Java - CountDownLatch與CyclicBarrier的區別

類型	定義	例子	超時例子
CountDownLatch	一個（或多個）線程等待其他線程(彼此獨立)執行完成	每個人(其他線程)下班打卡（countDown）後，各回各家(彼此獨立)；等人都走光後，保衛大爺（等待線程）才關門(await)	保衛大爺（等待線程）到點就關門(await)，即使還有人沒下班
CyclicBarrier	多個線程互相等待，直到所有線程到達同一個同步點。並且可以重複使用（節能環保）	開會時，一般需要參會人（多個線程）都到會議室（await同步點）後，才能開會	參會人（多個線程）等待（await）一定時間後，即使有人缺席，還是要開會

CountDownLatch示例

代碼

     public static void main(String[] args) throws InterruptedException {
        CountDownLatch countDownLatch = new CountDownLatch(3);
        ExecutorService threadPool = Executors.newFixedThreadPool(10);
        int threadCount = 3;
        for (int i = 0; i < threadCount; i++) {
            final int threadNum = i;
            Thread.sleep(1000);
            threadPool.execute(() -> {
                try {
                    System.out.printf("開始上班::threadNum=%d time=%s %n", threadNum, getTime());
                    // 上班8毫秒（黑人問號？？？）
                    Thread.sleep(8);
                } catch (InterruptedException e) {
                    e.printStackTrace();
                }
                System.out.printf("打卡下班::threadNum=%d time=%s %n", threadNum, getTime());
                countDownLatch.countDown();
                System.out.printf("打卡完畢，各回各家::threadNum=%d time=%s %n", threadNum, getTime());
                System.out.println("**********\n");
            });
        }

        System.out.println("保衛大爺等待關門...");
        countDownLatch.await();
        threadPool.shutdown();
        System.out.println("保衛大爺關門回家！");
    }

    private static String getTime() {
        SimpleDateFormat simpleDateFormat = new SimpleDateFormat("HH:mm:ss S");
        return simpleDateFormat.format(new Date());
    }

結果

CyclicBarrier示例

代碼

    public static void main(String[] args) throws InterruptedException {
        CyclicBarrier cyclicBarrier = new CyclicBarrier(3);
        ExecutorService threadPool = Executors.newFixedThreadPool(10);
        int threadCount = 3;
        for (int i = 0; i < threadCount; i++) {
            final int threadNum = i;
            Thread.sleep(1000);
            threadPool.execute(() -> {
                try {
                    System.out.printf("到達會議室::threadNum=%d time=%s %n", threadNum, getTime());
                    System.out.printf("已等待人數::%d 人%n", cyclicBarrier.getNumberWaiting());
                    System.out.println("**********\n");
                    cyclicBarrier.await();
                    System.out.printf("開始開會::threadNum=%d time=%s %n", threadNum, getTime());
                } catch (InterruptedException | BrokenBarrierException e) {
                    e.printStackTrace();
                }
            });
        }
        threadPool.shutdown();
    }

    private static String getTime() {
        SimpleDateFormat simpleDateFormat = new SimpleDateFormat("HH:mm:ss S");
        return simpleDateFormat.format(new Date());
    }

結果