java - 了解多线程等待和通知

Question

这个问题已经有答案了:

How to use wait and notify in Java without IllegalMonitorStateException? (12 个回答)

已关闭 5 年前。

我正在学习多线程，我试图了解如何使用 Object 类的等待和通知方法。我已经浏览过这个链接https://www.journaldev.com/1037/java-thread-wait-notify-and-notifyall-example并编写了以下程序

服务员

public class Waiter implements Runnable {
    private Message m;
    public Waiter(Message m) {
         this.m = m;
    }
    public void run() {
        String name = Thread.currentThread().getName();
        System.out.println(t1 + " thread waiting for message");
        synchronized (m) {
            try {
                m.wait();
                System.out.println(t1 + " " + m.getText());
            } catch (InterruptedException e) {
                e.printStackTrace();
            }
        }
        System.out.println(t1 + " thread waiting for message");
    }
}

通知者

public class Notifier implements Runnable {
    private Message m;
    public Notifier(Message m) {
         this.m = m;
    }
    public void run() {
        synchronized (m) {
            try {
                Thread.sleep(2000);
                m.notifyAll();
            } catch (InterruptedException e) {
                e.printStackTrace();
            }
        }
    }
}

测试

public class WaitNotifyTest {
    public static void main(String[] str) {
        Message m = new Message("hello");
        new Thread(new Waiter(m), "t1").start();
        new Thread(new Waiter(m), "t2").start();
        new Thread(new Notifier(m)).start();
    }
}

当我执行程序时，它有时会正确终止，有时会无限期等待，有时一个线程终止而另一个线程无限期等待。谁能告诉我这里出了什么问题吗？

我还想了解一些等待和通知方法实时应用的例子。

Answer 1

当您进行等待时，最佳实践是在带有条件的 while 循环中执行。可能存在线程发出通知，然后其他线程进入等待状态的情况。因此线程将始终处于等待状态

修改后的代码:

public class Waiter implements Runnable {
    private Message m;

    public Waiter(Message m) {
        this.m = m;
    }

    public void run() {
        String name = Thread.currentThread().getName();
        System.out.println(name + " thread waiting for message");
        synchronized (m) {
            try {
                while (m.getText() == null) {
                    m.wait();
                }
                System.out.println(name + " " + m.getText());
            } catch (InterruptedException e) {
                e.printStackTrace();
            }
        }
        System.out.println(name + " thread waiting for message");
    }
}

public class Notifier implements Runnable {
    private Message m;

    public Notifier(Message m) {
        this.m = m;
    }

    public void run() {
        synchronized (m) {

            m.setText("hello");
            m.notifyAll();

        }
    }
}

public class WaitNotifyTest {
    public static void main(String[] str) {
        Message m = new Message();
        new Thread(new Waiter(m), "t1").start();
        new Thread(new Waiter(m), "t2").start();
        new Thread(new Notifier(m)).start();
    }
}