1 /*
2 * Copyright (c) 1998, 2010, Oracle and/or its affiliates. All rights reserved.
3 * DO NOT ALTER OR REMOVE COPYRIGHT NOTICES OR THIS FILE HEADER.
4 *
5 * This code is free software; you can redistribute it and/or modify it
6 * under the terms of the GNU General Public License version 2 only, as
7 * published by the Free Software Foundation. Oracle designates this
8 * particular file as subject to the "Classpath" exception as provided
9 * by Oracle in the LICENSE file that accompanied this code.
10 *
11 * This code is distributed in the hope that it will be useful, but WITHOUT
12 * ANY WARRANTY; without even the implied warranty of MERCHANTABILITY or
13 * FITNESS FOR A PARTICULAR PURPOSE. See the GNU General Public License
14 * version 2 for more details (a copy is included in the LICENSE file that
15 * accompanied this code).
16 *
17 * You should have received a copy of the GNU General Public License version
18 * 2 along with this work; if not, write to the Free Software Foundation,
19 * Inc., 51 Franklin St, Fifth Floor, Boston, MA 02110-1301 USA.
20 *
21 * Please contact Oracle, 500 Oracle Parkway, Redwood Shores, CA 94065 USA
22 * or visit www.oracle.com if you need additional information or have any
23 * questions.
24 */
25
26 package java.util;
27 import java.lang.ref.WeakReference;
28 import java.lang.ref.ReferenceQueue;
29
30
31 /**
32 * Hash table based implementation of the <tt>Map</tt> interface, with
33 * <em>weak keys</em>.
34 * An entry in a <tt>WeakHashMap</tt> will automatically be removed when
35 * its key is no longer in ordinary use. More precisely, the presence of a
36 * mapping for a given key will not prevent the key from being discarded by the
37 * garbage collector, that is, made finalizable, finalized, and then reclaimed.
38 * When a key has been discarded its entry is effectively removed from the map,
39 * so this class behaves somewhat differently from other <tt>Map</tt>
40 * implementations.
41 *
42 * <p> Both null values and the null key are supported. This class has
43 * performance characteristics similar to those of the <tt>HashMap</tt>
44 * class, and has the same efficiency parameters of <em>initial capacity</em>
45 * and <em>load factor</em>.
46 *
47 * <p> Like most collection classes, this class is not synchronized.
48 * A synchronized <tt>WeakHashMap</tt> may be constructed using the
49 * {@link Collections#synchronizedMap Collections.synchronizedMap}
50 * method.
51 *
52 * <p> This class is intended primarily for use with key objects whose
53 * <tt>equals</tt> methods test for object identity using the
54 * <tt>==</tt> operator. Once such a key is discarded it can never be
55 * recreated, so it is impossible to do a lookup of that key in a
56 * <tt>WeakHashMap</tt> at some later time and be surprised that its entry
57 * has been removed. This class will work perfectly well with key objects
58 * whose <tt>equals</tt> methods are not based upon object identity, such
59 * as <tt>String</tt> instances. With such recreatable key objects,
60 * however, the automatic removal of <tt>WeakHashMap</tt> entries whose
61 * keys have been discarded may prove to be confusing.
62 *
63 * <p> The behavior of the <tt>WeakHashMap</tt> class depends in part upon
64 * the actions of the garbage collector, so several familiar (though not
65 * required) <tt>Map</tt> invariants do not hold for this class. Because
66 * the garbage collector may discard keys at any time, a
67 * <tt>WeakHashMap</tt> may behave as though an unknown thread is silently
68 * removing entries. In particular, even if you synchronize on a
69 * <tt>WeakHashMap</tt> instance and invoke none of its mutator methods, it
70 * is possible for the <tt>size</tt> method to return smaller values over
71 * time, for the <tt>isEmpty</tt> method to return <tt>false</tt> and
72 * then <tt>true</tt>, for the <tt>containsKey</tt> method to return
73 * <tt>true</tt> and later <tt>false</tt> for a given key, for the
74 * <tt>get</tt> method to return a value for a given key but later return
75 * <tt>null</tt>, for the <tt>put</tt> method to return
76 * <tt>null</tt> and the <tt>remove</tt> method to return
77 * <tt>false</tt> for a key that previously appeared to be in the map, and
78 * for successive examinations of the key set, the value collection, and
79 * the entry set to yield successively smaller numbers of elements.
80 *
81 * <p> Each key object in a <tt>WeakHashMap</tt> is stored indirectly as
82 * the referent of a weak reference. Therefore a key will automatically be
83 * removed only after the weak references to it, both inside and outside of the
84 * map, have been cleared by the garbage collector.
