001 /*
002 * Copyright (C) 2007 The Guava Authors
003 *
004 * Licensed under the Apache License, Version 2.0 (the "License");
005 * you may not use this file except in compliance with the License.
006 * You may obtain a copy of the License at
007 *
008 * http://www.apache.org/licenses/LICENSE-2.0
009 *
010 * Unless required by applicable law or agreed to in writing, software
011 * distributed under the License is distributed on an "AS IS" BASIS,
012 * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
013 * See the License for the specific language governing permissions and
014 * limitations under the License.
015 */
016
017 package com.google.common.collect;
018
019 import static com.google.common.base.Preconditions.checkArgument;
020 import static com.google.common.base.Preconditions.checkNotNull;
021 import static com.google.common.base.Preconditions.checkState;
022 import static com.google.common.collect.Multisets.checkNonnegative;
023
024 import com.google.common.annotations.Beta;
025 import com.google.common.annotations.VisibleForTesting;
026 import com.google.common.collect.Serialization.FieldSetter;
027 import com.google.common.math.IntMath;
028 import com.google.common.primitives.Ints;
029
030 import java.io.IOException;
031 import java.io.ObjectInputStream;
032 import java.io.ObjectOutputStream;
033 import java.io.Serializable;
034 import java.util.Collection;
035 import java.util.Iterator;
036 import java.util.List;
037 import java.util.Map;
038 import java.util.Set;
039 import java.util.concurrent.ConcurrentHashMap;
040 import java.util.concurrent.ConcurrentMap;
041 import java.util.concurrent.atomic.AtomicInteger;
042
043 import javax.annotation.Nullable;
044
045 /**
046 * A multiset that supports concurrent modifications and that provides atomic versions of most
047 * {@code Multiset} operations (exceptions where noted). Null elements are not supported.
048 *
049 * <p>See the Guava User Guide article on <a href=
050 * "http://code.google.com/p/guava-libraries/wiki/NewCollectionTypesExplained#Multiset">
051 * {@code Multiset}</a>.
052 *
053 * @author Cliff L. Biffle
054 * @author mike nonemacher
055 * @since 2.0 (imported from Google Collections Library)
056 */
057 public final class ConcurrentHashMultiset<E> extends AbstractMultiset<E> implements Serializable {
058
059 /*
060 * The ConcurrentHashMultiset's atomic operations are implemented primarily in terms of
061 * AtomicInteger's atomic operations, with some help from ConcurrentMap's atomic operations on
062 * creation and removal (including automatic removal of zeroes). If the modification of an
063 * AtomicInteger results in zero, we compareAndSet the value to zero; if that succeeds, we remove
064 * the entry from the Map. If another operation sees a zero in the map, it knows that the entry is
065 * about to be removed, so this operation may remove it (often by replacing it with a new
066 * AtomicInteger).
067 */
068
069 /** The number of occurrences of each element. */
070 private final transient ConcurrentMap<E, AtomicInteger> countMap;
071
072 // This constant allows the deserialization code to set a final field. This holder class
073 // makes sure it is not initialized unless an instance is deserialized.
074 private static class FieldSettersHolder {
075 static final FieldSetter<ConcurrentHashMultiset> COUNT_MAP_FIELD_SETTER =
076 Serialization.getFieldSetter(ConcurrentHashMultiset.class, "countMap");
077 }
078
079 /**
080 * Creates a new, empty {@code ConcurrentHashMultiset} using the default
081 * initial capacity, load factor, and concurrency settings.
082 */
083 public static <E> ConcurrentHashMultiset<E> create() {
084 // TODO(schmoe): provide a way to use this class with other (possibly arbitrary)
085 // ConcurrentMap implementors. One possibility is to extract most of this class into
086 // an AbstractConcurrentMapMultiset.
087 return new ConcurrentHashMultiset<E>(new ConcurrentHashMap<E, AtomicInteger>());
088 }
089
090 /**
091 * Creates a new {@code ConcurrentHashMultiset} containing the specified elements, using
092 * the default initial capacity, load factor, and concurrency settings.
093 *
094 * <p>This implementation is highly efficient when {@code elements} is itself a {@link Multiset}.
095 *
096 * @param elements the elements that the multiset should contain
097 */
098 public static <E> ConcurrentHashMultiset<E> create(Iterable<? extends E> elements) {
099 ConcurrentHashMultiset<E> multiset = ConcurrentHashMultiset.create();
100 Iterables.addAll(multiset, elements);
101 return multiset;
102 }
103
104 /**
105 * Creates a new, empty {@code ConcurrentHashMultiset} using {@code mapMaker}
106 * to construct the internal backing map.
