问怎么判断asynctask线程执行结束？

package com.example.qiongju_1.Repository;

import android.content.Context;
import android.os.AsyncTask;
import android.util.Log;

import com.example.qiongju_1.Dao.UserDao;
import com.example.qiongju_1.Database.MyDatabase;
import com.example.qiongju_1.entity.User;

public class UserRepository {

    public User aUser;
    private User bUser;
    private UserDao userDao;
    //声明另一个全局变量,获取用户的状态


    public UserRepository(){}
    public UserRepository(Context context) {
        MyDatabase myDatabase = MyDatabase.getDatabase(context.getApplicationContext());
        userDao = myDatabase.getUserDao();
    }
    //外访问数据库操作接口
    public void inserUser(User...users){
        new InsertUserAsyncTask(userDao).execute(users[0]);
    }

    public User findUser(String...strings){

        QueryUserAsyncTask queryUserAsyncTask = new QueryUserAsyncTask(userDao);
        queryUserAsyncTask.execute(strings);
        queryUserAsyncTask.setListener(new QueryUserAsyncTask.OnResponseListener<User>(){
            @Override
            public void onResponse(User user) {
                //回调获得数据
                aUser = user;

                //aUser这里可以正常被赋值
                if (aUser != null){
                    Log.d("CF","在Repository.fingUser().listener方法里："+aUser.getUserAccount());
                }else{
                    Log.d("CF","在Repository.fingUser().listener方法里：用户为空");
                }
            }
        });


        //aUser在这里就是null的
        if (aUser != null){
            Log.d("CF","在Repository.fingUser()方法里："+aUser.getUserAccount());
        }else{
            Log.d("CF","在Repository.fingUser()方法里：用户为空");
        }

        return aUser;
    }

    //单例模式的类要声明static, 后台执行数据库操作
    //插入用户
    static class InsertUserAsyncTask extends AsyncTask<User,Void,Void> {
        private UserDao userDao;
        public InsertUserAsyncTask(UserDao userDao) {
            this.userDao = userDao;
        }
        @Override
        protected Void doInBackground(User...users) {
            userDao.insertUser(users[0]);
            return null;
        }
    }
//    根据账号查找用户
    static class QueryUserAsyncTask extends AsyncTask<String,Void,User>{
        private UserDao userDao;
        public User user;
        private OnResponseListener<User> listener;

        public QueryUserAsyncTask(UserDao userDao) {
            this.userDao = userDao;
        }
        public void setListener(OnResponseListener<User> listener){
            this.listener = listener;
        }
        @Override
        protected User doInBackground(String... strings) {
            return userDao.findUserByAccount(strings[0]);
        }
        @Override
        protected void onPostExecute(User user) {
            super.onPostExecute(user);
            if(listener!=null){
                //调用接口方法，返回结果
                listener.onResponse(user);
            }
        }
        //接口，类似一个监听事件
        public interface OnResponseListener<T>{
            void onResponse(T t);
        }
    }
}