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Hibernate definition of the Java Persistence 2.0 (JSR 317) API.

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// $Id: MapKey.java 17752 2009-10-15 01:19:21Z [email protected] $
// EJB3 Specification Copyright 2004-2009 Sun Microsystems, Inc.
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;

/**
 * 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({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.
     */
    String name() default "";
}