85 *
86 * <p> <strong>Implementation note:</strong> The value objects in a
87 * <tt>WeakHashMap</tt> are held by ordinary strong references. Thus care
88 * should be taken to ensure that value objects do not strongly refer to their
89 * own keys, either directly or indirectly, since that will prevent the keys
90 * from being discarded. Note that a value object may refer indirectly to its
91 * key via the <tt>WeakHashMap</tt> itself; that is, a value object may
92 * strongly refer to some other key object whose associated value object, in
93 * turn, strongly refers to the key of the first value object. One way
94 * to deal with this is to wrap values themselves within
95 * <tt>WeakReferences</tt> before
96 * inserting, as in: <tt>m.put(key, new WeakReference(value))</tt>,
97 * and then unwrapping upon each <tt>get</tt>.
98 *
99 * <p>The iterators returned by the <tt>iterator</tt> method of the collections
100 * returned by all of this class's "collection view methods" are
101 * <i>fail-fast</i>: if the map is structurally modified at any time after the
102 * iterator is created, in any way except through the iterator's own
103 * <tt>remove</tt> method, the iterator will throw a {@link
104 * ConcurrentModificationException}. Thus, in the face of concurrent
105 * modification, the iterator fails quickly and cleanly, rather than risking
106 * arbitrary, non-deterministic behavior at an undetermined time in the future.
107 *
108 * <p>Note that the fail-fast behavior of an iterator cannot be guaranteed
109 * as it is, generally speaking, impossible to make any hard guarantees in the
110 * presence of unsynchronized concurrent modification. Fail-fast iterators
111 * throw <tt>ConcurrentModificationException</tt> on a best-effort basis.
112 * Therefore, it would be wrong to write a program that depended on this
113 * exception for its correctness: <i>the fail-fast behavior of iterators
114 * should be used only to detect bugs.</i>
115 *
116 * <p>This class is a member of the
117 * <a href="{@docRoot}/../technotes/guides/collections/index.html">
118 * Java Collections Framework</a>.
119 *
120 * @param <K> the type of keys maintained by this map
121 * @param <V> the type of mapped values
122 *
123 * @author Doug Lea
124 * @author Josh Bloch
125 * @author Mark Reinhold
126 * @since 1.2
127 * @see java.util.HashMap
128 * @see java.lang.ref.WeakReference
129 */
130 public class WeakHashMap<K,V>
131 extends AbstractMap<K,V>
132 implements Map<K,V> {
133
134 /**
135 * The default initial capacity -- MUST be a power of two.
136 */
137 private static final int DEFAULT_INITIAL_CAPACITY = 16;
138
139 /**
140 * The maximum capacity, used if a higher value is implicitly specified
141 * by either of the constructors with arguments.
142 * MUST be a power of two <= 1<<30.
143 */
144 private static final int MAXIMUM_CAPACITY = 1 << 30;
145
146 /**
147 * The load factor used when none specified in constructor.
148 */
149 private static final float DEFAULT_LOAD_FACTOR = 0.75f;
150
151 /**
152 * The table, resized as necessary. Length MUST Always be a power of two.
153 */
154 Entry<K,V>[] table;
155
156 /**
157 * The number of key-value mappings contained in this weak hash map.
158 */
159 private int size;
160
161 /**
162 * The next size value at which to resize (capacity * load factor).
163 */
164 private int threshold;
165
166 /**
167 * The load factor for the hash table.
168 */
169 private final float loadFactor;
170
171 /**
172 * Reference queue for cleared WeakEntries
173 */
174 private final ReferenceQueue<Object> queue = new ReferenceQueue<>();
175
176 /**
177 * The number of times this WeakHashMap has been structurally modified.
178 * Structural modifications are those that change the number of
179 * mappings in the map or otherwise modify its internal structure
180 * (e.g., rehash). This field is used to make iterators on
181 * Collection-views of the map fail-fast.
182 *
183 * @see ConcurrentModificationException
184 */
185 int modCount;
186
187 /**
188 * The default threshold of capacity above which alternate hashing is
189 * used. Alternative hashing reduces the incidence of collisions due to
190 * weak hash code calculation.
191 * <p/>
192 * This value may be overridden by defining the system property
193 * {@code java.util.althashing.threshold} to an integer value. A property
194 * value of {@code 1} forces alternative hashing to be used at all times
195 * whereas {@code 2147483648 } ({@code Integer.MAX_VALUE}) value ensures
196 * that alternative hashing is never used.
197 */
198 static final int ALTERNATE_HASHING_THRESHOLD_DEFAULT = 0;
199
200 /**
201 * holds values which can't be initialized until after VM is booted.
202 */
203 private static class Holder {
204
205 /**
206 * Table capacity above which to switch to use alternate hashing.