107 *
108 * <p>If this {@link MapMaker} is configured to use entry eviction of any kind, this eviction
109 * applies to all occurrences of a given element as a single unit. However, most updates to the
110 * multiset do not count as map updates at all, since we're usually just mutating the value
111 * stored in the map, so {@link MapMaker#expireAfterAccess} makes sense (evict the entry that
112 * was queried or updated longest ago), but {@link MapMaker#expireAfterWrite} doesn't, because
113 * the eviction time is measured from when we saw the first occurrence of the object.
114 *
115 * <p>The returned multiset is serializable but any serialization caveats
116 * given in {@code MapMaker} apply.
117 *
118 * <p>Finally, soft/weak values can be used but are not very useful: the values are created
119 * internally and not exposed externally, so no one else will have a strong reference to the
120 * values. Weak keys on the other hand can be useful in some scenarios.
121 *
122 * @since 7.0
123 */
124 @Beta
125 public static <E> ConcurrentHashMultiset<E> create(
126 GenericMapMaker<? super E, ? super Number> mapMaker) {
127 return new ConcurrentHashMultiset<E>(mapMaker.<E, AtomicInteger>makeMap());
128 }
129
130 /**
131 * Creates an instance using {@code countMap} to store elements and their counts.
132 *
133 * <p>This instance will assume ownership of {@code countMap}, and other code
134 * should not maintain references to the map or modify it in any way.
135 *
136 * @param countMap backing map for storing the elements in the multiset and
137 * their counts. It must be empty.
138 * @throws IllegalArgumentException if {@code countMap} is not empty
139 */
140 @VisibleForTesting ConcurrentHashMultiset(ConcurrentMap<E, AtomicInteger> countMap) {
141 checkArgument(countMap.isEmpty());
142 this.countMap = countMap;
143 }
144
145 // Query Operations
146
147 /**
148 * Returns the number of occurrences of {@code element} in this multiset.
149 *
150 * @param element the element to look for
151 * @return the nonnegative number of occurrences of the element
152 */
153 @Override public int count(@Nullable Object element) {
154 AtomicInteger existingCounter = safeGet(element);
155 return (existingCounter == null) ? 0 : existingCounter.get();
156 }
157
158 /**
159 * Depending on the type of the underlying map, map.get may throw NullPointerException or
160 * ClassCastException, if the object is null or of the wrong type. We usually just want to treat
161 * those cases as if the element isn't in the map, by catching the exceptions and returning null.
162 */
163 private AtomicInteger safeGet(Object element) {
164 try {
165 return countMap.get(element);
166 } catch (NullPointerException e) {
167 return null;
168 } catch (ClassCastException e) {
169 return null;
170 }
171 }
172
173 /**
174 * {@inheritDoc}
175 *
176 * <p>If the data in the multiset is modified by any other threads during this method,
177 * it is undefined which (if any) of these modifications will be reflected in the result.
178 */
179 @Override public int size() {
180 long sum = 0L;
181 for (AtomicInteger value : countMap.values()) {
182 sum += value.get();
183 }
184 return Ints.saturatedCast(sum);
185 }
186
187 /*
188 * Note: the superclass toArray() methods assume that size() gives a correct
189 * answer, which ours does not.
190 */
191
192 @Override public Object[] toArray() {
193 return snapshot().toArray();
194 }
195
196 @Override public <T> T[] toArray(T[] array) {
197 return snapshot().toArray(array);
198 }
199
200 /*
201 * We'd love to use 'new ArrayList(this)' or 'list.addAll(this)', but
202 * either of these would recurse back to us again!
203 */
204 private List<E> snapshot() {
205 List<E> list = Lists.newArrayListWithExpectedSize(size());
206 for (Multiset.Entry<E> entry : entrySet()) {
207 E element = entry.getElement();
208 for (int i = entry.getCount(); i > 0; i--) {
209 list.add(element);
210 }
211 }
212 return list;
213 }
214
215 // Modification Operations
216
217 /**
218 * Adds a number of occurrences of the specified element to this multiset.
