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Glassfish example source code file (LruCache.java)

This example Glassfish source code file (LruCache.java) is included in the DevDaily.com "Java Source Code Warehouse" project. The intent of this project is to help you "Learn Java by Example" TM.

Java - Glassfish tags/keywords

arraylist, basecache, cacheitem, cacheitem, lrucache, lrucache, lrucacheitem, lrucacheitem, map, no_timeout, object, object, properties, util

The Glassfish LruCache.java source code

/*
 * DO NOT ALTER OR REMOVE COPYRIGHT NOTICES OR THIS HEADER.
 *
 * Copyright (c) 1997-2010 Oracle and/or its affiliates. All rights reserved.
 *
 * The contents of this file are subject to the terms of either the GNU
 * General Public License Version 2 only ("GPL") or the Common Development
 * and Distribution License("CDDL") (collectively, the "License").  You
 * may not use this file except in compliance with the License.  You can
 * obtain a copy of the License at
 * https://glassfish.dev.java.net/public/CDDL+GPL_1_1.html
 * or packager/legal/LICENSE.txt.  See the License for the specific
 * language governing permissions and limitations under the License.
 *
 * When distributing the software, include this License Header Notice in each
 * file and include the License file at packager/legal/LICENSE.txt.
 *
 * GPL Classpath Exception:
 * Oracle designates this particular file as subject to the "Classpath"
 * exception as provided by Oracle in the GPL Version 2 section of the License
 * file that accompanied this code.
 *
 * Modifications:
 * If applicable, add the following below the License Header, with the fields
 * enclosed by brackets [] replaced by your own identifying information:
 * "Portions Copyright [year] [name of copyright owner]"
 *
 * Contributor(s):
 * If you wish your version of this file to be governed by only the CDDL or
 * only the GPL Version 2, indicate your decision by adding "[Contributor]
 * elects to include this software in this distribution under the [CDDL or GPL
 * Version 2] license."  If you don't indicate a single choice of license, a
 * recipient has the option to distribute your version of this file under
 * either the CDDL, the GPL Version 2 or to extend the choice of license to
 * its licensees as provided above.  However, if you add GPL Version 2 code
 * and therefore, elected the GPL Version 2 license, then the option applies
 * only if the new code is made subject to such option by the copyright
 * holder.
 */

package com.sun.appserv.util.cache;

import java.util.ArrayList;
import java.util.Map;
import java.util.Properties;
import java.util.ResourceBundle;

/**
 * LRUCache
 * in-memory bounded cache with an LRU list
 */
public class LruCache extends BaseCache {

    // the item never expires
    public static final long NO_TIMEOUT = -1;

    // LRU list
    protected LruCacheItem head;
    protected LruCacheItem tail;

    // the number of times the cache was trimmed
    protected int trimCount;

    protected int listSize;
    protected long timeout = NO_TIMEOUT;

    protected int defaultMaxEntries = Constants.DEFAULT_MAX_ENTRIES;
    protected boolean isUnbounded = false;

    /**
     * default constructor
     */
    public LruCache() { }

    /**
     * constructor with specified max entries.
     * @param defaultMaxEntries specifies the default max entries
     *        to use when the maxEntries is <= 0.
     */
    public LruCache(int defaultMaxEntries) {
        this.defaultMaxEntries = defaultMaxEntries;
    }

    /**
     * initialize the cache
     * @param maxEntries maximum number of entries expected in the cache
     * @param loadFactor the load factor
     * @param timeout to be used to trim the expired entries
     * @param props opaque list of properties for a given cache implementation
     * @throws a generic Exception if the initialization failed
     */
    public void init(int maxEntries, long timeout, float loadFactor, Properties props) {

        // if the max entries is <= 0 then set the default max entries
        if (maxEntries <= 0) {
            maxEntries = defaultMaxEntries;

            // mark this cache unbounded
            isUnbounded = true;
        }
        setTimeout(timeout);

        super.init(maxEntries, loadFactor, props);
    }

    /**
     * sets the timeout value
     * @param timeout to be used to trim the expired entries
     */
    public void setTimeout(long timeout) {
        // accept a positive timeout
        if (timeout > 0)
            this.timeout = timeout;
    }

    /**
     * create new item
     * @param hashCode for the entry
     * @param key <code>Object key 
     * @param value <code>Object value
     * @param size size in bytes of the item
     * 
     * subclasses may override to provide their own CacheItem extensions
     * e.g. one that permits persistence.
     */
    protected CacheItem createItem(int hashCode, Object key, 
                                        Object value, int size) {
        return new LruCacheItem(hashCode, key, value, size);
    }

    /**
     * trim one item from the LRU list
     * @param currentTime of this operation
     * @return the item trimmed from cache
     *
     * list synchronization is handled by the caller
     */
    protected CacheItem trimLru(long currentTime) {

        LruCacheItem trimItem = tail;

        if (tail != head) {
            tail = trimItem.lPrev;
            if(tail == null) {
                //TODO:: print out a warning message here
                tail = head = null;
            } else {
                tail.lNext = null;
                trimItem.lPrev = null;
            }
        } else {
            tail = head = null;
        }

        if (trimItem != null) {
            trimItem.isTrimmed = true;
            trimItem.lPrev = null;
            trimCount++;
            listSize--;
        }

        return trimItem;
    }

    /**
    /**
     * this item is just added to the cache
     * @param item <code>CacheItem that was created
     * @return a overflow item; may be null
     *
     * this function checks if adding the new item results in a overflow;
     * if so, it returns the item to be removed.
     *
     * Cache bucket is already synchronized by the caller
     */
    protected CacheItem itemAdded(CacheItem item) {
        boolean updateThreshold = false;
        CacheItem overflow = null;
        LruCacheItem lc = (LruCacheItem) item;