207 */
208 static final int ALTERNATE_HASHING_THRESHOLD;
209
210 static {
211 String altThreshold = java.security.AccessController.doPrivileged(
212 new sun.security.action.GetPropertyAction(
213 "jdk.map.althashing.threshold"));
214
215 int threshold;
216 try {
217 threshold = (null != altThreshold)
218 ? Integer.parseInt(altThreshold)
219 : ALTERNATE_HASHING_THRESHOLD_DEFAULT;
220
221 if(threshold == -1) {
222 threshold = Integer.MAX_VALUE;
223 }
224
225 if(threshold < 0) {
226 throw new IllegalArgumentException("value must be positive integer.");
227 }
228 } catch(IllegalArgumentException failed) {
229 throw new Error("Illegal value for 'jdk.map.althashing.threshold'", failed);
230 }
231 ALTERNATE_HASHING_THRESHOLD = threshold;
232 }
233 }
234
235 /**
236 * If {@code true} then perform alternate hashing to reduce the incidence of
237 * collisions due to weak hash code calculation.
238 */
239 transient boolean useAltHashing;
240
241 /**
242 * A randomizing value associated with this instance that is applied to
243 * hash code of keys to make hash collisions harder to find.
244 */
245 transient final int hashSeed = sun.misc.Hashing.randomHashSeed(this);
246
247 @SuppressWarnings("unchecked")
248 private Entry<K,V>[] newTable(int n) {
249 return (Entry<K,V>[]) new Entry[n];
250 }
251
252 /**
253 * Constructs a new, empty <tt>WeakHashMap</tt> with the given initial
254 * capacity and the given load factor.
255 *
256 * @param initialCapacity The initial capacity of the <tt>WeakHashMap</tt>
257 * @param loadFactor The load factor of the <tt>WeakHashMap</tt>
258 * @throws IllegalArgumentException if the initial capacity is negative,
259 * or if the load factor is nonpositive.
260 */
261 public WeakHashMap(int initialCapacity, float loadFactor) {
262 if (initialCapacity < 0)
263 throw new IllegalArgumentException("Illegal Initial Capacity: "+
264 initialCapacity);
265 if (initialCapacity > MAXIMUM_CAPACITY)
266 initialCapacity = MAXIMUM_CAPACITY;
267
268 if (loadFactor <= 0 || Float.isNaN(loadFactor))
269 throw new IllegalArgumentException("Illegal Load factor: "+
270 loadFactor);
271 int capacity = 1;
272 while (capacity < initialCapacity)
273 capacity <<= 1;
274 table = newTable(capacity);
275 this.loadFactor = loadFactor;
276 threshold = (int)(capacity * loadFactor);
277 useAltHashing = sun.misc.VM.isBooted() &&
278 (capacity >= Holder.ALTERNATE_HASHING_THRESHOLD);
279 }
280
281 /**
282 * Constructs a new, empty <tt>WeakHashMap</tt> with the given initial
283 * capacity and the default load factor (0.75).
284 *
285 * @param initialCapacity The initial capacity of the <tt>WeakHashMap</tt>
286 * @throws IllegalArgumentException if the initial capacity is negative
287 */
288 public WeakHashMap(int initialCapacity) {
289 this(initialCapacity, DEFAULT_LOAD_FACTOR);
290 }
291
292 /**
293 * Constructs a new, empty <tt>WeakHashMap</tt> with the default initial
294 * capacity (16) and load factor (0.75).
295 */
296 public WeakHashMap() {
297 this(DEFAULT_INITIAL_CAPACITY, DEFAULT_LOAD_FACTOR);
298 }
299
300 /**
301 * Constructs a new <tt>WeakHashMap</tt> with the same mappings as the
302 * specified map. The <tt>WeakHashMap</tt> is created with the default
303 * load factor (0.75) and an initial capacity sufficient to hold the
304 * mappings in the specified map.
305 *
306 * @param m the map whose mappings are to be placed in this map
307 * @throws NullPointerException if the specified map is null
308 * @since 1.3
309 */
310 public WeakHashMap(Map<? extends K, ? extends V> m) {
311 this(Math.max((int) (m.size() / DEFAULT_LOAD_FACTOR) + 1,
312 DEFAULT_INITIAL_CAPACITY),
313 DEFAULT_LOAD_FACTOR);
314 putAll(m);
315 }
316
317 // internal utilities
318
319 /**
320 * Value representing null keys inside tables.
321 */
322 private static final Object NULL_KEY = new Object();
323
324 /**
325 * Use NULL_KEY for key if it is null.
326 */
327 private static Object maskNull(Object key) {
328 return (key == null) ? NULL_KEY : key;
329 }
330
331 /**
332 * Returns internal representation of null key back to caller as null.
