must specify `hibernate.transaction.manager_lookup_class`.

### 21.2.6. Cache-provider/concurrency-strategy compatibility

## Important

None of the cache providers support all of the cache concurrency strategies.

The following table shows which providers are compatible with which concurrency strategies.

Table 21.2. Cache Concurrency Strategy Support

Cache | read-only | nonstrict-read-write | read-write | transactional
--- | --- | --- | --- | ---
ConcurrentHashMap (not intended for production use) | yes | yes | yes |
EHCache | yes | yes | yes | yes
Infinispan | yes |  |  | yes

## 21.3. Managing the caches

Whenever you pass an object to `save()`, `update()` or `saveOrUpdate()`, and whenever you retrieve an object using `load()`, `get()`, `list()`, `iterate()` or `scroll()`, that object is added to the internal cache of the `Session`.

When `flush()` is subsequently called, the state of that object will be synchronized with the database. If you do not want this synchronization to occur, or if you are processing a huge number of objects and need to manage memory efficiently, the `evict()` method can be used to remove the object and its collections from the first-level cache.

Example 21.9. Explcitly evicting a cached instance from the first level cache using `Session.evict()`

    ScrollableResult cats = sess.createQuery("from Cat as cat").scroll(); //a huge result set
    while ( cats.next() ) {
        Cat cat = (Cat) cats.get(0);
        doSomethingWithACat(cat);
        sess.evict(cat);
    }

The `Session` also provides a `contains()` method to determine if an instance belongs to the session cache.

To evict all objects from the session cache, call `Session.clear()`

For the second-level cache, there are methods defined on `SessionFactory` for evicting the cached state of an instance, entire class, collection instance or entire collection role.

Example 21.10. Second-level cache eviction via `SessionFactoty.evict()` and `SessionFacyory.evictCollection()`

    sessionFactory.evict(Cat.class, catId); //evict a particular Cat
    sessionFactory.evict(Cat.class);  //evict all Cats
    sessionFactory.evictCollection("Cat.kittens", catId); //evict a particular collection of kittens
    sessionFactory.evictCollection("Cat.kittens"); //evict all kitten collections

The `CacheMode` controls how a particular session interacts with the second-level cache:

- `CacheMode.NORMAL`: will read items from and write items to the second-level cache

- `CacheMode.GET`: will read items from the second-level cache. Do not write to the second-level cache except when updating data

- `CacheMode.PUT`: will write items to the second-level cache. Do not read from the second-level cache

- `CacheMode.REFRESH`: will write items to the second-level cache. Do not read from the second-level cache. Bypass the effect of `hibernate.cache.use_minimal_puts` forcing a refresh of the second-level cache for all items read from the database

To browse the contents of a second-level or query cache region, use the `Statistics` API:

Example 21.11. Browsing the second-level cache entries via the `Statistics` API

    Map cacheEntries = sessionFactory.getStatistics()
            .getSecondLevelCacheStatistics(regionName)
            .getEntries();

You will need to enable statistics and, optionally, force Hibernate to keep the cache entries in a more readable format:

Example 21.12. Enabling Hibernate statistics

    hibernate.generate_statistics true
    hibernate.cache.use_structured_entries true

## 21.4. The Query Cache

Query result sets can also be cached. This is only useful for queries that are run frequently with the same parameters.

### 21.4.1. Enabling query caching

Caching of query results introduces some overhead in terms of your applications normal transactional processing. For example, if you cache results of a query against Person Hibernate will need to keep track of when those results should be invalidated because changes have been committed against Person. That, coupled with the fact that most applications simply gain no benefit from caching query results, leads Hibernate to disable caching of query results by default. To use query caching, you will first need to enable the query cache:

    hibernate.cache.use_query_cache true

This setting creates two new cache regions:

- `org.hibernate.cache.internal.StandardQueryCache`, holding the cached query results

- `org.hibernate.cache.spi.UpdateTimestampsCache`, holding timestamps of the most recent updates to queryable tables. These are used to validate the results as they are served from the query cache.

## Important

If you configure your underlying cache implementation to use expiry or timeouts is very important that the cache timeout of the underlying cache region for the UpdateTimestampsCache be set to a higher value than the timeouts of any of the query caches. In fact, we recommend that the the UpdateTimestampsCache region not be configured for expiry at all. Note, in particular, that an LRU cache expiry policy is never appropriate.

As mentioned above, most queries do not benefit from caching or their results. So by default, individual queries are not cached even after enabling query caching. To enable results caching for a particular query, call `org.hibernate.Query.setCacheable(true)`. This call allows the query to look for existing cache results or add its results to the cache when it is executed.

## Note

The query cache does not cache the state of the actual entities in the cache; it caches only identifier values and results of value type. For this reaso, the query cache should always be used in conjunction with the second-level cache for those entities expected to be cached as part of a query result cache (just as with collection caching).

### 21.4.2. Query cache regions

If you require fine-grained control over query cache expiration policies, you can specify a named cache region for a particular query by calling `Query.setCacheRegion()`.

    List blogs = sess.createQuery("from Blog blog where blog.blogger = :blogger")
            .setEntity("blogger", blogger)
            .setMaxResults(15)
            .setCacheable(true)
            .setCacheRegion("frontpages")
            .list();

If you want to force the query cache to refresh one of its regions (disregard any cached results it finds there) you can use `org.hibernate.Query.setCacheMode(CacheMode.REFRESH)`. In conjunction with the region you have defined for the given query, Hibernate will selectively force the results cached in that particular region to be refreshed. This is particularly useful in cases where underlying data may have been updated via a separate process and is a far more efficient alternative to bulk eviction of the region via `org.hibernate.SessionFactory.evictQueries()`.

## 21.5. Understanding Collection performance

In the previous sections we have covered collections and their applications. In this section we explore some more issues in relation to collections at runtime.

### 21.5.1. Taxonomy

Hibernate defines three basic kinds of collections:

- collections of values

- one-to-many associations

- many-to-many associations

This classification distinguishes the various table and foreign key relationships but does not tell us quite everything we need to know about the relational model. To fully understand the relational structure and performance characteristics, we must also consider the structure of the primary key that is used by Hibernate to update or delete collection rows. This suggests the following classification:

- indexed collections

- sets

- bags

All indexed collections (maps, lists, and arrays) have a primary key consisting of the `<key>` and `<index>` columns. In this case, collection updates are extremely efficient. The primary key can be efficiently indexed and a particular row can be efficiently located when Hibernate tries to update or delete it.

Sets have a primary key consisting of `<key>` and element columns. This can be less efficient for some types of collection element, particularly composite elements or large text or binary fields, as the database may not be able to index a complex primary key as efficiently. However, for one-to-many or many-to-many associations, particularly in the case of synthetic identifiers, it is likely to be just as efficient. If you want `SchemaExport` to actually create the primary key of a `<set>`, you must declare all columns as `not-null="true"`.

`<idbag>` mappings define a surrogate key, so they are efficient to update. In fact, they are the best case.

Bags are the worst case since they permit duplicate element values and, as they have no index column, no primary key can be defined. Hibernate has no way of distinguishing between duplicate rows. Hibernate resolves this problem by completely removing in a single `DELETE` and recreating the collection whenever it changes. This can be inefficient.

For a one-to-many association, the "primary key" may not be the physical primary key of the database table. Even in this case, the above classification is still useful. It reflects how Hibernate "locates" individual rows of the collection.

### 21.5.2. Lists, maps, idbags and sets are the most efficient collections to update

From the discussion above, it should be clear that indexed collections and sets allow the most efficient operation in terms of adding, removing and updating elements.

There is, arguably, one more advantage that indexed collections have over sets for many-to-many associations or collections of values. Because of the structure of a `Set`, Hibernate does not `UPDATE` a row when an element is "changed". Changes to a `Set` always work via `INSERT` and `DELETE` of individual rows. Once again, this consideration does not apply to one-to-many associations.

After observing that arrays cannot be lazy, you can conclude that lists, maps and idbags are the most performant (non-inverse) collection types, with sets not far behind. You can expect sets to be the most common kind of collection in Hibernate applications. This is because the "set" semantics are most natural in the relational model.

However, in well-designed Hibernate domain models, most collections are in fact one-to-many associations with `inverse="true"`. For these associations, the update is handled by the many-to-one end of the association, and so considerations of collection update performance simply do not apply.

