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Specifying a Composite Primary Key For an Entity or Mapped Superclass Using @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.
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, 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 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).
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 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); } }
Example: @IdClass Annotation
@IdClass (EmployeePK.class) @Entity public class Employee implements Serializable{ @Id String empName; @Id Date birthDay; ... }
For more information, see Section 9.1.15 "IdClass Annotation" in the JPA Specification.