code: private TreeControl getTreeControl(HttpServletRequest request) throws
        Exception {
      HttpSession session = request.getSession();
      TreeControl control = (TreeControl) session.getAttribute(TreeControlTag.
          TREE_NAME);
      try{
        //第一次调用
        if ( (control == null) || (request.getParameterMap().isEmpty())) {
          Debug.logVerbose("[JdonFramework] get tree from db..", module);
          ViewNode root =  getRootViewNode(request);
          control = new TreeControl(root);
          fetchRootChildern(request, root);
          session.setAttribute(TreeControlTag.TREE_NAME, control);
        }
      }catch(Exception ex){
        Debug.logError("[JdonFramework] getTreeControl error: " + ex, module);
      }
      return  control;
    }
docs: 当调用无参数，看成是初始化，重新再从后台获得root
  
  root node 双向关联　control
  每一个Root Node 对应一个control
  
  @param request HttpServletRequest
  @throws Exception
  @return TreeControl