Hibernate: different object with the same identifier value was already associated with the session

Question

when I use the DAO.update(userbean), session.SaveOrUpdate(e);throw the Exception:different object with the same identifier value was already associated with the session

the function is like next:

    public E save(E e) {
    Session session = null;
    try {
        session = sessionFactory.openSession();
        log.debug("session="+session.hashCode()+" save "+e);
        session.SaveOrUpdate(e);  //here throws exception 
        session.flush();
    }
    catch (Exception e1) {
        log.err("Cannot open hibernate session "+ e1.getMessage()+" cause : "+e1.getCause());
        e1.printStackTrace();
    }
    finally { if ( session != null ) session.close(); session = null;}
    return e ;
}

the userbean is an instance of class UserBean

public class UserBean{
   private List<GroupBean> groups = new ArrayList<GroupBean> ();
   private List<RoleBean> roles = new ArrayList<RoleBean> ();
}

public class GroupBean{
private List<RoleBean> roles = new ArrayList<RoleBean> ();
}

every groupbean has a list of roles, which are not changed.

in database, group and role is many-to-many mapping,

for example,

we have a groupbean#1, it's roles: rolebean#1, rolebean#2;

groupbean#2, which roles are rolebean#1.

now I create a new userbean#1, it's groups is groupbean#1 and if I want to add the rolebean#1 to userbean#1, it will throws the exception like the title descript

I look the server.log, and find that when I user DAO.save, the saveOrUpdate order is:

userbean#1
|---|-----------***userbean.groups
|     |     groupbean#1
|     |         groupbean.roles
|     |             rolebean#1  # save relebean#1 the first time
|     |             ---done rolebean#1
|     |         ------done all rolebeans of group.roles
|     |     ---done groupbean#1
|     |-----------done all groupbeans of userbean.groups
|---|-----------***userbean.roles
     |      rolebean#1          # save rolebean#1 the second time, and throws exception here!
     |      ----done rolebean#1
     |      .....
     |-----------done all rolebeans of userbean.roles

the cause of the exception is rolebean#1 has been saved twice in a session, and their identity is the same.

In the function save(E e), If I use

session.merge(e);

replace

session.SaveOrUpdate(e);

will not throw exception, but the rolebean#1 is not assocaited to userbean#1

anyone can give some suggestions about this? thank you !