Source Code Cross Referenced for WeakHashSet.java in  » GIS » GeoTools-2.4.1 » org » geotools » util » Java Source Code / Java DocumentationJava Source Code and Java Documentation

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Java Source Code / Java Documentation » GIS » GeoTools 2.4.1 » org.geotools.util 
Source Cross Referenced  Class Diagram Java Document (Java Doc) 


001:        /*
002:         *    GeoTools - OpenSource mapping toolkit
003:         *    http://geotools.org
004:         *    (C) 2003-2006, Geotools Project Managment Committee (PMC)
005:         *    (C) 2001, Institut de Recherche pour le Développement
006:         *
007:         *    This library is free software; you can redistribute it and/or
008:         *    modify it under the terms of the GNU Lesser General Public
009:         *    License as published by the Free Software Foundation; either
010:         *    version 2.1 of the License, or (at your option) any later version.
011:         *
012:         *    This library is distributed in the hope that it will be useful,
013:         *    but WITHOUT ANY WARRANTY; without even the implied warranty of
014:         *    MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the GNU
015:         *    Lesser General Public License for more details.
016:         */
017:        package org.geotools.util;
018:
019:        // Collections and references
020:        import java.lang.ref.WeakReference;
021:        import java.util.AbstractSet;
022:        import java.util.Arrays;
023:        import java.util.Iterator;
024:        import java.util.Set;
025:        import java.util.WeakHashMap;
026:        import java.util.logging.Level;
027:        import java.util.logging.LogRecord;
028:        import java.util.logging.Logger;
029:
030:        // Geotools dependencies
031:        import org.geotools.resources.XArray;
032:        import org.geotools.util.logging.Logging;
033:
034:        /**
035:         * A set of objects hold by weak references. An entry in a {@code WeakHashSet}
036:         * will automatically be removed when it is no longer in ordinary use. More precisely,
037:         * the presence of an entry will not prevent the entry from being discarded by the
038:         * garbage collector, that is, made finalizable, finalized, and then reclaimed.
039:         * When an entry has been discarded it is effectively removed from the set, so
040:         * this class behaves somewhat differently than other {@link Set} implementations.
041:         * <p>
042:         * If you would like to use {@code WeakHashSet} as inside a factory to prevent creating
043:         * duplicate immutable objects, please look at the {@link CanonicalSet} subclass.
044:         * <p>
045:         * The {@code WeakHashSet} class is thread-safe.
046:         *
047:         * @since 2.0
048:         * @source $URL: http://svn.geotools.org/geotools/tags/2.4.1/modules/library/metadata/src/main/java/org/geotools/util/WeakHashSet.java $
049:         * @version $Id: WeakHashSet.java 27862 2007-11-12 19:51:19Z desruisseaux $
050:         * @author Martin Desruisseaux
051:         *
052:         * @see WeakHashMap
053:         */
054:        public class WeakHashSet extends AbstractSet {
055:            /**
056:             * Minimal capacity for {@link #table}.
057:             */
058:            private static final int MIN_CAPACITY = 7;
059:
060:            /**
061:             * Load factor. Control the moment
062:             * where {@link #table} must be rebuild.
063:             */
064:            private static final float LOAD_FACTOR = 0.75f;
065:
066:            /**
067:             * A weak reference to an element.
068:             */
069:            private final class Entry extends WeakReference {
070:                /**
071:                 * The next entry, or {@code null} if there is none.
072:                 */
073:                Entry next;
074:
075:                /**
076:                 * Index for this element in {@link #table}. This index
077:                 * must be updated at every {@link #rehash} call.
078:                 */
079:                int index;
080:
081:                /**
082:                 * Construct a new weak reference.
083:                 */
084:                Entry(final Object obj, final Entry next, final int index) {
085:                    super (obj, WeakCollectionCleaner.DEFAULT.referenceQueue);
086:                    this .next = next;
087:                    this .index = index;
088:                }
089:
090:                /**
091:                 * Clear the reference.
092:                 */
093:                public void clear() {
094:                    super .clear();
095:                    removeEntry(this );
096:                }
097:            }
098:
099:            /**
100:             * Table of weak references.
