Spring-data:如何使用规范查询子类型

我有三个这样的人:

@Entity
@Inheritance(strategy = InheritanceType.JOINED)
@DiscriminatorColumn(name = "TYPE")
public abstract class A
{
    // fields, getters, setters
}

@Entity
@DiscriminatorValue("B")
public class B extends A
{
    // fields, getters, setters
}

@Entity
@DiscriminatorValue("C")
public class C extends A
{
    // fields, getters, setters
}

我也有这样的存储库:

@Repository
public interface ADao extends JpaRepository, JpaSpecificationExecutor 
{
}

List findAll(Specification s)