Revision as of 11:04, 8 June 2011

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Key API

• @Id
• @IdClass
• @GeneratedValue

@Id

Use the @Id annotation or <id> XML element to designate one or more persistent fields or properties as the entity's primary key.

For each entity, you must designate at least one of the following:

• one @Id, or
• one @EmbeddedId, or
• multiple @Id and an @IdClass

The @Id annotation does not have attributes.

By default, the entities Id must be set by the application, normally before the persist is called. A @GeneratedValue can be used to have EclipseLink generate the Id value.

If the Id (or part of the Id) is also a foreign key from a reference to another entity, the @Id annotation should be placed on the @OneToOne or @ManyToOne attribute. The entity's Id is then composed of the Id of the referenced entity.

This example shows how to use this annotation to designate the persistent field empID as the primary key of the Employee table.

Example: @Id Annotation

@Entity
public class Employee implements Serializable {
    @Id
    private int empID;
    ...
}

Example: Id XML

<entity class="Employee">
    <attributes>
        <id name="empID"/>
        ...
    </attributes>
</entity>

The @Id annotation supports the use of EclipseLink converters. See Using EclipseLink JPA Converters.

It is possible to generate the value for an Id using the prePersist event, or through triggers when using the Oracle database platform. See Returning.

For more information, see Section 11.1.18 "Id Annotation" in the JPA Specification.

Configuring Entities	Configuring Entities	@IdClass
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