Java中notifyAll()无法唤醒wait()暂停的线程如何解决？

问题背景 #

多人传球Socket游戏逻辑如下：

• 客户端连接服务端后先判断自身是否持有球（hasBall）
• 持有球可调用方法将球传给其他客户端
• 未持有球则调用lock.wait()进入等待接收球的状态

问题现象 #

调用lock.notifyAll()后，无法唤醒处于lock.wait()状态的线程。具体表现为：
第一个加入的客户端默认持有球，第二个加入的客户端无球进入等待状态；客户端1将球传给客户端2后，客户端1更新为无球状态，客户端2始终没有更新为持有球状态。

问题根因 #

1. wait/notify是同JVM进程内的线程同步机制：每个客户端是独立运行的进程，进程1内调用的notifyAll只能唤醒进程1内的等待线程，完全无法影响进程2内的等待线程，跨进程的状态同步不能依赖这套API。
2. 状态值缓存未更新：原代码中hasBall的取值是在while(true)循环最开始计算的，进入synchronized块的等待循环后，哪怕线程被唤醒，判断条件用的还是循环开始时缓存的旧值，永远不会更新为最新的持球状态，导致线程被唤醒后又立刻进入等待。
3. 通知触发时机错误：原代码只有本客户端执行传球操作时才会调用notifyAll，当其他客户端传球给当前客户端时，当前客户端没有任何触发notifyAll的逻辑，等待线程永远不会被主动唤醒。

修复方案 #

1. 新增服务端推送逻辑：服务端收到任意客户端的传球请求、更新完全局持球人信息后，要主动给所有在线客户端推送最新的持球人数据，不能等客户端轮询拉取。
2. 修正状态判断逻辑：等待循环的判断条件每次都要拉取最新的持球人信息，禁止使用循环开头缓存的旧值，修改后代码如下：

try{
    synchronized (lock){
        // 每次判断都拉取服务端最新的持球状态，不使用外层缓存值
        while (!Objects.equals(client.getPlayerId(), client.getPlayerWithBall())){
            receive();
            lock.wait();
            System.out.println("resumed");
        }
    }
}catch (Exception e){
    e.printStackTrace();
}

3. 新增客户端收到推送后的唤醒逻辑：每个客户端收到服务端推送的最新持球人信息后，立刻调用lock.notifyAll()唤醒自身进程内的等待线程，重新判断持球状态。

原问题相关代码 #

import java.util.*;
import java.util.concurrent.CountDownLatch;

public class ClientInterface extends Thread {

    private final Client client;
    final static Object lock = new Object();


    public ClientInterface(Client client){
        this.client = client;
    }


    public void pass(String playerId) throws Exception {
            Scanner scanner = new Scanner(System.in);
            System.out.println("\nYou currently have the ball.");
            System.out.println("Enter ID of player you wish to pass to...");
            String line = scanner.nextLine();
            client.passBall(playerId, line);
            System.out.println("Pass ball to player #" + line);

    }

    public void receive() throws InterruptedException {
            System.out.println("waiting");
            System.out.println("\nPlayer #" + client.getPlayerWithBall() + " currently has the ball.");
    }


    @Override
    public void run() {
        try {

                System.out.println("Connected to Ball Game successfully.\n");

                while (true) {

                    // Get the list of all players currently connected to the server
                    String[] players = client.getPlayers();
                    // Get the ID generated for this client
                    String playerId = client.getPlayerId();
                    boolean hasBall = Objects.equals(playerId, client.getPlayerWithBall());

                    System.out.println("Your ID is : #" + playerId + '\n');
                    System.out.println("List of current in-game players:");
                    int count = 1;
                    //Print the ID strings of each player
                    for (String player : players) {
                        System.out.println("[" + count + "] ID: #" + player);
                        count++;
                    }

                    // Check if the player currently has the ball or not
                    // If they don't have the ball, print waiting, and call wait on object lock
                    try{
                        synchronized (lock){
                            while (!hasBall){
                                receive();
                                lock.wait();
                                System.out.println("resumed");
                            }
                        }
                    }catch (Exception e){
                        e.printStackTrace();
                    }

                    // Allow player with ball to send ball to another client
                    // call notify on object lock so thread can unlock if they now have the ball
                    synchronized (lock){
                        try {
                            pass(playerId);
                            lock.notifyAll();
                        }catch (Exception e){
                            e.printStackTrace();
                        }
                    }

                }

        } catch (Exception e) {
            System.out.println(e.getMessage());
            e.printStackTrace();
        }
    }


    public static void main(String[] args) throws Exception {
        Client client = new Client();
        ClientInterface c = new ClientInterface(client);
        if(client.getPlayers().length > 0){
            c.start();
        }
    }
}

内容的提问来源于stack exchange，提问作者Sean2148