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
28 import java.lang.ref.WeakReference;
29 import java.lang.ref.ReferenceQueue;
30 import java.util.concurrent.ThreadLocalRandom;
31 import java.util.function.BiConsumer;
32 import java.util.function.BiFunction;
33 import java.util.function.Consumer;
34
35
36 /**
37 * Hash table based implementation of the <tt>Map</tt> interface, with
38 * <em>weak keys</em>.
39 * An entry in a <tt>WeakHashMap</tt> will automatically be removed when
40 * its key is no longer in ordinary use. More precisely, the presence of a
41 * mapping for a given key will not prevent the key from being discarded by the
42 * garbage collector, that is, made finalizable, finalized, and then reclaimed.
43 * When a key has been discarded its entry is effectively removed from the map,
44 * so this class behaves somewhat differently from other <tt>Map</tt>
45 * implementations.
46 *
47 * <p> Both null values and the null key are supported. This class has
48 * performance characteristics similar to those of the <tt>HashMap</tt>
49 * class, and has the same efficiency parameters of <em>initial capacity</em>
50 * and <em>load factor</em>.
51 *
52 * <p> Like most collection classes, this class is not synchronized.
53 * A synchronized <tt>WeakHashMap</tt> may be constructed using the
54 * {@link Collections#synchronizedMap Collections.synchronizedMap}
55 * method.
56 *
57 * <p> This class is intended primarily for use with key objects whose
58 * <tt>equals</tt> methods test for object identity using the
59 * <tt>==</tt> operator. Once such a key is discarded it can never be
60 * recreated, so it is impossible to do a lookup of that key in a
61 * <tt>WeakHashMap</tt> at some later time and be surprised that its entry
62 * has been removed. This class will work perfectly well with key objects
63 * whose <tt>equals</tt> methods are not based upon object identity, such
64 * as <tt>String</tt> instances. With such recreatable key objects,
65 * however, the automatic removal of <tt>WeakHashMap</tt> entries whose
66 * keys have been discarded may prove to be confusing.
67 *
68 * <p> The behavior of the <tt>WeakHashMap</tt> class depends in part upon
69 * the actions of the garbage collector, so several familiar (though not
70 * required) <tt>Map</tt> invariants do not hold for this class. Because
71 * the garbage collector may discard keys at any time, a
72 * <tt>WeakHashMap</tt> may behave as though an unknown thread is silently
73 * removing entries. In particular, even if you synchronize on a
74 * <tt>WeakHashMap</tt> instance and invoke none of its mutator methods, it
75 * is possible for the <tt>size</tt> method to return smaller values over
76 * time, for the <tt>isEmpty</tt> method to return <tt>false</tt> and
77 * then <tt>true</tt>, for the <tt>containsKey</tt> method to return
78 * <tt>true</tt> and later <tt>false</tt> for a given key, for the
79 * <tt>get</tt> method to return a value for a given key but later return
80 * <tt>null</tt>, for the <tt>put</tt> method to return
81 * <tt>null</tt> and the <tt>remove</tt> method to return
82 * <tt>false</tt> for a key that previously appeared to be in the map, and
83 * for successive examinations of the key set, the value collection, and
84 * the entry set to yield successively smaller numbers of elements.
85 *
86 * <p> Each key object in a <tt>WeakHashMap</tt> is stored indirectly as
87 * the referent of a weak reference. Therefore a key will automatically be
88 * removed only after the weak references to it, both inside and outside of the
89 * map, have been cleared by the garbage collector.
90 *
91 * <p> <strong>Implementation note:</strong> The value objects in a
92 * <tt>WeakHashMap</tt> are held by ordinary strong references. Thus care
93 * should be taken to ensure that value objects do not strongly refer to their
94 * own keys, either directly or indirectly, since that will prevent the keys
95 * from being discarded. Note that a value object may refer indirectly to its
96 * key via the <tt>WeakHashMap</tt> itself; that is, a value object may
97 * strongly refer to some other key object whose associated value object, in
98 * turn, strongly refers to the key of the first value object. If the values
99 * in the map do not rely on the map holding strong references to them, one way
100 * to deal with this is to wrap values themselves within
101 * <tt>WeakReferences</tt> before
102 * inserting, as in: <tt>m.put(key, new WeakReference(value))</tt>,
103 * and then unwrapping upon each <tt>get</tt>.
