java - 多线程 Java 的 Hello World

Question

我想了解如何使用关键字:wait、notify/All、synchronized，所以我决定尝试一个简单的示例。基本上我想做的是创建两个将打印字符串的线程。第一个线程具有字符串“Hello”，而第二个线程具有字符串“World”。

我想要达到的输出如下: 你好 世界 你好 世界 你好 世界 ...

这是我到目前为止编写的代码，但现在的输出是: 你好 你好 你好 ... 世界 世界 世界 ...

错误在哪里？谢谢。 :)

代码如下:

class MyThread implements Runnable {
    private SimpleSyncThread sync;
    private String s;

    public MyThread(SimpleSyncThread sync, String s) {
        this.sync = sync;
        this.s = s;
    }

    public static void pause(long time) {
        try {Thread.sleep(time); }
        catch (InterruptedException e) {Thread.currentThread().interrupt();}
    }

    @Override
    public void run() {
        synchronized (sync) {
            for (int i = 0; i < 10; i++) {
                sync.print(s);
            }
        }
    }
}

public class SimpleSyncThread {

    public void print(String s) {
        System.out.println(s);
        MyThread.pause(200);
    }

    public static void main(String[] args) {
        SimpleSyncThread sync = new SimpleSyncThread();
        MyThread st1 = new MyThread(sync, "Hello");
        MyThread st2 = new MyThread(sync, "World");

        Thread t1 = new Thread(st1);
        Thread t2 = new Thread(st2);

        t1.start();
        t2.start();
    }
}

Answer 1

您在这里持有锁，因此一次只有一个进程可以打印

   synchronized (sync) {
        for (int i = 0; i < 10; i++) {
            sync.print(s);
        }
    }

您可以使用以下方法暂时释放锁定，而不是这样做:

   synchronized (sync) {
        for (int i = 0; i < 10; i++) {
            sync.print(s);
            // I have done my bit, wake other threads.
            sync.notifyAll();
            try {
                // give up the lock and let another thread run.
                sync.wait(10);
            } catch(InterruptedException ie) {
                throw new AssertionError(ie);
            }
        }
    }

<小时/>

您可能想到的是我所说的乒乓球测试。您不会在真实的程序中执行此操作，但此模式可以提供有用的微基准。

public class PingPongMain {
    public static void main(String[] args) throws InterruptedException {
        boolean[] next = {false};
        AtomicInteger count = new AtomicInteger();
        Thread t1 = new Thread(() -> {
            try {
                synchronized (next) {
                    for(;;) {
                        // handle spurious wake ups.
                        while (next[0])
                            next.wait();

                        System.out.println("ping");

                        // state change before notify
                        next[0] = true;
                        next.notifyAll();
                    }
                }
            } catch (InterruptedException e) {
                // expected
            }
        });

        Thread t2 = new Thread(() -> {
            try {
                synchronized (next) {
                    for(;;) {
                        // handle spurious wake ups.
                        while (!next[0])
                            next.wait();

                        System.out.println("pong");

                        // state change before notify
                        next[0] = false;
                        next.notifyAll();

                        count.incrementAndGet();
                    }
                }
            } catch (InterruptedException e) {
                // expected
            }
        });

        t1.start();
        t2.start();

        Thread.sleep(5000);
        t1.interrupt();
        t2.interrupt();
        System.out.println("Ping ponged " + count + " times in 5 seconds");

    }
}

打印

ping
pong
ping
pong
.. deleted ...
Ping ponged 323596 times in 5 seconds