java并发中CountDownLatch的使用

java并发中CountDownLatch的使用

在java并发中，控制共享变量的访问非常重要，有时候我们也想控制并发线程的执行顺序，比如：等待所有线程都执行完毕之后再执行另外的线程，或者等所有线程都准备好了才开始所有线程的执行等。

这个时候我们就可以使用到CountDownLatch。

简单点讲，CountDownLatch存有一个放在QueuedSynchronizer中的计数器。当调用countdown() 方法时，该计数器将会减一。然后再调用await()来等待计数器归零。

private static final class Sync extends AbstractQueuedSynchronizer {...}private final Sync sync;public void countDown() {sync.releaseShared(1);}

public void await() throws InterruptedException {sync.acquireSharedInterruptibly(1);}public boolean await(long timeout, TimeUnit unit)throws InterruptedException {return sync.tryAcquireSharedNanos(1, unit.toNanos(timeout));}

下面我们举两个使用的例子:

主线程等待子线程全都结束之后再开始运行

这里我们定义子线程类，在子线程类里面，我们传入一个CountDownLatch用来计数，然后在子线程结束之前，调用该CountDownLatch的countDown方法。最后在主线程中调用await（）方法来等待子线程结束执行。

@Slf4jpublic class MainThreadWaitUsage implements Runnable {private List outputScraper;private CountDownLatch countDownLatch;public MainThreadWaitUsage(List outputScraper, CountDownLatch countDownLatch) {this.outputScraper = outputScraper;this.countDownLatch = countDownLatch;}@Overridepublic void run() {outputScraper.add("Counted down");countDownLatch.countDown();}}

看下怎么调用：

@Testpublic void testCountDownLatch()throws InterruptedException {List outputScraper = Collections.synchronizedList(new ArrayList());CountDownLatch countDownLatch = new CountDownLatch(5);List workers = Stream.generate(() -> new Thread(new MainThreadWaitUsage(outputScraper, countDownLatch))).limit(5).collect(toList());workers.forEach(Thread::start);countDownLatch.await();outputScraper.add("Latch released");log.info(outputScraper.toString());}

执行结果如下：

07:37:27.388 [main] INFO MainThreadWaitUsageTest - [Counted down, Counted down, Counted down, Counted down, Counted down, Latch released]

等待所有线程都准备好再一起执行

上面的例子中，我们是主线程等待子线程，那么在这个例子中，我们将会看看怎么子线程一起等待到准备好的状态，再一起执行。

思路也很简单，在子线程开始之后，将等待的子线程计数器减一，在主线程中await该计数器，等计数器归零之后，主线程再通知子线程运行。

public class ThreadWaitThreadUsage implements Runnable {private List outputScraper;private CountDownLatch readyThreadCounter;private CountDownLatch callingThreadBlocker;private CountDownLatch completedThreadCounter;public ThreadWaitThreadUsage(List outputScraper,CountDownLatch readyThreadCounter,CountDownLatch callingThreadBlocker,CountDownLatch completedThreadCounter) {this.outputScraper = outputScraper;this.readyThreadCounter = readyThreadCounter;this.callingThreadBlocker = callingThreadBlocker;this.completedThreadCounter = completedThreadCounter;}@Overridepublic void run() {readyThreadCounter.countDown();try {callingThreadBlocker.await();outputScraper.add("Counted down");} catch (InterruptedException e) {e.printStackTrace();} finally {completedThreadCounter.countDown();}}}

看下怎么调用：

@Testpublic void testCountDownLatch()throws InterruptedException {List outputScraper = Collections.synchronizedList(new ArrayList());CountDownLatch readyThreadCounter = new CountDownLatch(5);CountDownLatch callingThreadBlocker = new CountDownLatch(1);CountDownLatch completedThreadCounter = new CountDownLatch(5);List workers = Stream.generate(() -> new Thread(new ThreadWaitThreadUsage(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");log.info(outputScraper.toString());}

输出结果如下：

07:41:47.861 [main] INFO ThreadWaitThreadUsageTest - [Workers ready, Counted down, Counted down, Counted down, Counted down, Counted down, Workers complete]

停止CountdownLatch的await

如果我们调用await（）方法，该方法将会等待一直到count=0才结束。但是如果在线程执行过程中出现了异常，可能导致countdown方法执行不了。那么await（）方法可能会出现无限等待的情况。

这个时候我们可以使用：

public boolean await(long timeout, TimeUnit unit)throws InterruptedException {return sync.tryAcquireSharedNanos(1, unit.toNanos(timeout));}

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