The given object has a null identifier: entity.Teacher有关问题_J2EE

求助 The given object has a null identifier: entity.Teacher问题
在网上下了一个系统，结果出错连连
下面是教师修改的问题:update失败，出现 The given object has a null identifier: entity.Teacher  我查了网上的，加入了s:hidden也不行

其中:
addteacher.java里面的主要段落为:
public String updataTeacher(){
         Teacher t=new Teacher();
      TeacherDao td=new TeacherDao();
      t.setAge(age);
      t.setEmail(email);
      t.setTeaId(teaId);
      t.setTel(tel);
      t.setTname(tname);
      t.setTpassword(tpassword);
      t.setTsex(tsex);
      t.setTqq(tqq);
      t.setTinf(tinf);
      t.setTsubject(tsubject);
      if(t!=null){
          td.updateTeacher(t);
          return SUCCESS;
      }
      return INPUT;
    }//就是这个出错，学生的那边用另外一种方法是没错的，但如果改的话要从头改到尾

      teacherDAO里面更新的核心代码:
public void updateTeacher(Teacher tc)
    {
        session=HibernateSessionFactory.getSession();
        try{
            transaction=session.beginTransaction();
            session.update(tc);
            transaction.commit();
        }
        catch (Exception e){
            e.printStackTrace();
        }
        HibernateSessionFactory.closeSession();
    }

    struts中有关teacher的代码是这样的:

         <action name="tpageAction" class="AdminAction.TpageAction">
            <result type="redirect" >/allTeacher?pageNumber=${pageNumber}&totalPage=${totalPage}</result>
        </action>
        <action name="allTeacher" class="AdminAction.AllTeacher">
           <result name="success">/admin/allteacher.jsp</result>
        </action>
        
        <action name="getTeacher" class="AdminAction.DguTeacher" method="getTeacher">
        <result name="success">/admin/getTeacher.jsp</result>
        <result name="input">/admin/error.jsp</result>
        </action>