333 */
334 static Object unmaskNull(Object key) {
335 return (key == NULL_KEY) ? null : key;
336 }
337
338 /**
339 * Checks for equality of non-null reference x and possibly-null y. By
340 * default uses Object.equals.
341 */
342 private static boolean eq(Object x, Object y) {
343 return x == y || x.equals(y);
344 }
345
346 /**
347 * Retrieve object hash code and applies a supplemental hash function to the
348 * result hash, which defends against poor quality hash functions. This is
349 * critical because HashMap uses power-of-two length hash tables, that
350 * otherwise encounter collisions for hashCodes that do not differ
351 * in lower bits.
352 */
353 int hash(Object k) {
354
355 int h;
356 if (useAltHashing) {
357 h = hashSeed;
358 if (k instanceof String) {
359 return h ^ sun.misc.Hashing.stringHash32((String) k);
360 } else {
361 h ^= k.hashCode();
362 }
363 } else {
364 h = k.hashCode();
365 }
366
367 // This function ensures that hashCodes that differ only by
368 // constant multiples at each bit position have a bounded
369 // number of collisions (approximately 8 at default load factor).
370 h ^= (h >>> 20) ^ (h >>> 12);
371 return h ^ (h >>> 7) ^ (h >>> 4);
372 }
373
374 /**
375 * Returns index for hash code h.
376 */
377 private static int indexFor(int h, int length) {
378 return h & (length-1);
379 }
380
381 /**
382 * Expunges stale entries from the table.
383 */
384 private void expungeStaleEntries() {
385 for (Object x; (x = queue.poll()) != null; ) {
386 synchronized (queue) {
387 @SuppressWarnings("unchecked")
388 Entry<K,V> e = (Entry<K,V>) x;
389 int i = indexFor(e.hash, table.length);
390
391 Entry<K,V> prev = table[i];
392 Entry<K,V> p = prev;
393 while (p != null) {
394 Entry<K,V> next = p.next;
395 if (p == e) {
396 if (prev == e)
397 table[i] = next;
398 else
399 prev.next = next;
400 // Must not null out e.next;
401 // stale entries may be in use by a HashIterator
402 e.value = null; // Help GC
403 size--;
404 break;
405 }
406 prev = p;
407 p = next;
408 }
409 }
410 }
411 }
412
413 /**
414 * Returns the table after first expunging stale entries.
415 */
416 private Entry<K,V>[] getTable() {
417 expungeStaleEntries();
418 return table;
419 }
420
421 /**
422 * Returns the number of key-value mappings in this map.
423 * This result is a snapshot, and may not reflect unprocessed
424 * entries that will be removed before next attempted access
425 * because they are no longer referenced.
426 */
427 public int size() {
428 if (size == 0)
429 return 0;
430 expungeStaleEntries();
431 return size;
432 }
433
434 /**
435 * Returns <tt>true</tt> if this map contains no key-value mappings.
436 * This result is a snapshot, and may not reflect unprocessed
437 * entries that will be removed before next attempted access
438 * because they are no longer referenced.
439 */
440 public boolean isEmpty() {
441 return size() == 0;
442 }
443
444 /**
445 * Returns the value to which the specified key is mapped,
446 * or {@code null} if this map contains no mapping for the key.
447 *
448 * <p>More formally, if this map contains a mapping from a key
449 * {@code k} to a value {@code v} such that {@code (key==null ? k==null :
450 * key.equals(k))}, then this method returns {@code v}; otherwise
451 * it returns {@code null}. (There can be at most one such mapping.)
452 *
453 * <p>A return value of {@code null} does not <i>necessarily</i>
454 * indicate that the map contains no mapping for the key; it's also
455 * possible that the map explicitly maps the key to {@code null}.
456 * The {@link #containsKey containsKey} operation may be used to
457 * distinguish these two cases.
458 *
459 * @see #put(Object, Object)
460 */
461 public V get(Object key) {
462 Object k = maskNull(key);
463 int h = hash(k);
464 Entry<K,V>[] tab = getTable();
465 int index = indexFor(h, tab.length);
466 Entry<K,V> e = tab[index];
467 while (e != null) {
468 if (e.hash == h && eq(k, e.get()))
469 return e.value;
470 e = e.next;
471 }
472 return null;
473 }
474
475 /**
476 * Returns <tt>true</tt> if this map contains a mapping for the
477 * specified key.
478 *
479 * @param key The key whose presence in this map is to be tested
480 * @return <tt>true</tt> if there is a mapping for <tt>key</tt>;
481 * <tt>false</tt> otherwise
482 */
483 public boolean containsKey(Object key) {
484 return getEntry(key) != null;
485 }
486
487 /**
488 * Returns the entry associated with the specified key in this map.
