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();
        }
    }