com.lrodriguez.SoftLimitMRUCache.java Source code

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Here is the source code for com.lrodriguez.SoftLimitMRUCache.java

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/*
 * Hibernate, Relational Persistence for Idiomatic Java
 *
 * Copyright (c) 2008, Red Hat Middleware LLC or third-party contributors as
 * indicated by the @author tags or express copyright attribution
 * statements applied by the authors.  All third-party contributions are
 * distributed under license by Red Hat Middleware LLC.
 *
 * This copyrighted material is made available to anyone wishing to use, modify,
 * copy, or redistribute it subject to the terms and conditions of the GNU
 * Lesser General Public License, as published by the Free Software Foundation.
 *
 * This program is distributed in the hope that it will be useful,
 * but WITHOUT ANY WARRANTY; without even the implied warranty of MERCHANTABILITY
 * or FITNESS FOR A PARTICULAR PURPOSE.  See the GNU Lesser General Public License
 * for more details.
 *
 * You should have received a copy of the GNU Lesser General Public License
 * along with this distribution; if not, write to:
 * Free Software Foundation, Inc.
 * 51 Franklin Street, Fifth Floor
 * Boston, MA  02110-1301  USA
 *
 */
package com.lrodriguez;

import org.apache.commons.collections.map.ReferenceMap;
import org.apache.commons.collections.map.LRUMap;

import java.io.Serializable;
import java.io.IOException;

/**
 * Cache following a "Most Recently Used" (MRY) algorithm for maintaining a
 * bounded in-memory size; the "Least Recently Used" (LRU) entry is the first
 * available for removal from the cache.
 * <p/>
 * This implementation uses a "soft limit" to the in-memory size of the cache,
 * meaning that all cache entries are kept within a completely
 * {@link java.lang.ref.SoftReference}-based map with the most recently utilized
 * entries additionally kept in a hard-reference manner to prevent those cache
 * entries soft references from becoming enqueued by the garbage collector.
 * Thus the actual size of this cache impl can actually grow beyond the stated
 * max size bound as long as GC is not actively seeking soft references for
 * enqueuement.
 *
 * @author Steve Ebersole
 */
@SuppressWarnings("serial")
public class SoftLimitMRUCache implements Serializable {

    public static final int DEFAULT_STRONG_REF_COUNT = 128;

    private final int strongReferenceCount;

    // actual cache of the entries.  soft references are used for both the keys and the
    // values here since the values pertaining to the MRU entries are kept in a
    // seperate hard reference cache (to avoid their enqueuement/garbage-collection).
    private transient ReferenceMap softReferenceCache = new ReferenceMap(ReferenceMap.SOFT, ReferenceMap.SOFT);
    // the MRU cache used to keep hard references to the most recently used query plans;
    // note : LRU here is a bit of a misnomer, it indicates that LRU entries are removed, the
    // actual kept entries are the MRU entries
    private transient LRUMap strongReferenceCache;

    public SoftLimitMRUCache() {
        this(DEFAULT_STRONG_REF_COUNT);
    }

    public SoftLimitMRUCache(int strongRefCount) {
        this.strongReferenceCount = strongRefCount;
        init();
    }

    public synchronized Object get(Object key) {
        Object result = softReferenceCache.get(key);
        if (result != null) {
            strongReferenceCache.put(key, result);
        }
        return result;
    }

    public synchronized Object put(Object key, Object value) {
        softReferenceCache.put(key, value);
        return strongReferenceCache.put(key, value);
    }

    public synchronized int size() {
        return strongReferenceCache.size();
    }

    public synchronized int softSize() {
        return softReferenceCache.size();
    }

    private void init() {
        strongReferenceCache = new LRUMap(strongReferenceCount);
    }

    private void readObject(java.io.ObjectInputStream in) throws IOException, ClassNotFoundException {
        in.defaultReadObject();
        init();
    }

    public synchronized void clear() {
        strongReferenceCache.clear();
        softReferenceCache.clear();
    }
}