AJAX 前后端交互 验证信息是否正确

1、前段：

 function checkPtCode(obj){
     $.ajax({
         type: "post",
         url: "xxxxxxx",
         data: {"xx":obj.value},
         cache: false,
         async : true,
         dataType:"json",
         type:"POST",
         success: function (result){
         if(result.flag == "No"){
             jAlert("编码库不存在此编码，请修正！");
         }
          }
      });
 }       

2、后端

 public void checkPtCode(HttpServletRequest request,
             HttpServletResponse response) throws IOException {
         PrintWriter out = response.getWriter();
         JSONObject json = new JSONObject();
         try {
             response.setContentType("text/html;charset=UTF-8");
             response.setContentType("text/html");
             response.setHeader("Cache-Control", "no-store");
             response.setHeader("Pragma", "no-cache");
             response.setDateHeader("Expires", 0);
             String ptCode = request.getParameter("ptCode");

             if(ptCode != null && ptCode.trim().length() != 0){
                 // 编码库校验
                 List<DrpProductType> list = this.controlRelationService.checkCode(ptCode);
                 if(list != null && list.size() != 0){
                     json.put("flag", "ok");
                     out.print(json.toString());
                 }else {
                     json.put("flag", "No");
                     out.write(json.toString());
                 }
             }
         } finally {
             out.close();
         }
     }