文章目錄

• • 主線程等待子線程全都結束之后再開始運行
  • 等待所有線程都準備好再一起執行
  • 停止CountdownLatch的await

java并發中CountDownLatch的使用

在java并發中，控制共享變量的訪問非常重要，有時候我們也想控制并發線程的執行順序，比如：等待所有線程都執行完畢之后再執行另外的線程，或者等所有線程都準備好了才開始所有線程的執行等。

這個時候我們就可以使用到CountDownLatch。

簡單點講，CountDownLatch存有一個放在QueuedSynchronizer中的計數器。當調用countdown() 方法時，該計數器將會減一。然后再調用await()來等待計數器歸零。

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));}

下面我們舉兩個使用的例子:

主線程等待子線程全都結束之后再開始運行

這里我們定義子線程類，在子線程類里面，我們傳入一個CountDownLatch用來計數，然后在子線程結束之前，調用該CountDownLatch的countDown方法。最后在主線程中調用await（）方法來等待子線程結束執行。

@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;}@Overridepublic void run() {outputScraper.add("Counted down");countDownLatch.countDown();} }

看下怎么調用：

@Testpublic 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());}

執行結果如下：

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

等待所有線程都準備好再一起執行

上面的例子中，我們是主線程等待子線程，那么在這個例子中，我們將會看看怎么子線程一起等待到準備好的狀態，再一起執行。

思路也很簡單，在子線程開始之后，將等待的子線程計數器減一，在主線程中await該計數器，等計數器歸零之后，主線程再通知子線程運行。

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

看下怎么調用：

@Testpublic 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());}

輸出結果如下：

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

如果我們調用await（）方法，該方法將會等待一直到count=0才結束。但是如果在線程執行過程中出現了異常，可能導致countdown方法執行不了。那么await（）方法可能會出現無限等待的情況。

這個時候我們可以使用：

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

本文的例子可以參考https://github.com/ddean2009/learn-java-concurrency/tree/master/CountDownLatch

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更多教程請參考 flydean的博客

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