并发(三) CountDownLatch

CountDownLatch

和CyclicBarrier的区别是，CyclicBarrier可以循环使用，CountDownLatch不可以；CyclicBarrier可以有一个Runnable参数，在所有子任务执行完成后触发一个动作。两者在模型上近似，都是等待一批固定数量的子任务执行完成后才能执行主任务。

流程如下：

示例代码如下：

    @Test
    public void test3() throws InterruptedException {
        CountDownLatch countDownLatch = new CountDownLatch(5);

        ExecutorService executorService = Executors.newCachedThreadPool();

        for(int i = 0; i < 5; i++) {
            executorService.execute(new Task(countDownLatch));
        }

        countDownLatch.await();

        executorService.shutdown();
    }

    public static class Task implements Runnable {

        CountDownLatch countDownLatch;

        public Task(CountDownLatch countDownLatch) {
            this.countDownLatch = countDownLatch;
        }

        @Override
        public void run() {
            System.err.println("当前完成的任务数目：" + countDownLatch.getCount());
            countDownLatch.countDown();
        }
    }