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There is a newer version: 2.2.1

/*******************************************************************************
 * 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.FIELD;
import static java.lang.annotation.ElementType.METHOD;
import static java.lang.annotation.RetentionPolicy.RUNTIME;

/**
 * Designates a ManyToOne or
 * OneToOne relationship attribute that provides the
 * mapping for an {@link EmbeddedId} primary key, an attribute within
 * an EmbeddedId primary key, or a simple primary key of
 * the parent entity. The value element specifies the
 * attribute within a composite key to which the relationship
 * attribute corresponds. If the entity's primary key is of the same
 * Java type as the primary key of the entity referenced by the
 * relationship, the value attribute is not specified.
 * 
 *  *    Example:
 *
 *    // parent entity has simple primary key
 *
 *    @Entity
 *    public class Employee {
 *       @Id long empId;
 *       String name;
 *       ...
 *    } 
 *
 *    // dependent entity uses EmbeddedId for composite key
 *
 *    @Embeddable
 *    public class DependentId {
 *       String name;
 *       long empid;   // corresponds to primary key type of Employee
 *    }
 *
 *    @Entity
 *    public class Dependent {
 *       @EmbeddedId DependentId id;
 *        ...
 *       @MapsId("empid")  //  maps the empid attribute of embedded id
 *       @ManyToOne Employee emp;
 *    }
 * 
 *
 * @since Java Persistence 2.0
 */
@Target( { METHOD, FIELD })
@Retention(RUNTIME)
public @interface MapsId {

    /**
     * (Optional) The name of the attribute within the composite key
     * to which the relationship attribute corresponds.  If not
     * supplied, the relationship maps the entity's primary
     * key.
     */
   String value() default ""; }