How do I override the GenerationType strategy using Hibernate/JPA annotations?

Question

I'm considering using Annotations to define my Hibernate mappings but have run into a problem: I want to use a base entity class to define common fields (including the ID field) but I want different tables to have different ID generation strategies:

@MappedSuperclass
public abstract class Base implements Serializable {
    @Id
    @Column(name="ID", nullable = false)
    private Integer id;
    public Integer getId(){return id;}
    public void setId(Integer id){this.id = id;}
    ...
}

@Entity
@Table(name="TABLE_A")
public class TableA extends Base {
    // Table_A wants to set an application-defined value for ID
    ...
}

@Entity
@Table(name="TABLE_B")
public class TableB extends Base {
    // How do I specify @GeneratedValue(strategy = AUTO) for ID here?
    ...
}

Is there some way to do this? I've tried including the following into TableB but hibernate objected to my having the same column twice and it seems wrong:

@Override // So that we can set Generated strategy
@Id
@GeneratedValue(strategy = AUTO)
public Integer getId() {
    return super.getId();
}

Answer 1

In the code above, it looks like you're mixing annotations on fields (superclass) and methods (subclass). The Hibernate reference documentation recommends avoiding this, and I suspect it might be causing the problem. In my experience with Hibernate, it's safer and more flexible to annotate getter/setter methods instead of fields anyway, so I suggest sticking to that design if you can.

As a solution to your problem, I recommend removing the id field from your Base superclass altogether. Instead, move that field into the subclasses, and create abstract getId() and setId() methods in your Base class. Then override/implement the getId() and setId() methods in your subclasses and annotate the getters with the desired generation strategy.

Hope this helps.

Answer 2

On the method in the child dont add the second @Id tag.

@Override // So that we can set Generated strategy
@GeneratedValue(strategy = AUTO)
public Integer getId() {
    return super.getId();
}

Answer 3

If you put your annotations on the getter rather than the field, when you override the method in the subclass, the annotations placed there will be used rather than the ones in the superclass.