在多线程中某段代码只执行一次

使用Automic包中的AtomicBoolean结合CAS来实现。当只希望需要某段代码在多线程中只执行一次的时候，此时你会发现boolean这个变量类型是多么有用。

代码如下

public class AtomicBoolean1{

    private static AtomicBoolean isHappened = new AtomicBoolean(false); 
    //初始值设定为fasle，当在多线程环境运行时只希望某段代码只执行一次，则可调用实例isHappened的CAS来仅执行一次。
//    private static AtomicInteger isHappened = new AtomicInteger(0);
    // 请求总数
    public static int clientTotal = 5000;

    // 同时并发执行的线程数
    public static int threadTotal = 200;

    public static void main(String[] args) throws Exception {
        ExecutorService executorService = Executors.newCachedThreadPool();
        final Semaphore semaphore = new Semaphore(threadTotal);
        final CountDownLatch countDownLatch = new CountDownLatch(clientTotal);
        for (int i = 0; i < clientTotal ; i++) {
            executorService.execute(() -> {
                try {
                    semaphore.acquire();
                    test();
                    semaphore.release();
                } catch (Exception e) {
                    log.error("exception", e);
                }
                countDownLatch.countDown();
            });
        }
        countDownLatch.await();
        executorService.shutdown();
        log.info("isHappened:{}", isHappened.get());
    }

    private static void test() {
//        log.info("execute");
        //当前实例的值为false时将其uopdate为true并执行对应代码，否则不执行。
        if (isHappened.compareAndSet(false, true)) {
            log.info("execute");
        }
    }
}