java - notifyAll() 不通知所有线程

Question

这是我的代码，每次运行代码时输出都会有所不同。有时，所有三个读者都会收到通知，输出为:

等待计算...

等待计算...

等待计算...

完成

总计:4950Thread-1

总计:4950Thread-2

总计:4950Thread-0

有时只有两名或一名读者会收到通知。有什么问题吗？

class Reader extends Thread {
    Calculator c;

    public Reader(Calculator calc) {
        c = calc;
    }

    public void run() {
        synchronized (c) {
            try {
                System.out.println("Waiting for calculation...");
                c.wait();
            } catch (InterruptedException e) {
            }
            System.out.println("Total is: " + c.total +Thread.currentThread().getName());
        }
    }

    public static void main(String[] args) {
        Calculator calculator = new Calculator();
        new Reader(calculator).start();
        new Reader(calculator).start();
        new Reader(calculator).start();
        new Thread(calculator).start();
    }
}

class Calculator implements Runnable {
    int total;

    public void run() {
        synchronized (this) {
            for (int i = 0; i < 100; i++) {
                total += i;
            }
            System.out.println("Finished");
            notifyAll();
        }
    }
}

根据元帖子，这个问题被认为是重复的，但两个被欺骗的“重复”根本不适用。 How to use wait and notify in Java?提醒用户，如果您确实想等待同一个对象，则必须对该对象进行同步。但这个解决方案已经在这样做了。 Java: notify() vs. notifyAll() all over again提醒用户 notify 和 notifyAll 之间的区别，这甚至离问题更远。

Answer 1

我能够重现该问题 - 基本上，如果您的计算机速度太快，或者调度不当，计算器可能会在读者有机会同步和等待之前完成。

c:\files\j>java Reader
Waiting for calculation...
Waiting for calculation...
Finished
Waiting for calculation...
Total is: 4950Thread-2
Total is: 4950Thread-0

为防止出现这种情况，您应该在执行计算之前验证所有读取器是否已准备好。

c:\files\j>java 阅读器等待计算...正在等待读者...目前 1等待计算...等待计算...完成的总计:4950Thread-1总计:4950Thread-2总计:4950Thread-0

这是我的代码

import java.util.concurrent.atomic.AtomicInteger; // corsiKa added import
class Reader extends Thread {

    static class Calculator implements Runnable {
        int total;
        AtomicInteger readers = new AtomicInteger(0); // corsiKa added atomicinteger

        public void run() {
            // corsiKa added while
            while(readers.get() < 3) {
                System.out.println("Waiting for readers... currently " + readers.get());
                try { Thread.sleep(100); } catch(InterruptedException e) { }
            }
            synchronized (this) {
                for (int i = 0; i < 100; i++) {
                    total += i;
                }
                System.out.println("Finished");
                notifyAll();
            }
        }
    }

    Calculator c;

    public Reader(Calculator calc) {
        c = calc;
    }

    public void run() {
        synchronized (c) {
            try {
                c.readers.incrementAndGet(); // corsiKa added increment
                System.out.println("Waiting for calculation...");
                c.wait();
            } catch (InterruptedException e) {
            }
            System.out.println("Total is: " + c.total +Thread.currentThread().getName());
        }
    }

    public static void main(String[] args) {
        Calculator calculator = new Calculator();
        new Reader(calculator).start();
        new Reader(calculator).start();
        new Reader(calculator).start();
        new Thread(calculator).start();
    }
}