我下面的代码没用Volatile 修饰flag，所以程序会一直运行，但是，我在while（flag）里面随便打印一句话，结果，结果程序就停了，这是为什么呢

 class ThreadVolatile1 extends Thread{
    public  boolean flag=true;
    @Override
    public void run() {
        // TODO Auto-generated method stub
        
        System.out.println("开始执行子线程");
        
        while(flag) {
            //System.out.println("输出");
        }
        System.out.println("线程停止");
    }
    public void setRunning(boolean flag) {
        this.flag=flag;
    }
}
public class ThreadVolatile{
    public static void main(String[] args) throws Exception{
        ThreadVolatile1 demo=new ThreadVolatile1();
        demo.start();
        Thread.sleep(3000);
        demo.setRunning(false);
        System.out.println("flag设置成false");
        Thread.sleep(1000);
        System.out.println(demo.flag);
    }
}