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();
}
显然,这不是我们想要的行为 – 应用程序继续比无限阻塞要好得多。
为了解决这个问题,我们在调用 await()
时添加一个超时参数。
boolean completed = countDownLatch.await(3L, TimeUnit.SECONDS);
assertThat(completed).isFalse();
然后,我们可以看到,测试最终会超时,await()
将返回 false
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