219 *
220 * @param element the element to add
221 * @param occurrences the number of occurrences to add
222 * @return the previous count of the element before the operation; possibly zero
223 * @throws IllegalArgumentException if {@code occurrences} is negative, or if
224 * the resulting amount would exceed {@link Integer#MAX_VALUE}
225 */
226 @Override public int add(E element, int occurrences) {
227 checkNotNull(element);
228 if (occurrences == 0) {
229 return count(element);
230 }
231 checkArgument(occurrences > 0, "Invalid occurrences: %s", occurrences);
232
233 while (true) {
234 AtomicInteger existingCounter = safeGet(element);
235 if (existingCounter == null) {
236 existingCounter = countMap.putIfAbsent(element, new AtomicInteger(occurrences));
237 if (existingCounter == null) {
238 return 0;
239 }
240 // existingCounter != null: fall through to operate against the existing AtomicInteger
241 }
242
243 while (true) {
244 int oldValue = existingCounter.get();
245 if (oldValue != 0) {
246 try {
247 int newValue = IntMath.checkedAdd(oldValue, occurrences);
248 if (existingCounter.compareAndSet(oldValue, newValue)) {
249 // newValue can't == 0, so no need to check & remove
250 return oldValue;
251 }
252 } catch (ArithmeticException overflow) {
253 throw new IllegalArgumentException("Overflow adding " + occurrences
254 + " occurrences to a count of " + oldValue);
255 }
256 } else {
257 // In the case of a concurrent remove, we might observe a zero value, which means another
258 // thread is about to remove (element, existingCounter) from the map. Rather than wait,
259 // we can just do that work here.
260 AtomicInteger newCounter = new AtomicInteger(occurrences);
261 if ((countMap.putIfAbsent(element, newCounter) == null)
262 || countMap.replace(element, existingCounter, newCounter)) {
263 return 0;
264 }
265 break;
266 }
267 }
268
269 // If we're still here, there was a race, so just try again.
270 }
271 }
272
273 /**
274 * Removes a number of occurrences of the specified element from this multiset. If the multiset
275 * contains fewer than this number of occurrences to begin with, all occurrences will be removed.
276 *
277 * @param element the element whose occurrences should be removed
278 * @param occurrences the number of occurrences of the element to remove
279 * @return the count of the element before the operation; possibly zero
280 * @throws IllegalArgumentException if {@code occurrences} is negative
281 */
282 /*
283 * TODO(cpovirk): remove and removeExactly currently accept null inputs only
284 * if occurrences == 0. This satisfies both NullPointerTester and
285 * CollectionRemoveTester.testRemove_nullAllowed, but it's not clear that it's
286 * a good policy, especially because, in order for the test to pass, the
287 * parameter must be misleadingly annotated as @Nullable. I suspect that
288 * we'll want to remove @Nullable, add an eager checkNotNull, and loosen up
289 * testRemove_nullAllowed.
290 */
291 @Override public int remove(@Nullable Object element, int occurrences) {
292 if (occurrences == 0) {
293 return count(element);
294 }
295 checkArgument(occurrences > 0, "Invalid occurrences: %s", occurrences);
296
297 AtomicInteger existingCounter = safeGet(element);
298 if (existingCounter == null) {
299 return 0;
300 }
301 while (true) {
302 int oldValue = existingCounter.get();
303 if (oldValue != 0) {
304 int newValue = Math.max(0, oldValue - occurrences);
305 if (existingCounter.compareAndSet(oldValue, newValue)) {
306 if (newValue == 0) {
307 // Just CASed to 0; remove the entry to clean up the map. If the removal fails,
308 // another thread has already replaced it with a new counter, which is fine.
309 countMap.remove(element, existingCounter);
310 }
311 return oldValue;
312 }
313 } else {
314 return 0;
315 }
316 }
317 }
318
319 /**
320 * Removes exactly the specified number of occurrences of {@code element}, or makes no
321 * change if this is not possible.
322 *
323 * <p>This method, in contrast to {@link #remove(Object, int)}, has no effect when the
324 * element count is smaller than {@code occurrences}.
325 *
326 * @param element the element to remove
327 * @param occurrences the number of occurrences of {@code element} to remove
328 * @return {@code true} if the removal was possible (including if {@code occurrences} is zero)
329 */
330 public boolean removeExactly(@Nullable Object element, int occurrences) {
331 if (occurrences == 0) {
332 return true;
333 }
334 checkArgument(occurrences > 0, "Invalid occurrences: %s", occurrences);
335
336 AtomicInteger existingCounter = safeGet(element);
337 if (existingCounter == null) {
338 return false;
339 }
340 while (true) {
341 int oldValue = existingCounter.get();
342 if (oldValue < occurrences) {
343 return false;
344 }
345 int newValue = oldValue - occurrences;
346 if (existingCounter.compareAndSet(oldValue, newValue)) {
347 if (newValue == 0) {
348 // Just CASed to 0; remove the entry to clean up the map. If the removal fails,
349 // another thread has already replaced it with a new counter, which is fine.
