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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 map key for associations of type {@link java.util.Map java.util.Map} when the map key is
 * itself the primary key or a persistent field or property of the entity that is the value of the map.
 * 
 * If a persistent field or property other than the primary key is used as a map key then it is expected to
 * have a uniqueness constraint associated with it.
 * 

 * The {@link MapKeyClass} annotation is not used when MapKey is specified and vice versa.
 *
 * 
 *
 *    Example 1:
 * 
 *    @Entity
 *    public class Department {
 *        ...
 *        @OneToMany(mappedBy="department")
 *        @MapKey  // map key is primary key
 *        public Map<Integer, Employee> getEmployees() {... }
 *        ...
 *    }
 * 
 *    @Entity
 *    public class Employee {
 *        ...
 *        @Id Integer getEmpId() { ... }
 *        @ManyToOne
 *        @JoinColumn(name="dept_id")
 *        public Department getDepartment() { ... }
 *        ...
 *    }
 * 
 *    Example 2:
 * 
 *    @Entity
 *        public class Department {
 *        ...
 *        @OneToMany(mappedBy="department")
 *        @MapKey(name="name")
 *        public Map<String, Employee> getEmployees() {... }
 *        ...
 *    }
 * 
 *    @Entity
 *        public class Employee {
 *        @Id public Integer getEmpId() { ... }
 *        ...
 *        @ManyToOne
 *        @JoinColumn(name="dept_id")
 *        public Department getDepartment() { ... }
 *        ...
 *    }
 * 
 * @since Java Persistence 1.0
 */
@Target({ANNOTATION_TYPE, METHOD, FIELD})
@Retention(RUNTIME)
public @interface MapKey
{

    /**
     * (Optional) The name of the persistent field or property of the associated entity that is used as the
     * map key.
     * 
     * Default: If the name element is not specified, the primary key of the associated entity is
     * used as the map key. If the primary key is a composite primary key and is mapped as
     * IdClass, an instance of the primary key class is used as the key.
     * @return The name
     */
    String name() default "";
}