volatile关键字，内存可见性问题

public class TestVolatile {
    public static void main(String[] args) {
        ThreadDemo td = new ThreadDemo();
        new Thread(td).start();
        while(true){
            if(td.isFlag()){
                System.out.println("!!!!!!!!!!!!!!");
                break;
            }
        }
    }
}
class ThreadDemo implements Runnable{
    private boolean flag = false;
    @Override
    public void run() {
        try {
            Thread.sleep(200);
        } catch (InterruptedException e) {
        }
        flag = true;
        System.out.println(flag);
    }
    public boolean isFlag(){
        return flag;
    }
}