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## Chapter 8. Data categorizations

Hibernate understands both the Java and JDBC representations of application data. The ability to read and write object data to a database is called marshalling, and is the function of a Hibernate `type`. A `type` is an implementation of the `org.hibernate.type.Type` interface. A Hibernate `type` describes various aspects of behavior of the Java type such as how to check for equality and how to clone values.

## Usage of the word type

A Hibernate `type` is neither a Java type nor a SQL datatype. It provides information about both of these.

When you encounter the term type in regards to Hibernate, it may refer to the Java type, the JDBC type, or the Hibernate type, depending on context.

Hibernate categorizes types into two high-level groups: Section 8.1, “Value types” and Section 8.2, “Entity Types”.

## 8.1. Value types

A value type does not define its own lifecycle. It is, in effect, owned by an Section 8.2, “Entity Types”, which defines its lifecycle. Value types are further classified into three sub-categories.

- Section 8.1.1, “Basic types”

- Section 8.1.2, “Composite types”

- Section 8.1.3, “Collection types”

### 8.1.1. Basic types

Basic value types usually map a single database value, or column, to a single, non-aggregated Java type. Hibernate provides a number of built-in basic types, which follow the natural mappings recommended in the JDBC specifications. You can override these mappings and provide and use alternative mappings. These topics are discussed further on.

Table 8.1. Basic Type Mappings

Hibernate type | Database type | JDBC type | Type registry
--- | --- | --- | ---
org.hibernate.type.StringType | string | VARCHAR | string, java.lang.String
org.hibernate.type.MaterializedClob | string | CLOB | materialized_clob
org.hibernate.type.TextType | string | LONGVARCHAR | text
org.hibernate.type.CharacterType | char, java.lang.Character | CHAR | char, java.lang.Character
org.hibernate.type.BooleanType | boolean | BIT | boolean, java.lang.Boolean
org.hibernate.type.NumericBooleanType | boolean | INTEGER, 0 is false, 1 is true | numeric_boolean
org.hibernate.type.YesNoType | boolean | CHAR, 'N'/'n' is false, 'Y'/'y' is true. The uppercase value is written to the database. | yes_no
org.hibernate.type.TrueFalseType | boolean | CHAR, 'F'/'f' is false, 'T'/'t' is true. The uppercase value is written to the database. | true_false
org.hibernate.type.ByteType | byte, java.lang.Byte | TINYINT | byte, java.lang.Byte
org.hibernate.type.ShortType | short, java.lang.Short | SMALLINT | short, java.lang.Short
org.hibernate.type.IntegerTypes | int, java.lang.Integer | INTEGER | int, java.lang.Integer
org.hibernate.type.LongType | long, java.lang.Long | BIGINT | long, java.lang.Long
org.hibernate.type.FloatType | float, java.lang.Float | FLOAT | float, java.lang.Float
org.hibernate.type.DoubleType | double, java.lang.Double | DOUBLE | double, java.lang.Double
org.hibernate.type.BigIntegerType | java.math.BigInteger | NUMERIC | big_integer
org.hibernate.type.BigDecimalType | java.math.BigDecimal | NUMERIC | big_decimal, java.math.bigDecimal
org.hibernate.type.TimestampType | java.sql.Timestamp | TIMESTAMP | timestamp, java.sql.Timestamp
org.hibernate.type.TimeType | java.sql.Time | TIME | time, java.sql.Time
org.hibernate.type.DateType | java.sql.Date | DATE | date, java.sql.Date
org.hibernate.type.CalendarType | java.util.Calendar | TIMESTAMP | calendar, java.util.Calendar
org.hibernate.type.CalendarDateType | java.util.Calendar | DATE | calendar_date
org.hibernate.type.CurrencyType | java.util.Currency | VARCHAR | currency, java.util.Currency
org.hibernate.type.LocaleType | java.util.Locale | VARCHAR | locale, java.utility.locale
org.hibernate.type.TimeZoneType | java.util.TimeZone | VARCHAR, using the TimeZone ID | timezone, java.util.TimeZone
org.hibernate.type.UrlType | java.net.URL | VARCHAR | url, java.net.URL
org.hibernate.type.ClassType | java.lang.Class | VARCHAR, using the class name | class, java.lang.Class
org.hibernate.type.BlobType | java.sql.Blob | BLOB | blog, java.sql.Blob
org.hibernate.type.ClobType | java.sql.Clob | CLOB | clob, java.sql.Clob
org.hibernate.type.BinaryType | primitive byte[] | VARBINARY | binary, byte[]
org.hibernate.type.MaterializedBlobType | primitive byte[] | BLOB | materized_blob
org.hibernate.type.ImageType | primitive byte[] | LONGVARBINARY | image
org.hibernate.type.BinaryType | java.lang.Byte[] | VARBINARY | wrapper-binary
org.hibernate.type.CharArrayType | char[] | VARCHAR | characters, char[]
org.hibernate.type.CharacterArrayType | java.lang.Character[] | VARCHAR | wrapper-characters, Character[], java.lang.Character[]
org.hibernate.type.UUIDBinaryType | java.util.UUID | BINARY | uuid-binary, java.util.UUID
org.hibernate.type.UUIDCharType | java.util.UUID | CHAR, can also read VARCHAR | uuid-char
org.hibernate.type.PostgresUUIDType | java.util.UUID | PostgreSQL UUID, through Types#OTHER, which complies to the PostgreSQL JDBC driver definition | pg-uuid
org.hibernate.type.SerializableType | implementors of java.lang.Serializable | VARBINARY | Unlike the other value types, multiple instances of this type are registered. It is registered once under java.io.Serializable, and registered under the specific java.io.Serializable implementation class names.

### 8.1.2. Composite types

Composite types, or embedded types, as they are called by the Java Persistence API, have traditionally been called components in Hibernate. All of these terms mean the same thing.

Components represent aggregations of values into a single Java type. An example is an `Address` class, which aggregates street, city, state, and postal code. A composite type behaves in a similar way to an entity. They are each classes written specifically for an application. They may both include references to other application-specific classes, as well as to collections and simple JDK types. The only distinguishing factors are that a component does not have its own lifecycle or define an identifier.

### 8.1.3. Collection types

A collection type refers to the data type itself, not its contents.

A Collection denotes a one-to-one or one-to-many relationship between tables of a database.

Refer to the chapter on Collections for more information on collections.

## 8.2. Entity Types

Entities are application-specific classes which correlate to rows in a table, using a unique identifier. Because of the requirement for a unique identifier, ntities exist independently and define their own lifecycle. As an example, deleting a Membership should not delete the User or the Group. For more information, see the chapter on Persistent Classes.

## 8.3. Implications of different data categorizations

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