Module jakarta.persistence
Package jakarta.persistence

Annotation Interface MapKeyClass

@Target({METHOD,FIELD}) @Retention(RUNTIME) public @interface MapKeyClass
Specifies the type of the map key for associations of type Map. The map key can be a basic type, an embeddable class, or an entity. If the map is specified using Java generics, the MapKeyClass annotation and associated type need not be specified; otherwise they must be specified.

The MapKeyClass annotation is used in conjunction with ElementCollection or one of the collection-valued relationship annotations (OneToMany or ManyToMany). The MapKey annotation is not used when MapKeyClass is specified and vice versa.

Example 1: 

@Entity
public class Item {
    @Id
    int id;
    ...
    @ElementCollection(targetClass = String.class)
    @MapKeyClass(String.class)
    Map images;  // map from image name to image filename
    ...
}

Example 2: 

// MapKeyClass and target type of relationship can be defaulted

@Entity
public class Item {
    @Id
    int id;
    ...
    @ElementCollection
    Map<String, String> images;
    ...
}

Example 3: 

@Entity
public class Company {
    @Id
    int id;
    ...
    @OneToMany(targetEntity = com.example.VicePresident.class)
    @MapKeyClass(com.example.Division.class)
    Map organization;
}

Example 4: 

// MapKeyClass and target type of relationship are defaulted

@Entity
public class Company {
    @Id
    int id;
    ...
    @OneToMany
    Map<Division, VicePresident> organization;
}
Since:
2.0
See Also: