Using @Id for multiple fields in same class

Question

I am a newbie to Hibernate and have to support an existing application that uses Hibernate It is a pure reporting application - no insert / update / delete - only selects

I can see existing POJO objects which have the @Id annotation used in more than one field in the same class

My understanding was that for a composite primary key - you need to use @Embeddable and @EmbeddedId

However this is not defined in my class and the strange thing is code compiles and runs just fine

Here is an example of my code :

package com.xyz.vo;

import java.io.Serializable;
import javax.persistence.*;
import org.hibernate.annotations.GenericGenerator;

@Entity(name="com.xyz.vo.Emp")
@Table(name = "TEmployee")
public class Emp implements Serializable    {
private java.lang.Integer  empId;
@Id 
@Column(name = "EMP_ID")   
public java.lang.Integer  getEmpId(){   
    return this.empId;
}

public  void  setEmpId(java.lang.Integer empId){    
    this.empId=empId;
}

private java.lang.Integer  empAge;
@Id 
@Column(name = "EMP_AGE")   
public java.lang.Integer  getEmpAge(){  
    return this.empAge;
}

    public  void  setEmpAge(java.lang.Integer empAge){  
    this.empAge=empAge;
}

    private String empName;
    @Column(name = "EMP_NAME")   
public String  getEmpName(){    
    return this.empName;
}

    public  void  setEmpName(String empName){   
    this.empName=empName;
}

and many more fields in the pojo - which do not have the @Id defined

NOTE - here two fields have the @Id annotation

I am confused why this compiles and works ? Thanks akila

Answer 1

According to the Hibernate docs, Hibernate seems to allow this but it is not however JPA compliant.

5.1.2.1.2. Multiple id properties without identifier type

Another, arguably more natural, approach is to place @Id on multiple properties of your entity. This approach is only supported by Hibernate (not JPA compliant) but does not require an extra embeddable component.

http://docs.jboss.org/hibernate/core/3.6/reference/en-US/html_single/#d0e4819

For a JPA compliant solution you would have to specify an additional @IDClass for this set-up (or use an EmbeddedId):

http://www.objectdb.com/java/jpa/entity/id