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EclipseLink/UserGuide/JPA/Basic JPA Development/Entities/Ids/GeneratedValue

@GeneratedValue Annotation

Use the @GeneratedValue annotation to enable EclipseLink persistence provider to generate unique identifiers for entity primary keys (see @Id).

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For more information, see Section 9.1.9 "GeneratedValue Annotation" in the JPA Specification.


This annotation lets you do the following:

  • override the type of identity value generation selected by the persistence provider for your database if you feel another generator type is more appropriate for your database or application;
  • override the primary key generator name selected by the persistence provider if this name is awkward, a reserved word, incompatible with a preexisting data model, or invalid as a primary key generator name in your database.

The @GeneratedValue annotation has the following attributes:

@GeneratedValue Attributes
Attribute Description Default Required?
generator The default value of this attribute is the name that EclipseLink persistence provider assigns to the primary key generator it selects.
If the generator name is awkward, a reserved word, incompatible with a preexisting data model, or invalid as a primary key generator name in your database, set the value of this attribute to the String generator name you want to use.
The name that EclipseLink persistence provider assigns to the primary key generator it selects No
strategy By default, EclipseLink persistence provider chooses the type of primary key generator that is most appropriate for the underlying database.
If you feel that another generator type is more appropriate for your database or application, set the value of this attribute to one of the following enumerated values of the GenerationType enumerated type:
  • AUTO (default) – specify that EclipseLink persistence provider should choose a primary key generator that is most appropriate for the underlying database.

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    Note: By default, EclipseLink chooses the TABLE strategy using a table named SEQ_GEN_TABLE, with SEQ_NAME and SEQ_COUNT columns, with allocationSize of 50 and pkColumnValue of SEQ_GEN. The default SEQUENCE used is database sequence SEQ_GEN_SEQUENCE with allocationSize of 50. Note that the database sequence increment must match the allocation size.

  • TABLE – specify that EclipseLink persistence provider assign primary keys for the entity using an underlying database table to ensure uniqueness (see @TableGenerator).
  • SEQUENCE – specify that EclipseLink persistence provider use a database sequence (see @SequenceGenerator).
  • IDENTITY – specify that EclipseLink persistence provider use a database identity column. Setting this value will indicate to the persistence provider that it must reread the inserted row from the table after an insert has occurred. This will allow it to obtain the newly generated identifier from the database and put it into the in-memory entity that was just persisted. The identity must be defined as part of the database schema for the primary key column. Identity generation may not be shared across multiple entity types.

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    Note: IDENTITY strategy is supported on Sybase, DB2, SQL Server, MySQL, Derby, JavaDB, Informix, and Postgres databases.

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    Note: There is a difference between using IDENTITY and other id generation strategies: the identifier will not be accessible until after the insert has occurred – it is the action of inserting that caused the identifier generation. Due to the fact that insertion of entities is most often deferred until the commit time, the identifier would not be available until after the transaction has been committed.

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    Note: We do not recommend using the IDENTITY strategy for it does not support preallocation.

The type of primary key generator that is most appropriate for the underlying database No

This example shows how to use automatic id generation. This will cause EclipseLink persistence provider to create an identifier value and insert it into the id field of each Employee entity that gets persisted.
Using Automatic Id Generation

 @Entity
 public class Employee implements Serializable {
 
     @Id
     @GeneratedValue(strategy=GenerationType.AUTO)
     private int id;
     ...
 }


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Warning: Be careful when using the automatic ID generation: the persistence provider has to pick its own strategy to store the identifiers, but it needs to have a persistent resource, such as a table or a sequence, to do so. The persistence provider cannot always rely upon the database connection that it obtains from the server to have permissions to create a table in the database. This is usually a privileged operation that is often restricted to the DBA. There will need to be a creation phase or schema generation to cause the resource to be created before the AUTO strategy can function.



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Version: 2.1.0
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