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

My resolution:

Refactor the Base class into:

@MappedSuperclass
abstract class SuperBase<K> {
    public abstract K getId();
}

@MappedSuperclass
class Base<K> extends SuperBase<K> {
    @Id @GeneratedValue(AUTO)
    public K getId() { ... }
}

Then, you can extends from Base for most of your entity classes, and if one needs to override the @GeneratedValue , just extend from SuperBase and define it.

Answer 4

如果将注释放在getter而不是字段上，当重写子类中的方法时，将使用放在那里的注释而不是超类中的注释。