104 *
105 * <p>The iterators returned by the <tt>iterator</tt> method of the collections
106 * returned by all of this class's "collection view methods" are
107 * <i>fail-fast</i>: if the map is structurally modified at any time after the
108 * iterator is created, in any way except through the iterator's own
109 * <tt>remove</tt> method, the iterator will throw a {@link
110 * ConcurrentModificationException}. Thus, in the face of concurrent
111 * modification, the iterator fails quickly and cleanly, rather than risking
112 * arbitrary, non-deterministic behavior at an undetermined time in the future.
113 *
114 * <p>Note that the fail-fast behavior of an iterator cannot be guaranteed
115 * as it is, generally speaking, impossible to make any hard guarantees in the
116 * presence of unsynchronized concurrent modification. Fail-fast iterators
117 * throw <tt>ConcurrentModificationException</tt> on a best-effort basis.
118 * Therefore, it would be wrong to write a program that depended on this
119 * exception for its correctness: <i>the fail-fast behavior of iterators
120 * should be used only to detect bugs.</i>
121 *
122 * <p>This class is a member of the
123 * <a href="{@docRoot}/../technotes/guides/collections/index.html">
124 * Java Collections Framework</a>.
125 *
126 * @param <K> the type of keys maintained by this map
127 * @param <V> the type of mapped values
128 *
129 * @author Doug Lea
130 * @author Josh Bloch
131 * @author Mark Reinhold
132 * @since 1.2
133 * @see java.util.HashMap
134 * @see java.lang.ref.WeakReference
135 */
136 public class WeakHashMap<K,V>
137 extends AbstractMap<K,V>
179 */
180 private final ReferenceQueue<Object> queue = new ReferenceQueue<>();
181
182 /**
183 * The number of times this WeakHashMap has been structurally modified.
184 * Structural modifications are those that change the number of
185 * mappings in the map or otherwise modify its internal structure
186 * (e.g., rehash). This field is used to make iterators on
187 * Collection-views of the map fail-fast.
188 *
189 * @see ConcurrentModificationException
190 */
191 int modCount;
192
193 @SuppressWarnings("unchecked")
194 private Entry<K,V>[] newTable(int n) {
195 return (Entry<K,V>[]) new Entry<?,?>[n];
196 }
197
198 /**
199 * Constructs a new, empty <tt>WeakHashMap</tt> with the given initial
200 * capacity and the given load factor.
201 *
202 * @param initialCapacity The initial capacity of the <tt>WeakHashMap</tt>
203 * @param loadFactor The load factor of the <tt>WeakHashMap</tt>
204 * @throws IllegalArgumentException if the initial capacity is negative,
205 * or if the load factor is nonpositive.
206 */
207 public WeakHashMap(int initialCapacity, float loadFactor) {
208 if (initialCapacity < 0)
209 throw new IllegalArgumentException("Illegal Initial Capacity: "+
210 initialCapacity);
211 if (initialCapacity > MAXIMUM_CAPACITY)
212 initialCapacity = MAXIMUM_CAPACITY;
213
214 if (loadFactor <= 0 || Float.isNaN(loadFactor))
215 throw new IllegalArgumentException("Illegal Load factor: "+
216 loadFactor);
217 int capacity = 1;
218 while (capacity < initialCapacity)
219 capacity <<= 1;
220 table = newTable(capacity);
221 this.loadFactor = loadFactor;
222 threshold = (int)(capacity * loadFactor);
223 }
224
225 /**
226 * Constructs a new, empty <tt>WeakHashMap</tt> with the given initial
227 * capacity and the default load factor (0.75).