101:             */
102:            private Entry[] table;
103:
104:            /**
105:             * Number of non-nul elements in {@link #table}.
106:             */
107:            private int count;
108:
109:            /**
110:             * The next size value at which to resize. This value should
111:             * be <code>{@link #table}.length*{@link #loadFactor}</code>.
112:             */
113:            private int threshold;
114:
115:            /**
116:             * The timestamp when {@link #table} was last rehashed. This information
117:             * is used to avoid too early table reduction. When the garbage collector
118:             * collected a lot of elements, we will wait at least 20 seconds before
119:             * rehashing {@link #table}. Too early table reduction leads to many cycles
120:             * like "reduce", "expand", "reduce", "expand", etc.
121:             */
122:            private long lastRehashTime;
123:
124:            /**
125:             * Number of millisecond to wait before to rehash
126:             * the table for reducing its size.
127:             */
128:            private static final long HOLD_TIME = 20 * 1000L;
129:
130:            /**
131:             * Constructs a {@code WeakHashSet}.
132:             */
133:            public WeakHashSet() {
134:                table = new Entry[MIN_CAPACITY];
135:                threshold = Math.round(table.length * LOAD_FACTOR);
136:                lastRehashTime = System.currentTimeMillis();
137:            }
138:
139:            /**
140:             * Invoked by {@link Entry} when an element has been collected
141:             * by the garbage collector. This method will remove the weak reference
142:             * from {@link #table}.
143:             */
144:            private synchronized void removeEntry(final Entry toRemove) {
145:                assert valid() : count;
146:                final int i = toRemove.index;
147:                // Index 'i' may not be valid if the reference 'toRemove'
148:                // has been already removed in a previous rehash.
149:                if (i < table.length) {
150:                    Entry prev = null;
151:                    Entry e = table[i];
152:                    while (e != null) {
153:                        if (e == toRemove) {
154:                            if (prev != null) {
155:                                prev.next = e.next;
156:                            } else {
157:                                table[i] = e.next;
158:                            }
159:                            count--;
160:                            assert valid();
161:
162:                            // If the number of elements has dimunished
163:                            // significatively, rehash the table.
164:                            if (count <= threshold / 4) {
165:                                rehash(false);
166:                            }
167:                            // We must not continue the loop, since
168:                            // variable 'e' is no longer valid.
169:                            return;
170:                        }
171:                        prev = e;
172:                        e = e.next;
173:                    }
174:                }
175:                assert valid();
176:                /*
177:                 * If we reach this point, its mean that reference 'toRemove' has not
178:                 * been found. This situation may occurs if 'toRemove' has already been
179:                 * removed in a previous run of {@link #rehash}.
180:                 */
181:            }
182:
183:            /**
184:             * Rehash {@link #table}.
185:             *
186:             * @param augmentation {@code true} if this method is invoked
187:             *        for augmenting {@link #table}, or {@code false} if
188:             *        it is invoked for making the table smaller.
189:             */
190:            private void rehash(final boolean augmentation) {
191:                assert Thread.holdsLock(this );
192:                assert valid();
193:                final long currentTime = System.currentTimeMillis();
194:                final int capacity = Math.max(Math.round(count
195:                        / (LOAD_FACTOR / 2)), count + MIN_CAPACITY);
196:                if (augmentation ? (capacity <= table.length)
197:                        : (capacity >= table.length || currentTime
198:                                - lastRehashTime < HOLD_TIME)) {
199:                    return;
200:                }
201:                lastRehashTime = currentTime;
202:                final Entry[] oldTable = table;
203:                table = new Entry[capacity];
204:                threshold = Math.round(capacity * LOAD_FACTOR);
205:                for (int i = 0; i < oldTable.length; i++) {
206:                    for (Entry old = oldTable[i]; old != null;) {
207:                        final Entry e = old;
208:                        old = old.next; // On retient 'next' tout de suite car sa valeur va changer...
