为什么休眠主线程会更改Java中正在运行的其他线程的行为？

Question

Here is my code:

/* User: koray@tugay.biz Date: 21/02/15 Time: 21:52 */

public class MyThread extends Thread {

    int x = 0;

    @Override
    public synchronized void run() {
        System.out.println("Thread running..");
        while(x == 0) {
        }
        while(x != 1) {
            System.out.println("x is" + x);
            // do something else time consuming
            try {
                sleep(2000);
            } catch (InterruptedException e) {
                e.printStackTrace();
            }
        }
    }

    public void foo(int y) {
        x = y;
    }

}

When I run TestClass like this:

/* User: koray@tugay.biz Date: 21/02/15 Time: 21:55 */

public class TestClass {

    public static void main(String[] args) throws InterruptedException {

        MyThread myThread = new MyThread();
        myThread.start();
        //Thread.sleep(1000);
        myThread.foo(3);

    }
}

I see in the console:

Thread running..
x is3
x is3
x is3
x is3

which is ok..

But when I uncomment the //Thread.sleep(1000); I expect the same behaviour, only after 1 second.. But all I see in the Console is.

Thread running..

No matter how long I wait..

Why?

Answer 1

您需要让Java知道x的值可能由另一个线程使用volatile关键字更新：

volatile int x;