Question

我有一个关于wait（）和notifyAll（）方法的问题。代码模拟两个线程的'Race'。

让我们看一下代码 - 问题是notifyAll（）方法对等待线程什么都不做，导致main方法首先获得锁定...简单的解决方法是设置一些延迟（参见注释行）。但这是一种不好的做法。这个问题的好方法是什么？我希望只使用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。

谢谢你的回答。那么，这个解决方案更好吗？

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

当您调用notifyAll（）时，您需要更改状态，当您等待（）时，您需要在循环中检查该状态。如果你不这样做

notifyAll（）只通知正在等待的线程（），而不是稍后开始等待的线程。
wait（）可以假装醒来。

在这种情况下，最简单的解决方案是不用担心。你正在等待一个长达100毫秒的随机延迟，所以尝试同步这样一个随机系统的开始是不太可能的。

当循环退出时，我认为“竞赛已完成”，您不需要if子句。

调用notifyAll的好方法是什么？

1 个答案: