十一、Java并发 Java CountDownLatch

云少2018-09-112018-09-11

十一、Java并发 Java CountDownLatch

master ，这是我的小站,欢迎访问哦~~

本章节我们来讨论下 java.util.concurrent.CountDownLatch 这个类，顺带演示下如何在一些实际例子中使用它。

CountDownLatch 类的作用呢？怎么说呢？简单来说，我们可以使用它来阻塞线程，直到其他线程完成给定任务。

并发编程中使用 CountDownLatch

简而言之，CountDownLatch 有一个计数器字段，我们可以根据需要减少它，因此，我们可以使用它来阻止调用线程，直到它被计数到零。

如果我们正在进行一些并行处理，我们可以使用与计数器相同的值来实例化 CountDownLatch，因为我们想要处理多个线程。然后，我们可以在每个线程完成后调用 countdown()，保证调用 await() 的依赖线程将阻塞，直到工作线程完成。

使用 CountDownLatch 等待线程池完成

我们通过创建一个 Worker 来尝试这个模式，并使用 CountDownLatch 字段来指示它何时完成

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("Counted down");
        countDownLatch.countDown();
    }
}

然后，我们创建一个测试，以证明我们可以让 CountDownLatch 等待 Worker 实例完成

@Test
public void whenParallelProcessing_thenMainThreadWillBlockUntilCompletion()
  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("Latch released");

      assertThat(outputScraper)
        .containsExactly(
          "Counted down",
          "Counted down",
          "Counted down",
          "Counted down",
          "Counted down",
          "Latch released"
        );
}

上面这个示例中，"Latch release" 将始终是最后一个输出 – 因为它取决于 CountDownLatch 的释放。

注意，如果我们没有调用 await() 方法，我们将无法保证线程执行的顺序，因此测试会随机失败。

在等待开始的线程池中使用 CountDownLatch

我们重用前面的示例，但是这次开启了了数千个线程而不是 5 个线程，很可能许多早期的线程在后面的线程上调用 start() 之前已经完成了处理。这可能会使尝试重现并发问题变得困难，因为我们无法让所有线程并行运行。

为了解决这个问题，我们让 CountdownLatch 的工作方式与上一个示例有所不同。在某些子线程完成之前，我们可以阻止每个子线程直到所有其他子线程都启动，而不是阻塞父线程。

我们把上一个示例的 run() 方法修改下，使其在处理之前阻塞

public class WaitingWorker implements Runnable {

    private List<String> outputScraper;
    private CountDownLatch readyThreadCounter;
    private CountDownLatch callingThreadBlocker;
    private CountDownLatch completedThreadCounter;

    public WaitingWorker(
      List<String> outputScraper,
      CountDownLatch readyThreadCounter,
      CountDownLatch callingThreadBlocker,
      CountDownLatch completedThreadCounter) {

        this.outputScraper = outputScraper;
        this.readyThreadCounter = readyThreadCounter;
        this.callingThreadBlocker = callingThreadBlocker;
        this.completedThreadCounter = completedThreadCounter;
    }

    @Override
    public void run() {
        readyThreadCounter.countDown();
        try {
            callingThreadBlocker.await();
            doSomeWork();
            outputScraper.add("Counted down");
        } catch (InterruptedException e) {
            e.printStackTrace();
        } finally {
            completedThreadCounter.countDown();
        }
    }
}

接下来，我们修改下测试，直到所有工人都已启动，解锁工人，然后阻止，直到工人完成

@Test
public void whenDoingLotsOfThreadsInParallel_thenStartThemAtTheSameTime()
 throws InterruptedException {

    List<String> outputScraper = Collections.synchronizedList(new ArrayList<>());
    CountDownLatch readyThreadCounter = new CountDownLatch(5);
    CountDownLatch callingThreadBlocker = new CountDownLatch(1);
    CountDownLatch completedThreadCounter = new CountDownLatch(5);
    List<Thread> workers = Stream
      .generate(() -> new Thread(new WaitingWorker(
        outputScraper, readyThreadCounter, callingThreadBlocker, completedThreadCounter)))
      .limit(5)
      .collect(toList());

    workers.forEach(Thread::start);
    readyThreadCounter.await(); 
    outputScraper.add("Workers ready");
    callingThreadBlocker.countDown(); 
    completedThreadCounter.await(); 
    outputScraper.add("Workers complete");

    assertThat(outputScraper)
      .containsExactly(
        "Workers ready",
        "Counted down",
        "Counted down",
        "Counted down",
        "Counted down",
        "Counted down",
        "Workers complete"
      );
}

这种模式对于尝试重现并发错误非常有用，可以用来强制数千个线程尝试并行执行某些逻辑。

让 CountdownLatch 尽早结束

有时，我们可能会遇到一个情况，即在 CountdownLatch 倒计时之前，Workers 已经终止了错误。这可能导致它永远不会达到零并且 await() 永远不会终止。

@Override
public void run() {
    if (true) {
        throw new RuntimeException("Oh dear, I'm a BrokenWorker");
    }
    countDownLatch.countDown();
    outputScraper.add("Counted down");
}

我们修改下之前的测试以使用 BrokenWorker，来演示 await() 将如何永久阻塞

@Test
public void whenFailingToParallelProcess_thenMainThreadShouldGetNotGetStuck()
  throws InterruptedException {

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

    workers.forEach(Thread::start);
    countDownLatch.await();
}