001 /**
002 * JDBM LICENSE v1.00
003 *
004 * Redistribution and use of this software and associated documentation
005 * ("Software"), with or without modification, are permitted provided
006 * that the following conditions are met:
007 *
008 * 1. Redistributions of source code must retain copyright
009 * statements and notices. Redistributions must also contain a
010 * copy of this document.
011 *
012 * 2. Redistributions in binary form must reproduce the
013 * above copyright notice, this list of conditions and the
014 * following disclaimer in the documentation and/or other
015 * materials provided with the distribution.
016 *
017 * 3. The name "JDBM" must not be used to endorse or promote
018 * products derived from this Software without prior written
019 * permission of Cees de Groot. For written permission,
020 * please contact cg@cdegroot.com.
021 *
022 * 4. Products derived from this Software may not be called "JDBM"
023 * nor may "JDBM" appear in their names without prior written
024 * permission of Cees de Groot.
025 *
026 * 5. Due credit should be given to the JDBM Project
027 * (http://jdbm.sourceforge.net/).
028 *
029 * THIS SOFTWARE IS PROVIDED BY THE JDBM PROJECT AND CONTRIBUTORS
030 * ``AS IS'' AND ANY EXPRESSED OR IMPLIED WARRANTIES, INCLUDING, BUT
031 * NOT LIMITED TO, THE IMPLIED WARRANTIES OF MERCHANTABILITY AND
032 * FITNESS FOR A PARTICULAR PURPOSE ARE DISCLAIMED. IN NO EVENT SHALL
033 * CEES DE GROOT OR ANY CONTRIBUTORS BE LIABLE FOR ANY DIRECT,
034 * INDIRECT, INCIDENTAL, SPECIAL, EXEMPLARY, OR CONSEQUENTIAL DAMAGES
035 * (INCLUDING, BUT NOT LIMITED TO, PROCUREMENT OF SUBSTITUTE GOODS OR
036 * SERVICES; LOSS OF USE, DATA, OR PROFITS; OR BUSINESS INTERRUPTION)
037 * HOWEVER CAUSED AND ON ANY THEORY OF LIABILITY, WHETHER IN CONTRACT,
038 * STRICT LIABILITY, OR TORT (INCLUDING NEGLIGENCE OR OTHERWISE)
039 * ARISING IN ANY WAY OUT OF THE USE OF THIS SOFTWARE, EVEN IF ADVISED
040 * OF THE POSSIBILITY OF SUCH DAMAGE.
041 *
042 * Copyright 2000 (C) Cees de Groot. All Rights Reserved.
043 * Contributions are Copyright (C) 2000 by their associated contributors.
044 *
045 * $Id: MRU.java,v 1.8 2005/06/25 23:12:31 doomdark Exp $
046 */
047
048 package jdbm.helper;
049
050 import java.util.Enumeration;
051 import java.util.Hashtable;
052 import java.util.Vector;
053
054 import org.apache.directory.server.i18n.I18n;
055
056
057 /**
058 * MRU - Most Recently Used cache policy.
059 *
060 * Methods are *not* synchronized, so no concurrent access is allowed.
061 *
062 * @author <a href="mailto:boisvert@intalio.com">Alex Boisvert</a>
063 * @version $Id: MRU.java,v 1.8 2005/06/25 23:12:31 doomdark Exp $
064 */
065 public class MRU implements CachePolicy
066 {
067
068 /** Cached object hashtable */
069 Hashtable _hash = new Hashtable();
070
071 /**
072 * Maximum number of objects in the cache.
073 */
074 int _max;
075
076 /**
077 * Beginning of linked-list of cache elements. First entry is element
078 * which has been used least recently.
079 */
080 CacheEntry _first;
081
082 /**
083 * End of linked-list of cache elements. Last entry is element
084 * which has been used most recently.
085 */
086 CacheEntry _last;
087
088
089 /**
090 * Cache eviction listeners
091 */
092 Vector listeners = new Vector();
093
094
095 /**
096 * Construct an MRU with a given maximum number of objects.
097 */
098 public MRU(int max) {
099 if (max <= 0) {
100 throw new IllegalArgumentException( I18n.err( I18n.ERR_528 ));
101 }
102 _max = max;
103 }
104
105
106 /**
107 * Place an object in the cache.
