使用ajax提交表单，回调函数执行error方法

RT,
js：

    <script>
        $(function() {
            $('#login_button').unbind("click").bind("click",function() {
                                $.ajax({
                                    type : 'POST',
                                    url : '${pageContext.request.contextPath}/userBiz?action=login',
                                    data : $('#login_form').serialize(),                                                                                
                                    success : function(data) {
                                                            if(data==1){
                                                                alert("登陆成功");
                                                                location.reload();  
                                                            }
                                                            if (data== 2) {
                                                                alert("用户名或密码错误，请重新登录");                                                                                                                                
                                                            }                                                           
                                                            },
                                     error :function(XMLHttpRequest, textStatus, errorThrown,dataType){
                                        alert(XMLHttpRequest.status); 
                                        alert(XMLHttpRequest.readyState); 
                                        alert(textStatus); 
                                        alert(errorThrown);
                                        alert(dataType);
                                    }
                                        })
                            })
        })
    </script>

servlet:

    private void userLogin(HttpServletRequest request, HttpServletResponse response){
        PrintWriter out = null;
        User user = new User();
        System.out.println("login执行中。。。。。。。。。。。。。。。。。。。。。。。。。");
        user.setUser_account(request.getParameter("user_account"));
        user.setUser_password(MD5Util.MD5(request.getParameter("user_password")));
        UserService service = new UserServiceImpl();
        user=service.userLogin(user);
        System.out.println("执行登录");
        if(user != null){
            try {               
                out =response.getWriter();
                out.print(1);
                System.out.println("执行");
                request.getSession().setAttribute("user",user);
            } catch (Exception e) {
                System.out.println(e);
            }
        }else{
            try {
                out =response.getWriter();
                out.print(2);
            } catch (IOException e) {
                e.printStackTrace();
            }
        }

    }

 error :function(XMLHttpRequest, textStatus, errorThrown,dataType){
alert(XMLHttpRequest.responseText);//看服务器返回什么内容

用抓包工具查看数据包