### 21.5.3. Bags and lists are the most efficient inverse collections

There is a particular case, however, in which bags, and also lists, are much more performant than sets. For a collection with `inverse="true"`, the standard bidirectional one-to-many relationship idiom, for example, we can add elements to a bag or list without needing to initialize (fetch) the bag elements. This is because, unlike a `set`, `Collection.add()` or `Collection.addAll()` must always return true for a bag or `List`. This can make the following common code much faster:

    Parent p = (Parent) sess.load(Parent.class, id);
    Child c = new Child();
    c.setParent(p);
    p.getChildren().add(c);  //no need to fetch the collection!
    sess.flush();

### 21.5.4. One shot delete

Deleting collection elements one by one can sometimes be extremely inefficient. Hibernate knows not to do that in the case of an newly-empty collection (if you called `list.clear()`, for example). In this case, Hibernate will issue a single `DELETE`.

Suppose you added a single element to a collection of size twenty and then remove two elements. Hibernate will issue one `INSERT` statement and two `DELETE` statements, unless the collection is a bag. This is certainly desirable.

However, suppose that we remove eighteen elements, leaving two and then add thee new elements. There are two possible ways to proceed

- delete eighteen rows one by one and then insert three rows

- remove the whole collection in one SQL `DELETE` and insert all five current elements one by one

Hibernate cannot know that the second option is probably quicker. It would probably be undesirable for Hibernate to be that intuitive as such behavior might confuse database triggers, etc.

Fortunately, you can force this behavior (i.e. the second strategy) at any time by discarding (i.e. dereferencing) the original collection and returning a newly instantiated collection with all the current elements.

One-shot-delete does not apply to collections mapped `inverse="true"`.

## 21.6. Monitoring performance

Optimization is not much use without monitoring and access to performance numbers. Hibernate provides a full range of figures about its internal operations. Statistics in Hibernate are available per `SessionFactory`.

### 21.6.1. Monitoring a SessionFactory

You can access `SessionFactory` metrics in two ways. Your first option is to call `sessionFactory.getStatistics()` and read or display the `Statistics` yourself.

Hibernate can also use JMX to publish metrics if you enable the `StatisticsService` MBean. You can enable a single MBean for all your `SessionFactory` or one per factory. See the following code for minimalistic configuration examples:

    // MBean service registration for a specific SessionFactory
    Hashtable tb = new Hashtable();
    tb.put("type", "statistics");
    tb.put("sessionFactory", "myFinancialApp");
    ObjectName on = new ObjectName("hibernate", tb); // MBean object name

    StatisticsService stats = new StatisticsService(); // MBean implementation
    stats.setSessionFactory(sessionFactory); // Bind the stats to a SessionFactory
    server.registerMBean(stats, on); // Register the Mbean on the server

    // MBean service registration for all SessionFactory's
    Hashtable tb = new Hashtable();
    tb.put("type", "statistics");
    tb.put("sessionFactory", "all");
    ObjectName on = new ObjectName("hibernate", tb); // MBean object name

    StatisticsService stats = new StatisticsService(); // MBean implementation
    server.registerMBean(stats, on); // Register the MBean on the server

You can activate and deactivate the monitoring for a `SessionFactory`:

- at configuration time, set `hibernate.generate_statistics` to `false`

- at runtime: `sf.getStatistics().setStatisticsEnabled(true)` or `hibernateStatsBean.setStatisticsEnabled(true)`

Statistics can be reset programmatically using the `clear()` method. A summary can be sent to a logger (info level) using the `logSummary()` method.

### 21.6.2. Metrics

Hibernate provides a number of metrics, from basic information to more specialized information that is only relevant in certain scenarios. All available counters are described in the `Statistics` interface API, in three categories:

- Metrics related to the general `Session` usage, such as number of open sessions, retrieved JDBC connections, etc.

- Metrics related to the entities, collections, queries, and caches as a whole (aka global metrics).

- Detailed metrics related to a particular entity, collection, query or cache region.

For example, you can check the cache hit, miss, and put ratio of entities, collections and queries, and the average time a query needs. Be aware that the number of milliseconds is subject to approximation in Java. Hibernate is tied to the JVM precision and on some platforms this might only be accurate to 10 seconds.

Simple getters are used to access the global metrics (i.e. not tied to a particular entity, collection, cache region, etc.). You can access the metrics of a particular entity, collection or cache region through its name, and through its HQL or SQL representation for queries. Please refer to the `Statistics`, `EntityStatistics`, `CollectionStatistics`, `SecondLevelCacheStatistics`, and `QueryStatistics` API Javadoc for more information. The following code is a simple example:

    Statistics stats = HibernateUtil.sessionFactory.getStatistics();

    double queryCacheHitCount  = stats.getQueryCacheHitCount();
    double queryCacheMissCount = stats.getQueryCacheMissCount();
    double queryCacheHitRatio =
      queryCacheHitCount / (queryCacheHitCount + queryCacheMissCount);

    log.info("Query Hit ratio:" + queryCacheHitRatio);

    EntityStatistics entityStats =
      stats.getEntityStatistics( Cat.class.getName() );
    long changes =
            entityStats.getInsertCount()
            + entityStats.getUpdateCount()
            + entityStats.getDeleteCount();
    log.info(Cat.class.getName() + " changed " + changes + "times"  );

You can work on all entities, collections, queries and region caches, by retrieving the list of names of entities, collections, queries and region caches using the following methods: `getQueries()`, `getEntityNames()`, `getCollectionRoleNames()`, and `getSecondLevelCacheRegionNames()`.

## Chapter 22. Toolset Guide

Roundtrip engineering with Hibernate is possible using a set of Eclipse plugins, commandline tools, and Ant tasks.

Hibernate Tools currently include plugins for the Eclipse IDE as well as Ant tasks for reverse engineering of existing databases:

- Mapping Editor: an editor for Hibernate XML mapping files that supports auto-completion and syntax highlighting. It also supports semantic auto-completion for class names and property/field names, making it more versatile than a normal XML editor.

- Console: the console is a new view in Eclipse. In addition to a tree overview of your console configurations, you are also provided with an interactive view of your persistent classes and their relationships. The console allows you to execute HQL queries against your database and browse the result directly in Eclipse.

- Development Wizards: several wizards are provided with the Hibernate Eclipse tools. You can use a wizard to quickly generate Hibernate configuration (cfg.xml) files, or to reverse engineer an existing database schema into POJO source files and Hibernate mapping files. The reverse engineering wizard supports customizable templates.

Please refer to the Hibernate Tools package documentation for more information.

However, the Hibernate main package comes bundled with an integrated tool : SchemaExport aka `hbm2ddl`.It can even be used from "inside" Hibernate.

## 22.1. Automatic schema generation

DDL can be generated from your mapping files by a Hibernate utility. The generated schema includes referential integrity constraints, primary and foreign keys, for entity and collection tables. Tables and sequences are also created for mapped identifier generators.

You must specify a SQL `Dialect` via the `hibernate.dialect` property when using this tool, as DDL is highly vendor-specific.

First, you must customize your mapping files to improve the generated schema. The next section covers schema customization.

### 22.1.1. Customizing the schema

Many Hibernate mapping elements define optional attributes named `length`, `precision` and `scale`. You can set the length, precision and scale of a column with this attribute.

    <property name="zip" length="5"/>

    <property name="balance" precision="12" scale="2"/>

Some tags also accept a `not-null` attribute for generating a `NOT NULL` constraint on table columns, and a `unique` attribute for generating `UNIQUE` constraint on table columns.

    <many-to-one name="bar" column="barId" not-null="true"/>

    <element column="serialNumber" type="long" not-null="true" unique="true"/>

A `unique-key` attribute can be used to group columns in a single, unique key constraint. Currently, the specified value of the `unique-key` attribute is not used to name the constraint in the generated DDL. It is only used to group the columns in the mapping file.

    <many-to-one name="org" column="orgId" unique-key="OrgEmployeeId"/>
    <property name="employeeId" unique-key="OrgEmployee"/>

An `index` attribute specifies the name of an index that will be created using the mapped column or columns. Multiple columns can be grouped into the same index by simply specifying the same index name.

    <property name="lastName" index="CustName"/>
    <property name="firstName" index="CustName"/>

A `foreign-key` attribute can be used to override the name of any generated foreign key constraint.

    <many-to-one name="bar" column="barId" foreign-key="FKFooBar"/>

Many mapping elements also accept a child `<column>` element. This is particularly useful for mapping multi-column types:

    <property name="name" type="my.customtypes.Name"/>
        <column name="last" not-null="true" index="bar_idx" length="30"/>
        <column name="first" not-null="true" index="bar_idx" length="20"/>
        <column name="initial"/>
    </property>

The `default` attribute allows you to specify a default value for a column.You should assign the same value to the mapped property before saving a new instance of the mapped class.

    <property name="credits" type="integer" insert="false">
        <column name="credits" default="10"/>
    </property>

    <version name="version" type="integer" insert="false">
        <column name="version" default="0"/>
    </property>

The `sql-type` attribute allows the user to override the default mapping of a Hibernate type to SQL datatype.

