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The Java Persistence API (JPA) : a standard interface-based Java model abstraction of persistence, developed by the JCP.

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/*
 * Copyright (c) 2008, 2009, 2011 Oracle, Inc. 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.
 */
package javax.persistence;

import java.lang.annotation.Retention;
import java.lang.annotation.Target;

import static java.lang.annotation.ElementType.ANNOTATION_TYPE;
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({ANNOTATION_TYPE, 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 entitys primary key.
     * @return name
     */
    String value() default "";
}