java - 调用notifyAll有什么好的解决方案吗？

Question

我有一个关于 wait() 和 notifyAll() 方法的小问题。该代码模拟两个线程的“竞争”。

让我们看一下代码 - 问题是，notifyAll() 方法对等待线程不执行任何操作，导致 main 方法首先获得锁...简单的解决方案是设置一些延迟(请参阅注释行)。但这是一个不好的做法。这个问题有什么好的解决办法吗？我希望仅使用 wait/notifyAll/join 方法。

public class TestThreads {
  public static void main(String[] args) throws InterruptedException {
    System.out.println("_start main");
    Object lock = new Object();
    Thread t1 = new Thread(new Car("Red car", lock));
    Thread t2 = new Thread(new Car("Black car", lock));
    t1.start();
    t2.start();
    //Thread.sleep(10L);
    synchronized (lock){
      System.out.println("Let`s go!");
      lock.notifyAll();
    }
    t1.join();
    t2.join();
    System.out.println("_exiting from main...");
  }
}

class Car implements Runnable {
  private final String name;
  private final Object lock;

  public Car(String name, Object lock) {
    this.name = name;
    this.lock = lock;
  }

  @Override
  public void run() {
    int distance = 100;
    synchronized (lock){
      try{
        System.out.println(name + " waiting...");
        lock.wait();
      }catch (InterruptedException e){
        e.printStackTrace();
      }
    }
    System.out.println(name + " started...");
    while (distance != 0){
      try{
        Thread.sleep((long) (100 * Math.random()));
      }catch (InterruptedException e){
        e.printStackTrace();
        break;
      }
      distance--;
      if (distance % 20 == 0 && distance != 0){
        System.out.println(name + " " + distance+ " miles left");
      }

      else if (distance == 0){
        System.out.println(name + " finished race!!!");
      }
    }
    System.out.println("_exiting from thread of " + name + " move simulation...");
  }
}

PS。抱歉我的英语不好。

谢谢您的回答。那么，这个解决方案是不是更好呢？

public class TestThreads {
  public static void main(String[] args) throws InterruptedException {
    System.out.println("_start main");
    LightSignal lock = new LightSignal();
    Thread t1 = new Thread(new Car("Red car", lock));
    Thread t2 = new Thread(new Car("Black car", lock));
    t1.start();
    t2.start();
    synchronized (lock){
      Thread.sleep(1000L);
      lock.isGreen = true;
      System.out.println("Let`s go!");
      lock.notifyAll();
    }
    t1.join();
    t2.join();
    System.out.println("_exiting from main...");
  }
}

class Car implements Runnable {
  private final String name;
  private final LightSignal lock;

  public Car(String name, LightSignal lock) {
    this.name = name;
    this.lock = lock;
  }

  @Override
  public void run() {
    int distance = 100;
    synchronized (lock){
      try{
        while (!lock.isGreen){
          System.out.println(name + " waiting...");
          lock.wait();
        }
      }catch (InterruptedException e){
        e.printStackTrace();
      }
    }
    System.out.println(name + " started...");
    while (distance != 0){
      try{
        Thread.sleep((long) (100 * Math.random()));
      }catch (InterruptedException e){
        e.printStackTrace();
        break;
      }
      distance--;
      if (distance % 20 == 0 && distance != 0){
        System.out.println(name + " " + distance + " miles left");
      }
    }
    System.out.println(name + " finished race!!!");
    System.out.println("_exiting from thread of " + name + " move simulation...");
  }
}

class LightSignal {
  public boolean isGreen = false;
}

Answer 1

当你调用notifyAll()时，你需要改变一个状态，当你在waiting()时，你需要循环检查该状态。如果您不这样做

• notifyAll() 只通知正在 waiting() 的线程，而不通知稍后开始等待的线程。
• wait() 可能会被虚假唤醒。

在这种情况下，最简单的解决方案就是不要担心。您正在等待长达 100 毫秒的随机延迟，因此尝试同步此类随机系统的启动不太可能产生太大影响。

我认为当循环退出时“竞赛结束”，您不需要为此使用 if 子句。