209:                        final Object obj_e = e.get();
210:                        if (obj_e != null) {
211:                            final int index = (obj_e.hashCode() & 0x7FFFFFFF)
212:                                    % table.length;
213:                            e.index = index;
214:                            e.next = table[index];
215:                            table[index] = e;
216:                        } else {
217:                            count--;
218:                        }
219:                    }
220:                }
221:                final Logger logger = Logging.getLogger("org.geotools.util");
222:                final Level level = Level.FINEST;
223:                if (logger.isLoggable(level)) {
224:                    final LogRecord record = new LogRecord(level,
225:                            "Rehash from " + oldTable.length + " to "
226:                                    + table.length);
227:                    record.setSourceMethodName(augmentation ? "unique"
228:                            : "remove");
229:                    record.setSourceClassName(WeakHashSet.class.getName());
230:                    logger.log(record);
231:                }
232:                assert valid();
233:            }
234:
235:            /**
236:             * Check if this {@code WeakHashSet} is valid. This method counts the
237:             * number of elements and compare it to {@link #count}. If the check fails,
238:             * the number of elements is corrected (if we didn't, an {@link AssertionError}
239:             * would be thrown for every operations after the first error,  which make
240:             * debugging more difficult). The set is otherwise unchanged, which should
241:             * help to get similar behaviour as if assertions hasn't been turned on.
242:             */
243:            private boolean valid() {
244:                int n = 0;
245:                for (int i = 0; i < table.length; i++) {
246:                    for (Entry e = table[i]; e != null; e = e.next) {
247:                        n++;
248:                    }
249:                }
250:                if (n != count) {
251:                    count = n;
252:                    return false;
253:                } else {
254:                    return true;
255:                }
256:            }
257:
258:            /**
259:             * Returns the count of element in this set.
260:             */
261:            public synchronized int size() {
262:                assert valid();
263:                return count;
264:            }
265:
266:            /**
267:             * Returns {@code true} if this set contains the specified element.
268:             *
269:             * @param  obj Object to be checked for containment in this set.
270:             * @return {@code true} if this set contains the specified element.
271:             */
272:            public boolean contains(final Object obj) {
273:                return obj == null || get(obj) != null;
274:            }
275:
276:            /**
277:             * Returns an object equals to the specified object, if present. If
278:             * this set doesn't contains any object equals to {@code obj},
279:             * then this method returns {@code null}.
280:             *
281:             * @deprecated Moved to the {@link CanonicalSet} subclass.
282:             */
283:            public synchronized Object get(final Object obj) {
284:                return intern(obj, GET);
285:            }
286:
287:            /**
288:             * Removes a single instance of the specified element from this set,
289:             * if it is present
290:             *
291:             * @param  obj element to be removed from this set, if present.
292:             * @return {@code true} if the set contained the specified element.
293:             */
294:            public synchronized boolean remove(final Object obj) {
295:                return intern(obj, REMOVE) != null;
296:            }
297:
298:            /**
299:             * Adds the specified element to this set if it is not already present.
300:             * If this set already contains the specified element, the call leaves
301:             * this set unchanged and returns {@code false}.
302:             *
303:             * @param  obj Element to be added to this set.
304:             * @return {@code true} if this set did not already
305:             *         contain the specified element.
306:             */
307:            public synchronized boolean add(final Object obj) {
308:                return intern(obj, ADD) == null;
309:            }
310:
311:            // Arguments for the {@link #intern} method.
312:            /** The "remove" operation.  */
313:            static final int REMOVE = -1;
314:            /** The "get"    operation.  */
315:            static final int GET = 0;
316:            /** The "add"    operation.  */
317:            static final int ADD = +1;
318:            /** The "intern" operation.  */
319:            static final int INTERN = +2;
320:
321:            /**
322:             * Returns an object equals to {@code obj} if such an object already
323:             * exist in this {@code WeakHashSet}. Otherwise, add {@code obj}
324:             * to this {@code WeakHashSet}. This method is equivalents to the
325:             * following code:
326:             *
327:             * <blockquote><pre>
328:             * &nbsp;  if (object!=null) {
329:             * &nbsp;      final Object current = get(object);
330:             * &nbsp;      if (current != null) {
331:             * &nbsp;          return current;
332:             * &nbsp;      } else {
333:             * &nbsp;          add(object);
334:             * &nbsp;      }
335:             * &nbsp;  }
336:             * &nbsp;  return object;
337:             * </pre></blockquote>
338:             */
339:            final Object intern(final Object obj, final int operation) {
340:                assert Thread.holdsLock(this );
341:                assert WeakCollectionCleaner.DEFAULT.isAlive();
342:                assert valid() : count;
343:                if (obj != null) {
344:                    /*
345:                     * Check if {@code obj} is already contained in this
346:                     * {@code WeakHashSet}. If yes, returns the element.
