An LRU (Least Recently Used) cache replacement policy
/*
* $Id: CacheLRU.java,v 1.10 2003/11/07 20:16:25 dfs Exp $
*
*
* The Apache Software License, Version 1.1
*
* Copyright (c) 2000 The Apache Software Foundation. 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 acknowledgment:
* "This product includes software developed by the
* Apache Software Foundation (http://www.apache.org/)."
* Alternately, this acknowledgment may appear in the software itself,
* if and wherever such third-party acknowledgments normally appear.
*
* 4. The names "Apache" and "Apache Software Foundation", "Jakarta-Oro"
* must not be used to endorse or promote products derived from this
* software without prior written permission. For written
* permission, please contact apache@apache.org.
*
* 5. Products derived from this software may not be called "Apache"
* or "Jakarta-Oro", nor may "Apache" or "Jakarta-Oro" appear in their
* name, without prior written permission of the Apache Software Foundation.
*
* 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 THE APACHE SOFTWARE FOUNDATION OR
* ITS 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 many
* individuals on behalf of the Apache Software Foundation. For more
* information on the Apache Software Foundation, please see
* <http://www.apache.org/>.
*/
import java.util.*;
/**
* This class is a GenericCache subclass implementing an LRU
* (Least Recently Used) cache replacement policy. In other words,
* values are added to the cache until it becomes full. Once the
* cache is full, when a new value is added to the cache, it replaces
* the least recently used value currently in the cache. This is probably
* the best general purpose cache replacement policy.
*
* @version @version@
* @since 1.0
* @see GenericCache
*/
public final class CacheLRU extends GenericCache {
private int __head = 0, __tail = 0;
private int[] __next, __prev;
/**
* Creates a CacheLRU instance with a given cache capacity.
* <p>
* @param capacity The capacity of the cache.
*/
public CacheLRU(int capacity) {
super(capacity);
int i;
__next = new int[_cache.length];
__prev = new int[_cache.length];
for(i=0; i < __next.length; i++)
__next[i] = __prev[i] = -1;
}
/**
* Same as:
* <blockquote><pre>
* CacheLRU(GenericCache.DEFAULT_CAPACITY);
* </pre></blockquote>
*/
public CacheLRU(){
this(GenericCache.DEFAULT_CAPACITY);
}
private void __moveToFront(int index) {
int next, prev;
if(__head != index) {
next = __next[index];
prev = __prev[index];
// Only the head has a prev entry that is an invalid index so
// we don't check.
__next[prev] = next;
// Make sure index is valid. If it isn't, we're at the tail
// and don't set __prev[next].
if(next >= 0)
__prev[next] = prev;
else
__tail = prev;
__prev[index] = -1;
__next[index] = __head;
__prev[__head] = index;
__head = index;
}
}
public synchronized Object getElement(Object key) {
Object obj;
obj = _table.get(key);
if(obj != null) {
GenericCacheEntry entry;
entry = (GenericCacheEntry)obj;
// Maintain LRU property
__moveToFront(entry._index);
return entry._value;
}
return null;
}
/**
* Adds a value to the cache. If the cache is full, when a new value
* is added to the cache, it replaces the least recently used value
* in the cache (i.e., LRU).
* <p>
* @param key The key referencing the value added to the cache.
* @param value The value to add to the cache.
*/
public final synchronized void addElement(Object key, Object value) {
Object obj;
obj = _table.get(key);
if(obj != null) {
GenericCacheEntry entry;
// Just replace the value, but move it to the front.
entry = (GenericCacheEntry)obj;
entry._value = value;
entry._key = key;
__moveToFront(entry._index);
return;
}
// If we haven't filled the cache yet, place in next available spot
// and move to front.
if(!isFull()) {
if(_numEntries > 0) {
__prev[_numEntries] = __tail;
__next[_numEntries] = -1;
__moveToFront(_numEntries);
}
++_numEntries;
} else {
// We replace the tail of the list.
_table.remove(_cache[__tail]._key);
__moveToFront(__tail);
}
_cache[__head]._value = value;
_cache[__head]._key = key;
_table.put(key, _cache[__head]);
}
}
/*
* $Id: GenericCache.java,v 1.8 2003/11/07 20:16:25 dfs Exp $
*
*
* The Apache Software License, Version 1.1
*
* Copyright (c) 2000 The Apache Software Foundation. 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 acknowledgment:
* "This product includes software developed by the
* Apache Software Foundation (http://www.apache.org/)."
* Alternately, this acknowledgment may appear in the software itself,
* if and wherever such third-party acknowledgments normally appear.
*
* 4. The names "Apache" and "Apache Software Foundation", "Jakarta-Oro"
* must not be used to endorse or promote products derived from this
* software without prior written permission. For written
* permission, please contact apache@apache.org.
*
* 5. Products derived from this software may not be called "Apache"
* or "Jakarta-Oro", nor may "Apache" or "Jakarta-Oro" appear in their
* name, without prior written permission of the Apache Software Foundation.
*
* 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 THE APACHE SOFTWARE FOUNDATION OR
* ITS 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 many
* individuals on behalf of the Apache Software Foundation. For more
* information on the Apache Software Foundation, please see
* <http://www.apache.org/>.
*/
/**
* This is the base class for all cache implementations provided in the
* org.apache.oro.util package. To derive a subclass from GenericCache
* only the ... methods
* need be overridden.