489 * Returns null if the map contains no mapping for this key.
490 */
491 Entry<K,V> getEntry(Object key) {
492 Object k = maskNull(key);
493 int h = hash(k);
494 Entry<K,V>[] tab = getTable();
495 int index = indexFor(h, tab.length);
496 Entry<K,V> e = tab[index];
497 while (e != null && !(e.hash == h && eq(k, e.get())))
498 e = e.next;
499 return e;
500 }
501
502 /**
503 * Associates the specified value with the specified key in this map.
504 * If the map previously contained a mapping for this key, the old
505 * value is replaced.
506 *
507 * @param key key with which the specified value is to be associated.
508 * @param value value to be associated with the specified key.
509 * @return the previous value associated with <tt>key</tt>, or
510 * <tt>null</tt> if there was no mapping for <tt>key</tt>.
511 * (A <tt>null</tt> return can also indicate that the map
512 * previously associated <tt>null</tt> with <tt>key</tt>.)
513 */
514 public V put(K key, V value) {
515 Object k = maskNull(key);
516 int h = hash(k);
517 Entry<K,V>[] tab = getTable();
518 int i = indexFor(h, tab.length);
519
520 for (Entry<K,V> e = tab[i]; e != null; e = e.next) {
521 if (h == e.hash && eq(k, e.get())) {
522 V oldValue = e.value;
523 if (value != oldValue)
524 e.value = value;
525 return oldValue;
526 }
527 }
528
529 modCount++;
530 Entry<K,V> e = tab[i];
531 tab[i] = new Entry<>(k, value, queue, h, e);
532 if (++size >= threshold)
533 resize(tab.length * 2);
534 return null;
535 }
536
537 /**
538 * Rehashes the contents of this map into a new array with a
539 * larger capacity. This method is called automatically when the
540 * number of keys in this map reaches its threshold.
541 *
542 * If current capacity is MAXIMUM_CAPACITY, this method does not
543 * resize the map, but sets threshold to Integer.MAX_VALUE.
544 * This has the effect of preventing future calls.
545 *
546 * @param newCapacity the new capacity, MUST be a power of two;
547 * must be greater than current capacity unless current
548 * capacity is MAXIMUM_CAPACITY (in which case value
549 * is irrelevant).
550 */
551 void resize(int newCapacity) {
552 Entry<K,V>[] oldTable = getTable();
553 int oldCapacity = oldTable.length;
554 if (oldCapacity == MAXIMUM_CAPACITY) {
555 threshold = Integer.MAX_VALUE;
556 return;
557 }
558
559 Entry<K,V>[] newTable = newTable(newCapacity);
560 boolean oldAltHashing = useAltHashing;
561 useAltHashing |= sun.misc.VM.isBooted() &&
562 (newCapacity >= Holder.ALTERNATE_HASHING_THRESHOLD);
563 boolean rehash = oldAltHashing ^ useAltHashing;
564 transfer(oldTable, newTable, rehash);
565 table = newTable;
566
567 /*
568 * If ignoring null elements and processing ref queue caused massive
569 * shrinkage, then restore old table. This should be rare, but avoids
570 * unbounded expansion of garbage-filled tables.
571 */
572 if (size >= threshold / 2) {
573 threshold = (int)(newCapacity * loadFactor);
574 } else {
575 expungeStaleEntries();
576 transfer(newTable, oldTable, false);
577 table = oldTable;
578 }
579 }
580
581 /** Transfers all entries from src to dest tables */
582 private void transfer(Entry<K,V>[] src, Entry<K,V>[] dest, boolean rehash) {
583 for (int j = 0; j < src.length; ++j) {
584 Entry<K,V> e = src[j];
585 src[j] = null;
586 while (e != null) {
587 Entry<K,V> next = e.next;
588 Object key = e.get();
589 if (key == null) {
590 e.next = null; // Help GC
591 e.value = null; // " "
592 size--;
593 } else {
594 if(rehash) {
595 e.hash = hash(key);
596 }
597 int i = indexFor(e.hash, dest.length);
598 e.next = dest[i];
599 dest[i] = e;
600 }
601 e = next;
602 }
603 }
604 }
605
606 /**
607 * Copies all of the mappings from the specified map to this map.
608 * These mappings will replace any mappings that this map had for any
609 * of the keys currently in the specified map.
610 *
611 * @param m mappings to be stored in this map.
612 * @throws NullPointerException if the specified map is null.
613 */
614 public void putAll(Map<? extends K, ? extends V> m) {
615 int numKeysToBeAdded = m.size();
616 if (numKeysToBeAdded == 0)
617 return;
618
619 /*
620 * Expand the map if the map if the number of mappings to be added
621 * is greater than or equal to threshold. This is conservative; the
622 * obvious condition is (m.size() + size) >= threshold, but this
623 * condition could result in a map with twice the appropriate capacity,
624 * if the keys to be added overlap with the keys already in this map.