347:                     */
348:                    final int hash = obj.hashCode() & 0x7FFFFFFF;
349:                    int index = hash % table.length;
350:                    for (Entry e = table[index]; e != null; e = e.next) {
351:                        final Object candidate = e.get();
352:                        if (candidate != null) {
353:                            if (candidate.equals(obj)) {
354:                                if (operation == REMOVE) {
355:                                    e.clear();
356:                                }
357:                                return candidate;
358:                            }
359:                        }
360:                        // Do not remove the null element; lets ReferenceQueue do its job
361:                        // (it was a bug to remove element here as an "optimization")
362:                    }
363:                    if (operation >= ADD) {
364:                        /*
365:                         * Check if the table need to be rehashed,
366:                         * and add {@code obj} to the table.
367:                         */
368:                        if (count >= threshold) {
369:                            rehash(true);
370:                            index = hash % table.length;
371:                        }
372:                        table[index] = new Entry(obj, table[index], index);
373:                        count++;
374:                    }
375:                }
376:                assert valid();
377:                return (operation == INTERN) ? obj : null;
378:            }
379:
380:            /**
381:             * Returns an object equals to {@code object} if such an object already exist in this
382:             * {@code WeakHashSet}. Otherwise, adds {@code object} to this {@code WeakHashSet}.
383:             * This method is equivalents to the following code:
384:             *
385:             * <blockquote><pre>
386:             * &nbsp;  if (object != null) {
387:             * &nbsp;      Object current = get(object);
388:             * &nbsp;      if (current != null) {
389:             * &nbsp;          return current;
390:             * &nbsp;      } else {
391:             * &nbsp;          add(object);
392:             * &nbsp;      }
393:             * &nbsp;  }
394:             * &nbsp;  return object;
395:             * </pre></blockquote>
396:             *
397:             * @deprecated Moved to the {@link CanonicalSet} subclass.
398:             */
399:            public synchronized Object canonicalize(final Object object) {
400:                return intern(object, INTERN);
401:            }
402:
403:            /**
404:             * Iteratively call {@link #canonicalize(Object)} for an array of objects.
405:             * This method is equivalents to the following code:
406:             *
407:             * <blockquote><pre>
408:             * &nbsp;  for (int i=0; i<objects.length; i++) {
409:             * &nbsp;      objects[i] = canonicalize(objects[i]);
410:             * &nbsp;  }
411:             * </pre></blockquote>
412:             *
413:             * @deprecated Moved to the {@link CanonicalSet} subclass.
414:             */
415:            public synchronized void canonicalize(final Object[] objects) {
416:                for (int i = 0; i < objects.length; i++) {
417:                    objects[i] = intern(objects[i], INTERN);
418:                }
419:            }
420:
421:            /**
422:             * Removes all of the elements from this set.
423:             */
424:            public synchronized void clear() {
425:                Arrays.fill(table, null);
426:                count = 0;
427:            }
428:
429:            /**
430:             * Returns a view of this set as an array. Elements will be in an arbitrary
431:             * order. Note that this array contains strong reference.  Consequently, no
432:             * object reclamation will occurs as long as a reference to this array is hold.
433:             */
434:            public synchronized Object[] toArray() {
435:                assert valid();
436:                final Object[] elements = new Object[count];
437:                int index = 0;
438:                for (int i = 0; i < table.length; i++) {
439:                    for (Entry el = table[i]; el != null; el = el.next) {
440:                        if ((elements[index] = el.get()) != null) {
441:                            index++;
442:                        }
443:                    }
444:                }
445:                return XArray.resize(elements, index);
446:            }
447:
448:            /**
449:             * Returns an iterator over the elements contained in this collection.
450:             * No element from this set will be garbage collected as long as a
451:             * reference to the iterator is hold.
452:             */
453:            public Iterator iterator() {
454:                return Arrays.asList(toArray()).iterator();
455:            }
456:        }
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