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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;

/**
 * Specifies the type of the map key for associations of type java.util.Map. The map key can be a
 * basic type, an embeddable class, or an entity. If the map is specified using Java generics, the
 * MapKeyClass annotation and associated type need not be specified; otherwise they must be
 * specified.
 * 
 * The MapKeyClass annotation is used in conjunction with ElementCollection or one
 * of the collection-valued relationship annotations (OneToMany or ManyToMany). The
 * MapKey annotation is not used when MapKeyClass is specified and vice versa.
 *
 * 
 *
 *    Example 1:
 * 
 *    @Entity
 *    public class Item {
 *       @Id int id;
 *       ...
 *       @ElementCollection(targetClass=String.class)
 *       @MapKeyClass(String.class)
 *       Map images;  // map from image name to image filename
 *       ...
 *    }
 * 
 *    Example 2:
 * 
 *    // MapKeyClass and target type of relationship can be defaulted
 * 
 *    @Entity
 *    public class Item {
 *       @Id int id;
 *       ...
 *       @ElementCollection
 *       Map<String, String> images;
 *        ...
 *     }
 * 
 *     Example 3:
 * 
 *     @Entity
 *     public class Company {
 *        @Id int id;
 *        ...
 *        @OneToMany(targetEntity=com.example.VicePresident.class)
 *        @MapKeyClass(com.example.Division.class)
 *        Map organization;
 *     }
 * 
 *     Example 4:
 * 
 *     // MapKeyClass and target type of relationship are defaulted
 * 
 *     @Entity
 *     public class Company {
 *        @Id int id;
 *        ...
 *        @OneToMany
 *        Map<Division, VicePresident> organization;
 *     }
 *
 * 
 * @see ElementCollection
 * @see OneToMany
 * @see ManyToMany
 * @since Java Persistence 2.0
 */
@Target({ANNOTATION_TYPE, METHOD, FIELD})
@Retention(RUNTIME)
public @interface MapKeyClass
{
    /** 
     * (Required) The type of the map key.
     * @return The type
     */
    Class value();
}