350 countMap.remove(element, existingCounter);
351 }
352 return true;
353 }
354 }
355 }
356
357 /**
358 * Adds or removes occurrences of {@code element} such that the {@link #count} of the
359 * element becomes {@code count}.
360 *
361 * @return the count of {@code element} in the multiset before this call
362 * @throws IllegalArgumentException if {@code count} is negative
363 */
364 @Override public int setCount(E element, int count) {
365 checkNotNull(element);
366 checkNonnegative(count, "count");
367 while (true) {
368 AtomicInteger existingCounter = safeGet(element);
369 if (existingCounter == null) {
370 if (count == 0) {
371 return 0;
372 } else {
373 existingCounter = countMap.putIfAbsent(element, new AtomicInteger(count));
374 if (existingCounter == null) {
375 return 0;
376 }
377 // existingCounter != null: fall through
378 }
379 }
380
381 while (true) {
382 int oldValue = existingCounter.get();
383 if (oldValue == 0) {
384 if (count == 0) {
385 return 0;
386 } else {
387 AtomicInteger newCounter = new AtomicInteger(count);
388 if ((countMap.putIfAbsent(element, newCounter) == null)
389 || countMap.replace(element, existingCounter, newCounter)) {
390 return 0;
391 }
392 }
393 break;
394 } else {
395 if (existingCounter.compareAndSet(oldValue, count)) {
396 if (count == 0) {
397 // Just CASed to 0; remove the entry to clean up the map. If the removal fails,
398 // another thread has already replaced it with a new counter, which is fine.
399 countMap.remove(element, existingCounter);
400 }
401 return oldValue;
402 }
403 }
404 }
405 }
406 }
407
408 /**
409 * Sets the number of occurrences of {@code element} to {@code newCount}, but only if
410 * the count is currently {@code expectedOldCount}. If {@code element} does not appear
411 * in the multiset exactly {@code expectedOldCount} times, no changes will be made.
412 *
413 * @return {@code true} if the change was successful. This usually indicates
414 * that the multiset has been modified, but not always: in the case that
415 * {@code expectedOldCount == newCount}, the method will return {@code true} if
416 * the condition was met.
417 * @throws IllegalArgumentException if {@code expectedOldCount} or {@code newCount} is negative
418 */
419 @Override public boolean setCount(E element, int expectedOldCount, int newCount) {
420 checkNotNull(element);
421 checkNonnegative(expectedOldCount, "oldCount");
422 checkNonnegative(newCount, "newCount");
423
424 AtomicInteger existingCounter = safeGet(element);
425 if (existingCounter == null) {
426 if (expectedOldCount != 0) {
427 return false;
428 } else if (newCount == 0) {
429 return true;
430 } else {
431 // if our write lost the race, it must have lost to a nonzero value, so we can stop
432 return countMap.putIfAbsent(element, new AtomicInteger(newCount)) == null;
433 }
434 }
435 int oldValue = existingCounter.get();
436 if (oldValue == expectedOldCount) {
437 if (oldValue == 0) {
438 if (newCount == 0) {
439 // Just observed a 0; try to remove the entry to clean up the map
440 countMap.remove(element, existingCounter);
441 return true;
442 } else {
443 AtomicInteger newCounter = new AtomicInteger(newCount);
444 return (countMap.putIfAbsent(element, newCounter) == null)
445 || countMap.replace(element, existingCounter, newCounter);
446 }
447 } else {
448 if (existingCounter.compareAndSet(oldValue, newCount)) {
449 if (newCount == 0) {
450 // Just CASed to 0; remove the entry to clean up the map. If the removal fails,
451 // another thread has already replaced it with a new counter, which is fine.
