文章目录
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。
@Slf4j public class MainThreadWaitUsage implements Runnable { private List<String> outputScraper; private CountDownLatch countDownLatch; public MainThreadWaitUsage(List<String> outputScraper, CountDownLatch countDownLatch) { this.outputScraper = outputScraper; this.countDownLatch = countDownLatch; } @Override public void run() { outputScraper.add("Counted down"); countDownLatch.countDown(); } }
看下怎么调用：
@Test public void testCountDownLatch() throws InterruptedException { List<String> outputScraper = Collections.synchronizedList(new ArrayList<>()); CountDownLatch countDownLatch = new CountDownLatch(5); List<Thread> 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<String> outputScraper; private CountDownLatch readyThreadCounter; private CountDownLatch callingThreadBlocker; private CountDownLatch completedThreadCounter; public ThreadWaitThreadUsage( 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(); outputScraper.add("Counted down"); } catch (InterruptedException e) { e.printStackTrace(); } finally { completedThreadCounter.countDown(); } } }
看下怎么调用：
@Test public void testCountDownLatch() 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 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)); }
本文的例子可以参考https://github.com/ddean2009/learn-java-concurrency/tree/master/CountDownLatch
