有益的CountDownLatch和棘手的Java死锁

您是否曾经使用过java.util.concurrent.CountDownLatch ？

这是在两个或多个线程之间实现同步的非常方便的类，在该类中，一个或多个线程可以等待，直到在其他线程中执行的一组操作完成为止（请参阅javadoc和此文章）。 CountDownLatch在适当的情况下可以节省您的时间，您必须了解此类。

与往常一样，如果代码不好，线程同步会引发死锁。由于并发用例可能非常复杂，因此开发人员必须非常小心。在这里我不会描述复杂的并发问题，但是如果您不小心使用CountDownLatch ，可能会遇到一个简单的问题。

假设您有2个线程（线程1和线程2）共享一个java.util.concurrent.ArrayBlockingQueue，并且您想使用CountDownLatch对其进行同步。检查以下简单示例：

package gr.qiozas.simple.threads.countdownlatch;import java.util.concurrent.ArrayBlockingQueue;
import java.util.concurrent.CountDownLatch;public class DeadlockCaseCDL {public static void main(String[] args) throws InterruptedException {CountDownLatch c = new CountDownLatch(1);ArrayBlockingQueue b = new ArrayBlockingQueue(1);new Thread(new T1(c, b)).start();new Thread(new T2(c, b)).start();}private static class T1 implements Runnable {private CountDownLatch c;private ArrayBlockingQueue b;private T1(CountDownLatch c, ArrayBlockingQueue b) {this.c = c; this.b = b;}public void run() {try {b.put(234);b.put(654);doWork(T1.class);c.countDown();doWork(T1.class);} catch (InterruptedException ex) {}}}private static class T2 implements Runnable {private CountDownLatch c;private ArrayBlockingQueue b;private T2(CountDownLatch c, ArrayBlockingQueue b) {this.c = c; this.b = b;}public void run() {try {doWork(T2.class);c.await();doWork(T2.class);System.out.println("I just dequeue "+b.take());System.out.println("I just dequeue "+b.take());} catch (InterruptedException ex) {}}}private static void doWork(Class classz) {System.out.println(classz.getName()+" do the work");}
}

您在上面看到的是，主线程创建了一个计数为1的CountDownLatch。和一个容量为“ 1”的ArrayBlockingQueue 然后生成“ 2个线程”。 ArrayBlockingQueue将用于实际的“工作”（入队和出队），而CountDownLatch将用于同步线程（入队必须在出队前完成）。

线程1使2条消息入队，线程2希望使它们出队，但仅在线程1使两条消息入队之后。此同步由CountDownLatch保证。您认为此代码可以吗？不，这不是造成死锁的原因！

如果仔细看第10行，您将看到我初始化ArrayBlockingQueue的容量等于“ 1”。但是线程1要排队2条消息，然后释放（CountDownLatch）的锁，以便随后被线程2占用。容量“ 1？队列阻塞线程1，直到另一个线程将一个消息从队列中出队，然后再次尝试使第二条消息入队。不幸的是，只有线程2使消息从队列中出队，但是由于线程1拥有CountDownLatch锁，因此线程2无法使任何消息出队，因此被阻塞。因此，由于两个线程都被阻塞（等待获取不同的锁），我们确实有一个死锁。线程1等待ArrayBlockingQueue锁定，而线程2等待CountDownLatch锁定（您也可以在下面的相关线程转储中看到它）。

如果我们增加队列的容量，那么此代码将毫无问题地运行，但这不是本文的重点。您需要了解的是，必须谨慎使用CountDownLatch，以避免此类危险情况。您必须了解类的功能情况，向团队的其他开发人员详细说明该功能，编写有用的Javadoc，并始终编写在极端情况下（不仅对于开心路径而言）都可靠的代码。

您可能会帮助您的另一点是，现代JVM无法检测到此死锁。尝试一下。

如您所知，现代JVM（Hotspot和JRockit）都能够检测到简单的死锁，并在线程转储中报告它们。请参阅从Hotspot JVM检测到的简单死锁示例：

Found one Java-level deadlock:
=============================
"Thread-6":
waiting to lock monitor 0x00a891ec (object 0x06c616e0, a java.lang.String),
which is held by "Thread-9"
"Thread-9":
waiting to lock monitor 0x00a8950c (object 0x06c61708, a java.lang.String),
which is held by "Thread-6"

您可以尝试DeadlockCaseCDL并获得线程转储（在GNU / Linux上仅运行“ kill -3 ‹jvm_pid›”）。您将看到线程转储看起来很正常，JVM没有检测到死锁，但是您处于死锁状态！！！因此，请注意，JVM无法检测到这种死锁。

