app/src/main/java/org/apache/roller/weblogger/util/cache/LRUCacheImpl.java

/*
 * Licensed to the Apache Software Foundation (ASF) under one or more
 *  contributor license agreements.  The ASF licenses this file to You
 * 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.  For additional information regarding
 * copyright in this work, please see the NOTICE file in the top level
 * directory of this distribution.
 */

package org.apache.roller.weblogger.util.cache;

import java.util.Date;
import java.util.HashMap;
import java.util.LinkedHashMap;
import java.util.Map;
import org.apache.roller.util.RollerConstants;


/**
 * A simple LRU Cache.
 */
public class LRUCacheImpl implements Cache {
    
    private final String id;
    private final Map cache;
    
    // for metrics
    protected double hits = 0;
    protected double misses = 0;
    protected double puts = 0;
    protected double removes = 0;
    protected Date startTime = new Date();
    
    
    protected LRUCacheImpl(String id) {
        
        this(id, 100);
    }
    
    
    protected LRUCacheImpl(String id, int maxsize) {
        
        this.id = id;
        this.cache = new LRULinkedHashMap(maxsize);
    }
    
    
    @Override
    public String getId() {
        return this.id;
    }
    
    
    /**
     * Store an entry in the cache.
     */
    @Override
    public synchronized void put(String key, Object value) {
        
        this.cache.put(key, value);
        puts++;
    }
    
    
    /**
     * Retrieve an entry from the cache.
     */
    @Override
    public synchronized Object get(String key) {
        
        Object obj = this.cache.get(key);
        
        // for metrics
        if(obj == null) {
            misses++;
        } else {
            hits++;
        }
        
        return obj;
    }
    
    
    @Override
    public synchronized void remove(String key) {
        
        this.cache.remove(key);
        removes++;
    }
    
    
    @Override
    public synchronized void clear() {
        
        this.cache.clear();
        
        // clear metrics
        hits = 0;
        misses = 0;
        puts = 0;
        removes = 0;
        startTime = new Date();
    }
    
    
    @Override
    public Map<String, Object> getStats() {
        
        Map<String, Object> stats = new HashMap<>();
        stats.put("startTime", this.startTime);
        stats.put("hits", this.hits);
        stats.put("misses", this.misses);
        stats.put("puts", this.puts);
        stats.put("removes", this.removes);
        
        // calculate efficiency
        if((misses - removes) > 0) {
            double efficiency = hits / (misses + hits);
            stats.put("efficiency", efficiency * RollerConstants.PERCENT_100);
        }
        
        return stats;
    }
    
    
    // David Flanaghan: http://www.davidflanagan.com/blog/000014.html
    private static class LRULinkedHashMap extends LinkedHashMap {
        protected int maxsize;
        
        public LRULinkedHashMap(int maxsize) {
            super(maxsize * 4 / 3 + 1, 0.75f, true);
            this.maxsize = maxsize;
        }
        
        @Override
        protected boolean removeEldestEntry(Map.Entry eldest) {
            return this.size() > this.maxsize;
        }
    }
    
}