    <property name="balance" type="float">
        <column name="balance" sql-type="decimal(13,3)"/>
    </property>

The `check` attribute allows you to specify a check constraint.

    <property name="foo" type="integer">
        <column name="foo" check="foo > 10"/>
    </property>

    <class name="Foo" table="foos" check="bar < 100.0">
        ...
        <property name="bar" type="float"/>
    </class>

The following table summarizes these optional attributes.

Table 22.1. Summary

Attribute | Values | Interpretation
--- | --- | ---
`length` | number | column length
`precision` | number | column decimal precision
`scale` | number | column decimal scale
`not-null` | `true|false` | specifies that the column should be non-nullable
`unique` | `true|false` | specifies that the column should have a unique constraint
`index` | `index_name` | specifies the name of a (multi-column) index
`unique-key` | `unique_key_name` | specifies the name of a multi-column unique constraint
`foreign-key` | `foreign_key_name` | specifies the name of the foreign key constraint generated for an association, for a `<one-to-one>`, `<many-to-one>`, `<key>`, or `<many-to-many>` mapping element. Note that `inverse="true"` sides will not be considered by `SchemaExport`.
`sql-type` | `SQL column type` | overrides the default column type (attribute of `<column>` element only)
`default` | SQL expression | specify a default value for the column
`check` | SQL expression | create an SQL check constraint on either column or table

The `<comment>` element allows you to specify comments for the generated schema.

    <class name="Customer" table="CurCust">
        <comment>Current customers only</comment>
        ...
    </class>

    <property name="balance">
        <column name="bal">
            <comment>Balance in USD</comment>
        </column>
    </property>

This results in a `comment on table` or `comment on column` statement in the generated DDL where supported.

### 22.1.2. Running the tool

The `SchemaExport` tool writes a DDL script to standard out and/or executes the DDL statements.

The following table displays the `SchemaExport` command line options

`java -cp` hibernate_classpaths `org.hibernate.tool.hbm2ddl.SchemaExport` options mapping_files

Table 22.2. `SchemaExport` Command Line Options

Option | Description
--- | ---
`--quiet` | do not output the script to stdout
`--drop` | only drop the tables
`--create` | only create the tables
`--text` | do not export to the database
`--output=my_schema.ddl` | output the ddl script to a file
`--naming=eg.MyNamingStrategy` | select a `NamingStrategy`
`--config=hibernate.cfg.xml` | read Hibernate configuration from an XML file
`--properties=hibernate.properties` | read database properties from a file
`--format` | format the generated SQL nicely in the script
`--delimiter=;` | set an end of line delimiter for the script

You can even embed `SchemaExport` in your application:

    Configuration cfg = ....;
    new SchemaExport(cfg).create(false, true);

### 22.1.3. Properties

Database properties can be specified:

- as system properties with `-D`<property>

- in `hibernate.properties`

- in a named properties file with `--properties`

The needed properties are:

Table 22.3. SchemaExport Connection Properties

Property Name | Description
--- | ---
`hibernate.connection.driver_class` | jdbc driver class
`hibernate.connection.url` | jdbc url
`hibernate.connection.username` | database user
`hibernate.connection.password` | user password
`hibernate.dialect` | dialect

### 22.1.4. Using Ant

You can call `SchemaExport` from your Ant build script:

    <target name="schemaexport">
        <taskdef name="schemaexport"
            classname="org.hibernate.tool.hbm2ddl.SchemaExportTask"
            classpathref="class.path"/>

        <schemaexport
            properties="hibernate.properties"
            quiet="no"
            text="no"
            drop="no"
            delimiter=";"
            output="schema-export.sql">
            <fileset dir="src">
                <include name="**/*.hbm.xml"/>
            </fileset>
        </schemaexport>
    </target>

### 22.1.5. Incremental schema updates

The `SchemaUpdate` tool will update an existing schema with "incremental" changes. The `SchemaUpdate` depends upon the JDBC metadata API and, as such, will not work with all JDBC drivers.

`java -cp` hibernate_classpaths `org.hibernate.tool.hbm2ddl.SchemaUpdate` options mapping_files

Table 22.4. `SchemaUpdate` Command Line Options

Option | Description
--- | ---
`--quiet` | do not output the script to stdout
`--text` | do not export the script to the database
`--naming=eg.MyNamingStrategy` | select a `NamingStrategy`
`--properties=hibernate.properties` | read database properties from a file
`--config=hibernate.cfg.xml` | specify a `.cfg.xml` file

You can embed `SchemaUpdate` in your application:

    Configuration cfg = ....;
    new SchemaUpdate(cfg).execute(false);

### 22.1.6. Using Ant for incremental schema updates

You can call `SchemaUpdate` from the Ant script:

    <target name="schemaupdate">
        <taskdef name="schemaupdate"
            classname="org.hibernate.tool.hbm2ddl.SchemaUpdateTask"
            classpathref="class.path"/>

        <schemaupdate
            properties="hibernate.properties"
            quiet="no">
            <fileset dir="src">
                <include name="**/*.hbm.xml"/>
            </fileset>
        </schemaupdate>
    </target>

### 22.1.7. Schema validation

The `SchemaValidator` tool will validate that the existing database schema "matches" your mapping documents. The `SchemaValidator` depends heavily upon the JDBC metadata API and, as such, will not work with all JDBC drivers. This tool is extremely useful for testing.

`java -cp` hibernate_classpaths `org.hibernate.tool.hbm2ddl.SchemaValidator` options mapping_files

The following table displays the `SchemaValidator` command line options:

Table 22.5. `SchemaValidator` Command Line Options

Option | Description
--- | ---
`--naming=eg.MyNamingStrategy` | select a `NamingStrategy`
`--properties=hibernate.properties` | read database properties from a file
`--config=hibernate.cfg.xml` | specify a `.cfg.xml` file

You can embed `SchemaValidator` in your application:

    Configuration cfg = ....;
    new SchemaValidator(cfg).validate();

### 22.1.8. Using Ant for schema validation

You can call `SchemaValidator` from the Ant script:

    <target name="schemavalidate">
        <taskdef name="schemavalidator"
            classname="org.hibernate.tool.hbm2ddl.SchemaValidatorTask"
            classpathref="class.path"/>

        <schemavalidator
            properties="hibernate.properties">
            <fileset dir="src">
                <include name="**/*.hbm.xml"/>
            </fileset>
        </schemavalidator>
    </target>

## Chapter 23. Additional modules

Hibernate Core also offers integration with some external modules/projects. This includes Hibernate Validator the reference implementation of Bean Validation (JSR 303) and Hibernate Search.

## 23.1. Bean Validation

Bean Validation standardizes how to define and declare domain model level constraints. You can, for example, express that a property should never be null, that the account balance should be strictly positive, etc. These domain model constraints are declared in the bean itself by annotating its properties. Bean Validation can then read them and check for constraint violations. The validation mechanism can be executed in different layers in your application without having to duplicate any of these rules (presentation layer, data access layer). Following the DRY principle, Bean Validation and its reference implementation Hibernate Validator has been designed for that purpose.

The integration between Hibernate and Bean Validation works at two levels. First, it is able to check in-memory instances of a class for constraint violations. Second, it can apply the constraints to the Hibernate metamodel and incorporate them into the generated database schema.

Each constraint annotation is associated to a validator implementation responsible for checking the constraint on the entity instance. A validator can also (optionally) apply the constraint to the Hibernate metamodel, allowing Hibernate to generate DDL that expresses the constraint. With the appropriate event listener, you can execute the checking operation on inserts, updates and deletes done by Hibernate.

When checking instances at runtime, Hibernate Validator returns information about constraint violations in a set of `ConstraintViolation`s. Among other information, the `ConstraintViolation` contains an error description message that can embed the parameter values bundle with the annotation (eg. size limit), and message strings that may be externalized to a `ResourceBundle`.

### 23.1.1. Adding Bean Validation

To enable Hibernate's Bean Validation integration, simply add a Bean Validation provider (preferably Hibernate Validation 4) on your classpath.

### 23.1.2. Configuration

By default, no configuration is necessary.

The `Default` group is validated on entity insert and update and the database model is updated accordingly based on the `Default` group as well.

You can customize the Bean Validation integration by setting the validation mode. Use the `javax.persistence.validation.mode` property and set it up for example in your `persistence.xml` file or your `hibernate.cfg.xml` file. Several options are possible:

- `auto` (default): enable integration between Bean Validation and Hibernate (callback and ddl generation) only if Bean Validation is present in the classpath.

- `none`: disable all integration between Bean Validation and Hibernate

- `callback`: only validate entities when they are either inserted, updated or deleted. An exception is raised if no Bean Validation provider is present in the classpath.

