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"
        }
    ]
}