        // set the timestamp
        lc.lastAccessed = System.currentTimeMillis();

        // update the LRU
        synchronized (this) {
            if (head != null) {
                head.lPrev = lc;
                lc.lNext = head;
                lc.lPrev = null;
                head = lc;
            }
            else {
                head = tail = lc;
                lc.lPrev = lc.lNext = null;
            }

            listSize++;

            if (isThresholdReached()) {
                if (!isUnbounded) 
                    overflow = trimLru(lc.lastAccessed);
                else
                    updateThreshold = true;
            }
        }

        // update the base cache threshold if needed
        if (updateThreshold)
            super.handleOverflow(); 

        return overflow;
    }

    /**
     * this item is accessed 
     * @param item <code>CacheItem accessed
     *
     * Cache bucket is already synchronized by the caller
     */
    protected void itemAccessed(CacheItem item) {
        LruCacheItem lc = (LruCacheItem) item;

        synchronized (this) {

            // if the item is already trimmed from the LRU list, nothing to do.
            if (lc.isTrimmed)
                return;

            // update the timestamp
            lc.lastAccessed = System.currentTimeMillis();

            LruCacheItem prev = lc.lPrev;
            LruCacheItem next = lc.lNext;
            // update the LRU list
            if (prev != null) {
                // put the item at the head of LRU list
                lc.lPrev = null;
                lc.lNext = head;
                head.lPrev = lc;
                head = lc;
    
                // patch up the previous neighbors
                prev.lNext = next;
                if (next != null)
                    next.lPrev = prev;
                else
                    tail = prev;
           }
        }
    }

    /**
     * item value has been refreshed
     * @param item <code>CacheItem that was refreshed
     * @param oldSize size of the previous value that was refreshed
     * Cache bucket is already synchronized by the caller
     */
    protected void itemRefreshed(CacheItem item, int oldSize) {
        itemAccessed(item);   
    }

    /**
     * item value has been removed from the cache
     * @param item <code>CacheItem that was just removed
     *
     * Cache bucket is already synchronized by the caller
     */
    protected void itemRemoved(CacheItem item) {
        LruCacheItem l = (LruCacheItem) item;

        // remove the item from the LRU list
        synchronized (this) {
            LruCacheItem prev = l.lPrev;
            LruCacheItem next = l.lNext;

            // if the item is already trimmed from the LRU list, nothing to do.
            if (l.isTrimmed)
                return;

            // patch up the neighbors and make sure head/tail are correct
            if (prev != null)
                prev.lNext = next;
            else
                head = next;

            if (next != null)
                next.lPrev = prev;
            else
                tail = prev;

            l.lPrev = l.lNext = null;
            listSize--;
        }
    }

    /**
     * trim the expired entries from the cache.
     * @param maxCount maximum number of invalid entries to trim
     *        specify Integer.MAX_VALUE to trim all invalid entries
     * This call is to be scheduled by a thread managed by the container.
     *
     * NOTE: this algorithm assumes that all the entries in the cache have
     * identical timeout (otherwise traversing from tail won't be right).
     */
    public void trimExpiredEntries(int maxCount) {
        
        int count = 0;
        LruCacheItem item;
        long currentTime = System.currentTimeMillis();
        ArrayList list = new ArrayList();

        synchronized (this) {
            // traverse LRU list till we reach a valid item; remove them at once
            for (item = tail; item != null && count < maxCount;
                                                item = item.lPrev) {

                if (timeout != NO_TIMEOUT && 
                    (item.lastAccessed + timeout) <= currentTime) {
                    item.isTrimmed = true;
		    list.add(item);

                    count++;
                } else
                    break;
            }

            // if there was at least one invalid item then item != tail.
            if (item != tail) {
                if (item != null)
                    item.lNext = null;
                else
                    head = null;

                tail = item;
            }
            listSize -= count;
            trimCount += count;
        }
        
        // trim the items from the BaseCache from the old tail backwards
        for (int index=list.size()-1; index >= 0; index--) {
            trimItem((LruCacheItem) list.get(index));
        }
    }


    /**
     * get generic stats from subclasses 
     */

    /**
     * get the desired statistic counter
     * @param key to corresponding stat
     * @return an Object corresponding to the stat
     * See also: Constant.java for the key
     */
    public Object getStatByName(String key) {
        Object stat = super.getStatByName(key);

        if (stat == null && key != null) {
            if (key.equals(Constants.STAT_LRUCACHE_LIST_LENGTH))
                stat = Integer.valueOf(listSize);
            else if (key.equals(Constants.STAT_LRUCACHE_TRIM_COUNT))
                stat = Integer.valueOf(trimCount);
        }
        return stat;
    }

    public Map getStats() {
        Map stats = super.getStats();
        stats.put(Constants.STAT_LRUCACHE_LIST_LENGTH,
                  Integer.valueOf(listSize));
        stats.put(Constants.STAT_LRUCACHE_TRIM_COUNT,
                  Integer.valueOf(trimCount));

        return stats;
    }

    /** default CacheItem class implementation  ***/
    protected static class LruCacheItem extends CacheItem {

        // double linked LRU list
        protected LruCacheItem lNext;
        protected LruCacheItem lPrev;
        protected boolean isTrimmed;
        protected long lastAccessed;

        protected LruCacheItem(int hashCode, Object key, Object value, int size) {
            super(hashCode, key, value, size);
        }
    }
}

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