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## Chapter 12. Criteria

Criteria queries offer a type-safe alternative to HQL, JPQL and native-sql queries.

## Important

Hibernate offers an older, legacy `org.hibernate.Criteria` API which should be considered deprecated. No feature development will target those APIs. Eventually, Hibernate-specific criteria features will be ported as extensions to the JPA `javax.persistence.criteria.CriteriaQuery`. For details on the `org.hibernate.Criteria` API, see ???.

This chapter will focus on the JPA APIs for declaring type-safe criteria queries.

Criteria queries are a programmatic, type-safe way to express a query. They are type-safe in terms of using interfaces and classes to represent various structural parts of a query such as the query itself, or the select clause, or an order-by, etc. They can also be type-safe in terms of referencing attributes as we will see in a bit. Users of the older Hibernate `org.hibernate.Criteria` query API will recognize the general approach, though we believe the JPA API to be superior as it represents a clean look at the lessons learned from that API.

Criteria queries are essentially an object graph, where each part of the graph represents an increasing (as we navigate down this graph) more atomic part of query. The first step in performing a criteria query is building this graph. The `javax.persistence.criteria.CriteriaBuilder` interface is the first thing with which you need to become acquainted to begin using criteria queries. Its role is that of a factory for all the individual pieces of the criteria. You obtain a `javax.persistence.criteria.CriteriaBuilder` instance by calling the `getCriteriaBuilder` method of either `javax.persistence.EntityManagerFactory` or `javax.persistence.EntityManager`.

The next step is to obtain a `javax.persistence.criteria.CriteriaQuery`. This is accomplished using one of the 3 methods on `javax.persistence.criteria.CriteriaBuilder` for this purpose:

    <T> CriteriaQuery<T> createQuery(Class<T> resultClass);
    CriteriaQuery<Tuple> createTupleQuery();
    CriteriaQuery<Object> createQuery();

Each serves a different purpose depending on the expected type of the query results.

## Note

Chapter 6 Criteria API of the JPA Specification already contains a decent amount of reference material pertaining to the various parts of a criteria query. So rather than duplicate all that content here, lets instead look at some of the more widely anticipated usages of the API.

## 12.1. Typed criteria queries

The type of the criteria query (aka the <T>) indicates the expected types in the query result. This might be an entity, an Integer, or any other object.

### 12.1.1. Selecting an entity

This is probably the most common form of query. The application wants to select entity instances.

Example 12.1. Selecting the root entity

    CriteriaQuery<Person> criteria = builder.createQuery( Person.class );
    Root<Person> personRoot = criteria.from( Person.class );
    criteria.select( personRoot );
    criteria.where( builder.equal( personRoot.get( Person_.eyeColor ), "brown" ) );

    List<Person> people = em.createQuery( criteria ).getResultList();
    for ( Person person : people ) {
        ...
    }

The example uses `createQuery` passing in the `Person` class reference as the results of the query will be Person objects.

## Note

The call to the `CriteriaQuery.select` method in this example is unnecessary because personRoot will be the implied selection since we have only a single query root. It was done here only for completeness of an example.

The Person_.eyeColor reference is an example of the static form of JPA metamodel reference. We will use that form exclusively in this chapter. See the documentation for the Hibernate JPA Metamodel Generator for additional details on the JPA static metamodel.

### 12.1.2. Selecting an expression

The simplest form of selecting an expression is selecting a particular attribute from an entity. But this expression might also represent an aggregation, a mathematical operation, etc.

Example 12.2. Selecting an attribute

    CriteriaQuery<Integer> criteria = builder.createQuery( Integer.class );
    Root<Person> personRoot = criteria.from( Person.class );
    criteria.select( personRoot.get( Person_.age ) );
    criteria.where( builder.equal( personRoot.get( Person_.eyeColor ), "brown" ) );

    List<Integer> ages = em.createQuery( criteria ).getResultList();
    for ( Integer age : ages ) {
    	...
    }

In this example, the query is typed as `java.lang.Integer` because that is the anticipated type of the results (the type of the `Person#age` attribute is `java.lang.Integer`). Because a query might contain multiple references to the Person entity, attribute references always need to be qualified. This is accomplished by the `Root#get` method call.

### 12.1.3. Selecting multiple values

There are actually a few different ways to select multiple values using criteria queries. We will explore 2 options here, but an alternative recommended approach is to use tuples as described in Section 12.2, “Tuple criteria queries”. Or consider a wrapper query; see Section 12.1.4, “Selecting a wrapper” for details.

Example 12.3. Selecting an array

    CriteriaQuery<Object[]> criteria = builder.createQuery( Object[].class );
    Root<Person> personRoot = criteria.from( Person.class );
    Path<Long> idPath = personRoot.get( Person_.id );
    Path<Integer> agePath = personRoot.get( Person_.age );
    criteria.select( builder.array( idPath, agePath ) );
    criteria.where( builder.equal( personRoot.get( Person_.eyeColor ), "brown" ) );

    List<Object[]> valueArray = em.createQuery( criteria ).getResultList();
    for ( Object[] values : valueArray ) {
    	final Long id = (Long) values[0];
    	final Integer age = (Integer) values[1];
    	...
    }

Technically this is classified as a typed query, but you can see from handling the results that this is sort of misleading. Anyway, the expected result type here is an array.