从我的本地执行中检查以下线程转储示例：

Full thread dump Java HotSpot(TM) Server VM (17.1-b03 mixed mode):"DestroyJavaVM" prio=10 tid=0x0946e800 nid=0x5382 waiting on condition [0x00000000]java.lang.Thread.State: RUNNABLE"Thread-1" prio=10 tid=0x094b1400 nid=0x5393 waiting on condition [0x7c79a000]java.lang.Thread.State: WAITING (parking)at sun.misc.Unsafe.park(Native Method)- parking to wait for   (a java.util.concurrent.CountDownLatch$Sync)at java.util.concurrent.locks.LockSupport.park(LockSupport.java:158)at java.util.concurrent.locks.AbstractQueuedSynchronizer.parkAndCheckInterrupt(AbstractQueuedSynchronizer.java:811)at java.util.concurrent.locks.AbstractQueuedSynchronizer.doAcquireSharedInterruptibly(AbstractQueuedSynchronizer.java:969)at java.util.concurrent.locks.AbstractQueuedSynchronizer.acquireSharedInterruptibly(AbstractQueuedSynchronizer.java:1281)at java.util.concurrent.CountDownLatch.await(CountDownLatch.java:207)at gr.qiozas.simple.threads.countdownlatch.DeadlockCaseCDL$T2.run(DeadlockCaseCDL.java:50)at java.lang.Thread.run(Thread.java:662)"Thread-0" prio=10 tid=0x094afc00 nid=0x5392 waiting on condition [0x7c7eb000]java.lang.Thread.State: WAITING (parking)at sun.misc.Unsafe.park(Native Method)- parking to wait for   (a java.util.concurrent.locks.AbstractQueuedSynchronizer$ConditionObject)at java.util.concurrent.locks.LockSupport.park(LockSupport.java:158)at java.util.concurrent.locks.AbstractQueuedSynchronizer$ConditionObject.await(AbstractQueuedSynchronizer.java:1987)at java.util.concurrent.ArrayBlockingQueue.put(ArrayBlockingQueue.java:252)at gr.qiozas.simple.threads.countdownlatch.DeadlockCaseCDL$T1.run(DeadlockCaseCDL.java:29)at java.lang.Thread.run(Thread.java:662)"Low Memory Detector" daemon prio=10 tid=0x0947f800 nid=0x5390 runnable [0x00000000]java.lang.Thread.State: RUNNABLE"CompilerThread1" daemon prio=10 tid=0x7cfa9000 nid=0x538f waiting on condition [0x00000000]java.lang.Thread.State: RUNNABLE"CompilerThread0" daemon prio=10 tid=0x7cfa7000 nid=0x538e waiting on condition [0x00000000]java.lang.Thread.State: RUNNABLE"Signal Dispatcher" daemon prio=10 tid=0x7cfa5800 nid=0x538d waiting on condition [0x00000000]java.lang.Thread.State: RUNNABLE"Finalizer" daemon prio=10 tid=0x7cf96000 nid=0x538c in Object.wait() [0x7cd15000]java.lang.Thread.State: WAITING (on object monitor)at java.lang.Object.wait(Native Method)- waiting on  (a java.lang.ref.ReferenceQueue$Lock)at java.lang.ref.ReferenceQueue.remove(ReferenceQueue.java:118)- locked  (a java.lang.ref.ReferenceQueue$Lock)at java.lang.ref.ReferenceQueue.remove(ReferenceQueue.java:134)at java.lang.ref.Finalizer$FinalizerThread.run(Finalizer.java:159)"Reference Handler" daemon prio=10 tid=0x7cf94800 nid=0x538b in Object.wait() [0x7cd66000]java.lang.Thread.State: WAITING (on object monitor)at java.lang.Object.wait(Native Method)- waiting on  (a java.lang.ref.Reference$Lock)at java.lang.Object.wait(Object.java:485)at java.lang.ref.Reference$ReferenceHandler.run(Reference.java:116)- locked  (a java.lang.ref.Reference$Lock)"VM Thread" prio=10 tid=0x7cf92000 nid=0x538a runnable"GC task thread#0 (ParallelGC)" prio=10 tid=0x09475c00 nid=0x5383 runnable"GC task thread#1 (ParallelGC)" prio=10 tid=0x09477000 nid=0x5384 runnable"GC task thread#2 (ParallelGC)" prio=10 tid=0x09478800 nid=0x5385 runnable"GC task thread#3 (ParallelGC)" prio=10 tid=0x0947a000 nid=0x5387 runnable"VM Periodic Task Thread" prio=10 tid=0x09489800 nid=0x5391 waiting on conditionJNI global references: 854HeapPSYoungGen      total 14976K, used 1029K [0xa2dd0000, 0xa3e80000, 0xb39d0000)eden space 12864K, 8% used [0xa2dd0000,0xa2ed1530,0xa3a60000)from space 2112K, 0% used [0xa3c70000,0xa3c70000,0xa3e80000)to   space 2112K, 0% used [0xa3a60000,0xa3a60000,0xa3c70000)PSOldGen        total 34304K, used 0K [0x815d0000, 0x83750000, 0xa2dd0000)object space 34304K, 0% used [0x815d0000,0x815d0000,0x83750000)PSPermGen       total 16384K, used 1739K [0x7d5d0000, 0x7e5d0000, 0x815d0000)object space 16384K, 10% used [0x7d5d0000,0x7d782e90,0x7e5d0000)

参考： 有益的CountDownLatch和我们的JCG合作伙伴 Adrianos Dadis 遇到的棘手的Java僵局，位于“ Java，集成和源代码的美德”博客上。

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