CountDownLatch简单使用

CountDownLatch是一个倒计时锁存器，用来实现线程之间的同步操作

1.创建一个runable实现类

public class Worker implements Runnable {
    private List<String> outputScraper;
    private CountDownLatch countDownLatch;

    public Worker(List<String> outputScraper, CountDownLatch countDownLatch) {
        this.outputScraper = outputScraper;
        this.countDownLatch = countDownLatch;
    }

    @Override
    public void run() {
        doSomeWork();
        outputScraper.add("countDown");
        countDownLatch.countDown();
    }
}

2.创建测试类

@Test
public void test() throws InterruptedException {

    List<String> outputScraper = Collections.synchronizedList(new ArrayList<>());
    CountDownLatch countDownLatch = new CountDownLatch(5);
    List<Thread> workers = Stream
    .generate(() -> new Thread(new Worker(outputScraper, countDownLatch)))
    .limit(5)
    .collect(toList());

    workers.forEach(Thread::start);
    countDownLatch.await(); 
    outputScraper.add("latchReleased");

    assertThat(outputScraper).containsExactly("countDown","countDown","countDown","countDown","countDown","latchReleased");
}

此方法中创建了一个线程安全List和一个倒计时锁存器，锁存计数值为5，再创建5个线程执行Worker任务，传入的参数是同一个List和CountDownLatch，然后启动全部线程，在任务的run方法的最后调用countDownLatch.countDown()使计数器-1，在主线程中调用countDownLatch.await()等待计数器为0，然后再继续执行后面代码，保证了outputScraper.add(“latchReleased”)是在所有的outputScraper.add(“countDown”)之后才添加的。

异常处理

如果任务中途出现异常导致计数器锁存器不能正常-1，导致主线程一直等不到计数器为0，解决办法：

boolean completed = countDownLatch.await(3L, TimeUnit.SECONDS);

设置超时时间，如果规定时间内等待不到计数器清零则返回false。