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/**
* Copyright Terracotta, Inc.
*
* 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 net.sf.ehcache.concurrent;
import java.util.Arrays;
import java.util.Collections;
import java.util.List;
import java.util.concurrent.locks.ReadWriteLock;
import net.sf.ehcache.CacheException;
/**
* Provides a number of Sync which allow fine-grained concurrency. Rather than locking a cache or a store,
* the individual elements or constituent objects can be locked. This dramatically increases
* the possible concurrency.
*
* The more stripes, the higher the concurrency. To be threadsafe, the instance of CacheLockProvider needs to be
* maintained for the entire life of the cache or store, so there is some added memory use.
*
* Though a new class, this code has been refactored from BlockingCache, where it has been in use
* in highly concurrent production environments for years.
*
* Based on the lock striping concept from Brian Goetz. See Java Concurrency in Practice 11.4.3
* @author Alex Snaps
*/
public class StripedReadWriteLockSync implements StripedReadWriteLock {
/**
* The default number of locks to use. Must be a power of 2.
*
* The choice of 2048 enables 2048 concurrent operations per cache or cache store, which should be enough for most
* uses.
*/
public static final int DEFAULT_NUMBER_OF_MUTEXES = 2048;
private final ReadWriteLockSync[] mutexes;
private final List mutexesAsList;
/**
* Constructs a striped mutex with the default 2048 stripes.
*/
public StripedReadWriteLockSync() {
this(DEFAULT_NUMBER_OF_MUTEXES);
}
/**
* Constructs a striped mutex with the default 2048 stripes.
*
* The number of stripes determines the number of concurrent operations per cache or cache store.
*
* @param numberOfStripes - must be a factor of two
*/
public StripedReadWriteLockSync(int numberOfStripes) {
if ((numberOfStripes & (numberOfStripes - 1)) != 0) {
throw new CacheException("Cannot create a CacheLockProvider with a non power-of-two number of stripes");
}
if (numberOfStripes == 0) {
throw new CacheException("A zero size CacheLockProvider does not have useful semantics.");
}
mutexes = new ReadWriteLockSync[numberOfStripes];
for (int i = 0; i < mutexes.length; i++) {
mutexes[i] = new ReadWriteLockSync();
}
mutexesAsList = Collections.unmodifiableList(Arrays.asList(mutexes));
}
/**
* Gets the Sync Stripe to use for a given key.
*
* This lookup must always return the same Sync for a given key.
*
* @param key the key
* @return one of a limited number of Sync's.
*/
public ReadWriteLockSync getSyncForKey(final Object key) {
int lockNumber = ConcurrencyUtil.selectLock(key, mutexes.length);
return mutexes[lockNumber];
}
/**
* Gets the RWL Stripe to use for a given key.
*
* This lookup must always return the same RWL for a given key.
*
* @param key the key
* @return one of a limited number of RWLs.
*/
public ReadWriteLock getLockForKey(final Object key) {
int lockNumber = ConcurrencyUtil.selectLock(key, mutexes.length);
return mutexes[lockNumber].getReadWriteLock();
}
/**
* Returns all internal syncs
* @return all internal syncs
*/
public List getAllSyncs() {
return mutexesAsList;
}
}