public @Interface MapsId

The value element specifies the attribute within a composite key to which the relationship attribute corresponds. If the primary key of the entity is of the same Java type as the primary key of the entity referenced by the relationship, the value attribute is not specified.

In this example, the parent entity has simple primary key:

@Entity
public class Employee {
    @Id
    long empId;
    String name;
    ...
}

And then the dependent entity uses EmbeddedId to declare its composite primary key:

@Embeddable
public class DependentId {
    String name;
    long empid;  // corresponds to primary key type of Employee
}

@Entity
public class Dependent {
    @EmbeddedId
    DependentId id;
    ...
    @MapsId("empid")  // maps the empid attribute of embedded id
    @ManyToOne
    Employee emp;
}

If a ManyToOne or OneToOne relationship declared by a dependent entity is annotated MapsId, an instance of the entity cannot be made persistent until the relationship has been assigned a reference to an instance of the parent entity, since the identity of the dependent entity declaring the relationship is derived from the referenced parent entity.