228 *
229 * @param initialCapacity The initial capacity of the <tt>WeakHashMap</tt>
230 * @throws IllegalArgumentException if the initial capacity is negative
231 */
232 public WeakHashMap(int initialCapacity) {
233 this(initialCapacity, DEFAULT_LOAD_FACTOR);
234 }
235
236 /**
237 * Constructs a new, empty <tt>WeakHashMap</tt> with the default initial
238 * capacity (16) and load factor (0.75).
239 */
240 public WeakHashMap() {
241 this(DEFAULT_INITIAL_CAPACITY, DEFAULT_LOAD_FACTOR);
242 }
243
244 /**
245 * Constructs a new <tt>WeakHashMap</tt> with the same mappings as the
246 * specified map. The <tt>WeakHashMap</tt> is created with the default
247 * load factor (0.75) and an initial capacity sufficient to hold the
248 * mappings in the specified map.
249 *
250 * @param m the map whose mappings are to be placed in this map
251 * @throws NullPointerException if the specified map is null
252 * @since 1.3
253 */
254 public WeakHashMap(Map<? extends K, ? extends V> m) {
255 this(Math.max((int) (m.size() / DEFAULT_LOAD_FACTOR) + 1,
256 DEFAULT_INITIAL_CAPACITY),
257 DEFAULT_LOAD_FACTOR);
258 putAll(m);
259 }
260
261 // internal utilities
262
263 /**
264 * Value representing null keys inside tables.
265 */
266 private static final Object NULL_KEY = new Object();
348 */
349 private Entry<K,V>[] getTable() {
350 expungeStaleEntries();
351 return table;
352 }
353
354 /**
355 * Returns the number of key-value mappings in this map.
356 * This result is a snapshot, and may not reflect unprocessed
357 * entries that will be removed before next attempted access
358 * because they are no longer referenced.
359 */
360 public int size() {
361 if (size == 0)
362 return 0;
363 expungeStaleEntries();
364 return size;
365 }
366
367 /**
368 * Returns <tt>true</tt> if this map contains no key-value mappings.
369 * This result is a snapshot, and may not reflect unprocessed
370 * entries that will be removed before next attempted access
371 * because they are no longer referenced.
372 */
373 public boolean isEmpty() {
374 return size() == 0;
375 }
376
377 /**
378 * Returns the value to which the specified key is mapped,
379 * or {@code null} if this map contains no mapping for the key.
380 *
381 * <p>More formally, if this map contains a mapping from a key
382 * {@code k} to a value {@code v} such that {@code (key==null ? k==null :
383 * key.equals(k))}, then this method returns {@code v}; otherwise
384 * it returns {@code null}. (There can be at most one such mapping.)
385 *
386 * <p>A return value of {@code null} does not <i>necessarily</i>
387 * indicate that the map contains no mapping for the key; it's also
388 * possible that the map explicitly maps the key to {@code null}.
389 * The {@link #containsKey containsKey} operation may be used to
390 * distinguish these two cases.
391 *
392 * @see #put(Object, Object)
393 */
394 public V get(Object key) {
395 Object k = maskNull(key);
396 int h = hash(k);
397 Entry<K,V>[] tab = getTable();
398 int index = indexFor(h, tab.length);
399 Entry<K,V> e = tab[index];
400 while (e != null) {
401 if (e.hash == h && eq(k, e.get()))
402 return e.value;
403 e = e.next;
404 }
405 return null;
406 }
407
408 /**
409 * Returns <tt>true</tt> if this map contains a mapping for the
410 * specified key.
411 *
412 * @param key The key whose presence in this map is to be tested
413 * @return <tt>true</tt> if there is a mapping for <tt>key</tt>;
414 * <tt>false</tt> otherwise
415 */
416 public boolean containsKey(Object key) {
417 return getEntry(key) != null;
418 }
419
420 /**
421 * Returns the entry associated with the specified key in this map.
422 * Returns null if the map contains no mapping for this key.
423 */
424 Entry<K,V> getEntry(Object key) {
425 Object k = maskNull(key);
426 int h = hash(k);
427 Entry<K,V>[] tab = getTable();
428 int index = indexFor(h, tab.length);
429 Entry<K,V> e = tab[index];
430 while (e != null && !(e.hash == h && eq(k, e.get())))
431 e = e.next;
432 return e;
433 }
434
435 /**
436 * Associates the specified value with the specified key in this map.
