hibernate多对多操作

SQL(MY SQL):
create table tgroup (
   group_id int not null auto_increment,
   name varchar(16) not null default '',
   primary key   (group_id)
) type=innodb;
create table trole (
   role_id int not null auto_increment,
   name varchar(16) not null default '',  
   primary key   (role_id)
) type=innodb;
create table tgroup_role (
   group_id int not null,
   role_id int not null,
   primary key(group_id, role_id),
   index (group_id),
   foreign key (group_id) references tgroup(group_id),
   index (role_id),
   foreign key (role_id) references trole(role_id)
) type=innodb;
Group 类：
package ivan.hibernate.many2many;
public class Group {
    private int id;
    private String name;
    private Set roles = new HashSet();
    ...
}
Group.hbm.xml：
<hibernate-mapping>
     <class name="ivan.hibernate.many2many.Group" table="tgroup">
         <id name="id" column="group_id" unsaved-value="0">
             <generator class="increment"/>
         </id>
         <property name="name" type="string"/>       
         <set name="roles"
              table="tgroup_role"
             cascade="save-update">             
              <key column="group_id"/>
              <many-to-many class="ivan.hibernate.many2many.Role"
                            column="role_id"/>
        </set>
     </class>
</hibernate-mapping>
Role 类：
package ivan.hibernate.many2many;
public class Role {
    private int id;
    private String name;
    private Set groups = new HashSet();
    ...
}
Role.hbm.xml：
<hibernate-mapping>
     <class name="ivan.hibernate.many2many.Role" table="trole">
         <id name="id" column="role_id" unsaved-value="0">
             <generator class="increment"/>
         </id>
         <property name="name" type="string"/>       
         <set name="groups"
              table="tgroup_role"
             inverse="true"
             cascade="save-update">             
              <key column="role_id"/>
              <many-to-many class="ivan.hibernate.many2many.Group"
                            column="group_id"/>
   </set>
     </class>
</hibernate-mapping>
hibernate.cfg.xml
public class Test {
public static void main(String[] args) throws HibernateException {
     
   Role role1 = new Role();
   role1.setName("Role1");
   Role role2 = new Role();
   role2.setName("Role2");
   Role role3 = new Role();
   role3.setName("Role3");
   Group group1 = new Group();
   group1.setName("group1");
   Group group2 = new Group();
   group2.setName("group2");
   Group group3 = new Group();
   group3.setName("group3");
   group1.getRoles().add(role1);
   group1.getRoles().add(role2);
   group2.getRoles().add(role2);
   group2.getRoles().add(role3);
   group3.getRoles().add(role1);
   group3.getRoles().add(role3);
   role1.getGroups().add(group1);
   role1.getGroups().add(group3);
   role2.getGroups().add(group1);
   role2.getGroups().add(group2);
   role3.getGroups().add(group2);
   role3.getGroups().add(group3);
   SessionFactory sessionFactory = new Configuration().configure().buildSessionFactory();
   Session session = sessionFactory.openSession();
   Transaction tx= session.beginTransaction();
   session.save(role1);
   session.save(role2);
   session.save(role3);
   session.save(group1);
   session.save(group2);
   session.save(group3);
   tx.commit();
   session.close();
   sessionFactory.close();
}
}