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。

@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

更多教程请参考 flydean的博客

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