- `ddl`: only apply constraints to the database schema when generated by Hibernate. An exception is raised if no Bean Validation provider is present in the classpath. This value is not defined by the Java Persistence spec and is specific to Hibernate.

## Note

You can use both `callback` and `ddl` together by setting the property to `callback, dll`

    <persistence ...>

      <persistence-unit ...>

        ...

        <properties>

          <property name="javax.persistence.validation.mode"

                    value="callback, ddl"/>

        </properties>

      </persistence-unit>

    </persistence>

This is equivalent to `auto` except that if no Bean Validation provider is present, an exception is raised.

If you want to validate different groups during insertion, update and deletion, use:

- `javax.persistence.validation.group.pre-persist`: groups validated when an entity is about to be persisted (default to `Default`)

- `javax.persistence.validation.group.pre-update`: groups validated when an entity is about to be updated (default to `Default`)

- `javax.persistence.validation.group.pre-remove`: groups validated when an entity is about to be deleted (default to no group)

- `org.hibernate.validator.group.ddl`: groups considered when applying constraints on the database schema (default to `Default`)

Each property accepts the fully qualified class names of the groups validated separated by a comma (,)

Example 23.1. Using custom groups for validation

    <persistence ...>

      <persistence-unit ...>

        ...

        <properties>

          <property name="javax.persistence.validation.group.pre-update"

                    value="javax.validation.group.Default, com.acme.group.Strict"/>

          <property name="javax.persistence.validation.group.pre-remove"

                    value="com.acme.group.OnDelete"/>

          <property name="org.hibernate.validator.group.ddl"

                    value="com.acme.group.DDL"/>

        </properties>

      </persistence-unit>

    </persistence>

## Note

You can set these properties in `hibernate.cfg.xml`, `hibernate.properties` or programmatically.

### 23.1.3. Catching violations

If an entity is found to be invalid, the list of constraint violations is propagated by the `ConstraintViolationException` which exposes the set of `ConstraintViolation`s.

This exception is wrapped in a `RollbackException` when the violation happens at commit time. Otherwise the `ConstraintViolationException` is returned (for example when calling `flush()`. Note that generally, catchable violations are validated at a higher level (for example in Seam / JSF 2 via the JSF - Bean Validation integration or in your business layer by explicitly calling Bean Validation).

An application code will rarely be looking for a `ConstraintViolationException` raised by Hibernate. This exception should be treated as fatal and the persistence context should be discarded (`EntityManager` or `Session`).

### 23.1.4. Database schema

Hibernate uses Bean Validation constraints to generate an accurate database schema:

- `@NotNull` leads to a not null column (unless it conflicts with components or table inheritance)

- `@Size.max` leads to a `varchar(max)` definition for Strings

- `@Min`, `@Max` lead to column checks (like `value <= max`)

- `@Digits` leads to the definition of precision and scale (ever wondered which is which? It's easy now with `@Digits` :) )

These constraints can be declared directly on the entity properties or indirectly by using constraint composition.

For more information check the Hibernate Validator reference documentation at http://docs.jboss.org/hibernate/stable/validator/reference/en-US/html/

## 23.2. Hibernate Search

### 23.2.1. Description

Full text search engines like Apache Lucene are a very powerful technology to bring free text/efficient queries to applications. If suffers several mismatches when dealing with a object domain model (keeping the index up to date, mismatch between the index structure and the domain model, querying mismatch...) Hibernate Search indexes your domain model thanks to a few annotations, takes care of the database / index synchronization and brings you back regular managed objects from free text queries. Hibernate Search is using Apache Lucene under the covers.

### 23.2.2. Integration with Hibernate Annotations

Hibernate Search integrates with Hibernate Core transparently provided that the Hibernate Search jar is present on the classpath. If you do not wish to automatically register Hibernate Search event listeners, you can set `hibernate.search.autoregister_listeners` to false. Such a need is very uncommon and not recommended.

Check the Hibernate Search reference documentation ( http://docs.jboss.org/hibernate/stable/validator/reference/en-US/html/ ) for more information.

## Chapter 24. Example: Parent/Child

One of the first things that new users want to do with Hibernate is to model a parent/child type relationship. There are two different approaches to this. The most convenient approach, especially for new users, is to model both `Parent` and `Child` as entity classes with a `<one-to-many>` association from `Parent` to `Child`. The alternative approach is to declare the `Child` as a `<composite-element>`. The default semantics of a one-to-many association in Hibernate are much less close to the usual semantics of a parent/child relationship than those of a composite element mapping. We will explain how to use a bidirectional one-to-many association with cascades to model a parent/child relationship efficiently and elegantly.

## 24.1. A note about collections

Hibernate collections are considered to be a logical part of their owning entity and not of the contained entities. Be aware that this is a critical distinction that has the following consequences:

- When you remove/add an object from/to a collection, the version number of the collection owner is incremented.

- If an object that was removed from a collection is an instance of a value type (e.g. a composite element), that object will cease to be persistent and its state will be completely removed from the database. Likewise, adding a value type instance to the collection will cause its state to be immediately persistent.

- Conversely, if an entity is removed from a collection (a one-to-many or many-to-many association), it will not be deleted by default. This behavior is completely consistent; a change to the internal state of another entity should not cause the associated entity to vanish. Likewise, adding an entity to a collection does not cause that entity to become persistent, by default.

Adding an entity to a collection, by default, merely creates a link between the two entities. Removing the entity will remove the link. This is appropriate for all sorts of cases. However, it is not appropriate in the case of a parent/child relationship. In this case, the life of the child is bound to the life cycle of the parent.

## 24.2. Bidirectional one-to-many

Suppose we start with a simple `<one-to-many>` association from `Parent` to `Child`.

    <set name="children">
        <key column="parent_id"/>
        <one-to-many class="Child"/>
    </set>

If we were to execute the following code:

    Parent p = .....;
    Child c = new Child();
    p.getChildren().add(c);
    session.save(c);
    session.flush();

Hibernate would issue two SQL statements:

- an `INSERT` to create the record for `c`

- an `UPDATE` to create the link from `p` to `c`

This is not only inefficient, but also violates any `NOT NULL` constraint on the `parent_id` column. You can fix the nullability constraint violation by specifying `not-null="true"` in the collection mapping:

    <set name="children">
        <key column="parent_id" not-null="true"/>
        <one-to-many class="Child"/>
    </set>

However, this is not the recommended solution.

The underlying cause of this behavior is that the link (the foreign key `parent_id`) from `p` to `c` is not considered part of the state of the `Child` object and is therefore not created in the `INSERT`. The solution is to make the link part of the `Child` mapping.

    <many-to-one name="parent" column="parent_id" not-null="true"/>

You also need to add the `parent` property to the `Child` class.

Now that the `Child` entity is managing the state of the link, we tell the collection not to update the link. We use the `inverse` attribute to do this:

    <set name="children" inverse="true">
        <key column="parent_id"/>
        <one-to-many class="Child"/>
    </set>

The following code would be used to add a new `Child`:

    Parent p = (Parent) session.load(Parent.class, pid);
    Child c = new Child();
    c.setParent(p);
    p.getChildren().add(c);
    session.save(c);
    session.flush();

Only one SQL `INSERT` would now be issued.

You could also create an `addChild()` method of `Parent`.

    public void addChild(Child c) {
        c.setParent(this);
        children.add(c);
    }

The code to add a `Child` looks like this:

    Parent p = (Parent) session.load(Parent.class, pid);
    Child c = new Child();
    p.addChild(c);
    session.save(c);
    session.flush();

## 24.3. Cascading life cycle

You can address the frustrations of the explicit call to `save()` by using cascades.

    <set name="children" inverse="true" cascade="all">
        <key column="parent_id"/>
        <one-to-many class="Child"/>
    </set>

This simplifies the code above to:

    Parent p = (Parent) session.load(Parent.class, pid);
    Child c = new Child();
    p.addChild(c);
    session.flush();

Similarly, we do not need to iterate over the children when saving or deleting a `Parent`. The following removes `p` and all its children from the database.

    Parent p = (Parent) session.load(Parent.class, pid);
    session.delete(p);
    session.flush();

However, the following code:

    Parent p = (Parent) session.load(Parent.class, pid);
    Child c = (Child) p.getChildren().iterator().next();
    p.getChildren().remove(c);
    c.setParent(null);
    session.flush();

will not remove `c` from the database. In this case, it will only remove the link to `p` and cause a `NOT NULL` constraint violation. You need to explicitly `delete()` the `Child`.

    Parent p = (Parent) session.load(Parent.class, pid);
    Child c = (Child) p.getChildren().iterator().next();
    p.getChildren().remove(c);
    session.delete(c);
    session.flush();

In our case, a `Child` cannot exist without its parent. So if we remove a `Child` from the collection, we do want it to be deleted. To do this, we must use `cascade="all-delete-orphan"`.

