等待多个子线程执行完毕

1.CountDownLatch
(1) new CountDownLatch(X)
(2) countDownLatch.countDown();
一个线程执行完毕后，X减少1
(3)countDownLatch.await();
等待直至X为0

2.Java CompletableFuture：allOf等待所有异步线程任务结束

List testList = Lists.newArrayList();
        testList.add("cf1"); testList.add("cf2");
        CompletableFuture[] cfArr = testList.stream().
            map(t -> CompletableFuture
                    .supplyAsync(() -> pause(t), executor)
                    .whenComplete((result, th) -> {
                        System.out.println("hello" + result);
                    })).toArray(CompletableFuture[]::new);
        // 开始等待所有任务执行完成
        CompletableFuture.allOf(cfArr).join();