java - 尝试同步两个线程时遇到问题

Question

我试图同步到两个线程，一个打印“ping”，另一个打印“pong”，打印类似“ping pong ping pong ping....”的内容，我编写了以下程序来实现这一目标，但无法得不到预期的结果。我想知道我犯了什么错误。互联网上可以找到相同的解决方案，但我认为在寻找现成的答案之前我应该​​自己尝试一下。

class MyThread implements Runnable{
  String val = null;
  MyThread(String val) {
      this.val = val;
  }

    public void run() {
        while(true){
            PingPong.pintMessage(val);
        }
    }
}

public class PingPong {
   static Integer turn = 1;

   public static void main(String args []) {
       Thread t1 = new Thread(new MyThread("ping"));
       Thread t2 = new Thread(new MyThread("pong"));
       t1.start();
       t2.start();

   }
   public static void pintMessage(String msg) {
       synchronized (turn) {
         if(turn==1) {
             System.out.println(Thread.currentThread().getName()+" "+msg);
             turn=2;
         }
         else {
             System.out.println(Thread.currentThread().getName()+" "+msg);
             turn = 1;
         }
    }
   }
}

输出:

Thread-0 ping
Thread-1 pong
Thread-1 pong
Thread-1 pong
Thread-0 ping
Thread-0 ping
Thread-0 ping
Thread-1 pong
Thread-1 pong
Thread-1 pong
Thread-0 ping
Thread-0 ping
Thread-0 ping
Thread-1 pong
Thread-1 pong
Thread-1 pong
Thread-0 ping
Thread-0 ping
Thread-0 ping
Thread-1 pong
Thread-1 pong
Thread-1 pong
Thread-0 ping
Thread-0 ping
Thread-0 ping
Thread-1 pong
Thread-1 pong
Thread-1 pong
Thread-0 ping
Thread-0 ping
Thread-0 ping
Thread-1 pong

请告诉我我在这里犯了什么基本错误，以及是否有更好的方法来实现相同的行为。谢谢!!

进行建议的更改后:我尝试使用等待/通知来使用线程通信，但收到 IllegalMonitorStateException。我想我已经在我正在获取锁定的对象上使用了等待/通知(在同步上下文中使用了等待/通知)。请帮助我理解这个概念的微妙之处。谢谢!

class MyThread implements Runnable{
    String val = null;

    MyThread(String val) {
        this.val = val;
    }

    public void run() {
        while(true){
            try {
                PingPong.printMessage(val);
            } catch (InterruptedException e) {
                e.printStackTrace();
            }

        }
    }
}

public class PingPong {
    static Integer turn = 1;

    public static void main(String args []) {
        Thread t1 = new Thread(new MyThread("ping"));
        Thread t2 = new Thread(new MyThread("pong"));
        t1.start();
        t2.start();
    }
    public static void printMessage(String msg) throws InterruptedException {
        synchronized (turn) {
            if(turn==1) {
                System.out.println(Thread.currentThread().getName()+" "+msg);
                turn=2;
            }
            else {
                System.out.println(Thread.currentThread().getName()+" "+msg);
                turn = 1;
            }
            turn.notifyAll();
            turn.wait();
        }
    }
}

Output :
Thread-0 ping
Exception in thread "Thread-0" Thread-1 pong
java.lang.IllegalMonitorStateException
    at java.lang.Object.notifyAll(Native Method)
    at PingPong.printMessage(PingPong.java:40)
    at MyThread.run(PingPong.java:12)
    at java.lang.Thread.run(Unknown Source)

Answer 1

你对同步的概念理解错误了。

JVM 确保始终只有一个进程执行同步 block ，但它不控制排队进程，这意味着它只按照调用的顺序处理调用 block 。

在您的情况下:每个线程都有机会多次执行循环，然后 JVM 将其设置为后台以供另一个线程执行。

如果您想确保特定的顺序，则必须使用 wait() 和 notify()/notifyAll():([编辑]:此代码可能无法正常工作(未经测试)，它只是为了演示原理...)

class MyThread implements Runnable{
  private Object other;
  public void setOther (MyThread other){
    this.other = other;

  }
  //no changes
       while(true){
            PingPong.pintMessage(val);
            notifyAll();
            other.wait();
        }

}

public static void pintMessage(String msg) {
    synchronized (turn) {
     // no changes
      // MyThread.semaphore.notifyAll();
 }
}    

请参阅此处了解更多信息 https://docs.oracle.com/javase/tutorial/essential/concurrency/