https://github.com/javaee/jpa-spec: Java(TM) Persistence API 
Copyright (c) 2008 - 2013 Oracle Corporation. All rights reserved.
This program and the accompanying materials are made available under the
terms of the Eclipse Public License v1.0 and Eclipse Distribution License v. 1.0
which accompanies this distribution.
The Eclipse Public License is available at http://www.eclipse.org/legal/epl-v10.html
and the Eclipse Distribution License is available at
http://www.eclipse.org/org/documents/edl-v10.php.
Contributors:
    Linda DeMichiel - Java Persistence 2.1
    Linda DeMichiel - Java Persistence 2.0

/*******************************************************************************
 * Copyright (c) 2008 - 2013 Oracle Corporation. All rights reserved.
 *
 * This program and the accompanying materials are made available under the
 * terms of the Eclipse Public License v1.0 and Eclipse Distribution License v. 1.0
 * which accompanies this distribution.
 * The Eclipse Public License is available at http://www.eclipse.org/legal/epl-v10.html
 * and the Eclipse Distribution License is available at
 * http://www.eclipse.org/org/documents/edl-v10.php.
 *
 * Contributors:
 *     Linda DeMichiel - Java Persistence 2.1
 *     Linda DeMichiel - Java Persistence 2.0
 *
 ******************************************************************************/
package javax.persistence;

import java.lang.annotation.Target;
import java.lang.annotation.Retention;
import static java.lang.annotation.ElementType.TYPE;
import static java.lang.annotation.RetentionPolicy.RUNTIME;


Specifies a composite primary key class that is mapped to 
multiple fields or properties of the entity.  

 The names of the fields or properties in the primary key 
class and the primary key fields or properties of the entity 
must correspond and their types must be the same.

  Example:
  @IdClass(com.acme.EmployeePK.class)
  @Entity
  public class Employee {
     @Id String empName;
     @Id Date birthDay;
     ...
  }



Since:Java Persistence 1.0
/**
 * Specifies a composite primary key class that is mapped to 
 * multiple fields or properties of the entity.  
 *
 * <p> The names of the fields or properties in the primary key 
 * class and the primary key fields or properties of the entity 
 * must correspond and their types must be the same.
 *
 * <pre>
 *
 *   Example:
 *
 *   &#064;IdClass(com.acme.EmployeePK.class)
 *   &#064;Entity
 *   public class Employee {
 *      &#064;Id String empName;
 *      &#064;Id Date birthDay;
 *      ...
 *   }
 * </pre>
 *
 * @since Java Persistence 1.0
 */
@Target({TYPE})
@Retention(RUNTIME)

public @interface IdClass {

    
Primary key class 
/** Primary key class */
    Class value();
}