我有多个消费者线程使用await() 在大小为 1 的 CountDownLatch 上等待。我有一个生产者线程，它在成功完成时调用countDown()。
在没有错误的情况下效果很好。
但是，如果生产者检测到错误，我希望它能够向消费者线程发出错误信号。理想情况下，我可以让生产者调用 abortCountDown() 之类的东西，并让所有消费者收到 InterruptedException 或其他一些异常。我不想调用countDown()，因为这需要我所有的消费者线程在调用await() 后进行额外的手动检查是否成功。我宁愿他们只收到一个他们已经知道如何处理的异常。
我知道CountDownLatch 中没有中止功能。是否有另一个同步原语可以轻松适应以有效地创建支持中止倒计时的CountDownLatch？
【问题讨论】：
标签： java concurrency java.util.concurrent
JB Nizet 给出了一个很好的答案。我拿了他的，稍微打磨了一下。结果是一个名为 AbortableCountDownLatch 的 CountDownLatch 的子类，它向该类添加了一个“abort()”方法，这将导致所有等待锁存器的线程接收 AbortException（InterruptedException 的子类）。
此外，与 JB 的类不同，AbortableCountDownLatch 将在中止时立即中止所有阻塞线程，而不是等待倒计时达到零（对于使用计数>1 的情况）。
import java.util.concurrent.CountDownLatch;
import java.util.concurrent.TimeUnit;
public class AbortableCountDownLatch extends CountDownLatch {
protected boolean aborted = false;
public AbortableCountDownLatch(int count) {
super(count);
}
/**
* Unblocks all threads waiting on this latch and cause them to receive an
* AbortedException. If the latch has already counted all the way down,
* this method does nothing.
*/
public void abort() {
if( getCount()==0 )
return;
this.aborted = true;
while(getCount()>0)
countDown();
}
@Override
public boolean await(long timeout, TimeUnit unit) throws InterruptedException {
final boolean rtrn = super.await(timeout,unit);
if (aborted)
throw new AbortedException();
return rtrn;
}
@Override
public void await() throws InterruptedException {
super.await();
if (aborted)
throw new AbortedException();
}
public static class AbortedException extends InterruptedException {
public AbortedException() {
}
public AbortedException(String detailMessage) {
super(detailMessage);
}
}
}
【讨论】：
countDown()abort()，则abort() 的while 循环中存在潜在问题。
在内部使用 CountDownLatch 将此行为封装在特定的更高级别的类中：
public class MyLatch {
private CountDownLatch latch;
private boolean aborted;
...
// called by consumers
public void await() throws AbortedException {
latch.await();
if (aborted) {
throw new AbortedException();
}
}
// called by producer
public void abort() {
this.aborted = true;
latch.countDown();
}
// called by producer
public void succeed() {
latch.countDown();
}
}
【讨论】：
您可以围绕CountDownLatch 创建一个包装器，以提供取消服务员的功能。它需要跟踪等待的线程并在它们超时时释放它们，并记住闩锁已被取消，因此将来对await 的调用将立即中断。
public class CancellableCountDownLatch
{
final CountDownLatch latch;
final List<Thread> waiters;
boolean cancelled = false;
public CancellableCountDownLatch(int count) {
latch = new CountDownLatch(count);
waiters = new ArrayList<Thread>();
}
public void await() throws InterruptedException {
try {
addWaiter();
latch.await();
}
finally {
removeWaiter();
}
}
public boolean await(long timeout, TimeUnit unit) throws InterruptedException {
try {
addWaiter();
return latch.await(timeout, unit);
}
finally {
removeWaiter();
}
}
private synchronized void addWaiter() throws InterruptedException {
if (cancelled) {
Thread.currentThread().interrupt();
throw new InterruptedException("Latch has already been cancelled");
}
waiters.add(Thread.currentThread());
}
private synchronized void removeWaiter() {
waiters.remove(Thread.currentThread());
}
public void countDown() {
latch.countDown();
}
public synchronized void cancel() {
if (!cancelled) {
cancelled = true;
for (Thread waiter : waiters) {
waiter.interrupt();
}
waiters.clear();
}
}
public long getCount() {
return latch.getCount();
}
@Override
public String toString() {
return latch.toString();
}
}
【讨论】：
您可以使用允许访问其受保护的getWaitingThreads 方法的ReentrantLock 推出自己的CountDownLatch。
例子：
public class FailableCountDownLatch {
private static class ConditionReentrantLock extends ReentrantLock {
private static final long serialVersionUID = 2974195457854549498L;
@Override
public Collection<Thread> getWaitingThreads(Condition c) {
return super.getWaitingThreads(c);
}
}
private final ConditionReentrantLock lock = new ConditionReentrantLock();
private final Condition countIsZero = lock.newCondition();
private long count;
public FailableCountDownLatch(long count) {
this.count = count;
}
public void await() throws InterruptedException {
lock.lock();
try {
if (getCount() > 0) {
countIsZero.await();
}
} finally {
lock.unlock();
}
}
public boolean await(long time, TimeUnit unit) throws InterruptedException {
lock.lock();
try {
if (getCount() > 0) {
return countIsZero.await(time, unit);
}
} finally {
lock.unlock();
}
return true;
}
public long getCount() {
lock.lock();
try {
return count;
} finally {
lock.unlock();
}
}
public void countDown() {
lock.lock();
try {
if (count > 0) {
count--;
if (count == 0) {
countIsZero.signalAll();
}
}
} finally {
lock.unlock();
}
}
public void abortCountDown() {
lock.lock();
try {
for (Thread t : lock.getWaitingThreads(countIsZero)) {
t.interrupt();
}
} finally {
lock.unlock();
}
}
}
您可能希望更改此类以在取消对 await 的新调用时抛出 InterruptedException。如果您需要该功能，您甚至可以让此类扩展 CountDownLatch。
【讨论】：
从 Java 8 开始，您可以为此使用 CompletableFuture。一个或多个线程可以调用阻塞的 get() 方法：
CompletableFuture<Void> cf = new CompletableFuture<>();
try {
cf.get();
} catch (ExecutionException e) {
//act on error
}
另一个线程可以使用cf.complete(null) 成功完成它，或者使用cf.completeExceptionally(new MyException()) 异常完成它
【讨论】：