437 * If the map previously contained a mapping for this key, the old
438 * value is replaced.
439 *
440 * @param key key with which the specified value is to be associated.
441 * @param value value to be associated with the specified key.
442 * @return the previous value associated with <tt>key</tt>, or
443 * <tt>null</tt> if there was no mapping for <tt>key</tt>.
444 * (A <tt>null</tt> return can also indicate that the map
445 * previously associated <tt>null</tt> with <tt>key</tt>.)
446 */
447 public V put(K key, V value) {
448 Object k = maskNull(key);
449 int h = hash(k);
450 Entry<K,V>[] tab = getTable();
451 int i = indexFor(h, tab.length);
452
453 for (Entry<K,V> e = tab[i]; e != null; e = e.next) {
454 if (h == e.hash && eq(k, e.get())) {
455 V oldValue = e.value;
456 if (value != oldValue)
457 e.value = value;
458 return oldValue;
459 }
460 }
461
462 modCount++;
463 Entry<K,V> e = tab[i];
464 tab[i] = new Entry<>(k, value, queue, h, e);
465 if (++size >= threshold)
551 * By using the conservative calculation, we subject ourself
552 * to at most one extra resize.
553 */
554 if (numKeysToBeAdded > threshold) {
555 int targetCapacity = (int)(numKeysToBeAdded / loadFactor + 1);
556 if (targetCapacity > MAXIMUM_CAPACITY)
557 targetCapacity = MAXIMUM_CAPACITY;
558 int newCapacity = table.length;
559 while (newCapacity < targetCapacity)
560 newCapacity <<= 1;
561 if (newCapacity > table.length)
562 resize(newCapacity);
563 }
564
565 for (Map.Entry<? extends K, ? extends V> e : m.entrySet())
566 put(e.getKey(), e.getValue());
567 }
568
569 /**
570 * Removes the mapping for a key from this weak hash map if it is present.
571 * More formally, if this map contains a mapping from key <tt>k</tt> to
572 * value <tt>v</tt> such that <code>(key==null ? k==null :
573 * key.equals(k))</code>, that mapping is removed. (The map can contain
574 * at most one such mapping.)
575 *
576 * <p>Returns the value to which this map previously associated the key,
577 * or <tt>null</tt> if the map contained no mapping for the key. A
578 * return value of <tt>null</tt> does not <i>necessarily</i> indicate
579 * that the map contained no mapping for the key; it's also possible
580 * that the map explicitly mapped the key to <tt>null</tt>.
581 *
582 * <p>The map will not contain a mapping for the specified key once the
583 * call returns.
584 *
585 * @param key key whose mapping is to be removed from the map
586 * @return the previous value associated with <tt>key</tt>, or
587 * <tt>null</tt> if there was no mapping for <tt>key</tt>
588 */
589 public V remove(Object key) {
590 Object k = maskNull(key);
591 int h = hash(k);
592 Entry<K,V>[] tab = getTable();
593 int i = indexFor(h, tab.length);
594 Entry<K,V> prev = tab[i];
595 Entry<K,V> e = prev;
596
597 while (e != null) {
598 Entry<K,V> next = e.next;
599 if (h == e.hash && eq(k, e.get())) {
600 modCount++;
601 size--;
602 if (prev == e)
603 tab[i] = next;
604 else
605 prev.next = next;
606 return e.value;
607 }
647 * The map will be empty after this call returns.
648 */
649 public void clear() {
650 // clear out ref queue. We don't need to expunge entries
651 // since table is getting cleared.
652 while (queue.poll() != null)
653 ;
654
655 modCount++;
656 Arrays.fill(table, null);
657 size = 0;
658
659 // Allocation of array may have caused GC, which may have caused
660 // additional entries to go stale. Removing these entries from the
661 // reference queue will make them eligible for reclamation.
662 while (queue.poll() != null)
663 ;
664 }
665
666 /**
667 * Returns <tt>true</tt> if this map maps one or more keys to the
668 * specified value.
