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Ehcache is an open source, standards-based cache used to boost performance, offload the database and simplify scalability. Ehcache is robust, proven and full-featured and this has made it the most widely-used Java-based cache.

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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;
    }
}