src/main/java/net/floodlightcontroller/util/TimedCache.java

/**
 *    Copyright 2011, Big Switch Networks, Inc.
 *    Originally created by David Erickson, Stanford University
 *
 *    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.floodlightcontroller.util;

import java.util.Set;
import java.util.concurrent.ConcurrentMap;

import com.googlecode.concurrentlinkedhashmap.ConcurrentLinkedHashMap;

/**
 * The key is any object/hash-code
 * The value is time-stamp in milliseconds
 * The time interval denotes the interval for which the entry should remain in the hashmap.
 * If an entry is present in the Linkedhashmap, it does not mean that it's valid (recently seen)
 *
 * @param <K> Type of the values in this cache
 */
public class TimedCache<K> {
    private final long timeoutInterval;    //specified in milliseconds.
    private ConcurrentMap<K, Long> cache;

    /**
     * @param capacity   the maximum number of entries in the cache before the
     *                   oldest entry is evicted.
     * @param timeToLive specified in milliseconds
     */
    public TimedCache(int capacity, int timeToLive) {
        cache = new ConcurrentLinkedHashMap.Builder<K, Long>()
                .maximumWeightedCapacity(capacity)
                .build();
        this.timeoutInterval = timeToLive;
    }

    public long getTimeoutInterval() {
        return this.timeoutInterval;
    }

    /**
     * Always try to update the cache and set the last-seen value for this key.
     * <p/>
     * Return true, if a valid existing field was updated, else return false.
     * (note: if multiple threads update simultaneously, one of them will succeed,
     * other wills return false)
     *
     * @param key
     * @return boolean
     */
    public boolean update(K key) {
        long curr = System.currentTimeMillis();
        Long prev = cache.putIfAbsent(key, curr);

        if (prev == null) {
            return false;
        }

        if (curr - prev > this.timeoutInterval) {
            if (cache.replace(key, prev, curr)) {
                return false;
            }
        }

        return true;
    }

    /**
     * Gets all the cached entries.
     * <p/>
     * Modification to the returned set will be reflected to the cache.
     *
     * @return cached entries
     */
    Set<K> getCachedEntries() {
        return cache.keySet();
    }
}