fifomemorystore.java

EasyDBO v0.4 是一个非常适合中小型软件数据库开发的数据持久层框架

/* ====================================================================
 * The Apache Software License, Version 1.1
 *
 * Copyright (c) 2003 - 2004 Greg Luck.  All rights reserved.
 *
 * Redistribution and use in source and binary forms, with or without
 * modification, are permitted provided that the following conditions
 * are met:
 *
 * 1. Redistributions of source code must retain the above copyright
 *    notice, this list of conditions and the following disclaimer.
 *
 * 2. Redistributions in binary form must reproduce the above copyright
 *    notice, this list of conditions and the following disclaimer in
 *    the documentation and/or other materials provided with the
 *    distribution.
 *
 * 3. The end-user documentation included with the redistribution, if
 *    any, must include the following acknowlegement:
 *       "This product includes software developed by Greg Luck
 *       (http://sourceforge.net/users/gregluck) and contributors.
 *       See http://sourceforge.net/project/memberlist.php?group_id=93232
 *       for a list of contributors"
 *    Alternately, this acknowledgement may appear in the software itself,
 *    if and wherever such third-party acknowlegements normally appear.
 *
 * 4. The names "EHCache" must not be used to endorse or promote products
 *    derived from this software without prior written permission. For written
 *    permission, please contact Greg Luck (gregluck at users.sourceforge.net).
 *
 * 5. Products derived from this software may not be called "EHCache"
 *    nor may "EHCache" appear in their names without prior written
 *    permission of Greg Luck.
 *
 * THIS SOFTWARE IS PROVIDED ``AS IS'' AND ANY EXPRESSED OR IMPLIED
 * WARRANTIES, INCLUDING, BUT NOT LIMITED TO, THE IMPLIED WARRANTIES
 * OF MERCHANTABILITY AND FITNESS FOR A PARTICULAR PURPOSE ARE
 * DISCLAIMED.  IN NO EVENT SHALL GREG LUCK OR OTHER
 * CONTRIBUTORS BE LIABLE FOR ANY DIRECT, INDIRECT, INCIDENTAL,
 * SPECIAL, EXEMPLARY, OR CONSEQUENTIAL DAMAGES (INCLUDING, BUT NOT
 * LIMITED TO, PROCUREMENT OF SUBSTITUTE GOODS OR SERVICES; LOSS OF
 * USE, DATA, OR PROFITS; OR BUSINESS INTERRUPTION) HOWEVER CAUSED AND
 * ON ANY THEORY OF LIABILITY, WHETHER IN CONTRACT, STRICT LIABILITY,
 * OR TORT (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY OUT
 * OF THE USE OF THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF
 * SUCH DAMAGE.
 * ====================================================================
 *
 * This software consists of voluntary contributions made by contributors
 * individuals on behalf of the EHCache project.  For more
 * information on EHCache, please see <http://ehcache.sourceforge.net/>.
 *
 */
package com.easyjf.cache.store;
/**
 * 这里直接使用的Apache开源缓存ECache中的算法
 */
import com.easyjf.cache.*;

import org.apache.commons.collections.SequencedHashMap;
import org.apache.log4j.Logger;

import java.io.Serializable;
import java.util.Iterator;
import java.util.LinkedHashMap;
import java.util.Set;

/**
 * First-In-First-Out (FIFO) implementation of MemoryStore.
 *
 * @author <a href="mailto:[e-mail]">Surya Suravarapu</a>
 * @version $Id: FifoMemoryStore.java,v 1.9 2005/10/15 17:32:30 gregluck Exp $
 */
public class FifoMemoryStore extends MemoryStore {
	private final static Logger logger = Logger.getLogger(FifoMemoryStore.class);
    private final static int SEQUENCED_HASH_MAP = 1;
    private final static int LINKED_HASH_MAP = 2;
    /**
     * One of the above declared static collection types
     */
    private int collectionType;

    /**
     * Constructor for the FifoMemoryStore object.
     * <p/>
     * First tries to use {@link java.util.LinkedHashMap}. If not found uses
     * Jakarta Commons collections.
     */
    public FifoMemoryStore(ICache cache) {
        super(cache);
        // Use LinkedHashMap for JDK 1.4
        try {
            Class.forName("java.util.LinkedHashMap");
            map = new LinkedHashMap();
            collectionType = LINKED_HASH_MAP;
        } catch (ClassNotFoundException e) {
            // If not JDK 1.4 use the commons collections
            try {
                Class.forName("org.apache.commons.collections.SequencedHashMap");
                map = new SequencedHashMap();
                collectionType = SEQUENCED_HASH_MAP;
            } catch (ClassNotFoundException ee) {
                logger.error(ee.getMessage());
            }
        }
    }

    /**
     * Allow specialised actions over adding the element to the map
     *
     * @param element
     */
    protected void doPut(Element element) {
        if (isFull()) {
            removeFirstElement();
        }
    }

    /**
     * Returns the first eligible element that can be taken out of the cache
     * based on the FIFO policy
     */
    public synchronized Element getFirstElement() {
        if (map.size() == 0) {
            return null;
        }

        Element element = null;
        Serializable key = null;

        if (collectionType == LINKED_HASH_MAP) {
            Set keySet = map.keySet();
            Iterator itr = keySet.iterator();
            // The first element is the candidate to remove
            if (itr.hasNext()) {
                key = (Serializable) itr.next();
                element = (Element) map.get(key);
            }
        } else if (collectionType == SEQUENCED_HASH_MAP) {
            key = (Serializable) ((SequencedHashMap) map).getFirstKey();
            element = (Element) map.get(key);
        }

        return element;
    }

    /**
     * Remove the first element that is eligible to removed from the store
     * based on the FIFO policy
     */
    private void removeFirstElement() {
        Element element = getFirstElement();

        if (cache.isExpired(element)) {
            remove(element.getKey());        
            return;
        }
        remove(element.getKey());
    
    }
}