Revision as of 11:11, 8 June 2011

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Key API

• @IdClass

@IdClass

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.

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.

The Nonembedded Composite Primary Key Class example, below, 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 @IdClass Annotation example, below, 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).

Example: 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 Date getBirthDay() {
         return this.birthDay;
     }
 
     public void setBirthDay(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;
         EmployeePK pk = (EmployeePK) obj;
         return pk.birthDay.equals(this.birthDay) && pk.empName.equals(this.empName);
     }
 }

Example: @IdClass Annotation

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

For more information, see Section 11.1.19 "IdClass Annotation" in the JPA Specification.

@Id	Configuring Entities	@EmbeddedId
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