| /* |
| * Copyright (c) 2003, 2006, Oracle and/or its affiliates. All rights reserved. |
| * DO NOT ALTER OR REMOVE COPYRIGHT NOTICES OR THIS FILE HEADER. |
| * |
| * This code is free software; you can redistribute it and/or modify it |
| * under the terms of the GNU General Public License version 2 only, as |
| * published by the Free Software Foundation. Oracle designates this |
| * particular file as subject to the "Classpath" exception as provided |
| * by Oracle in the LICENSE file that accompanied this code. |
| * |
| * This code is distributed in the hope that it will be useful, but WITHOUT |
| * ANY WARRANTY; without even the implied warranty of MERCHANTABILITY or |
| * FITNESS FOR A PARTICULAR PURPOSE. See the GNU General Public License |
| * version 2 for more details (a copy is included in the LICENSE file that |
| * accompanied this code). |
| * |
| * You should have received a copy of the GNU General Public License version |
| * 2 along with this work; if not, write to the Free Software Foundation, |
| * Inc., 51 Franklin St, Fifth Floor, Boston, MA 02110-1301 USA. |
| * |
| * Please contact Oracle, 500 Oracle Parkway, Redwood Shores, CA 94065 USA |
| * or visit www.oracle.com if you need additional information or have any |
| * questions. |
| */ |
| |
| package com.sun.jmx.remote.util; |
| |
| import java.lang.ref.SoftReference; |
| import java.util.Iterator; |
| import java.util.LinkedList; |
| import java.util.List; |
| import java.util.WeakHashMap; |
| |
| import com.sun.jmx.mbeanserver.Util; |
| |
| /** |
| * <p>Like WeakHashMap, except that the keys of the <em>n</em> 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.</p> |
| */ |
| public class CacheMap<K, V> extends WeakHashMap<K, V> { |
| /** |
| * <p>Create a <code>CacheMap</code> that can keep up to |
| * <code>nSoftReferences</code> as soft references.</p> |
| * |
| * @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 <code>nSoftReferences</code>, so this value |
| * should not be too great. |
| * |
| * @throws IllegalArgumentException if |
| * <code>nSoftReferences</code> is negative. |
| */ |
| public CacheMap(int nSoftReferences) { |
| if (nSoftReferences < 0) { |
| throw new IllegalArgumentException("nSoftReferences = " + |
| nSoftReferences); |
| } |
| this.nSoftReferences = nSoftReferences; |
| } |
| |
| public V put(K key, V value) { |
| cache(key); |
| return super.put(key, value); |
| } |
| |
| public V get(Object key) { |
| cache(Util.<K>cast(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 behavior is not wrong if |
| they are in fact removed -- the caching is just less |
| performant. */ |
| |
| private void cache(K key) { |
| Iterator<SoftReference<K>> it = cache.iterator(); |
| while (it.hasNext()) { |
| SoftReference<K> 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<K>(key)); |
| } |
| |
| /* 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<SoftReference<K>> cache = |
| new LinkedList<SoftReference<K>>(); |
| private final int nSoftReferences; |
| } |