@Slf4j
public class SimpleHappenBefore {

    private static int a = 0;

    private static boolean flag = false;

    public static void main(String[] args) throws InterruptedException {
        for (int i = 0; i < 1000; i++) {
            ThreadA threadA = new ThreadA();
            ThreadB threadB 
 
= new ThreadB();
            threadA.start();
            threadB.start();

            threadA.join();
            threadB.join();
            a = 0;
            flag = false;
        }
    }

    static class ThreadA extends Thread {
        @Override
        public void run() {  //這裏的指令重排序
 
            a = 1;
            flag = true;   
        }
    }

    static class ThreadB extends Thread {
        @Override
        public void run() {
            if (flag) {
                a = a * 1;
            }
            if (a == 0) {
                System.out.println("ha,a==0");
            }
        }
    }
}
 

指令重排序