108 */
109 public void put(Object key, Object value) throws CacheEvictionException {
110 CacheEntry entry = (CacheEntry)_hash.get(key);
111 if (entry != null) {
112 entry.setValue(value);
113 touchEntry(entry);
114 } else {
115
116 if (_hash.size() == _max) {
117 // purge and recycle entry
118 entry = purgeEntry();
119 entry.setKey(key);
120 entry.setValue(value);
121 } else {
122 entry = new CacheEntry(key, value);
123 }
124 addEntry(entry);
125 _hash.put(entry.getKey(), entry);
126 }
127 }
128
129
130 /**
131 * Obtain an object in the cache
132 */
133 public Object get(Object key) {
134 CacheEntry entry = (CacheEntry)_hash.get(key);
135 if (entry != null) {
136 touchEntry(entry);
137 return entry.getValue();
138 } else {
139 return null;
140 }
141 }
142
143
144 /**
145 * Remove an object from the cache
146 */
147 public void remove(Object key) {
148 CacheEntry entry = (CacheEntry)_hash.get(key);
149 if (entry != null) {
150 removeEntry(entry);
151 _hash.remove(entry.getKey());
152 }
153 }
154
155
156 /**
157 * Remove all objects from the cache
158 */
159 public void removeAll() {
160 _hash = new Hashtable();
161 _first = null;
162 _last = null;
163 }
164
165
166 /**
167 * Enumerate elements' values in the cache
168 */
169 public Enumeration elements() {
170 return new MRUEnumeration(_hash.elements());
171 }
172
173 /**
174 * Add a listener to this cache policy
175 *
176 * @param listener Listener to add to this policy
177 */
178 public void addListener(CachePolicyListener listener) {
179 if (listener == null) {
180 throw new IllegalArgumentException( I18n.err( I18n.ERR_539 ) );
181 }
182 if ( ! listeners.contains(listener)) {
183 listeners.addElement(listener);
184 }
185 }
186
187 /**
188 * Remove a listener from this cache policy
189 *
190 * @param listener Listener to remove from this policy
191 */
192 public void removeListener(CachePolicyListener listener) {
193 listeners.removeElement(listener);
194 }
195
196 /**
197 * Add a CacheEntry. Entry goes at the end of the list.
198 */
199 protected void addEntry(CacheEntry entry) {
200 if (_first == null) {
201 _first = entry;
202 _last = entry;
203 } else {
204 _last.setNext(entry);
205 entry.setPrevious(_last);
206 _last = entry;
207 }
208 }
209
210
211 /**
212 * Remove a CacheEntry from linked list
213 */
214 protected void removeEntry(CacheEntry entry) {
215 if (entry == _first) {
216 _first = entry.getNext();
217 }
218 if (_last == entry) {
219 _last = entry.getPrevious();
220 }
221 CacheEntry previous = entry.getPrevious();
222 CacheEntry next = entry.getNext();
223 if (previous != null) {
224 previous.setNext(next);
225 }
226 if (next != null) {
227 next.setPrevious(previous);
228 }
229 entry.setPrevious(null);
230 entry.setNext(null);
231 }
232
233 /**
234 * Place entry at the end of linked list -- Most Recently Used
235 */
236 protected void touchEntry(CacheEntry entry) {
237 if (_last == entry) {
238 return;
239 }
240 removeEntry(entry);
241 addEntry(entry);
242 }
243
244 /**
245 * Purge least recently used object from the cache
246 *
247 * @return recyclable CacheEntry
248 */
249 protected CacheEntry purgeEntry() throws CacheEvictionException {
250 CacheEntry entry = _first;
251
252 // Notify policy listeners first. if any of them throw an
253 // eviction exception, then the internal data structure
254 // remains untouched.
255 CachePolicyListener listener;
256 for (int i=0; i<listeners.size(); i++) {
257 listener = (CachePolicyListener)listeners.elementAt(i);
258 listener.cacheObjectEvicted(entry.getValue());
259 }
260
261 removeEntry(entry);
262 _hash.remove(entry.getKey());
263
264 entry.setValue(null);
265 return entry;
266 }
267
268 }
269
270 /**
271 * State information for cache entries.
272 */
273 class CacheEntry {
274 private Object _key;
275 private Object _value;
276
277 private CacheEntry _previous;
278 private CacheEntry _next;
279
280 CacheEntry(Object key, Object value) {
281 _key = key;
282 _value = value;
283 }
284
285 Object getKey() {
286 return _key;
287 }
288
289 void setKey(Object obj) {
290 _key = obj;
291 }
292
293 Object getValue() {
294 return _value;
295 }
296
297 void setValue(Object obj) {
298 _value = obj;
299 }
300
301 CacheEntry getPrevious() {
302 return _previous;
303 }
304
305 void setPrevious(CacheEntry entry) {
306 _previous = entry;
307 }
308
309 CacheEntry getNext() {
310 return _next;
311 }
312
313 void setNext(CacheEntry entry) {
314 _next = entry;
315 }
316 }
317
318 /**
319 * Enumeration wrapper to return actual user objects instead of
320 * CacheEntries.
321 */
322 class MRUEnumeration implements Enumeration {
323 Enumeration _enum;
324
325 MRUEnumeration(Enumeration enume) {
326 _enum = enume;
327 }
328
329 public boolean hasMoreElements() {
330 return _enum.hasMoreElements();
331 }
332
333 public Object nextElement() {
334 CacheEntry entry = (CacheEntry)_enum.nextElement();
335 return entry.getValue();
336 }
337 }