i have a generic class which is supper class of some non-generic class and those are just setting its generic parameter like this:
@Inheritance(strategy = InheritanceType.TABLE_PER_CLASS)
class A<T>{
@Id
getId(){..}
setID(int id){..}
int id
T t;
T getT(){...}
setT(T t){...}
}
and
@Entity
class B extends A<Integer>{}
but hibernate says that B does not have an identifier what should I do?
If A won't be directly persisted, but you do want it's subclasses to pick up some (or all) of its Hibernate annotations, you should use @MappedSuperclass:
@MappedSuperclass
@Inheritance(strategy = InheritanceType.TABLE_PER_CLASS)
class A<T>{
@Id
getId(){..}
setID(int id){..}
int id
T t;
T getT(){...}
setT(T t){...}
}
You need to add the @Entity annotation to class A as well. The @Transient annotation on attribute t should help with your second exception
@Entity
@Inheritance(strategy = InheritanceType.TABLE_PER_CLASS)
class A<T> {
@Id
getId(){..}
setID(int id){..}
int id
@Transient
T t;
T getT(){...}
setT(T t){...}
}
I agree with reply No. 1, use @MappedSuperclass for A - don't make something abstract an Entity. You should probably make this class specifically abstract too.
@MappedSuperclass
@Inheritance(strategy = InheritanceType.TABLE_PER_CLASS)
abstract class A<T>{
@Id
getId(){..}
setID(int id){..}
int id
T t;
T getT(){...}
setT(T t){...}
}
A table-per-class strategy often requires this kind of abstract base. Then the subclass specifies the table name, and additional fields.
@Entity
@Table(name="MY_INTEGERS")
class B extends A<Integer>{}
(Personally I would move this variable type into the subclass, but I don't know what you're trying to achieve).
After lots of testing, trying to get Java parameterisation working with an abstract parent (Single-table inheritance), and an abstract child table (one-table-per-class inheritance), I've given up.
It may be possible, but often you get problems where Hibernate tries to instantiate an abstract (parameterised) class as an entity. this is when you get the error "A has an unbound type and no explicit target entity." It means Hibernate doesn't have a parameter value for a parameterised type. I found that tests for the extending classes were fine, but tests around parent entities would break.
I would suggest rewriting it using the JPA inheritance, moving the parameterised stuff down into extending classes. That way you get the same polymorphism back from the database.
@MappedSuperclass
@Inheritance(strategy = InheritanceType.SINGLE_TABLE)
@DiscriminatorColumn(name = "CLASS_TYPE", discriminatorType = DiscriminatorType.STRING)
public abstract class ClassA {
[...]
}
extension B:
@Entity
@DiscriminatorValue=("B")
public class ClassB extends ClassA {
@OneToOne
@JoinColumn(name = "mycolumn_id")
private Integer instance;
[...]
}
extension C:
@Entity
@DiscriminatorValue=("C")
public class ClassC extends ClassA {
@OneToOne
@JoinColumn(name = "mycolumn_id")
private String instance;
[...]
}