java后台接收json数据，报错com.alibaba.fastjson.JSONObject cannot be cast to xxx

　　从前台接收json封装的list数据，在后台接收时一直报错，com.alibaba.fastjson.JSONObject cannot be cast to xxx，

　　使用这种方式接收可以接收

 @RequestMapping(value = "/insertUser", method = RequestMethod.POST)
 public ResultT insertUser(@RequestBody Map<String,List<User>> map){
     List<User> userList = map.get("insertUserList");
 }

 //前台json格式
 {
     "insertUserList": [
         {
             "firstName": "Brett",
             "lastName": "McLaughlin",
             "email": "aaaa"
         },
         {
             "firstName": "Jason",
             "lastName": "Hunter",
             "email": "bbbb"
         },
         {
             "firstName": "Elliotte",
             "lastName": "Harold",
             "email": "cccc"
         }
     ]
 }