java中怎么对指令进行重新排序

public class SimpleHappenBefore {  private static int a=0;  private static boolean flag=false; public static void main(String[] args) throws InterruptedException {  //由于多线程情况下未必会试出重排序的结论,所以多试一些次  for(int i=0;i<1000;i++){   ThreadA threadA=new ThreadA();   ThreadB threadB=new ThreadB();   threadA.start();   threadB.start();   //这里等待线程结束后,重置共享变量,以使验证结果的工作变得简单些.   threadA.join();   threadB.join();   a=0;   flag=false;  } } static class ThreadA extends Thread{  public void run(){  a=1;  flag=true;  } } static class ThreadB extends Thread{  public void run(){   if(flag){   a=a*1;   }   if(a==0){   System.out.println("ha,a==0");   }  } }}