* Although 4 subclasses of GenericCache are provided with this
* package, users may not derive subclasses from this class.
* Rather, users should create their own implmentations of the
* {@link Cache} interface.
*
* @version @version@
* @since 1.0
* @see Cache
* @see CacheLRU
* @see CacheFIFO
* @see CacheFIFO2
* @see CacheRandom
*/
abstract class GenericCache implements Cache, java.io.Serializable {
/**
* The default capacity to be used by the GenericCache subclasses
* provided with this package. Its value is 20.
*/
public static final int DEFAULT_CAPACITY = 20;
int _numEntries;
GenericCacheEntry[] _cache;
HashMap _table;
/**
* The primary constructor for GenericCache. It has default
* access so it will only be used within the package. It initializes
* _table to a Hashtable of capacity equal to the capacity argument,
* _cache to an array of size equal to the capacity argument, and
* _numEntries to 0.
* <p>
* @param capacity The maximum capacity of the cache.
*/
GenericCache(int capacity) {
_numEntries = 0;
_table = new HashMap(capacity);
_cache = new GenericCacheEntry[capacity];
while(--capacity >= 0)
_cache[capacity] = new GenericCacheEntry(capacity);
}
public abstract void addElement(Object key, Object value);
public synchronized Object getElement(Object key) {
Object obj;
obj = _table.get(key);
if(obj != null)
return ((GenericCacheEntry)obj)._value;
return null;
}
public final Iterator keys() {
return _table.keySet().iterator();
}
/**
* Returns the number of elements in the cache, not to be confused with
* the {@link #capacity()} which returns the number
* of elements that can be held in the cache at one time.
* <p>
* @return The current size of the cache (i.e., the number of elements
* currently cached).
*/
public final int size() { return _numEntries; }
/**
* Returns the maximum number of elements that can be cached at one time.
* <p>
* @return The maximum number of elements that can be cached at one time.
*/
public final int capacity() { return _cache.length; }
public final boolean isFull() { return (_numEntries >= _cache.length); }
}
/*
* $Id: Cache.java,v 1.7 2003/11/07 20:16:25 dfs Exp $
*
*
* The Apache Software License, Version 1.1
*
* Copyright (c) 2000 The Apache Software Foundation. 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 acknowledgment:
* "This product includes software developed by the
* Apache Software Foundation (http://www.apache.org/)."
* Alternately, this acknowledgment may appear in the software itself,
* if and wherever such third-party acknowledgments normally appear.
*
* 4. The names "Apache" and "Apache Software Foundation", "Jakarta-Oro"
* must not be used to endorse or promote products derived from this
* software without prior written permission. For written
* permission, please contact apache@apache.org.
*
* 5. Products derived from this software may not be called "Apache"
* or "Jakarta-Oro", nor may "Apache" or "Jakarta-Oro" appear in their
* name, without prior written permission of the Apache Software Foundation.
*
* 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 THE APACHE SOFTWARE FOUNDATION OR
* ITS 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 many
* individuals on behalf of the Apache Software Foundation. For more
* information on the Apache Software Foundation, please see
* <http://www.apache.org/>.
*/
/**
* An interface defining the basic functions of a cache.
*
* @version @version@
* @since 1.0
*/
interface Cache {
public void addElement(Object key, Object value);
public Object getElement(Object key);
/**
* Returns the number of elements in the cache, not to be confused with
* the {@link #capacity()} which returns the number
* of elements that can be held in the cache at one time.
* <p>
* @return The current size of the cache (i.e., the number of elements
* currently cached).
*/
public int size();
/**
* Returns the maximum number of elements that can be cached at one time.
* <p>
* @return The maximum number of elements that can be cached at one time.
*/
public int capacity();
}
/*
* $Id: GenericCacheEntry.java,v 1.7 2003/11/07 20:16:25 dfs Exp $
*
*
* The Apache Software License, Version 1.1
*
* Copyright (c) 2000 The Apache Software Foundation. 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 acknowledgment:
* "This product includes software developed by the
* Apache Software Foundation (http://www.apache.org/)."
* Alternately, this acknowledgment may appear in the software itself,
* if and wherever such third-party acknowledgments normally appear.
*
* 4. The names "Apache" and "Apache Software Foundation", "Jakarta-Oro"
* must not be used to endorse or promote products derived from this
* software without prior written permission. For written
* permission, please contact apache@apache.org.
*
* 5. Products derived from this software may not be called "Apache"
* or "Jakarta-Oro", nor may "Apache" or "Jakarta-Oro" appear in their
* name, without prior written permission of the Apache Software Foundation.
*
* 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 THE APACHE SOFTWARE FOUNDATION OR
* ITS 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 many
* individuals on behalf of the Apache Software Foundation. For more
* information on the Apache Software Foundation, please see
* <http://www.apache.org/>.
*/
/**
* A structure used to store values in a GenericCache. It
* is declared with default access to limit it to use only within the
* package.
*
* @version @version@
* @since 1.0
*/
final class GenericCacheEntry implements java.io.Serializable {
/** The cache array index of the entry. */
int _index;
/** The value stored at this entry. */
Object _value;
/** The key used to store the value. */
Object _key;
GenericCacheEntry(int index) {
_index = index;
_value = null;
_key = null;
}
}
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