669 *
670 * @param value value whose presence in this map is to be tested
671 * @return <tt>true</tt> if this map maps one or more keys to the
672 * specified value
673 */
674 public boolean containsValue(Object value) {
675 if (value==null)
676 return containsNullValue();
677
678 Entry<K,V>[] tab = getTable();
679 for (int i = tab.length; i-- > 0;)
680 for (Entry<K,V> e = tab[i]; e != null; e = e.next)
681 if (value.equals(e.value))
682 return true;
683 return false;
684 }
685
686 /**
687 * Special-case code for containsValue with null argument
688 */
689 private boolean containsNullValue() {
690 Entry<K,V>[] tab = getTable();
691 for (int i = tab.length; i-- > 0;)
838 public K next() {
839 return nextEntry().getKey();
840 }
841 }
842
843 private class EntryIterator extends HashIterator<Map.Entry<K,V>> {
844 public Map.Entry<K,V> next() {
845 return nextEntry();
846 }
847 }
848
849 // Views
850
851 private transient Set<Map.Entry<K,V>> entrySet;
852
853 /**
854 * Returns a {@link Set} view of the keys contained in this map.
855 * The set is backed by the map, so changes to the map are
856 * reflected in the set, and vice-versa. If the map is modified
857 * while an iteration over the set is in progress (except through
858 * the iterator's own <tt>remove</tt> operation), the results of
859 * the iteration are undefined. The set supports element removal,
860 * which removes the corresponding mapping from the map, via the
861 * <tt>Iterator.remove</tt>, <tt>Set.remove</tt>,
862 * <tt>removeAll</tt>, <tt>retainAll</tt>, and <tt>clear</tt>
863 * operations. It does not support the <tt>add</tt> or <tt>addAll</tt>
864 * operations.
865 */
866 public Set<K> keySet() {
867 Set<K> ks = keySet;
868 return (ks != null ? ks : (keySet = new KeySet()));
869 }
870
871 private class KeySet extends AbstractSet<K> {
872 public Iterator<K> iterator() {
873 return new KeyIterator();
874 }
875
876 public int size() {
877 return WeakHashMap.this.size();
878 }
879
880 public boolean contains(Object o) {
881 return containsKey(o);
882 }
883
887 return true;
888 }
889 else
890 return false;
891 }
892
893 public void clear() {
894 WeakHashMap.this.clear();
895 }
896
897 public Spliterator<K> spliterator() {
898 return new KeySpliterator<>(WeakHashMap.this, 0, -1, 0, 0);
899 }
900 }
901
902 /**
903 * Returns a {@link Collection} view of the values contained in this map.
904 * The collection is backed by the map, so changes to the map are
905 * reflected in the collection, and vice-versa. If the map is
906 * modified while an iteration over the collection is in progress
907 * (except through the iterator's own <tt>remove</tt> operation),
908 * the results of the iteration are undefined. The collection
909 * supports element removal, which removes the corresponding
910 * mapping from the map, via the <tt>Iterator.remove</tt>,
911 * <tt>Collection.remove</tt>, <tt>removeAll</tt>,
912 * <tt>retainAll</tt> and <tt>clear</tt> operations. It does not
913 * support the <tt>add</tt> or <tt>addAll</tt> operations.
914 */
915 public Collection<V> values() {
916 Collection<V> vs = values;
917 return (vs != null) ? vs : (values = new Values());
918 }
919
920 private class Values extends AbstractCollection<V> {
921 public Iterator<V> iterator() {
922 return new ValueIterator();
923 }
924
925 public int size() {
926 return WeakHashMap.this.size();
927 }
928
929 public boolean contains(Object o) {
930 return containsValue(o);
931 }
932
933 public void clear() {
934 WeakHashMap.this.clear();
935 }
936
937 public Spliterator<V> spliterator() {
938 return new ValueSpliterator<>(WeakHashMap.this, 0, -1, 0, 0);
939 }
940 }
941
942 /**
943 * Returns a {@link Set} view of the mappings contained in this map.
