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A modification of the OpenDMK JMX remote implementation. Licensed under the Apache Software License v2.0 as permitted by the original CDDL license
/*
* JPPF.
* Copyright (C) 2005-2016 JPPF Team.
* http://www.jppf.org
*
* 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.
*/
/*
* @(#)CacheMap.java 1.3
*
*
* DO NOT ALTER OR REMOVE COPYRIGHT NOTICES OR THIS HEADER.
*
* Copyright (c) 2007 Sun Microsystems, Inc. All Rights Reserved.
*
* 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 http://opendmk.dev.java.net/legal_notices/licenses.txt or in the
* LEGAL_NOTICES folder that accompanied this code. See the License for the
* specific language governing permissions and limitations under the License.
*
* When distributing the software, include this License Header Notice in each
* file and include the License file found at
* http://opendmk.dev.java.net/legal_notices/licenses.txt
* or in the LEGAL_NOTICES folder that accompanied this code.
* Sun designates this particular file as subject to the "Classpath" exception
* as provided by Sun in the GPL Version 2 section of the License file that
* accompanied this code.
*
* If applicable, add the following below the License Header, with the fields
* enclosed by brackets [] replaced by your own identifying information:
*
* "Portions Copyrighted [year] [name of copyright owner]"
*
* Contributor(s):
*
* If you wish your version of this file to be governed by only the CDDL or
* only the GPL Version 2, indicate your decision by adding
*
* "[Contributor] elects to include this software in this distribution
* under the [CDDL or GPL Version 2] license."
*
* If you don't indicate a single choice of license, a recipient has the option
* to distribute your version of this file under either the CDDL or the GPL
* Version 2, or to extend the choice of license to its licensees as provided
* above. However, if you add GPL Version 2 code and therefore, elected the
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package com.sun.jmx.remote.opt.util;
import java.lang.ref.SoftReference;
import java.util.*;
/**
* Like WeakHashMap, except that the keys of the n most recently-accessed entries are kept as {@link SoftReference soft references}.
* Accessing an element means creating it, or retrieving it with {@link #get(Object) get}. Because these entries are kept with soft references,
* they will tend to remain even if their keys are not referenced elsewhere. But if memory is short, they will be removed.
* @param the type of the keys in this map.
* @param the type of the values in this map.
*/
public class CacheMap extends WeakHashMap {
/**
* List of soft references for the most-recently referenced keys.
* The list is in most-recently-used order, i.e. the first element is the most-recently referenced key.
* There are never more than nSoftReferences elements of this list.
* If we didn't care about J2SE 1.3 compatibility, we could use LinkedHashSet in conjunction with a subclass of SoftReference whose equals and hashCode reflect the referent.
*/
private final LinkedList> cache = new LinkedList<>();
/**
*
*/
private final int nSoftReferences;
/**
* Create a CacheMap that can keep up to nSoftReferences as soft references.
* @param nSoftReferences Maximum number of keys to keep as soft references. Access times for {@link #get(Object) get}
* and {@link #put(Object, Object) put} have a component that scales linearly with nSoftReferences, so this value should not be too great.
* @throws IllegalArgumentException if nSoftReferences is negative.
*/
public CacheMap(final int nSoftReferences) {
if (nSoftReferences < 0) throw new IllegalArgumentException("nSoftReferences = " + nSoftReferences);
this.nSoftReferences = nSoftReferences;
}
@Override
public V put(final K key, final V value) {
cache(key);
return super.put(key, value);
}
@Override
@SuppressWarnings("unchecked")
public V get(final Object key) {
cache((K) key);
return super.get(key);
}
/**
* We don't override remove(Object) or try to do something with the map's iterators to detect removal.
* So we may keep useless entries in the soft reference list for keys that have since been removed.
* The assumption is that entries are added to the cache but never removed. But the behaviour is not wrong if they are in fact removed -- the caching is just less performant.
* @param key the key to cache.
*/
private void cache(final K key) {
Iterator> it = cache.iterator();
while (it.hasNext()) {
SoftReference sref = it.next();
K key1 = sref.get();
if (key1 == null) it.remove();
else if (key.equals(key1)) {
// Move this element to the head of the LRU list
it.remove();
cache.add(0, sref);
return;
}
}
int size = cache.size();
if (size == nSoftReferences) {
if (size == 0) return; // degenerate case, equivalent to WeakHashMap
it.remove();
}
cache.add(0, new SoftReference<>(key));
}
}
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