前台：

   $.ajax({
   type: ‘POST‘,
     url: ‘/admin/mall/category-add-edit.htm?ajax=‘+"ajax",
     data: "categoryId=" + categoryFormid+"&city="+city,
     success: function(msg){
      //alert(msg);
      if(msg==undefined || msg==null ||msg==""){
       return ;
      }
      var jo=eval(‘(‘+msg+‘)‘);
      //1百分比  2固定值
      $("#charge_type").val(jo.charge_type);
      if(jo.charge_type==1){
           document.getElementById(‘type1‘).style.display="";
           document.getElementById(‘type2‘).style.display="none";
          }
          if(jo.charge_type==2){
           document.getElementById(‘type1‘).style.display="none";
           document.getElementById(‘type2‘).style.display="";
          }
     }
  })

后台：

      Long categoryId = ControllerUtils.getLongParameter(map, "categoryId", request);
      response.setContentType("text/html;charset=GBK");
      PrintWriter out = response.getWriter();
      JSONObject jsonO=new JSONObject();
      GoodsCategoryExt charge_type=extService.getExt(categoryId, "charge_type");
      GoodsCategoryExt fixTip=extService.getExt(categoryId, "fixTip");
      GoodsCategoryExt fee=extService.getExt(categoryId, "fee");
      jsonO.put("charge_type", charge_type.getExt_str());
      jsonO.put("fixTip", fixTip.getExt_str());
      jsonO.put("fee", fee.getExt_str());
      out.print(jsonO.toString());
      out.flush();
      out.close();
      return null; //这里必须return null; 否则会return mv等信息，对返回的信息进行干扰

或者

          response.setCharacterEncoding("UTF-8");
          response.getWriter().write("exist");
          response.flushBuffer();
          return null;

Ajax