944 * The set is backed by the map, so changes to the map are
945 * reflected in the set, and vice-versa. If the map is modified
946 * while an iteration over the set is in progress (except through
947 * the iterator's own <tt>remove</tt> operation, or through the
948 * <tt>setValue</tt> operation on a map entry returned by the
949 * iterator) the results of the iteration are undefined. The set
950 * supports element removal, which removes the corresponding
951 * mapping from the map, via the <tt>Iterator.remove</tt>,
952 * <tt>Set.remove</tt>, <tt>removeAll</tt>, <tt>retainAll</tt> and
953 * <tt>clear</tt> operations. It does not support the
954 * <tt>add</tt> or <tt>addAll</tt> operations.
955 */
956 public Set<Map.Entry<K,V>> entrySet() {
957 Set<Map.Entry<K,V>> es = entrySet;
958 return es != null ? es : (entrySet = new EntrySet());
959 }
960
961 private class EntrySet extends AbstractSet<Map.Entry<K,V>> {
962 public Iterator<Map.Entry<K,V>> iterator() {
963 return new EntryIterator();
964 }
965
966 public boolean contains(Object o) {
967 if (!(o instanceof Map.Entry))
968 return false;
969 Map.Entry<?,?> e = (Map.Entry<?,?>)o;
970 Entry<K,V> candidate = getEntry(e.getKey());
971 return candidate != null && candidate.equals(e);
972 }
973
974 public boolean remove(Object o) {
|
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
28 import java.lang.ref.WeakReference;
29 import java.lang.ref.ReferenceQueue;
30 import java.util.concurrent.ThreadLocalRandom;
31 import java.util.function.BiConsumer;
32 import java.util.function.BiFunction;
33 import java.util.function.Consumer;
34
35
36 /**
37 * Hash table based implementation of the {@code Map} interface, with
38 * <em>weak keys</em>.
39 * An entry in a {@code WeakHashMap} will automatically be removed when
40 * its key is no longer in ordinary use. More precisely, the presence of a
41 * mapping for a given key will not prevent the key from being discarded by the
42 * garbage collector, that is, made finalizable, finalized, and then reclaimed.
43 * When a key has been discarded its entry is effectively removed from the map,
44 * so this class behaves somewhat differently from other {@code Map}
45 * implementations.
46 *
47 * <p> Both null values and the null key are supported. This class has
48 * performance characteristics similar to those of the {@code HashMap}
49 * class, and has the same efficiency parameters of <em>initial capacity</em>
50 * and <em>load factor</em>.
51 *
52 * <p> Like most collection classes, this class is not synchronized.
53 * A synchronized {@code WeakHashMap} may be constructed using the
54 * {@link Collections#synchronizedMap Collections.synchronizedMap}
55 * method.
56 *
57 * <p> This class is intended primarily for use with key objects whose
58 * {@code equals} methods test for object identity using the
59 * {@code ==} operator. Once such a key is discarded it can never be
60 * recreated, so it is impossible to do a lookup of that key in a
61 * {@code WeakHashMap} at some later time and be surprised that its entry
62 * has been removed. This class will work perfectly well with key objects
63 * whose {@code equals} methods are not based upon object identity, such
64 * as {@code String} instances. With such recreatable key objects,
65 * however, the automatic removal of {@code WeakHashMap} entries whose
66 * keys have been discarded may prove to be confusing.
67 *
68 * <p> The behavior of the {@code WeakHashMap} class depends in part upon
69 * the actions of the garbage collector, so several familiar (though not
70 * required) {@code Map} invariants do not hold for this class. Because
71 * the garbage collector may discard keys at any time, a
72 * {@code WeakHashMap} may behave as though an unknown thread is silently
73 * removing entries. In particular, even if you synchronize on a
74 * {@code WeakHashMap} instance and invoke none of its mutator methods, it
75 * is possible for the {@code size} method to return smaller values over
76 * time, for the {@code isEmpty} method to return {@code false} and
77 * then {@code true}, for the {@code containsKey} method to return
78 * {@code true} and later {@code false} for a given key, for the
79 * {@code get} method to return a value for a given key but later return
80 * {@code null}, for the {@code put} method to return
81 * {@code null} and the {@code remove} method to return
82 * {@code false} for a key that previously appeared to be in the map, and
83 * for successive examinations of the key set, the value collection, and
84 * the entry set to yield successively smaller numbers of elements.
