线程通过flag停止问题

Question

 public class VolatileTest {

    /*volatile*/ boolean flag = true;

    public static void main(String[] args) {
        new VolatileTest().test();
    }

    private void test() {

        Thread t1 = new Thread(new Runnable() {
            @Override
            public void run() {
                System.out.println(Thread.currentThread().getName() + " start!");
                while (flag) {
                    /*try {
                        Thread.sleep(100);
                    } catch (InterruptedException e) {
                        e.printStackTrace();
                    }*/
                }
                System.out.println(Thread.currentThread().getName() + " stop!");
            }
        });
        t1.setName("t1");
        t1.start();

        Thread t2 = new Thread(new Runnable() {
            @Override
            public void run() {
                System.out.println(Thread.currentThread().getName() + " start!");
                try {
                    Thread.sleep(1000);
                    flag = false;
                } catch (InterruptedException e) {
                    e.printStackTrace();
                }
                System.out.println(Thread.currentThread().getName() + " stop!");
            }
        });
        t2.setName("t2");
        t2.start();
    }
}

不加volatile的情况下，t1中如果加入sleep，则线程t1可停止。不加，则无法停止。sleep是让t1进入timewait的状态，为什么会出现这种情况。求大佬来解答

Answer 1

volatile不加。就flag变量会有优化。一个线程修改了数值，另一个线程读取不到新的数值。读取的旧值