The example then uses the `array` method of `javax.persistence.criteria.CriteriaBuilder` which explicitly combines individual selections into a `javax.persistence.criteria.CompoundSelection`.

Example 12.4. Selecting an array (2)

    CriteriaQuery<Object[]> criteria = builder.createQuery( Object[].class );
    Root<Person> personRoot = criteria.from( Person.class );
    Path<Long> idPath = personRoot.get( Person_.id );
    Path<Integer> agePath = personRoot.get( Person_.age );
    criteria.multiselect( idPath, agePath );
    criteria.where( builder.equal( personRoot.get( Person_.eyeColor ), "brown" ) );

    List<Object[]> valueArray = em.createQuery( criteria ).getResultList();
    for ( Object[] values : valueArray ) {
        final Long id = (Long) values[0];
        final Integer age = (Integer) values[1];
        ...
    }

Just as we saw in Example 12.3, “Selecting an array” we have a typed criteria query returning an Object array. Both queries are functionally equivalent. This second example uses the `multiselect` method which behaves slightly differently based on the type given when the criteria query was first built, but in this case it says to select and return an Object[].

### 12.1.4. Selecting a wrapper

Another alternative to Section 12.1.3, “Selecting multiple values” is to instead select an object that will “wrap” the multiple values. Going back to the example query there, rather than returning an array of [Person#id, Person#age] instead declare a class that holds these values and instead return that.

Example 12.5. Selecting an wrapper

    public class PersonWrapper {
    	private final Long id;
    	private final Integer age;
    	public PersonWrapper(Long id, Integer age) {
    		this.id = id;
    		this.age = age;
    	}
    	...
    }

    ...

    CriteriaQuery<PersonWrapper> criteria = builder.createQuery( PersonWrapper.class );
    Root<Person> personRoot = criteria.from( Person.class );
    criteria.select(
    		builder.construct(
    			PersonWrapper.class,
    			personRoot.get( Person_.id ),
    			personRoot.get( Person_.age )
    		)
    );
    criteria.where( builder.equal( personRoot.get( Person_.eyeColor ), "brown" ) );

    List<PersonWrapper> people = em.createQuery( criteria ).getResultList();
    for ( PersonWrapper person : people ) {
        ...
    }

First we see the simple definition of the wrapper object we will be using to wrap our result values. Specifically notice the constructor and its argument types. Since we will be returning `PersonWrapper` objects, we use `PersonWrapper` as the type of our criteria query.

This example illustrates the use of the `javax.persistence.criteria.CriteriaBuilder` method `construct` which is used to build a wrapper expression. For every row in the result we are saying we would like a PersonWrapper instantiated with the remaining arguments by the matching constructor. This wrapper expression is then passed as the select.

## 12.2. Tuple criteria queries

A better approach to Section 12.1.3, “Selecting multiple values” is to use either a wrapper (which we just saw in Section 12.1.4, “Selecting a wrapper”) or using the `javax.persistence.Tuple` contract.

Example 12.6. Selecting a tuple

    CriteriaQuery<Tuple> criteria = builder.createTupleQuery();
    Root<Person> personRoot = criteria.from( Person.class );
    Path<Long> idPath = personRoot.get( Person_.id );
    Path<Integer> agePath = personRoot.get( Person_.age );
    criteria.multiselect( idPath, agePath );
    criteria.where( builder.equal( personRoot.get( Person_.eyeColor ), "brown" ) );

    List<Tuple> tuples = em.createQuery( criteria ).getResultList();
    for ( Tuple tuple : valueArray ) {
        assert tuple.get( 0 ) == tuple.get( idPath );
    	assert tuple.get( 1 ) == tuple.get( agePath );
    	...
    }

This example illustrates accessing the query results through the `javax.persistence.Tuple` interface. The example uses the explicit `createTupleQuery` of `javax.persistence.criteria.CriteriaBuilder`. An alternate approach is to use `createQuery` passing `Tuple.class`.

Again we see the use of the `multiselect` method, just like in Example 12.4, “Selecting an array (2)”. The difference here is that the type of the `javax.persistence.criteria.CriteriaQuery` was defined as `javax.persistence.Tuple` so the compound selections in this case are interpreted to be the tuple elements.

The `javax.persistence.Tuple` contract provides 3 forms of access to the underlying elements:

typed

  The Example 12.6, “Selecting a tuple” example illustrates this form of access in the `tuple.get( idPath )` and `tuple.get( agePath )` calls. This allows typed access to the underlying tuple values based on the `javax.persistence.TupleElement` expressions used to build the criteria.

positional

  Allows access to the underlying tuple values based on the position. The simple Object get(int position) form is very similar to the access illustrated in Example 12.3, “Selecting an array” and Example 12.4, “Selecting an array (2)”. The <X> X get(int position, Class<X> type form allows typed positional access, but based on the explicitly supplied type which the tuple value must be type-assignable to.

aliased

  Allows access to the underlying tuple values based an (optionally) assigned alias. The example query did not apply an alias. An alias would be applied via the `alias` method on `javax.persistence.criteria.Selection`. Just like `positional` access, there is both a typed (Object get(String alias)) and an untyped (<X> X get(String alias, Class<X> type form.