452 countMap.remove(element, existingCounter);
453 }
454 return true;
455 }
456 }
457 }
458 return false;
459 }
460
461 // Views
462
463 @Override Set<E> createElementSet() {
464 final Set<E> delegate = countMap.keySet();
465 return new ForwardingSet<E>() {
466 @Override protected Set<E> delegate() {
467 return delegate;
468 }
469 @Override public boolean remove(Object object) {
470 try {
471 return delegate.remove(object);
472 } catch (NullPointerException e) {
473 return false;
474 } catch (ClassCastException e) {
475 return false;
476 }
477 }
478 @Override public boolean removeAll(Collection<?> c) {
479 return standardRemoveAll(c);
480 }
481 };
482 }
483
484 private transient EntrySet entrySet;
485
486 @Override public Set<Multiset.Entry<E>> entrySet() {
487 EntrySet result = entrySet;
488 if (result == null) {
489 entrySet = result = new EntrySet();
490 }
491 return result;
492 }
493
494 @Override int distinctElements() {
495 return countMap.size();
496 }
497
498 @Override public boolean isEmpty() {
499 return countMap.isEmpty();
500 }
501
502 @Override Iterator<Entry<E>> entryIterator() {
503 // AbstractIterator makes this fairly clean, but it doesn't support remove(). To support
504 // remove(), we create an AbstractIterator, and then use ForwardingIterator to delegate to it.
505 final Iterator<Entry<E>> readOnlyIterator =
506 new AbstractIterator<Entry<E>>() {
507 private Iterator<Map.Entry<E, AtomicInteger>> mapEntries = countMap.entrySet().iterator();
508
509 @Override protected Entry<E> computeNext() {
510 while (true) {
511 if (!mapEntries.hasNext()) {
512 return endOfData();
513 }
514 Map.Entry<E, AtomicInteger> mapEntry = mapEntries.next();
515 int count = mapEntry.getValue().get();
516 if (count != 0) {
517 return Multisets.immutableEntry(mapEntry.getKey(), count);
518 }
519 }
520 }
521 };
522
523 return new ForwardingIterator<Entry<E>>() {
524 private Entry<E> last;
525
526 @Override protected Iterator<Entry<E>> delegate() {
527 return readOnlyIterator;
528 }
529
530 @Override public Entry<E> next() {
531 last = super.next();
532 return last;
533 }
534
535 @Override public void remove() {
536 checkState(last != null);
537 ConcurrentHashMultiset.this.setCount(last.getElement(), 0);
538 last = null;
539 }
540 };
541 }
542
543 @Override public void clear() {
544 countMap.clear();
545 }
546
547 private class EntrySet extends AbstractMultiset<E>.EntrySet {
548 @Override ConcurrentHashMultiset<E> multiset() {
549 return ConcurrentHashMultiset.this;
550 }
551
552 /*
553 * Note: the superclass toArray() methods assume that size() gives a correct
554 * answer, which ours does not.
555 */
556
557 @Override public Object[] toArray() {
558 return snapshot().toArray();
559 }
560
561 @Override public <T> T[] toArray(T[] array) {
562 return snapshot().toArray(array);
563 }
564
565 private List<Multiset.Entry<E>> snapshot() {
566 List<Multiset.Entry<E>> list = Lists.newArrayListWithExpectedSize(size());
567 // Not Iterables.addAll(list, this), because that'll forward right back here.
568 Iterators.addAll(list, iterator());
569 return list;
570 }
571
572 @Override public boolean remove(Object object) {
573 if (object instanceof Multiset.Entry) {
574 Multiset.Entry<?> entry = (Multiset.Entry<?>) object;
575 Object element = entry.getElement();
576 int entryCount = entry.getCount();
577 if (entryCount != 0) {
578 // Safe as long as we never add a new entry, which we won't.
579 @SuppressWarnings("unchecked")
580 Multiset<Object> multiset = (Multiset) multiset();
581 return multiset.setCount(element, entryCount, 0);
582 }
583 }
584 return false;
585 }
586 }
587
588 /**
589 * @serialData the ConcurrentMap of elements and their counts.
590 */
591 private void writeObject(ObjectOutputStream stream) throws IOException {
592 stream.defaultWriteObject();
593 stream.writeObject(countMap);
594 }
595
596 private void readObject(ObjectInputStream stream) throws IOException, ClassNotFoundException {
597 stream.defaultReadObject();
598 @SuppressWarnings("unchecked") // reading data stored by writeObject
599 ConcurrentMap<E, Integer> deserializedCountMap =
600 (ConcurrentMap<E, Integer>) stream.readObject();
601 FieldSettersHolder.COUNT_MAP_FIELD_SETTER.set(this, deserializedCountMap);
602 }
603
604 private static final long serialVersionUID = 1;
605 }