85 *
86 * <p> Each key object in a {@code WeakHashMap} is stored indirectly as
87 * the referent of a weak reference. Therefore a key will automatically be
88 * removed only after the weak references to it, both inside and outside of the
89 * map, have been cleared by the garbage collector.
90 *
91 * <p> <strong>Implementation note:</strong> The value objects in a
92 * {@code WeakHashMap} are held by ordinary strong references. Thus care
93 * should be taken to ensure that value objects do not strongly refer to their
94 * own keys, either directly or indirectly, since that will prevent the keys
95 * from being discarded. Note that a value object may refer indirectly to its
96 * key via the {@code WeakHashMap} itself; that is, a value object may
97 * strongly refer to some other key object whose associated value object, in
98 * turn, strongly refers to the key of the first value object. If the values
99 * in the map do not rely on the map holding strong references to them, one way
100 * to deal with this is to wrap values themselves within
101 * {@code WeakReferences} before
102 * inserting, as in: {@code m.put(key, new WeakReference(value))},
103 * and then unwrapping upon each {@code get}.
104 *
105 * <p>The iterators returned by the {@code iterator} method of the collections
106 * returned by all of this class's "collection view methods" are
107 * <i>fail-fast</i>: if the map is structurally modified at any time after the
108 * iterator is created, in any way except through the iterator's own
109 * {@code remove} method, the iterator will throw a {@link
110 * ConcurrentModificationException}. Thus, in the face of concurrent
111 * modification, the iterator fails quickly and cleanly, rather than risking
112 * arbitrary, non-deterministic behavior at an undetermined time in the future.
113 *
114 * <p>Note that the fail-fast behavior of an iterator cannot be guaranteed
115 * as it is, generally speaking, impossible to make any hard guarantees in the
116 * presence of unsynchronized concurrent modification. Fail-fast iterators
117 * throw {@code ConcurrentModificationException} on a best-effort basis.
118 * Therefore, it would be wrong to write a program that depended on this
119 * exception for its correctness: <i>the fail-fast behavior of iterators
120 * should be used only to detect bugs.</i>
121 *
122 * <p>This class is a member of the
123 * <a href="{@docRoot}/../technotes/guides/collections/index.html">
124 * Java Collections Framework</a>.
125 *
126 * @param <K> the type of keys maintained by this map
127 * @param <V> the type of mapped values
128 *
129 * @author Doug Lea
130 * @author Josh Bloch
131 * @author Mark Reinhold
132 * @since 1.2
133 * @see java.util.HashMap
134 * @see java.lang.ref.WeakReference
135 */
136 public class WeakHashMap<K,V>
137 extends AbstractMap<K,V>
179 */
180 private final ReferenceQueue<Object> queue = new ReferenceQueue<>();
181
182 /**
183 * The number of times this WeakHashMap has been structurally modified.
184 * Structural modifications are those that change the number of
185 * mappings in the map or otherwise modify its internal structure
186 * (e.g., rehash). This field is used to make iterators on
187 * Collection-views of the map fail-fast.
188 *
189 * @see ConcurrentModificationException
190 */
191 int modCount;
192
193 @SuppressWarnings("unchecked")
194 private Entry<K,V>[] newTable(int n) {
195 return (Entry<K,V>[]) new Entry<?,?>[n];
196 }
197
198 /**
199 * Constructs a new, empty {@code WeakHashMap} with the given initial
200 * capacity and the given load factor.
201 *
202 * @param initialCapacity The initial capacity of the {@code WeakHashMap}
203 * @param loadFactor The load factor of the {@code WeakHashMap}
204 * @throws IllegalArgumentException if the initial capacity is negative,
205 * or if the load factor is nonpositive.
