同步，wait/notifyAll 必须在同一个对象上，但是为什么呢？

Question

当我尝试使用wait() 和synchronized 进行简单演示时，我突然想到了一件有趣的事情，以下演示给了我意外的输出。

public class WaitZero {
    private static AtomicInteger num = new AtomicInteger(0);
    private static boolean consumed = false;

    public static void main(String... args) throws Exception {
        ThreadPoolExecutor threadPoolExecutor = getMyCachedThreadPool();
        for (int i = 0; i < 5; i++) {
            threadPoolExecutor.submit(WaitZero::send);
            threadPoolExecutor.submit(WaitZero::receive);
        }
        threadPoolExecutor.shutdown();
        threadPoolExecutor.awaitTermination(60, TimeUnit.SECONDS);
    }

    private static synchronized void send() {
        try {
            while (!isConsumed()) {
                num.wait();
            }
        } catch (InterruptedException ignored) {
            ignored.printStackTrace();
        }
        num.incrementAndGet();
        System.out.println(Thread.currentThread().getName() + " number updated: " + num);
        setConsumed(false);
        num.notifyAll();
    }

    private static synchronized void receive() {
        try {
            while (isConsumed()) {
                num.wait();
            }
        } catch (InterruptedException ignored) {
            ignored.printStackTrace();
        }
        System.out.println(Thread.currentThread().getName() + " number received: " + num);
        setConsumed(true);
        num.notifyAll(); // ToDo: when to use notify?
        // ToDo: what is monitor?
    }

    private static boolean isConsumed() {
        return consumed;
    }

    private static void setConsumed(boolean consumed) {
        WaitZero.consumed = consumed;
    }
}

它的输出不稳定，但典型的一种可以

shared-pool-0 number received: 0
shared-pool-1 number updated: 1
shared-pool-0 number received: 1
shared-pool-1 number updated: 2
shared-pool-1 number received: 2
shared-pool-2 number updated: 3

而我所期待的是

shared-pool-1 number received: 0
shared-pool-0 number updated: 1
shared-pool-3 number received: 1
shared-pool-2 number updated: 2
shared-pool-1 number received: 2
shared-pool-0 number updated: 3
shared-pool-2 number received: 3
shared-pool-3 number updated: 4
shared-pool-5 number received: 4
shared-pool-4 number updated: 5

当我在wait()/notifyAll() 上使用WaitZero.class 而不是num 时检索到正确的结果。

我已经阅读过，似乎总是必须在同一个对象上使用它们中的三个以确保正确性。

我的猜测：如果不是所有这些都在同一个对象上，notifyAll() 和同步锁之间存在特殊情况。但它是什么？

任何帮助将不胜感激;)

Answer 1

在@JB Nizet、@Amardeep Bhowmick 等人提出了很多天真的问题和大力帮助之后，我从How to work with wait(), notify() and notifyAll() in Java? 中找到了一句精辟的句子，准确地解释了原因。

wait() 方法被设计/用于放弃锁（因为某些条件不满足）让其他线程工作/合作；典型的用例是发送者/接收者或生产者/消费者。

wait()

它告诉调用线程放弃锁并进入睡眠状态，直到其他线程进入同一个监视器并调用notify()...@987654328 @ 方法实际上与同步锁紧密集成，使用了同步机制无法直接提供的功能。

synchronized(lockObject) {
    while( ! condition ) {
        lockObject.wait();
    }
    //take the action here;
}

在这种情况下，问题可以简单地修复如下，或者只使用WaitZero.class 代替wait/notifyAll。

public class WaitZero {
    private static AtomicInteger num = new AtomicInteger(0);
    private static boolean consumed = false;

    public static void main(String... args) throws Exception {
        ThreadPoolExecutor threadPoolExecutor = getMyCachedThreadPool();
        for (int i = 0; i < 5; i++) {
            threadPoolExecutor.submit(WaitZero::send);
            threadPoolExecutor.submit(WaitZero::receive);
        }
        threadPoolExecutor.shutdown();
        threadPoolExecutor.awaitTermination(60, TimeUnit.SECONDS);
    }

    private static void send() {
        synchronized (num) {
            try {
                while (!isConsumed()) {
                    num.wait();
                }
            } catch (InterruptedException ignored) {
                ignored.printStackTrace();
            }
            num.incrementAndGet();
            System.out.println(Thread.currentThread().getName() + " number updated: " + num);
            setConsumed(false);
            num.notifyAll();
        }
    }

    private static void receive() {
        synchronized (num) {
            try {
                while (isConsumed()) {
                    num.wait();
                }
            } catch (InterruptedException ignored) {
                ignored.printStackTrace();
            }
            System.out.println(Thread.currentThread().getName() + " number received: " + num);
            setConsumed(true);
            num.notifyAll(); // ToDo: when to use notify?
            // ToDo: what is monitor?
        }
    }

    private static boolean isConsumed() {
        return consumed;
    }

    private static void setConsumed(boolean consumed) {
        WaitZero.consumed = consumed;
    }
}