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/*
* Licensed to the Apache Software Foundation (ASF) under one or more
* contributor license agreements. See the NOTICE file distributed with
* this work for additional information regarding copyright ownership.
* The ASF licenses this file to You 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 org.apache.tomcat.util.threads;
import java.util.Collection;
import java.util.concurrent.atomic.AtomicLong;
import java.util.concurrent.locks.AbstractQueuedSynchronizer;
import org.apache.juli.logging.Log;
import org.apache.juli.logging.LogFactory;
/**
* Shared latch that allows the latch to be acquired a limited number of times
* after which all subsequent requests to acquire the latch will be placed in a
* FIFO queue until one of the shares is returned.
*/
public class LimitLatch {
private static final Log log = LogFactory.getLog(LimitLatch.class);
private class Sync extends AbstractQueuedSynchronizer {
private static final long serialVersionUID = 1L;
public Sync() {
}
@Override
protected int tryAcquireShared(int ignored) {
long newCount = count.incrementAndGet();
if (!released && newCount > limit) {
// Limit exceeded
count.decrementAndGet();
return -1;
} else {
return 1;
}
}
@Override
protected boolean tryReleaseShared(int arg) {
count.decrementAndGet();
return true;
}
}
private final Sync sync;
private final AtomicLong count;
private volatile long limit;
private volatile boolean released = false;
/**
* Instantiates a LimitLatch object with an initial limit.
* @param limit - maximum number of concurrent acquisitions of this latch
*/
public LimitLatch(long limit) {
this.limit = limit;
this.count = new AtomicLong(0);
this.sync = new Sync();
}
/**
* Returns the current count for the latch
* @return the current count for latch
*/
public long getCount() {
return count.get();
}
/**
* Obtain the current limit.
*/
public long getLimit() {
return limit;
}
/**
* Sets a new limit. If the limit is decreased there may be a period where
* more shares of the latch are acquired than the limit. In this case no
* more shares of the latch will be issued until sufficient shares have been
* returned to reduce the number of acquired shares of the latch to below
* the new limit. If the limit is increased, threads currently in the queue
* may not be issued one of the newly available shares until the next
* request is made for a latch.
*
* @param limit The new limit
*/
public void setLimit(long limit) {
this.limit = limit;
}
/**
* Acquires a shared latch if one is available or waits for one if no shared
* latch is current available.
*/
public void countUpOrAwait() throws InterruptedException {
if (log.isDebugEnabled()) {
log.debug("Counting up["+Thread.currentThread().getName()+"] latch="+getCount());
}
sync.acquireSharedInterruptibly(1);
}
/**
* Releases a shared latch, making it available for another thread to use.
* @return the previous counter value
*/
public long countDown() {
sync.releaseShared(0);
long result = getCount();
if (log.isDebugEnabled()) {
log.debug("Counting down["+Thread.currentThread().getName()+"] latch="+result);
}
return result;
}
/**
* Releases all waiting threads and causes the {@link #limit} to be ignored
* until {@link #reset()} is called.
*/
public boolean releaseAll() {
released = true;
return sync.releaseShared(0);
}
/**
* Resets the latch and initializes the shared acquisition counter to zero.
* @see #releaseAll()
*/
public void reset() {
this.count.set(0);
released = false;
}
/**
* Returns <code>true</code> if there is at least one thread waiting to
* acquire the shared lock, otherwise returns <code>false</code>.
*/
public boolean hasQueuedThreads() {
return sync.hasQueuedThreads();
}
/**
* Provide access to the list of threads waiting to acquire this limited
* shared latch.
*/
public Collection<Thread> getQueuedThreads() {
return sync.getQueuedThreads();
}
}
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