625 * By using the conservative calculation, we subject ourself
626 * to at most one extra resize.
627 */
628 if (numKeysToBeAdded > threshold) {
629 int targetCapacity = (int)(numKeysToBeAdded / loadFactor + 1);
630 if (targetCapacity > MAXIMUM_CAPACITY)
631 targetCapacity = MAXIMUM_CAPACITY;
632 int newCapacity = table.length;
633 while (newCapacity < targetCapacity)
634 newCapacity <<= 1;
635 if (newCapacity > table.length)
636 resize(newCapacity);
637 }
638
639 for (Map.Entry<? extends K, ? extends V> e : m.entrySet())
640 put(e.getKey(), e.getValue());
641 }
642
643 /**
644 * Removes the mapping for a key from this weak hash map if it is present.
645 * More formally, if this map contains a mapping from key <tt>k</tt> to
646 * value <tt>v</tt> such that <code>(key==null ? k==null :
647 * key.equals(k))</code>, that mapping is removed. (The map can contain
648 * at most one such mapping.)
649 *
650 * <p>Returns the value to which this map previously associated the key,
651 * or <tt>null</tt> if the map contained no mapping for the key. A
652 * return value of <tt>null</tt> does not <i>necessarily</i> indicate
653 * that the map contained no mapping for the key; it's also possible
654 * that the map explicitly mapped the key to <tt>null</tt>.
655 *
656 * <p>The map will not contain a mapping for the specified key once the
657 * call returns.
658 *
659 * @param key key whose mapping is to be removed from the map
660 * @return the previous value associated with <tt>key</tt>, or
661 * <tt>null</tt> if there was no mapping for <tt>key</tt>
662 */
663 public V remove(Object key) {
664 Object k = maskNull(key);
665 int h = hash(k);
666 Entry<K,V>[] tab = getTable();
667 int i = indexFor(h, tab.length);
668 Entry<K,V> prev = tab[i];
669 Entry<K,V> e = prev;
670
671 while (e != null) {
672 Entry<K,V> next = e.next;
673 if (h == e.hash && eq(k, e.get())) {
674 modCount++;
675 size--;
676 if (prev == e)
677 tab[i] = next;
678 else
679 prev.next = next;
680 return e.value;
681 }
682 prev = e;
683 e = next;
684 }
685
686 return null;
687 }
688
689 /** Special version of remove needed by Entry set */
690 boolean removeMapping(Object o) {
691 if (!(o instanceof Map.Entry))
692 return false;
693 Entry<K,V>[] tab = getTable();
694 Map.Entry<?,?> entry = (Map.Entry<?,?>)o;
695 Object k = maskNull(entry.getKey());
696 int h = hash(k);
697 int i = indexFor(h, tab.length);
698 Entry<K,V> prev = tab[i];
699 Entry<K,V> e = prev;
700
701 while (e != null) {
702 Entry<K,V> next = e.next;
703 if (h == e.hash && e.equals(entry)) {
704 modCount++;
705 size--;
706 if (prev == e)
707 tab[i] = next;
708 else
709 prev.next = next;
710 return true;
711 }
712 prev = e;
713 e = next;
714 }
715
716 return false;
717 }
718
719 /**
720 * Removes all of the mappings from this map.
721 * The map will be empty after this call returns.
722 */
723 public void clear() {
724 // clear out ref queue. We don't need to expunge entries
725 // since table is getting cleared.
726 while (queue.poll() != null)
727 ;
728
729 modCount++;
730 Arrays.fill(table, null);
731 size = 0;
732
733 // Allocation of array may have caused GC, which may have caused
734 // additional entries to go stale. Removing these entries from the
735 // reference queue will make them eligible for reclamation.
736 while (queue.poll() != null)
737 ;
738 }
739
740 /**
741 * Returns <tt>true</tt> if this map maps one or more keys to the
742 * specified value.
743 *
744 * @param value value whose presence in this map is to be tested
745 * @return <tt>true</tt> if this map maps one or more keys to the
746 * specified value
747 */
748 public boolean containsValue(Object value) {
749 if (value==null)
750 return containsNullValue();
751
752 Entry<K,V>[] tab = getTable();
753 for (int i = tab.length; i-- > 0;)
754 for (Entry<K,V> e = tab[i]; e != null; e = e.next)
755 if (value.equals(e.value))
756 return true;
757 return false;
758 }
759
760 /**
761 * Special-case code for containsValue with null argument
762 */
763 private boolean containsNullValue() {
764 Entry<K,V>[] tab = getTable();
765 for (int i = tab.length; i-- > 0;)
766 for (Entry<K,V> e = tab[i]; e != null; e = e.next)
767 if (e.value==null)
768 return true;
769 return false;
770 }
771
772 /**
773 * The entries in this hash table extend WeakReference, using its main ref
774 * field as the key.
