Mapping multi-Level inheritance in Hibernate with Annotations

Question

Take the situation listed in this question:

http://stackoverflow.com/questions/261407/mapping-multi-level-inheritance-in-hibernate/1883829

How would this mapping be done with Annotations rather than an hbm file?

Answer 1

Inheritance between @Entity annotated classes is picked up automatically. So if you class A is annotated and class B is also annotated with @Entity and extends A, and C extends B and is also @Entity annotated all will be picked up.

I have not used joined subclasses combined with discriminator values, but I'm sure it will be very similar to what is done in XML (using @DiscriminatorColumn and @DiscriminatorValue)

Answer 2

The answer is here and it seems pretty clear, it's not possible using annotations.

Answer 3

What specifically are you having trouble with? Mapping class hierarchy via joined subclasses is pretty straightforward:

@Entity
@Inheritance(strategy=InheritanceType.JOINED)
public class A implements Serializable { ... }

@Entity
public class B extends A { ... }

@Entity
@PrimaryKeyJoinColumn(name="A_ID")
public class C extends A { ... }

@Entity
@PrimaryKeyJoinColumn(name="B_ID")
public class D extends B { ... }

---

Update (based on Michal's comment).

In case you do want to use discriminators (and you should have a good reason to do so), it's possible to do so by mixing table-per-class-hierarchy strategy with secondary tables:

@Entity
@Table(name="A_table")
@Inheritance(strategy=InheritanceType.SINGLE_TABLE)
@DiscriminatorColumn(name="entity_type")
@DiscriminatorValue("A")
public class A implements Serializable { ... }

@Entity
@SecondaryTable(name="B_table")
public class B extends A { ... }

@Entity
@SecondaryTable(name="C_table", pkJoinColumns={
    @PrimaryKeyJoinColumn(name="A_ID", referencedColumnName="ID")
))
public class C extends A { ... }

@Entity
@SecondaryTable(name="D_table", pkJoinColumns={
    @PrimaryKeyJoinColumn(name="B_ID", referencedColumnName="ID")
))
public class D extends B { ... }

The downside to this approach is you'll have to explicitly specify the table for each property mapped:

public class D extends B {
  @Column(table="D_table")
  private String someProperty;

  ...
}