为什么 volatile 关键字在 java 代码中没有按预期工作？

Question

我正在学习Java中的并发知识。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，它似乎不像预期的那样工作。实现Runnable的类中的run()方法永远不会停止。

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main方法中有两个任务实例:

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 
}
}

你将一个传递给Thread构造函数，然后在另一个上调用cancel。需要对传递给Thread的实例调用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();
}
}