如何检查json对象是否为空

我正在尝试使用jquery验证插件验证表单。我正在尝试是否有emailId在表中，如果不允许注册，否则不是。但它显示的是json对象总是大于0。我尝试使用getjson，它不工作在本地主机。那么，我哪里做错了?

$('input#emailId').blur( function() {
                    $.ajax({
                        url:'/getUserByEmailOrPhone-'+$('#emailId').val()+'-'+$('#contactNumber').val(),
                        type:'GET',
                        data: 'json',
                        success:function(json){
                            if (json.length == 0)
                                {
                                    $('.email-check').append('<span class="glyphicon glyphicon-ok form-control-feedback" aria-hidden="true" id="tt2" clientidmode="Static" runat="server" Visible="False"></span>');
                                  }
                            else{
                                  alert('user exists');
                                  $('.email-check').append('<span class="glyphicon glyphicon-remove form-control-feedback" aria-hidden="true" id="tt2" clientidmode="Static" runat="server" Visible="False"></span>');
                            }
                        }
                });
            });

AppController.java

@RequestMapping(value={"/getUserByEmailOrPhone-{emailId}-{contactNumber}"}, method = RequestMethod.GET)
public @ResponseBody User getUserByEmailOrPhone(@PathVariable("contactNumber") String contactNumber,
        @PathVariable("emailId") String emailId,ModelMap model){
        return service.getUserByEmailOrPhone(emailId, contactNumber);
}