一、是什么？

CountdownLatch 用来进行线程同步协作，等待所有线程完成倒计时。
其中构造参数用来初始化等待计数值，await() 用来等待计数归零，countDown() 用来让计数减一

二、demo演示

public class TestCountDownLatch {public static void main(String[] args) throws InterruptedException, ExecutionException {test5();}private static void test5() {CountDownLatch latch = new CountDownLatch(3);ExecutorService service = Executors.newFixedThreadPool(4);service.submit(() -> {log.debug("begin...");sleep(1);latch.countDown();log.debug("end...{}", latch.getCount());});service.submit(() -> {log.debug("begin...");sleep(1.5);latch.countDown();log.debug("end...{}", latch.getCount());});service.submit(() -> {log.debug("begin...");sleep(2);latch.countDown();log.debug("end...{}", latch.getCount());});service.submit(()->{try {log.debug("waiting...");latch.await();log.debug("wait end...");} catch (InterruptedException e) {e.printStackTrace();}});}
}

三、应用之同步等待多线程准备完毕

 private static void test2() throws InterruptedException {AtomicInteger num = new AtomicInteger(0);ExecutorService service = Executors.newFixedThreadPool(10, (r) -> {return new Thread(r, "t" + num.getAndIncrement());});CountDownLatch latch = new CountDownLatch(10);String[] all = new String[10];Random r = new Random();for (int j = 0; j < 10; j++) {int x = j;service.submit(() -> {for (int i = 0; i <= 100; i++) {try {Thread.sleep(r.nextInt(100));} catch (InterruptedException e) {}all[x] = Thread.currentThread().getName() + "(" + (i + "%") + ")";System.out.print("\r" + Arrays.toString(all));}latch.countDown();});}latch.await();System.out.println("\n游戏开始...");service.shutdown();}

四、 应用之同步等待多个远程调用结束

    private static void test3() throws InterruptedException, ExecutionException {RestTemplate restTemplate = new RestTemplate();log.debug("begin");ExecutorService service = Executors.newCachedThreadPool();CountDownLatch latch = new CountDownLatch(4);service.submit(() -> {Map<String, Object> response = restTemplate.getForObject("http://localhost:8080/order/{1}", Map.class, 1);log.debug("{}",response);latch.countDown();});service.submit(() -> {Map<String, Object> response1 = restTemplate.getForObject("http://localhost:8080/product/{1}", Map.class, 1);log.debug("{}",response1);latch.countDown();});service.submit(() -> {Map<String, Object> response2 = restTemplate.getForObject("http://localhost:8080/product/{1}", Map.class, 2);log.debug("{}",response2);latch.countDown();});service.submit(() -> {Map<String, Object> response3 = restTemplate.getForObject("http://localhost:8080/logistics/{1}", Map.class, 1);log.debug("{}",response3);latch.countDown();});latch.await();log.debug("执行完毕");service.shutdown();}

五、CountDownLatch 原理

AQS