Unknown entity issue for joined subclass when using annotation

I met exception when using annotated joined subclass, i don't know how to correct it, please help.

Exception is: org.hibernate.MappingException: Unknown entity: B

Code:

Class A

@Entity
@Table(name="table_a")
@Inheritance(strategy=InheritanceType.JOINED)   
public class A {
    // uses composite key
    @Id
    @Column(name="sid")
    private String sid;

    @Id
    @Column(name="uid")
    private String uid;

    .....
}

Class B

@Entity
@Table(name="table_b")
public class B extends A {
    // inherited sid and uid from A
    @Id
    @Column(name="xid")
    private String xid;

    @Column(name="name")
    private String name;
    ......
}

Tables

create table_a(sid varchar, uid varchar);
create table_b(sid varchar, uid varchar, xid varchar, name varchar);

Hibernate.cfg.xml

<hibernate-configuration>
    <session-factory>
        .....
        <mapping class="A"/>
        <!-- no need to map B here, right?
        <mapping class="B"/>
        -->
        .....
    </session-factory>
</hibernate-configuration>

TestClass

public class HibernateTest {
    public static void main(String[] args) throws Exception {
        SessionFactory sessionFactory = HibernateUtil.getSessionFactory();
        Session session = sessionFactory.openSession();
        session.beginTransaction();
        B temp = new B();
        temp.setSid(1);
        temp.setUid(2);
        temp.setXid(3);
        B target = session.get(B.class, temp);
        System.out.println("-开发者_如何学Python--------------" + target.getName());
        session.getTransaction().commit();
        session.close();
   }
}

Exception:

Exception in thread "main" org.hibernate.MappingException: Unknown entity: B

You have <mapping class="B"/> commented out with a question asking if it's needed. You need it.