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

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Revision as of 07:42, 17 June 2010 by (Talk | contribs) (New page: =@IdClass= Use the <tt>@IdClass</tt> annotation to specify a composite primary key class (usually made up of two or more primitive, JDK object types or Entity types) for an entity or Mapp...)

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Use the @IdClass annotation to specify a composite primary key class (usually made up of two or more primitive, JDK object types or Entity types) for an entity or MappedSuperclass.

Note: Composite primary keys typically arise during mapping from legacy databases when the database key is comprised of several columns.

A composite primary key class has the following characteristics:

  • It is a POJO class.
  • It is a public class with a public no-argument constructor.
  • If you use property-based access, the properties of the primary key class are public or protected.
  • It is serializable.
  • It defines equals and hashCode methods. The semantics of value equality for these methods must be consistent with the database equality for the database types to which the key is mapped.
  • Its fields or properties must correspond in type and name to the entity primary key fields or properties annotated with @Id.

Alternatively, you can make the composite primary key class an embedded class owned by the entity (see @EmbeddedId).

The @IdClass annotation has a required attribute value that you set to the class to specify this class as a composite primary key class (see @AttributeOverride).

The Nonembedded Composite Primary Key Class example shows a nonembedded composite primary key class. In this class, fields empName and birthDay must correspond in name and type to properties in the entity class. The Usage of @IdClass Annotation example shows how to configure an entity with this nonembedded composite primary key class using the @IdClass annotation. Because entity class fields empName and birthDay are used in the primary key, you must also annotate them using the @Id annotation (see @Id).

Nonembedded Composite Primary Key Class

 public class EmployeePK implements Serializable {
     private String empName;
     private Date birthDay;
     public EmployeePK() {
     public String getName() {
         return this.empName;
     public void setName(String name) {
         this.empName = name;
     public long getDateOfBirth() {
         return this.birthDay;
     public void setDateOfBirth(Date date) {
         this.birthDay = date;
     public int hashCode() {
         return (int)this.empName.hashCode();
     public boolean equals(Object obj) {
         if (obj == this) return true;
         if (!(obj instanceof EmployeePK)) return false;
         if (obj == null) return false;
         EmployeePK pk = (EmployeePK) obj;
         return pk.birthDay = this.birthDay && pk.empName.equals(this.empName);

Usage of @IdClass Annotation

  @IdClass (EmployeePK.class)
 public class Employee implements Serializable{
     @Id String empName;
     @Id Date birthDay;

Elug javaspec icon.gif

For more information, see Section 9.1.8 "Id Annotation9.1.15 "IdClass Annotation"" in the JPA Specification.

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