code: public Object getUserFromPrincipal(HttpServletRequest request) {
      Debug.logVerbose("[JdonFramework]--> enter getUserFromPrincipal", module);
      Principal principal = request.getUserPrincipal();
      if (principal == null) {
        Debug.logVerbose("[JdonFramework]principal is null, not login by JAAS", module);
        return null;
      }
      String username = principal.getName();
      Debug.logVerbose("[JdonFramework]--> find the logined username=" + username, module);
      HttpSession session = request.getSession();
      Object model =  session.getAttribute(USERMODEL);
      if (model == null) {
        model = findUserModelIFByName(request, username);
        session.setAttribute(USERMODEL, model);
      }
      return model;
    }
docs: 将UserModel从登陆的Principal中获取后，
  Principal当用户访问被授权区域，会由Web容器自动产生。
  保存在HttpSession中，供其它Web层组件使用。
  @param request