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/**
* Copyright 2011 Atlassian Pty Ltd
*
* Licensed under the Apache License, Version 2.0 (the "License");
* you may not use this file except in compliance with the License.
* You may obtain a copy of the License at
*
* http://www.apache.org/licenses/LICENSE-2.0
*
* Unless required by applicable law or agreed to in writing, software
* distributed under the License is distributed on an "AS IS" BASIS,
* WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
* See the License for the specific language governing permissions and
* limitations under the License.
*/
package io.atlassian.util.concurrent;
import net.jcip.annotations.ThreadSafe;
import java.util.concurrent.BlockingQueue;
import java.util.concurrent.Executor;
import java.util.concurrent.LinkedBlockingQueue;
import java.util.concurrent.RejectedExecutionException;
import java.util.concurrent.Semaphore;
import java.util.concurrent.ThreadPoolExecutor;
/**
* Class that limits the number submitted jobs to an Executor and stores the
* overflow in a queue. This is to get around the fact that the
* {@link ThreadPoolExecutor} can either be limited in pool size or be limited
* in queue size, but not both (it sort of can, but a pool will not grow until
* its queue is full).
*
* This class implements a limit on the pool size if using a cached thread pool
* and stores the overflow in its own queue.
*
* This class only makes sense in conjunction with a cached thread pool that
* uses a bound queue such as SynchronousQueue. This kind of pool
* is not for high-throughput jobs as submission depends on a thread being
* available to execute the job, otherwise a new thread is constructed. Wrapping
* this Executor around a fixed thread executor doesn't make sense as the limit
* is already applied by the available thread count.
*
* @since 1.0
*/
@ThreadSafe final class LimitedExecutor implements Executor {
private final Executor delegate;
private final BlockingQueue overflow = new LinkedBlockingQueue();
private final Semaphore semaphore;
LimitedExecutor(final Executor delegate, final int limit) {
this.delegate = delegate;
semaphore = new Semaphore(limit);
}
/** {@inheritDoc} */
@Override public final void execute(final Runnable command) {
if (semaphore.tryAcquire()) {
try {
delegate.execute(new Runner(command));
} catch (final RejectedExecutionException rej) {
semaphore.release();
throw rej;
}
} else {
overflow.add(command);
while (semaphore.availablePermits() > 0) {
if (!resubmit()) {
return;
}
}
}
}
private boolean resubmit() {
final Runnable next = overflow.poll();
if (next != null) {
execute(next);
return true;
}
return false;
}
class Runner implements Runnable {
private final Runnable delegate;
Runner(final Runnable delegate) {
this.delegate = delegate;
}
@Override public void run() {
try {
delegate.run();
} finally {
semaphore.release();
resubmit();
}
}
}
}