变量可见性问题

Question

public class Test {    public  static boolean flag=false;
    private static Runnable run=new Runnable() {
        @Override
        public void run() {
            while (true){
         //       System.out.println();  ①
                if (flag){
                    System.out.println("this is sub thread..........................");
                }
            }
        }
    };
    public static void main(String[] args) throws InterruptedException {
        new Thread(run).start();
        Thread.sleep(1000);
        flag=true;
    }
}
如题，子线程里的代码是不可能执行的，但是把①处的注释放开，子线程里的代码就可以执行。请问各路大神，是什么原因呢？

Answer 1

你主线程要等待i。不然直接进程退出了。可见性要用volatile

Answer 2

是一个死循环，没有显示是因为在死循环里面，显示的同样的内容。public class Test {
 
    public  static boolean flag=false;
    private static Runnable run=new Runnable() {
        @Override
        public void run() {
            while (true){
              System.out.println("***********"); 
                if (flag){
                    System.out.println("this is sub thread.........................."+new java.sql.Timestamp(System.currentTimeMillis()).toString());
                }
            }
        }
    };
    public static void main(String[] args) throws InterruptedException {
        new Thread(run).start();
        Thread.sleep(1000);
        flag=true;
    }
}
this is sub thread..........................2018-07-02 08:44:54.313
***********
this is sub thread..........................2018-07-02 08:44:54.313
***********
this is sub thread..........................2018-07-02 08:44:54.313
***********
this is sub thread..........................2018-07-02 08:44:54.313
***********
this is sub thread..........................2018-07-02 08:44:54.313
***********
this is sub thread..........................2018-07-02 08:44:54.313
***********
this is sub thread..........................2018-07-02 08:44:54.313
***********
this is sub thread..........................2018-07-02 08:44:54.313
***********
this is sub thread..........................2018-07-02 08:44:54.313
***********
this is sub thread..........................2018-07-02 08:44:54.313
***********
this is sub thread..........................2018-07-02 08:44:54.313
***********
this is sub thread..........................2018-07-02 08:44:54.313
***********
this is sub thread..........................2018-07-02 08:44:54.313
***********
this is sub thread..........................2018-07-02 08:44:54.313
***********
this is sub thread..........................2018-07-02 08:44:54.313
***********
this is sub thread..........................2018-07-02 08:4