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Ehcache is an open source, standards-based cache used to boost performance, offload the database and simplify scalability. Ehcache is robust, proven and full-featured and this has made it the most widely-used Java-based cache.

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/*
 * DO NOT ALTER OR REMOVE COPYRIGHT NOTICES OR THIS HEADER.
 *
 * Copyright (c) 2012 Oracle and/or its affiliates. All rights reserved.
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 * 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
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package org.glassfish.hk2.utilities.cache;

import org.glassfish.hk2.utilities.cache.internal.LRUCacheCheapRead;

/**
 * A cache that contains a certain number of entries, and whose oldest accessed
 * entries are removed when removal is necessary.
 *
 * @author jwells
 * @param  The key type for this cache
 * @param  The value type for this cache
 *
 */
public abstract class LRUCache {

    /**
     * Creates a cache with the given maximum cache size
     *
     * @param maxCacheSize The maximum number of entries in the cache, must be greater than 2
     * @return An LRUCache that can be used to quickly retrieve objects
     */
    public static  LRUCache createCache(int maxCacheSize) {
        return new LRUCacheCheapRead(maxCacheSize);
    }

    /**
     * Returns the value associated with the given key.  If there is no
     * value, returns null
     *
     * @param key Must be a non-null key, appropriate for use as the key to a hash map
     * @return The value associated with the key, or null if there is no such value
     */
    public abstract V get(K key);

    /**
     * Adds the given key and value pair into the cache
     *
     * @param key Must be a non-null key, appropriate for use as the key to a hash map
     * @param value Must be a non-null value
     * @return A cache entry that can be used to remove this entry from the cache.  Will not return null
     */
    public abstract CacheEntry put(K key, V value);

    /**
     * Clears all entries in the cache, for use when a known event makes the cache incorrect
     */
    public abstract void releaseCache();

    /**
     * Returns the maximum number of entries that will be stored in this cache
     *
     * @return The maximum number of entries that will be stored in this cache
     */
    public abstract int getMaxCacheSize();
    
    /**
     * This method will remove all cache entries for which this filter
     * matches
     * 
     * @param filter Entries in the cache that match this filter will
     * be removed from the cache.  If filter is null nothing
     * will be removed from the cache
     */
    public abstract void releaseMatching(CacheKeyFilter filter);
}