Java经常使用多线程辅助工具---countdownLatch

前言

        上一篇博文说到semaphore，一个增强版的synchronized，该多线程辅助工具适用于控制对资源操做或者访问的场景。如今有一张场景是，须要等各个线程都都执行完了再进行下一步的操做，好比须要批量对上传的文件进行压缩，上传十个文件，压缩成一个包，这时候，就须要等上传完十个之后再进行压缩操做，相似于阻塞，concurrent包提供了countdownLatch辅助工具。

        countdownLatch，中文翻译是门闩的意思，相似一个计数器，控制线程执行任务的时机，以组团的方式一块儿执行任务。该类会判断count计数不为0时，则呈wait状态，也就是屏障处等待，若是为0则继续执行。

入门

         开启两个线程进行计数，同时主线程在wait状态，最后实现组团执行后续任务：

@Test
    public void testCountDownLatch() throws InterruptedException {
        CountDownLatch latch = new CountDownLatch(2);
        for (int i = 0; i < 2; i++) {
            new Thread(() -> {
                log.info("开始count，当前是线程：{},时间是：{}",Thread.currentThread().getName(),LocalDateTime.now());
                try {
                    Thread.currentThread().sleep(2000);
                } catch (InterruptedException e) {
                    e.printStackTrace();
                }
                latch.countDown();
            }).start();
        }

        log.info("等待计数器count，如今是：{}", LocalDateTime.now());
        latch.await();
        log.info("结束计数器count，如今是：{}", LocalDateTime.now());
    }

    运行结果是：

比较复杂的使用案例

 模拟运动员从到场准备到开跑的过程，具体代码以下：

@Test
public void testCountDownLatch2() throws InterruptedException {
    CountDownLatch comingTag = new CountDownLatch(5);
    CountDownLatch waitTag = new CountDownLatch(1);
    CountDownLatch waitRunTag = new CountDownLatch(5);
    CountDownLatch beginTag = new CountDownLatch(1);
    CountDownLatch endTag = new CountDownLatch(5);

    for (int i = 0; i < 5; i++) {
        new Thread(() -> {
            try {
                Thread.sleep(5000);
                log.info("{}号运动员已经入场，time:{}", Thread.currentThread().getName(), LocalDateTime.now());
                comingTag.countDown();

                waitTag.await();

                log.info("{}号运动员开始准备，time:{}", Thread.currentThread().getName(), LocalDateTime.now());
                Thread.sleep(3000);
                waitRunTag.countDown();

                log.info("{}号运动员准备结束，time:{}", Thread.currentThread().getName(), LocalDateTime.now());
                beginTag.await();
                log.info("{}号运动员开跑，time:{}", Thread.currentThread().getName(), LocalDateTime.now());

                Thread.sleep(2000);
                log.info("{}号运动员跑完了，time:{}", Thread.currentThread().getName(), LocalDateTime.now());
                endTag.countDown();
            } catch (InterruptedException e) {
                e.printStackTrace();
            }
        }).start();
    }

    comingTag.await();
    log.info("运动员所有入场,time:{}", LocalDateTime.now());

    log.info("预备口令,time:{}", LocalDateTime.now());
    Thread.sleep(2000);
    waitTag.countDown();

    waitRunTag.await();
    log.info("远动员准备完毕，time:{}", LocalDateTime.now());

    log.info("发令枪响，time:{}", LocalDateTime.now());
    Thread.sleep(2000);
    beginTag.countDown();

    endTag.await();
    log.info("所有运动员到达终点，time:{}", LocalDateTime.now());
}

运行结果是：