Appendix A. Hibernate Types

Appendix A. Hibernate Types

Hibernate makes a fundamental distinction between two different kinds of data in terms of how they relate to the persistence service: entities and values .

An entity is something with its own independent existence, regardless of whether it's currently reachable by any object within a Java virtual machine. Entities can be retrieved from the database through queries, and they must be explicitly saved and deleted by the application. (If cascading relationships have been set up, the act of saving or deleting a parent entity can also save or delete its children, but this is still explicit at the level of the parent.)

Values are stored only as part of the persistent state of an entity. They have no independent existence. They might be primitives, collections, enumerations, and custom user types. Since they are entirely subordinated to the entity in which they exist, they cannot be independently versioned, nor can they be shared by more than one entity or collection.

Notice that a particular Java object might be either an entity or a value ”the difference is in how it is designed and presented to the persistence service. Primitive Java types are always values.

A.1 Basic Types

Hibernate's basic types fall into a number of groupings:

Simple numeric and Boolean types

These correspond to the primitive Java types that represent numbers , characters and Boolean values, or their wrapper classes. They get mapped to appropriate SQL column types (based on the SQL dialect in use). They are: boolean, byte, character, double, float, integer, long, short, true_false, and yes_no . The last two are alternate ways to represent a Boolean value within the database; true_false uses the values 'T' and 'F', while yes_no uses 'Y' and 'N'.

String type

The Hibernate type string maps from java.lang.String to the appropriate string column type for the SQL dialect (usually VARCHAR , but in Oracle VARCHAR2 is used).

Time types

Hibernate uses date , time , and timestamp to map from java.util.Date (and subclasses) to appropriate SQL types (e.g., DATE , TIME , TIMESTAMP ).

Arbitrary precision numeric

The Hibernate type big_decimal provides a mapping between java.math.BigDecimal to the appropriate SQL type (usually NUMERIC , but Oracle uses NUMBER ).

Localization values

The types locale , timezone , and currency are stored as strings ( VARCHAR or VARCHAR2 as noted above), and mapped to the Locale , TimeZone , and Currency classes in the java.util package. Locale and Currency are stored using their ISO codes, while TimeZone is stored using its ID property.

Class names

The type class maps instances of java.lang.Class using their fully qualified names, stored in a string column ( VARCHAR , or VARCHAR2 in Oracle).

Byte arrays

The type binary stores byte arrays in an appropriate SQL binary type.

Any serializable object

The type serializable can be used to map any serializable Java object into a SQL binary column. This is the fallback type used when attempting to persist an object that doesn't have a more specific appropriate mapping (and does not implement PersistentEnum ; see the next section).

JDBC large objects

The types blob and clob provide mappings for the Blob and Clob classes in the java.sql package. Note that there are rather severe restrictions on using these classes. Driver support is rather inconsistent in the first place, and they can't be reused past a single transaction.