为什么打印输出全是0,而第二种情况输出是42

Question

package com.my.current;

public class NoVisibility {

private static  volatile boolean ready;
private static   int number;
private static class ReaderThread extends Thread{
    public void run(){
        while(!ready){

            System.out.println(number);
        }
        //while(!ready);
        //System.out.println(number);
    }
}

public static void main(String[] args) throws InterruptedException{

    new ReaderThread().start();
    Thread.sleep(1000);
    number = 42;
    ready = true;
    Thread.sleep(1000);
}

}

Answer 1

 第一个是
while(!ready)
{
System.out.println(number);
}
第二个相当于
while(!ready)
{
}
System.out.println(number);

一个是ready以前输出，一个是以后输出。