    <set name="children" inverse="true" cascade="all-delete-orphan">
        <key column="parent_id"/>
        <one-to-many class="Child"/>
    </set>

Even though the collection mapping specifies `inverse="true"`, cascades are still processed by iterating the collection elements. If you need an object be saved, deleted or updated by cascade, you must add it to the collection. It is not enough to simply call `setParent()`.

## 24.4. Cascades and `unsaved-value`

Suppose we loaded up a `Parent` in one `Session`, made some changes in a UI action and wanted to persist these changes in a new session by calling `update()`. The `Parent` will contain a collection of children and, since the cascading update is enabled, Hibernate needs to know which children are newly instantiated and which represent existing rows in the database. We will also assume that both `Parent` and `Child` have generated identifier properties of type `Long`. Hibernate will use the identifier and version/timestamp property value to determine which of the children are new. (See Section 11.7, “Automatic state detection”.) In Hibernate3, it is no longer necessary to specify an `unsaved-value` explicitly.

The following code will update `parent` and `child` and insert `newChild`:

    //parent and child were both loaded in a previous session
    parent.addChild(child);
    Child newChild = new Child();
    parent.addChild(newChild);
    session.update(parent);
    session.flush();

This may be suitable for the case of a generated identifier, but what about assigned identifiers and composite identifiers? This is more difficult, since Hibernate cannot use the identifier property to distinguish between a newly instantiated object, with an identifier assigned by the user, and an object loaded in a previous session. In this case, Hibernate will either use the timestamp or version property, or will actually query the second-level cache or, worst case, the database, to see if the row exists.

## 24.5. Conclusion

The sections we have just covered can be a bit confusing. However, in practice, it all works out nicely. Most Hibernate applications use the parent/child pattern in many places.

We mentioned an alternative in the first paragraph. None of the above issues exist in the case of `<composite-element>` mappings, which have exactly the semantics of a parent/child relationship. Unfortunately, there are two big limitations with composite element classes: composite elements cannot own collections and they should not be the child of any entity other than the unique parent.

## Chapter 25. Example: Weblog Application

## 25.1. Persistent Classes

The persistent classes here represent a weblog and an item posted in a weblog. They are to be modelled as a standard parent/child relationship, but we will use an ordered bag, instead of a set:

    package eg;

    import java.util.List;

    public class Blog {
        private Long _id;
        private String _name;
        private List _items;

        public Long getId() {
            return _id;
        }
        public List getItems() {
            return _items;
        }
        public String getName() {
            return _name;
        }
        public void setId(Long long1) {
            _id = long1;
        }
        public void setItems(List list) {
            _items = list;
        }
        public void setName(String string) {
            _name = string;
        }
    }

    package eg;

    import java.text.DateFormat;
    import java.util.Calendar;

    public class BlogItem {
        private Long _id;
        private Calendar _datetime;
        private String _text;
        private String _title;
        private Blog _blog;

        public Blog getBlog() {
            return _blog;
        }
        public Calendar getDatetime() {
            return _datetime;
        }
        public Long getId() {
            return _id;
        }
        public String getText() {
            return _text;
        }
        public String getTitle() {
            return _title;
        }
        public void setBlog(Blog blog) {
            _blog = blog;
        }
        public void setDatetime(Calendar calendar) {
            _datetime = calendar;
        }
        public void setId(Long long1) {
            _id = long1;
        }
        public void setText(String string) {
            _text = string;
        }
        public void setTitle(String string) {
            _title = string;
        }
    }

## 25.2. Hibernate Mappings

The XML mappings are now straightforward. For example:

    <?xml version="1.0"?>
    <!DOCTYPE hibernate-mapping PUBLIC
        "-//Hibernate/Hibernate Mapping DTD 3.0//EN"
        "http://www.hibernate.org/dtd/hibernate-mapping-3.0.dtd">

    <hibernate-mapping package="eg">

        <class
            name="Blog"
            table="BLOGS">

            <id
                name="id"
                column="BLOG_ID">

                <generator class="native"/>

            </id>

            <property
                name="name"
                column="NAME"
                not-null="true"
                unique="true"/>

            <bag
                name="items"
                inverse="true"
                order-by="DATE_TIME"
                cascade="all">

                <key column="BLOG_ID"/>
                <one-to-many class="BlogItem"/>

            </bag>

        </class>

    </hibernate-mapping>

    <?xml version="1.0"?>
    <!DOCTYPE hibernate-mapping PUBLIC
        "-//Hibernate/Hibernate Mapping DTD 3.0//EN"
        "http://www.hibernate.org/dtd/hibernate-mapping-3.0.dtd">

    <hibernate-mapping package="eg">

        <class
            name="BlogItem"
            table="BLOG_ITEMS"
            dynamic-update="true">

            <id
                name="id"
                column="BLOG_ITEM_ID">

                <generator class="native"/>

            </id>

            <property
                name="title"
                column="TITLE"
                not-null="true"/>

            <property
                name="text"
                column="TEXT"
                not-null="true"/>

            <property
                name="datetime"
                column="DATE_TIME"
                not-null="true"/>

            <many-to-one
                name="blog"
                column="BLOG_ID"
                not-null="true"/>

        </class>

    </hibernate-mapping>

## 25.3. Hibernate Code

The following class demonstrates some of the kinds of things we can do with these classes using Hibernate:

    package eg;

    import java.util.ArrayList;
    import java.util.Calendar;
    import java.util.Iterator;
    import java.util.List;

    import org.hibernate.HibernateException;
    import org.hibernate.Query;
    import org.hibernate.Session;
    import org.hibernate.SessionFactory;
    import org.hibernate.Transaction;
    import org.hibernate.cfg.Configuration;
    import org.hibernate.tool.hbm2ddl.SchemaExport;

    public class BlogMain {

        private SessionFactory _sessions;

        public void configure() throws HibernateException {
            _sessions = new Configuration()
                .addClass(Blog.class)
                .addClass(BlogItem.class)
                .buildSessionFactory();
        }

        public void exportTables() throws HibernateException {
            Configuration cfg = new Configuration()
                .addClass(Blog.class)
                .addClass(BlogItem.class);
            new SchemaExport(cfg).create(true, true);
        }

        public Blog createBlog(String name) throws HibernateException {

            Blog blog = new Blog();
            blog.setName(name);
            blog.setItems( new ArrayList() );

            Session session = _sessions.openSession();
            Transaction tx = null;
            try {
                tx = session.beginTransaction();
                session.persist(blog);
                tx.commit();
            }
            catch (HibernateException he) {
                if (tx!=null) tx.rollback();
                throw he;
            }
            finally {
                session.close();
            }
            return blog;
        }

        public BlogItem createBlogItem(Blog blog, String title, String text)
                            throws HibernateException {

            BlogItem item = new BlogItem();
            item.setTitle(title);
            item.setText(text);
            item.setBlog(blog);
            item.setDatetime( Calendar.getInstance() );
            blog.getItems().add(item);

            Session session = _sessions.openSession();
            Transaction tx = null;
            try {
                tx = session.beginTransaction();
                session.update(blog);
                tx.commit();
            }
            catch (HibernateException he) {
                if (tx!=null) tx.rollback();
                throw he;
            }
            finally {
                session.close();
            }
            return item;
        }

        public BlogItem createBlogItem(Long blogid, String title, String text)
                            throws HibernateException {

            BlogItem item = new BlogItem();
            item.setTitle(title);
            item.setText(text);
            item.setDatetime( Calendar.getInstance() );

            Session session = _sessions.openSession();
            Transaction tx = null;
            try {
                tx = session.beginTransaction();
                Blog blog = (Blog) session.load(Blog.class, blogid);
                item.setBlog(blog);
                blog.getItems().add(item);
                tx.commit();
            }
            catch (HibernateException he) {
                if (tx!=null) tx.rollback();
                throw he;
            }
            finally {
                session.close();
            }
            return item;
        }

        public void updateBlogItem(BlogItem item, String text)
                        throws HibernateException {

            item.setText(text);

            Session session = _sessions.openSession();
            Transaction tx = null;
            try {
                tx = session.beginTransaction();
                session.update(item);
                tx.commit();
            }
            catch (HibernateException he) {
                if (tx!=null) tx.rollback();
                throw he;
            }
            finally {
                session.close();
            }
        }

        public void updateBlogItem(Long itemid, String text)
                        throws HibernateException {

            Session session = _sessions.openSession();
            Transaction tx = null;
            try {
                tx = session.beginTransaction();
                BlogItem item = (BlogItem) session.load(BlogItem.class, itemid);
                item.setText(text);
                tx.commit();
            }
            catch (HibernateException he) {
                if (tx!=null) tx.rollback();
                throw he;
            }
            finally {
                session.close();
            }
        }

        public List listAllBlogNamesAndItemCounts(int max)
                        throws HibernateException {

