@Target(value={METHOD,FIELD})
 @Retention(value=RUNTIME)
public @interface MapKey

Specifies the map key for associations of type 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 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() { ... }
        ...
    }