两个线程间变量可见性问题，求大神解答

Question

public class RunThread extends Thread{

private  boolean isRunning = true;
private void setRunning(boolean isRunning){
    this.isRunning = isRunning;
}

public void run(){
    System.out.println("进入run方法..");
    while(isRunning == true){
    }
    System.out.println("线程停止");
}

public static void main(String[] args) throws InterruptedException {
    RunThread rt = new RunThread();
    rt.start();
    Thread.sleep(1000);
    rt.setRunning(false);
    System.out.println("isRunning的值已经被设置了false");
}

}

---

public class RunThread {

private  boolean isRunning = true;
private void setRunning(boolean isRunning){
    this.isRunning = isRunning;
}
public void run1(){
    while(isRunning == true){
    }
}
public static void main(String[] args) throws InterruptedException {
    final RunThread rt = new RunThread();
    Thread t = new Thread(new Runnable() {
        @Override
        public void run() {
            rt.run1();
        }
    });
    Thread t2 = new Thread(new Runnable() {
        @Override
        public void run() {
            rt.setRunning(false);
        }
    });
    t.start();
    t2.start();
}

}

以上两个程序，对于isRunning变量的线程间可见性，是不一样的，上面的程序不可见，需要加volatile关键字，下面的程序却不需要volatile关键字即可见，这是为什么...

Answer 1

两个程序中的isRunning变量对线程一样，线程间变量都不保证可见性。你的第一个程序中在两个线程间使用Thread.sleep(1000)让主线程休眠了1s，第二个程序为什么没有，保持测试程序的一致性，效果是相同的。