ajax后台校验返回结果给前端

1.发送请求并传递参数
$.ajax({
    url: ctx + "mes/flowinfo/itemsExists?itemId="+row.itemId+"&progId="+progId,
type: 'post',//提交类型
//data: { "itemId": row.itemId, "progId": progId }, //参数都是以JSON方式传递
async: false,//true为异步，false为同步
contentType: "application/json",
dateType: "json",
success: function(data) {
        if ( data.EXISTS == 'Y' ) {
            $.modal.alertError("章节已经存在列表中!");
} else {
            $.operate.post(ctx + "dfm/configenum/addFmNode", { "itemId": row.itemId, "progId": progId } ,function () {
                //刷新列表
$.modal.msgSuccess("添加成功");
$.table.refresh();
$.modal.closeLoading();
});
}
    }
})

2.后台代码,返回JSON格式,这里用的是fastjson工具
@PostMapping({"/itemsExists"})
@ResponseBody
public JSONObject ifItemIdExists(Long itemId,Long progId) {
    FmFlowConfigInfo fmFlowConfigInfo = new FmFlowConfigInfo();
fmFlowConfigInfo.setItemId(itemId);
fmFlowConfigInfo.setProgId(progId);
List<FmFlowConfigInfo> fmFlowConfigList = this.fmFlowConfigInfoService.selectFmFlowConfigInfoList(fmFlowConfigInfo);
JSONObject jsonObject = new JSONObject();
String exists = fmFlowConfigList.size() > 0 ? "Y" : "N";
jsonObject.put("EXISTS",exists);
    return jsonObject;
}