            Session session = _sessions.openSession();
            Transaction tx = null;
            List result = null;
            try {
                tx = session.beginTransaction();
                Query q = session.createQuery(
                    "select blog.id, blog.name, count(blogItem) " +
                    "from Blog as blog " +
                    "left outer join blog.items as blogItem " +
                    "group by blog.name, blog.id " +
                    "order by max(blogItem.datetime)"
                );
                q.setMaxResults(max);
                result = q.list();
                tx.commit();
            }
            catch (HibernateException he) {
                if (tx!=null) tx.rollback();
                throw he;
            }
            finally {
                session.close();
            }
            return result;
        }

        public Blog getBlogAndAllItems(Long blogid)
                        throws HibernateException {

            Session session = _sessions.openSession();
            Transaction tx = null;
            Blog blog = null;
            try {
                tx = session.beginTransaction();
                Query q = session.createQuery(
                    "from Blog as blog " +
                    "left outer join fetch blog.items " +
                    "where blog.id = :blogid"
                );
                q.setParameter("blogid", blogid);
                blog  = (Blog) q.uniqueResult();
                tx.commit();
            }
            catch (HibernateException he) {
                if (tx!=null) tx.rollback();
                throw he;
            }
            finally {
                session.close();
            }
            return blog;
        }

        public List listBlogsAndRecentItems() throws HibernateException {

            Session session = _sessions.openSession();
            Transaction tx = null;
            List result = null;
            try {
                tx = session.beginTransaction();
                Query q = session.createQuery(
                    "from Blog as blog " +
                    "inner join blog.items as blogItem " +
                    "where blogItem.datetime > :minDate"
                );

                Calendar cal = Calendar.getInstance();
                cal.roll(Calendar.MONTH, false);
                q.setCalendar("minDate", cal);

                result = q.list();
                tx.commit();
            }
            catch (HibernateException he) {
                if (tx!=null) tx.rollback();
                throw he;
            }
            finally {
                session.close();
            }
            return result;
        }
    }

## Chapter 26. Example: Various Mappings

This chapters explores some more complex association mappings.

## 26.1. Employer/Employee

The following model of the relationship between `Employer` and `Employee` uses an entity class (`Employment`) to represent the association. You can do this when there might be more than one period of employment for the same two parties. Components are used to model monetary values and employee names.

Here is a possible mapping document:

    <hibernate-mapping>

        <class name="Employer" table="employers">
            <id name="id">
                <generator class="sequence">
                    <param name="sequence">employer_id_seq</param>
                </generator>
            </id>
            <property name="name"/>
        </class>

        <class name="Employment" table="employment_periods">

            <id name="id">
                <generator class="sequence">
                    <param name="sequence">employment_id_seq</param>
                </generator>
            </id>
            <property name="startDate" column="start_date"/>
            <property name="endDate" column="end_date"/>

            <component name="hourlyRate" class="MonetaryAmount">
                <property name="amount">
                    <column name="hourly_rate" sql-type="NUMERIC(12, 2)"/>
                </property>
                <property name="currency" length="12"/>
            </component>

            <many-to-one name="employer" column="employer_id" not-null="true"/>
            <many-to-one name="employee" column="employee_id" not-null="true"/>

        </class>

        <class name="Employee" table="employees">
            <id name="id">
                <generator class="sequence">
                    <param name="sequence">employee_id_seq</param>
                </generator>
            </id>
            <property name="taxfileNumber"/>
            <component name="name" class="Name">
                <property name="firstName"/>
                <property name="initial"/>
                <property name="lastName"/>
            </component>
        </class>

    </hibernate-mapping>

Here is the table schema generated by `SchemaExport`.

    create table employers (
        id BIGINT not null,
        name VARCHAR(255),
        primary key (id)
    )

    create table employment_periods (
        id BIGINT not null,
        hourly_rate NUMERIC(12, 2),
        currency VARCHAR(12),
        employee_id BIGINT not null,
        employer_id BIGINT not null,
        end_date TIMESTAMP,
        start_date TIMESTAMP,
        primary key (id)
    )

    create table employees (
        id BIGINT not null,
        firstName VARCHAR(255),
        initial CHAR(1),
        lastName VARCHAR(255),
        taxfileNumber VARCHAR(255),
        primary key (id)
    )

    alter table employment_periods
        add constraint employment_periodsFK0 foreign key (employer_id) references employers
    alter table employment_periods
        add constraint employment_periodsFK1 foreign key (employee_id) references employees
    create sequence employee_id_seq
    create sequence employment_id_seq
    create sequence employer_id_seq

## 26.2. Author/Work

Consider the following model of the relationships between `Work`, `Author` and `Person`. In the example, the relationship between `Work` and `Author` is represented as a many-to-many association and the relationship between `Author` and `Person` is represented as one-to-one association. Another possibility would be to have `Author` extend `Person`.

The following mapping document correctly represents these relationships:

    <hibernate-mapping>

        <class name="Work" table="works" discriminator-value="W">

            <id name="id" column="id">
                <generator class="native"/>
            </id>
            <discriminator column="type" type="character"/>

            <property name="title"/>
            <set name="authors" table="author_work">
                <key column name="work_id"/>
                <many-to-many class="Author" column name="author_id"/>
            </set>

            <subclass name="Book" discriminator-value="B">
                <property name="text"/>
            </subclass>

            <subclass name="Song" discriminator-value="S">
                <property name="tempo"/>
                <property name="genre"/>
            </subclass>

        </class>

        <class name="Author" table="authors">

            <id name="id" column="id">
                <!-- The Author must have the same identifier as the Person -->
                <generator class="assigned"/>
            </id>

            <property name="alias"/>
            <one-to-one name="person" constrained="true"/>

            <set name="works" table="author_work" inverse="true">
                <key column="author_id"/>
                <many-to-many class="Work" column="work_id"/>
            </set>

        </class>

        <class name="Person" table="persons">
            <id name="id" column="id">
                <generator class="native"/>
            </id>
            <property name="name"/>
        </class>

    </hibernate-mapping>

There are four tables in this mapping: `works`, `authors` and `persons` hold work, author and person data respectively. `author_work` is an association table linking authors to works. Here is the table schema, as generated by `SchemaExport`:

    create table works (
        id BIGINT not null generated by default as identity,
        tempo FLOAT,
        genre VARCHAR(255),
        text INTEGER,
        title VARCHAR(255),
        type CHAR(1) not null,
        primary key (id)
    )

    create table author_work (
        author_id BIGINT not null,
        work_id BIGINT not null,
        primary key (work_id, author_id)
    )

    create table authors (
        id BIGINT not null generated by default as identity,
        alias VARCHAR(255),
        primary key (id)
    )

    create table persons (
        id BIGINT not null generated by default as identity,
        name VARCHAR(255),
        primary key (id)
    )

    alter table authors
        add constraint authorsFK0 foreign key (id) references persons
    alter table author_work
        add constraint author_workFK0 foreign key (author_id) references authors
    alter table author_work
        add constraint author_workFK1 foreign key (work_id) references works

## 26.3. Customer/Order/Product

In this section we consider a model of the relationships between `Customer`, `Order`, `Line Item` and `Product`. There is a one-to-many association between `Customer` and `Order`, but how can you represent `Order` / `LineItem` / `Product`? In the example, `LineItem` is mapped as an association class representing the many-to-many association between `Order` and `Product`. In Hibernate this is called a composite element.

The mapping document will look like this:

    <hibernate-mapping>

        <class name="Customer" table="customers">
            <id name="id">
                <generator class="native"/>
            </id>
            <property name="name"/>
            <set name="orders" inverse="true">
                <key column="customer_id"/>
                <one-to-many class="Order"/>
            </set>
        </class>

        <class name="Order" table="orders">
            <id name="id">
                <generator class="native"/>
            </id>
            <property name="date"/>
            <many-to-one name="customer" column="customer_id"/>
            <list name="lineItems" table="line_items">
                <key column="order_id"/>
                <list-index column="line_number"/>
                <composite-element class="LineItem">
                    <property name="quantity"/>
                    <many-to-one name="product" column="product_id"/>
                </composite-element>
            </list>
        </class>

        <class name="Product" table="products">
            <id name="id">
                <generator class="native"/>
            </id>
            <property name="serialNumber"/>
        </class>

    </hibernate-mapping>

`customers`, `orders`, `line_items` and `products` hold customer, order, order line item and product data respectively. `line_items` also acts as an association table linking orders with products.

    create table customers (
        id BIGINT not null generated by default as identity,
        name VARCHAR(255),
        primary key (id)
    )

    create table orders (
        id BIGINT not null generated by default as identity,
        customer_id BIGINT,
        date TIMESTAMP,
        primary key (id)
    )

    create table line_items (
        line_number INTEGER not null,
        order_id BIGINT not null,
        product_id BIGINT,
        quantity INTEGER,
        primary key (order_id, line_number)
    )

    create table products (
        id BIGINT not null generated by default as identity,
        serialNumber VARCHAR(255),
        primary key (id)
    )

    alter table orders
        add constraint ordersFK0 foreign key (customer_id) references customers
    alter table line_items
        add constraint line_itemsFK0 foreign key (product_id) references products
    alter table line_items
        add constraint line_itemsFK1 foreign key (order_id) references orders

## 26.4. Miscellaneous example mappings

These examples are available from the Hibernate test suite. You will find many other useful example mappings there by searching in the `test` folder of the Hibernate distribution.

