測試邏輯
首先默認(rèn)為x = 0; y = 0; a = 0; b = 0;然后開啟兩個線程；
線程1執(zhí)行：a = 888;   x = b;
線程2執(zhí)行：b = 888;   y = a;
有且只有x = b，y = a兩個同時先執(zhí)行，才會出現(xiàn)x=y=0。

所以測試是否存在x=y=0觀察指令是否會出現(xiàn)重排現(xiàn)象。

public class OrderTest {
    private static int x = 0, y = 0;
    private static int a = 0, b = 0;

    public static void main(String[] args) throws InterruptedException{
        for(long i = 0; i < Long.MAX_VALUE; i++){
            x = 0; y = 0; a = 0; b = 0;
          
            CountDownLatch countDownLatch = new CountDownLatch(2);
          
          
          
            Thread one = new Thread(new Runnable() {
                @Override
                public void run() {
                    a = 888;
                    x = b;
                  
                    countDownLatch.countDown();
                }
            });

      
      
            Thread two = new Thread(new Runnable() {
                @Override
                public void run() {
              
                    b = 888;
                    y = a;
              
                    countDownLatch.countDown();
                }
            });

            one.start();
            two.start();


            //等待計數(shù)器變?yōu)?，即等待所有異步線程執(zhí)行完畢
            countDownLatch.await();
            if(x == 0 && y == 0){
                //x=y=0 只能是x = b;y = a;這兩個先執(zhí)行
                System.out.println("執(zhí)行次數(shù)"+i+"發(fā)現(xiàn)x=y=0");
                break;
            }

        }
    }

}