ajax 提交 “请求失败”

在前台指定的dataType，后台要和其保持一致，这个例子是在ssh整合视频的例子上做的{忘了是那的视频了}

 function queryGrade(param){//personright!save.action
    var url="/mytest/queryGrade!queryGrade.action";
   //alert(url); alert(param);
     $.ajax({
         url:url,  
         type: 'POST',
         dataType: 'xml', 
         data: {tabName:param},                
         error: function(){                    
           alert('操作失败');
         },
         success: function(xml){ 
           alert("操作成功");         
               
        }
     });
   
   
    }

后台action

 public String queryGrade() throws Exception
 {
  List<Grade> lst=this.service.findAllgrade(tabName);
  System.out.println("tabName is:"+tabName);
  System.out.println("size is:"+lst.size());
  
   StringBuilder sb=new StringBuilder();
   sb.append("<?xml version='1.0' encoding='UTF-8'?><table>");
  
  
  
     
        sb.append("</table>");
  
  HttpServletResponse rep=ServletActionContext.getResponse();
  rep.setContentType("text/xml");
  rep.setCharacterEncoding("utf-8");
  rep.getWriter().print(sb.toString());
  rep.getWriter().flush();
  rep.getWriter().close(); 
  return null;
 }