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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:
*
* @Entity
* public class Department {
* ...
* @OneToMany(mappedBy="department")
* @MapKey(name="empId")
* 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="empPK")
* public Map<EmployeePK, Employee> getEmployees() {... }
* ...
* }
*
* @Entity
* public class Employee {
* @EmbeddedId public EmployeePK getEmpPK() { ... }
* ...
* @ManyToOne
* @JoinColumn(name="dept_id")
* public Department getDepartment() { ... }
* ...
* }
*
* @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 "";
}