线程安全的可见性问题

public class VisibilityDemo2 {
    // 状态标识 (不用缓存)
    private boolean flag = true;
    
    public static void main(String[] args) throws InterruptedException {
        VisibilityDemo2 demo1 = new VisibilityDemo2();
        new Thread(new Runnable() {
            public void run() {
                int i = 0;
                while (demo1.flag) {
                    i++;
                }
                System.out.println(i);
            }
        }).start();

        TimeUnit.SECONDS.sleep(2);
        // 设置is为false，使上面的线程结束while循环
        demo1.flag = false;
        System.out.println("被置为false了.");
    }
}　　

// 指令重排序
if (demo1.flag){
    while (true) {
        i++;
    }
}