775 */
776 private static class Entry<K,V> extends WeakReference<Object> implements Map.Entry<K,V> {
777 V value;
778 int hash;
779 Entry<K,V> next;
780
781 /**
782 * Creates new entry.
783 */
784 Entry(Object key, V value,
785 ReferenceQueue<Object> queue,
786 int hash, Entry<K,V> next) {
787 super(key, queue);
788 this.value = value;
789 this.hash = hash;
790 this.next = next;
791 }
792
793 @SuppressWarnings("unchecked")
794 public K getKey() {
795 return (K) WeakHashMap.unmaskNull(get());
796 }
797
798 public V getValue() {
799 return value;
800 }
801
802 public V setValue(V newValue) {
803 V oldValue = value;
804 value = newValue;
805 return oldValue;
806 }
807
808 public boolean equals(Object o) {
809 if (!(o instanceof Map.Entry))
810 return false;
811 Map.Entry<?,?> e = (Map.Entry<?,?>)o;
812 K k1 = getKey();
813 Object k2 = e.getKey();
814 if (k1 == k2 || (k1 != null && k1.equals(k2))) {
815 V v1 = getValue();
816 Object v2 = e.getValue();
817 if (v1 == v2 || (v1 != null && v1.equals(v2)))
818 return true;
819 }
820 return false;
821 }
822
823 public int hashCode() {
824 K k = getKey();
825 V v = getValue();
826 return ((k==null ? 0 : k.hashCode()) ^
827 (v==null ? 0 : v.hashCode()));
828 }
829
830 public String toString() {
831 return getKey() + "=" + getValue();
832 }
833 }
834
835 private abstract class HashIterator<T> implements Iterator<T> {
836 private int index;
837 private Entry<K,V> entry = null;
838 private Entry<K,V> lastReturned = null;
839 private int expectedModCount = modCount;
840
841 /**
842 * Strong reference needed to avoid disappearance of key
843 * between hasNext and next
844 */
845 private Object nextKey = null;
846
847 /**
848 * Strong reference needed to avoid disappearance of key
849 * between nextEntry() and any use of the entry
850 */
851 private Object currentKey = null;
852
853 HashIterator() {
854 index = isEmpty() ? 0 : table.length;
855 }
856
857 public boolean hasNext() {
858 Entry<K,V>[] t = table;
859
860 while (nextKey == null) {
861 Entry<K,V> e = entry;
862 int i = index;
863 while (e == null && i > 0)
864 e = t[--i];
865 entry = e;
866 index = i;
867 if (e == null) {
868 currentKey = null;
869 return false;
870 }
871 nextKey = e.get(); // hold on to key in strong ref
872 if (nextKey == null)
873 entry = entry.next;
874 }
875 return true;
876 }
877
878 /** The common parts of next() across different types of iterators */
879 protected Entry<K,V> nextEntry() {
880 if (modCount != expectedModCount)
881 throw new ConcurrentModificationException();
882 if (nextKey == null && !hasNext())
883 throw new NoSuchElementException();
884
885 lastReturned = entry;
886 entry = entry.next;
887 currentKey = nextKey;
888 nextKey = null;
889 return lastReturned;
890 }
891
892 public void remove() {
893 if (lastReturned == null)
894 throw new IllegalStateException();
895 if (modCount != expectedModCount)
896 throw new ConcurrentModificationException();
897
898 WeakHashMap.this.remove(currentKey);
899 expectedModCount = modCount;
900 lastReturned = null;
901 currentKey = null;
902 }
903
904 }
905
906 private class ValueIterator extends HashIterator<V> {
907 public V next() {
908 return nextEntry().value;
909 }
910 }
911
912 private class KeyIterator extends HashIterator<K> {
913 public K next() {
914 return nextEntry().getKey();
915 }
916 }
917
918 private class EntryIterator extends HashIterator<Map.Entry<K,V>> {
919 public Map.Entry<K,V> next() {
920 return nextEntry();
921 }
922 }
923
924 // Views
925
926 private transient Set<Map.Entry<K,V>> entrySet = null;
927
928 /**
929 * Returns a {@link Set} view of the keys contained in this map.
