src/javax/persistence/MapKey.java

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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;

/**
 * Is used to specify the map key for associations of type 
 * {@link java.util.Map}.
 * 
 * <p> 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.
 *
 * <pre>
 *
 *    Example 1:
 *
 *    &#064;Entity
 *    public class Department {
 *        ...
 *        &#064;OneToMany(mappedBy="department")
 *        &#064;MapKey(name="empId")
 *        public Map<Integer, Employee> getEmployees() {... }
 *        ...
 *    }
 *
 *    &#064;Entity
 *    public class Employee {
 *        ...
 *        &#064;Id Integer getEmpid() { ... }
 *        &#064;ManyToOne
 *        &#064;JoinColumn(name="dept_id")
 *        public Department getDepartment() { ... }
 *        ...
 *    }
 *
 *    Example 2:
 *
 *    &#064;Entity
 *        public class Department {
 *        ...
 *        &#064;OneToMany(mappedBy="department")
 *        &#064;MapKey(name="empPK")
 *        public Map<EmployeePK, Employee> getEmployees() {... }
 *        ...
 *    }
 *
 *    &#064;Entity
 *        public class Employee {
 *        &#064;EmbeddedId public EmployeePK getEmpPK() { ... }
 *        ...
 *        &#064;ManyToOne
 *        &#064;JoinColumn(name="dept_id")
 *        public Department getDepartment() { ... }
 *        ...
 *    }
 *
 *    &#064;Embeddable
 *    public class EmployeePK {
 *        String name;
 *        Date bday;
 *    }
 * </pre>
 *
 * @since Java Persistence 1.0
 */
@Target({METHOD, FIELD}) 
@Retention(RUNTIME)

public @interface MapKey {

    /**
     * The name of the persistent field or property of the 
     * associated entity that is used as the map key. 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 {@link IdClass}, an instance of the primary key 
     * class is used as the key.
     */
    String name() default "";
}