## 12.3. FROM clause

| A CriteriaQuery object defines a query over one or more entity, embeddable, or basic abstract schema types. The root objects of the query are entities, from which the other types are reached by navigation. |
| --JPA Specification, section 6.5.2 Query Roots, pg 262

## Note

All the individual parts of the FROM clause (roots, joins, paths) implement the `javax.persistence.criteria.From` interface.

### 12.3.1. Roots

Roots define the basis from which all joins, paths and attributes are available in the query. A root is always an entity type. Roots are defined and added to the criteria by the overloaded `from` methods on `javax.persistence.criteria.CriteriaQuery`:

    <X> Root<X> from(Class<X>);

    <X> Root<X> from(EntityType<X>)

Example 12.7. Adding a root

    CriteriaQuery<Person> personCriteria = builder.createQuery( Person.class );
    // create and add the root
    person.from( Person.class );

Criteria queries may define multiple roots, the effect of which is to create a cartesian product between the newly added root and the others. Here is an example matching all single men and all single women:

Example 12.8. Adding multiple roots

    CriteriaQuery query = builder.createQuery();
    Root<Person> men = query.from( Person.class );
    Root<Person> women = query.from( Person.class );
    Predicate menRestriction = builder.and(
    		builder.equal( men.get( Person_.gender ), Gender.MALE ),
    		builder.equal( men.get( Person_.relationshipStatus ), RelationshipStatus.SINGLE )
    );
    Predicate womenRestriction = builder.and(
    		builder.equal( women.get( Person_.gender ), Gender.FEMALE ),
    		builder.equal( women.get( Person_.relationshipStatus ), RelationshipStatus.SINGLE )
    );
    query.where( builder.and( menRestriction, womenRestriction ) );

### 12.3.2. Joins

Joins allow navigation from other `javax.persistence.criteria.From` to either association or embedded attributes. Joins are created by the numerous overloaded `join` methods of the `javax.persistence.criteria.From` interface

Example 12.9. Example with Embedded and ManyToOne

    CriteriaQuery<Person> personCriteria = builder.createQuery( Person.class );
    Root<Person> personRoot = person.from( Person.class );
    // Person.address is an embedded attribute
    Join<Person,Address> personAddress = personRoot.join( Person_.address );
    // Address.country is a ManyToOne
    Join<Address,Country> addressCountry = personAddress.join( Address_.country );

Example 12.10. Example with Collections

    CriteriaQuery<Person> personCriteria = builder.createQuery( Person.class );
    Root<Person> personRoot = person.from( Person.class );
    Join<Person,Order> orders = personRoot.join( Person_.orders );
    Join<Order,LineItem> orderLines = orders.join( Order_.lineItems );

### 12.3.3. Fetches

Just like in HQL and JPQL, criteria queries can specify that associated data be fetched along with the owner. Fetches are created by the numerous overloaded `fetch` methods of the `javax.persistence.criteria.From` interface.

Example 12.11. Example with Embedded and ManyToOne

    CriteriaQuery<Person> personCriteria = builder.createQuery( Person.class );
    Root<Person> personRoot = person.from( Person.class );
    // Person.address is an embedded attribute
    Fetch<Person,Address> personAddress = personRoot.fetch( Person_.address );
    // Address.country is a ManyToOne
    Fetch<Address,Country> addressCountry = personAddress.fetch( Address_.country );

## Note

Technically speaking, embedded attributes are always fetched with their owner. However in order to define the fetching of Address#country we needed a `javax.persistence.criteria.Fetch` for its parent path.

Example 12.12. Example with Collections

    CriteriaQuery&lt;Person&gt; personCriteria = builder.createQuery( Person.class );
    Root<Person> personRoot = person.from( Person.class );
    Fetch<Person,Order> orders = personRoot.fetch( Person_.orders );
    Fetch<Order,LineItem> orderLines = orders.fetch( Order_.lineItems );

## 12.4. Path expressions

## Note

Roots, joins and fetches are themselves paths as well.

## 12.5. Using parameters

Example 12.13. Using parameters

    CriteriaQuery<Person> criteria = build.createQuery( Person.class );
    Root<Person> personRoot = criteria.from( Person.class );
    criteria.select( personRoot );
    ParameterExpression<String> eyeColorParam = builder.parameter( String.class );
    criteria.where( builder.equal( personRoot.get( Person_.eyeColor ), eyeColorParam ) );

    TypedQuery<Person> query = em.createQuery( criteria );
    query.setParameter( eyeColorParam, "brown" );
    List<Person> people = query.getResultList();

Use the `parameter` method of `javax.persistence.criteria.CriteriaBuilder` to obtain a parameter reference. Then use the parameter reference to bind the parameter value to the `javax.persistence.Query`

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