我有多个消费者线程正在等待CountDownLatch大小为 1 的await(). 我有一个生产者线程，countDown()它在成功完成时调用。
当没有错误时，这很有效。
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类似的东西，abortCountDown()并让所有消费者收到 InterruptedException 或其他一些异常。我不想调用countDown()，因为这需要我的所有消费者线程在调用await(). 我宁愿他们只收到一个他们已经知道如何处理的异常。
我知道中止设施在CountDownLatch. 是否有另一个同步原语可以轻松适应以有效创建CountDownLatch支持中止倒计时的同步原语？
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，该方法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);
}
}
}
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();
}
}
您可以创建一个包装器CountDownLatch，以提供取消服务员的能力。它将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，因此未来的调用await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();
}
}
CountDownLatch您可以使用ReentrantLock允许访问其受保护方法的 a推出自己的getWaitingThreads方法。
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后引发InterruptedException新的调用。如果您需要该功能，await您甚至可以扩展此类。CountDownLatch
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
}
另一个线程可以成功地完成它，也可以cf.complete(null)异常地完成它cf.completeExceptionally(new MyException())