java - 在java中同步两个线程

Question

我的 java 程序中有两个线程，一个是主线程，另一个线程是在主线程中生成的线程 A。现在我希望主线程启动线程 A 并等待直到线程 A 在 run 方法中执行了它的某些代码并且线程 A 应该挂起自己。然后主线程应该开始运行，运行几行代码，然后线程 A 应该从它停止的地方重新开始，反之亦然。这应该发生 n 次。我正在尝试如下:

线程A类:

public class ThreadA implements Runnable {
    boolean suspended = false;
    boolean stopped = false;
    synchronized void stop() {
        stopped = true;
        suspended = false;
        notify();
    }
    synchronized void suspend() {
        suspended = true;
    }
    synchronized void resume() {
        suspended = false;
        notify();
    }
    void job() throws InterruptedException {
        for (int i = 0; i < 5; i++)
            synchronized (this) {
                System.out.println("performing job.");
                suspend();
                while (suspended) {
                    notify();
                    suspended = false;
                }
            }
    }
    @Override
    public void run() {
        try {
            job();
        } catch (InterruptedException e) {
            e.printStackTrace();
        }
    }
}

主线程:

public class MainThread {
    public static void main(String[] args) throws InterruptedException {
        ThreadA a1=new ThreadA();
        Thread t1=new Thread(a1);
        synchronized (t1) {
            t1.start();
            for (int i = 0; i < 5; i++) {
                t1.wait();
                System.out.println("perform some action");
                a1.resume();
            }

        }
    }
}

预期输出:

performing job.
perform some action
performing job.
perform some action
performing job.
perform some action
performing job.
perform some action
performing job.
perform some action

实际输出:

performing job.
performing job.
performing job.
performing job.
performing job.
perform some action

我不知道为什么整个 for 循环都在线程 A 中执行，即使我已经在作业方法中发出了 notify() 信号。

Answer 1

这里有两个错误。

首先是你在同步和通知不同的对象。试试这个修改后的 main，我将 synchronized (t1) 更改为 synchronized (a1) 并将 t1.wait() 更改为 a1.wait()。

public static void main(String[] args) throws InterruptedException {
    ThreadA a1=new ThreadA();
    Thread t1=new Thread(a1);

    synchronized (a1) {    // CHANGED FROM t1 to a1
        t1.start();
        for (int i = 0; i < 5; i++) {
            a1.wait();    // CHANGED FROM t1 to a1
            System.out.println("perform some action");
            a1.resume();
        }

    }
}

第二个错误是在 job() 方法中，它调用了 notify() 但没有调用 wait()。这是一个固定版本:

void job() throws InterruptedException {
    for (int i = 0; i < 5; i++)
        synchronized (this) {
            System.out.println("performing job.");
            suspend();
            while (suspended) {
                notify();
                suspended = false;
                wait();    // ADDED
            }
        }
}

我的测试运行的输出是

performing job.
perform some action
performing job.
perform some action
performing job.
perform some action
performing job.
perform some action
performing job.
perform some action