为什么 volatile 关键字在 java 代码中不能按预期工作？

Question

我在Java学习并发知识。 关于 volatile 关键字，它应该使变量在不同的线程中可见。 但在我的演示代码中，它似乎没有按预期工作。 class 中实现Runnable的方法run()永远不会停止。

public class VisibilityDemo {

  public static void main(String[] args) throws InterruptedException {
    TimeConsumingTask timeConsumingTask = new TimeConsumingTask();
    Thread thread = new Thread(new TimeConsumingTask());
    thread.start();
    Thread.sleep(3000);
    timeConsumingTask.cancel();
  }
}

class TimeConsumingTask implements Runnable {
  private volatile boolean toCancel = false;

  @Override
  public void run() {
    while (! toCancel) {
      System.out.println("executing...");
      try {
        Thread.sleep(1000);
      } catch (InterruptedException e) {
        e.printStackTrace();
      }
    }
    if (toCancel) {
      System.out.println("Task was canceled.");
    } else {
      System.out.println("Task done.");
    }
  }

  public void cancel() {
    toCancel = true;
    System.out.println(this + " canceled.");
  }
}

Answer 1

在您的主要方法中，您有两个任务实例：

  public static void main(String[] args) throws InterruptedException {
    TimeConsumingTask timeConsumingTask = new TimeConsumingTask(); //<-- one
    Thread thread = new Thread(new TimeConsumingTask()); //<-- two
    thread.start();
    Thread.sleep(3000);
    timeConsumingTask.cancel(); //<-- cancel() on first 
  }
}

您将一个传递给Thread构造函数，然后在另一个上调用cancel 。 您需要在传递给Thread的实例上调用cancel ，如下所示：

  public static void main(String[] args) throws InterruptedException {
    TimeConsumingTask timeConsumingTask = new TimeConsumingTask(); 
    Thread thread = new Thread(timeConsumingTask); //<-- difference here
    thread.start();
    Thread.sleep(3000);
    timeConsumingTask.cancel();
  }
}