### 26.4.1. "Typed" one-to-one association

    <class name="Person">
        <id name="name"/>
        <one-to-one name="address"
                cascade="all">
            <formula>name</formula>
            <formula>'HOME'</formula>
        </one-to-one>
        <one-to-one name="mailingAddress"
                cascade="all">
            <formula>name</formula>
            <formula>'MAILING'</formula>
        </one-to-one>
    </class>

    <class name="Address" batch-size="2"
            check="addressType in ('MAILING', 'HOME', 'BUSINESS')">
        <composite-id>
            <key-many-to-one name="person"
                    column="personName"/>
            <key-property name="type"
                    column="addressType"/>
        </composite-id>
        <property name="street" type="text"/>
        <property name="state"/>
        <property name="zip"/>
    </class>

### 26.4.2. Composite key example

    <class name="Customer">

        <id name="customerId"
            length="10">
            <generator class="assigned"/>
        </id>

        <property name="name" not-null="true" length="100"/>
        <property name="address" not-null="true" length="200"/>

        <list name="orders"
                inverse="true"
                cascade="save-update">
            <key column="customerId"/>
            <index column="orderNumber"/>
            <one-to-many class="Order"/>
        </list>

    </class>

    <class name="Order" table="CustomerOrder" lazy="true">
        <synchronize table="LineItem"/>
        <synchronize table="Product"/>

        <composite-id name="id"
                class="Order$Id">
            <key-property name="customerId" length="10"/>
            <key-property name="orderNumber"/>
        </composite-id>

        <property name="orderDate"
                type="calendar_date"
                not-null="true"/>

        <property name="total">
            <formula>
                ( select sum(li.quantity*p.price)
                from LineItem li, Product p
                where li.productId = p.productId
                    and li.customerId = customerId
                    and li.orderNumber = orderNumber )
            </formula>
        </property>

        <many-to-one name="customer"
                column="customerId"
                insert="false"
                update="false"
                not-null="true"/>

        <bag name="lineItems"
                fetch="join"
                inverse="true"
                cascade="save-update">
            <key>
                <column name="customerId"/>
                <column name="orderNumber"/>
            </key>
            <one-to-many class="LineItem"/>
        </bag>

    </class>

    <class name="LineItem">

        <composite-id name="id"
                class="LineItem$Id">
            <key-property name="customerId" length="10"/>
            <key-property name="orderNumber"/>
            <key-property name="productId" length="10"/>
        </composite-id>

        <property name="quantity"/>

        <many-to-one name="order"
                insert="false"
                update="false"
                not-null="true">
            <column name="customerId"/>
            <column name="orderNumber"/>
        </many-to-one>

        <many-to-one name="product"
                insert="false"
                update="false"
                not-null="true"
                column="productId"/>

    </class>

    <class name="Product">
        <synchronize table="LineItem"/>

        <id name="productId"
            length="10">
            <generator class="assigned"/>
        </id>

        <property name="description"
            not-null="true"
            length="200"/>
        <property name="price" length="3"/>
        <property name="numberAvailable"/>

        <property name="numberOrdered">
            <formula>
                ( select sum(li.quantity)
                from LineItem li
                where li.productId = productId )
            </formula>
        </property>

    </class>

### 26.4.3. Many-to-many with shared composite key attribute

    <class name="User" table="`User`">
        <composite-id>
            <key-property name="name"/>
            <key-property name="org"/>
        </composite-id>
        <set name="groups" table="UserGroup">
            <key>
                <column name="userName"/>
                <column name="org"/>
            </key>
            <many-to-many class="Group">
                <column name="groupName"/>
                <formula>org</formula>
            </many-to-many>
        </set>
    </class>

    <class name="Group" table="`Group`">
        <composite-id>
            <key-property name="name"/>
            <key-property name="org"/>
        </composite-id>
        <property name="description"/>
        <set name="users" table="UserGroup" inverse="true">
            <key>
                <column name="groupName"/>
                <column name="org"/>
            </key>
            <many-to-many class="User">
                <column name="userName"/>
                <formula>org</formula>
            </many-to-many>
        </set>
    </class>

### 26.4.4. Content based discrimination

    <class name="Person"
        discriminator-value="P">

        <id name="id"
            column="person_id"
            unsaved-value="0">
            <generator class="native"/>
        </id>

        <discriminator
            type="character">
            <formula>
                case
                    when title is not null then 'E'
                    when salesperson is not null then 'C'
                    else 'P'
                end
            </formula>
        </discriminator>

        <property name="name"
            not-null="true"
            length="80"/>

        <property name="sex"
            not-null="true"
            update="false"/>

        <component name="address">
            <property name="address"/>
            <property name="zip"/>
            <property name="country"/>
        </component>

        <subclass name="Employee"
            discriminator-value="E">
                <property name="title"
                    length="20"/>
                <property name="salary"/>
                <many-to-one name="manager"/>
        </subclass>

        <subclass name="Customer"
            discriminator-value="C">
                <property name="comments"/>
                <many-to-one name="salesperson"/>
        </subclass>

    </class>

### 26.4.5. Associations on alternate keys

    <class name="Person">

        <id name="id">
            <generator class="hilo"/>
        </id>

        <property name="name" length="100"/>

        <one-to-one name="address"
            property-ref="person"
            cascade="all"
            fetch="join"/>

        <set name="accounts"
            inverse="true">
            <key column="userId"
                property-ref="userId"/>
            <one-to-many class="Account"/>
        </set>

        <property name="userId" length="8"/>

    </class>

    <class name="Address">

        <id name="id">
            <generator class="hilo"/>
        </id>

        <property name="address" length="300"/>
        <property name="zip" length="5"/>
        <property name="country" length="25"/>
        <many-to-one name="person" unique="true" not-null="true"/>

    </class>

    <class name="Account">
        <id name="accountId" length="32">
            <generator class="uuid"/>
        </id>

        <many-to-one name="user"
            column="userId"
            property-ref="userId"/>

        <property name="type" not-null="true"/>

    </class>

## Chapter 27. Best Practices

Write fine-grained classes and map them using `<component>`:

  Use an `Address` class to encapsulate `street`, `suburb`, `state`, `postcode`. This encourages code reuse and simplifies refactoring.

Declare identifier properties on persistent classes:

  Hibernate makes identifier properties optional. There are a range of reasons why you should use them. We recommend that identifiers be 'synthetic', that is, generated with no business meaning.

Identify natural keys:

  Identify natural keys for all entities, and map them using `<natural-id>`. Implement `equals()` and `hashCode()` to compare the properties that make up the natural key.

Place each class mapping in its own file:

  Do not use a single monolithic mapping document. Map `com.eg.Foo` in the file `com/eg/Foo.hbm.xml`. This makes sense, particularly in a team environment.

Load mappings as resources:

  Deploy the mappings along with the classes they map.

Consider externalizing query strings:

  This is recommended if your queries call non-ANSI-standard SQL functions. Externalizing the query strings to mapping files will make the application more portable.

Use bind variables.

  As in JDBC, always replace non-constant values by "?". Do not use string manipulation to bind a non-constant value in a query. You should also consider using named parameters in queries.

Do not manage your own JDBC connections:

  Hibernate allows the application to manage JDBC connections, but his approach should be considered a last-resort. If you cannot use the built-in connection providers, consider providing your own implementation of `org.hibernate.connection.ConnectionProvider`.

Consider using a custom type:

  Suppose you have a Java type from a library that needs to be persisted but does not provide the accessors needed to map it as a component. You should consider implementing `org.hibernate.UserType`. This approach frees the application code from implementing transformations to/from a Hibernate type.

Use hand-coded JDBC in bottlenecks:

  In performance-critical areas of the system, some kinds of operations might benefit from direct JDBC. Do not assume, however, that JDBC is necessarily faster. Please wait until you know something is a bottleneck. If you need to use direct JDBC, you can open a Hibernate `Session`, wrap your JDBC operation as a `org.hibernate.jdbc.Work` object and using that JDBC connection. This way you can still use the same transaction strategy and underlying connection provider.

Understand `Session` flushing:

  Sometimes the Session synchronizes its persistent state with the database. Performance will be affected if this process occurs too often. You can sometimes minimize unnecessary flushing by disabling automatic flushing, or even by changing the order of queries and other operations within a particular transaction.

In a three tiered architecture, consider using detached objects:

  When using a servlet/session bean architecture, you can pass persistent objects loaded in the session bean to and from the servlet/JSP layer. Use a new session to service each request. Use `Session.merge()` or `Session.saveOrUpdate()` to synchronize objects with the database.

In a two tiered architecture, consider using long persistence contexts:

  Database Transactions have to be as short as possible for best scalability. However, it is often necessary to implement long running application transactions, a single unit-of-work from the point of view of a user. An application transaction might span several client request/response cycles. It is common to use detached objects to implement application transactions. An appropriate alternative in a two tiered architecture, is to maintain a single open persistence contact session for the whole life cycle of the application transaction. Then simply disconnect from the JDBC connection at the end of each request and reconnect at the beginning of the subsequent request. Never share a single session across more than one application transaction or you will be working with stale data.

Do not treat exceptions as recoverable:

  This is more of a necessary practice than a "best" practice. When an exception occurs, roll back the `Transaction` and close the `Session`. If you do not do this, Hibernate cannot guarantee that in-memory state accurately represents the persistent state. For example, do not use `Session.load()` to determine if an instance with the given identifier exists on the database; use `Session.get()` or a query instead.

Prefer lazy fetching for associations:

  Use eager fetching sparingly. Use proxies and lazy collections for most associations to classes that are not likely to be completely held in the second-level cache. For associations to cached classes, where there is an a extremely high probability of a cache hit, explicitly disable eager fetching using `lazy="false"`. When join fetching is appropriate to a particular use case, use a query with a `left join fetch`.

Use the open session in view pattern, or a disciplined assembly phase to avoid problems with unfetched data:

  Hibernate frees the developer from writing tedious Data Transfer Objects (DTO). In a traditional EJB architecture, DTOs serve dual purposes: first, they work around the problem that entity beans are not serializable; second, they implicitly define an assembly phase where all data to be used by the view is fetched and marshalled into the DTOs before returning control to the presentation tier. Hibernate eliminates the first purpose. Unless you are prepared to hold the persistence context (the session) open across the view rendering process, you will still need an assembly phase. Think of your business methods as having a strict contract with the presentation tier about what data is available in the detached objects. This is not a limitation of Hibernate. It is a fundamental requirement of safe transactional data access.

Consider abstracting your business logic from Hibernate:

  Hide Hibernate data-access code behind an interface. Combine the DAO and Thread Local Session patterns. You can even have some classes persisted by handcoded JDBC associated to Hibernate via a `UserType`. This advice is, however, intended for "sufficiently large" applications. It is not appropriate for an application with five tables.

Do not use exotic association mappings:

  Practical test cases for real many-to-many associations are rare. Most of the time you need additional information stored in the "link table". In this case, it is much better to use two one-to-many associations to an intermediate link class. In fact, most associations are one-to-many and many-to-one. For this reason, you should proceed cautiously when using any other association style.

Prefer bidirectional associations:

  Unidirectional associations are more difficult to query. In a large application, almost all associations must be navigable in both directions in queries.

## Chapter 28. Database Portability Considerations

## 28.1. Portability Basics

One of the selling points of Hibernate (and really Object/Relational Mapping as a whole) is the notion of database portability. This could mean an internal IT user migrating from one database vendor to another, or it could mean a framework or deployable application consuming Hibernate to simultaneously target multiple database products by their users. Regardless of the exact scenario, the basic idea is that you want Hibernate to help you run against any number of databases without changes to your code, and ideally without any changes to the mapping metadata.

## 28.2. Dialect

The first line of portability for Hibernate is the dialect, which is a specialization of the `org.hibernate.dialect.Dialect` contract. A dialect encapsulates all the differences in how Hibernate must communicate with a particular database to accomplish some task like getting a sequence value or structuring a SELECT query. Hibernate bundles a wide range of dialects for many of the most popular databases. If you find that your particular database is not among them, it is not terribly difficult to write your own.

## 28.3. Dialect resolution

Originally, Hibernate would always require that users specify which dialect to use. In the case of users looking to simultaneously target multiple databases with their build that was problematic. Generally this required their users to configure the Hibernate dialect or defining their own method of setting that value.

Starting with version 3.2, Hibernate introduced the notion of automatically detecting the dialect to use based on the `java.sql.DatabaseMetaData` obtained from a `java.sql.Connection` to that database. This was much better, expect that this resolution was limited to databases Hibernate know about ahead of time and was in no way configurable or overrideable.

Starting with version 3.3, Hibernate has a fare more powerful way to automatically determine which dialect to should be used by relying on a series of delegates which implement the `org.hibernate.dialect.resolver.DialectResolver` which defines only a single method:

    public Dialect resolveDialect(DatabaseMetaData metaData) throws JDBCConnectionException

The basic contract here is that if the resolver 'understands' the given database metadata then it returns the corresponding Dialect; if not it returns null and the process continues to the next resolver. The signature also identifies `org.hibernate.exception.JDBCConnectionException` as possibly being thrown. A JDBCConnectionException here is interpreted to imply a "non transient" (aka non-recoverable) connection problem and is used to indicate an immediate stop to resolution attempts. All other exceptions result in a warning and continuing on to the next resolver.

The cool part about these resolvers is that users can also register their own custom resolvers which will be processed ahead of the built-in Hibernate ones. This might be useful in a number of different situations: it allows easy integration for auto-detection of dialects beyond those shipped with HIbernate itself; it allows you to specify to use a custom dialect when a particular database is recognized; etc. To register one or more resolvers, simply specify them (seperated by commas, tabs or spaces) using the 'hibernate.dialect_resolvers' configuration setting (see the `DIALECT_RESOLVERS` constant on `org.hibernate.cfg.Environment`).

## 28.4. Identifier generation

When considering portability between databases, another important decision is selecting the identifier generation stratagy you want to use. Originally Hibernate provided the native generator for this purpose, which was intended to select between a sequence, identity, or table strategy depending on the capability of the underlying database. However, an insidious implication of this approach comes about when targtetting some databases which support identity generation and some which do not. identity generation relies on the SQL definition of an IDENTITY (or auto-increment) column to manage the identifier value; it is what is known as a post-insert generation strategy becauase the insert must actually happen before we can know the identifier value. Because Hibernate relies on this identifier value to uniquely reference entities within a persistence context it must then issue the insert immediately when the users requests the entitiy be associated with the session (like via save() e.g.) regardless of current transactional semantics.

## Note

Hibernate was changed slightly once the implication of this was better understood so that the insert is delayed in cases where that is feasible.

The underlying issue is that the actual semanctics of the application itself changes in these cases.

Starting with version 3.2.3, Hibernate comes with a set of enhanced identifier generators targetting portability in a much different way.

## Note

There are specifically 2 bundled enhancedgenerators:

- `org.hibernate.id.enhanced.SequenceStyleGenerator`

- `org.hibernate.id.enhanced.TableGenerator`

The idea behind these generators is to port the actual semantics of the identifer value generation to the different databases. For example, the `org.hibernate.id.enhanced.SequenceStyleGenerator` mimics the behavior of a sequence on databases which do not support sequences by using a table.

## 28.5. Database functions

## Warning

This is an area in Hibernate in need of improvement. In terms of portability concerns, this function handling currently works pretty well from HQL; however, it is quite lacking in all other aspects.

SQL functions can be referenced in many ways by users. However, not all databases support the same set of functions. Hibernate, provides a means of mapping a logical function name to a delegate which knows how to render that particular function, perhaps even using a totally different physical function call.

## Important

Technically this function registration is handled through the `org.hibernate.dialect.function.SQLFunctionRegistry` class which is intended to allow users to provide custom function definitions without having to provide a custom dialect. This specific behavior is not fully completed as of yet.

It is sort of implemented such that users can programatically register functions with the `org.hibernate.cfg.Configuration` and those functions will be recognized for HQL.

## 28.6. Type mappings

This section scheduled for completion at a later date...

## References

[PoEAA] Patterns of Enterprise Application Architecture. 0-321-12742-0. Martin Fowler. Copyright © 2003 Pearson Education, Inc.. Addison-Wesley Publishing Company.

[JPwH] Java Persistence with Hibernate. Second Edition of Hibernate in Action. 1-932394-88-5. http://www.manning.com/bauer2 . Christian Bauer and Gavin King. Copyright © 2007 Manning Publications Co.. Manning Publications Co..