struts+hibernate 问题(急)

operate.java 代码
public class operate {
private Session session=null;
SessionFactory factory=new Configuration().configure().buildSessionFactory();
public static final ThreadLocal thread=new ThreadLocal();
    public Session currentsession(){
     session=(Session)thread.get();
     if(session==null){
     session=factory.openSession();
     thread.set(session);
     }
     return session;
    }
    
    public void closesession(){
     session=(Session)thread.get();
     if(session!=null)
     session.close();
     thread.set(null);
    }
}saveteam.java
public class saveteam {
Transaction tran=null; 
operate op=new operate();
 private Session session=null;
 ActionMessages errors=new ActionMessages();
 public void insert(Team team){
     try{
 session=op.currentsession();
     Transaction tran=session.beginTransaction();
     session.save(team);
     tran.commit();
    
     }catch(Exception ex){
      ex.printStackTrace();
      tran.rollback();
     }finally{
      op.closesession();
     }
   }
}
TeamAction.java
public ActionForward execute(ActionMapping mapping, ActionForm form,
HttpServletRequest request, HttpServletResponse response) {
TeamForm teamForm = (TeamForm) form;// TODO Auto-generated method stub
ActionMessages errors=new ActionMessages();
Team tm=new Team();
tm.setId(teamForm.getId());
tm.setTeamId(teamForm.getTeam_id());
System.out.println(tm.getId());
System.out.println(tm.getTeamId());
try{
saveteam stm=new saveteam();
stm.insert(tm);
}catch(Exception ex){
ex.printStackTrace(System.out);
errors.add(ActionMessages.GLOBAL_MESSAGE, new ActionMessage("error.insert.failed"));
}
if(!errors.isEmpty()){
saveMessages(request,errors);
return(new ActionForward(mapping.getInput()));
}
ActionMessages actionmessages=new ActionMessages();
actionmessages.add(ActionMessages.GLOBAL_MESSAGE, new ActionMessage("record.inserted"));
saveMessages(request,actionmessages);
return mapping.findForward("sucess");
}
}为什么插入数据的时候说
SQL Error: 515, SQLState: HY000
[Microsoft][SQLServer 2000 Driver for JDBC][SQLServer]无法将 NULL 值插入列 'id'，表 'piao.dbo.team'；该列不允许空值。INSERT 失败。
SQL Error: 3621, SQLState: HY000
[Microsoft][SQLServer 2000 Driver for JDBC][SQLServer]语句已终止。
org.hibernate.exception.GenericJDBCException: could not insert: [text.Team]
呢??
我打印出tm的id,和team_id都能打印出来.有值的啊!!