930 * The set is backed by the map, so changes to the map are
931 * reflected in the set, and vice-versa. If the map is modified
932 * while an iteration over the set is in progress (except through
933 * the iterator's own <tt>remove</tt> operation), the results of
934 * the iteration are undefined. The set supports element removal,
935 * which removes the corresponding mapping from the map, via the
936 * <tt>Iterator.remove</tt>, <tt>Set.remove</tt>,
937 * <tt>removeAll</tt>, <tt>retainAll</tt>, and <tt>clear</tt>
938 * operations. It does not support the <tt>add</tt> or <tt>addAll</tt>
939 * operations.
940 */
941 public Set<K> keySet() {
942 Set<K> ks = keySet;
943 return (ks != null ? ks : (keySet = new KeySet()));
944 }
945
946 private class KeySet extends AbstractSet<K> {
947 public Iterator<K> iterator() {
948 return new KeyIterator();
949 }
950
951 public int size() {
952 return WeakHashMap.this.size();
953 }
954
955 public boolean contains(Object o) {
956 return containsKey(o);
957 }
958
959 public boolean remove(Object o) {
960 if (containsKey(o)) {
961 WeakHashMap.this.remove(o);
962 return true;
963 }
964 else
965 return false;
966 }
967
968 public void clear() {
969 WeakHashMap.this.clear();
970 }
971 }
972
973 /**
974 * Returns a {@link Collection} view of the values contained in this map.
975 * The collection is backed by the map, so changes to the map are
976 * reflected in the collection, and vice-versa. If the map is
977 * modified while an iteration over the collection is in progress
978 * (except through the iterator's own <tt>remove</tt> operation),
979 * the results of the iteration are undefined. The collection
980 * supports element removal, which removes the corresponding
981 * mapping from the map, via the <tt>Iterator.remove</tt>,
982 * <tt>Collection.remove</tt>, <tt>removeAll</tt>,
983 * <tt>retainAll</tt> and <tt>clear</tt> operations. It does not
984 * support the <tt>add</tt> or <tt>addAll</tt> operations.
985 */
986 public Collection<V> values() {
987 Collection<V> vs = values;
988 return (vs != null) ? vs : (values = new Values());
989 }
990
991 private class Values extends AbstractCollection<V> {
992 public Iterator<V> iterator() {
993 return new ValueIterator();
994 }
995
996 public int size() {
997 return WeakHashMap.this.size();
998 }
999
1000 public boolean contains(Object o) {
1001 return containsValue(o);
1002 }
1003
1004 public void clear() {
1005 WeakHashMap.this.clear();
1006 }
1007 }
1008
1009 /**
1010 * Returns a {@link Set} view of the mappings contained in this map.
1011 * The set is backed by the map, so changes to the map are
1012 * reflected in the set, and vice-versa. If the map is modified
1013 * while an iteration over the set is in progress (except through
1014 * the iterator's own <tt>remove</tt> operation, or through the
1015 * <tt>setValue</tt> operation on a map entry returned by the
1016 * iterator) the results of the iteration are undefined. The set
1017 * supports element removal, which removes the corresponding
1018 * mapping from the map, via the <tt>Iterator.remove</tt>,
1019 * <tt>Set.remove</tt>, <tt>removeAll</tt>, <tt>retainAll</tt> and
1020 * <tt>clear</tt> operations. It does not support the
1021 * <tt>add</tt> or <tt>addAll</tt> operations.
1022 */
1023 public Set<Map.Entry<K,V>> entrySet() {
1024 Set<Map.Entry<K,V>> es = entrySet;
1025 return es != null ? es : (entrySet = new EntrySet());
1026 }
1027
1028 private class EntrySet extends AbstractSet<Map.Entry<K,V>> {
1029 public Iterator<Map.Entry<K,V>> iterator() {
1030 return new EntryIterator();
1031 }
1032
1033 public boolean contains(Object o) {
1034 if (!(o instanceof Map.Entry))
1035 return false;
1036 Map.Entry<?,?> e = (Map.Entry<?,?>)o;
1037 Entry<K,V> candidate = getEntry(e.getKey());
1038 return candidate != null && candidate.equals(e);
1039 }
1040
1041 public boolean remove(Object o) {
1042 return removeMapping(o);
1043 }
1044
1045 public int size() {
1046 return WeakHashMap.this.size();
1047 }
1048
1049 public void clear() {
1050 WeakHashMap.this.clear();
1051 }
1052
1053 private List<Map.Entry<K,V>> deepCopy() {
1054 List<Map.Entry<K,V>> list = new ArrayList<>(size());
1055 for (Map.Entry<K,V> e : this)
1056 list.add(new AbstractMap.SimpleEntry<>(e));
1057 return list;
1058 }
1059
1060 public Object[] toArray() {
1061 return deepCopy().toArray();
1062 }
1063
1064 public <T> T[] toArray(T[] a) {
1065 return deepCopy().toArray(a);
1066 }
1067 }
1068 }