public class TestVolatile {
    public static void main(String[] args) {
        ThreadDemo td = new ThreadDemo();
        new Thread(td).start();
        while (true) {
	        //   System.out.println();
            if (td.isFlag()) {
                System.out.println("**********");
                break;
            }
        }}}
class ThreadDemo implements Runnable {
    private boolean flag = false;
    @Override
    public void run() {
        try {
            Thread.sleep(3);
        } catch (InterruptedException e) {
            e.printStackTrace();
        }
        flag = true;
    }
    public boolean isFlag() {
        return flag;
    }
    public void setFlag(boolean flag) {
        this.flag = flag;
    }
}

老师，请问这个例子中，为什么循环中System.out.println();被注释之后，while循环就不会退出（即td.isFlag()的状态在主线程中没有被更新为true）。然而放开注释之后，while循环就能读取到flag = true