206 */
207 public WeakHashMap(int initialCapacity, float loadFactor) {
208 if (initialCapacity < 0)
209 throw new IllegalArgumentException("Illegal Initial Capacity: "+
210 initialCapacity);
211 if (initialCapacity > MAXIMUM_CAPACITY)
212 initialCapacity = MAXIMUM_CAPACITY;
213
214 if (loadFactor <= 0 || Float.isNaN(loadFactor))
215 throw new IllegalArgumentException("Illegal Load factor: "+
216 loadFactor);
217 int capacity = 1;
218 while (capacity < initialCapacity)
219 capacity <<= 1;
220 table = newTable(capacity);
221 this.loadFactor = loadFactor;
222 threshold = (int)(capacity * loadFactor);
223 }
224
225 /**
226 * Constructs a new, empty {@code WeakHashMap} with the given initial
227 * capacity and the default load factor (0.75).
228 *
229 * @param initialCapacity The initial capacity of the {@code WeakHashMap}
230 * @throws IllegalArgumentException if the initial capacity is negative
231 */
232 public WeakHashMap(int initialCapacity) {
233 this(initialCapacity, DEFAULT_LOAD_FACTOR);
234 }
235
236 /**
237 * Constructs a new, empty {@code WeakHashMap} with the default initial
238 * capacity (16) and load factor (0.75).
239 */
240 public WeakHashMap() {
241 this(DEFAULT_INITIAL_CAPACITY, DEFAULT_LOAD_FACTOR);
242 }
243
244 /**
245 * Constructs a new {@code WeakHashMap} with the same mappings as the
246 * specified map. The {@code WeakHashMap} is created with the default
247 * load factor (0.75) and an initial capacity sufficient to hold the
248 * mappings in the specified map.
249 *
250 * @param m the map whose mappings are to be placed in this map
251 * @throws NullPointerException if the specified map is null
252 * @since 1.3
253 */
254 public WeakHashMap(Map<? extends K, ? extends V> m) {
255 this(Math.max((int) (m.size() / DEFAULT_LOAD_FACTOR) + 1,
256 DEFAULT_INITIAL_CAPACITY),
257 DEFAULT_LOAD_FACTOR);
258 putAll(m);
259 }
260
261 // internal utilities
262
263 /**
264 * Value representing null keys inside tables.
265 */
266 private static final Object NULL_KEY = new Object();
348 */
349 private Entry<K,V>[] getTable() {
350 expungeStaleEntries();
351 return table;
352 }
353
354 /**
355 * Returns the number of key-value mappings in this map.
356 * This result is a snapshot, and may not reflect unprocessed
357 * entries that will be removed before next attempted access
358 * because they are no longer referenced.
359 */
360 public int size() {
361 if (size == 0)
362 return 0;
363 expungeStaleEntries();
364 return size;
365 }
366
367 /**
368 * Returns {@code true} if this map contains no key-value mappings.
369 * This result is a snapshot, and may not reflect unprocessed
370 * entries that will be removed before next attempted access
371 * because they are no longer referenced.
372 */
373 public boolean isEmpty() {
374 return size() == 0;
375 }
376
377 /**
378 * Returns the value to which the specified key is mapped,
379 * or {@code null} if this map contains no mapping for the key.
380 *
381 * <p>More formally, if this map contains a mapping from a key
382 * {@code k} to a value {@code v} such that {@code (key==null ? k==null :
383 * key.equals(k))}, then this method returns {@code v}; otherwise
384 * it returns {@code null}. (There can be at most one such mapping.)
385 *
386 * <p>A return value of {@code null} does not <i>necessarily</i>
387 * indicate that the map contains no mapping for the key; it's also
388 * possible that the map explicitly maps the key to {@code null}.
389 * The {@link #containsKey containsKey} operation may be used to
390 * distinguish these two cases.
391 *
392 * @see #put(Object, Object)
393 */
394 public V get(Object key) {
395 Object k = maskNull(key);
396 int h = hash(k);
397 Entry<K,V>[] tab = getTable();
398 int index = indexFor(h, tab.length);
399 Entry<K,V> e = tab[index];
400 while (e != null) {
401 if (e.hash == h && eq(k, e.get()))
402 return e.value;
403 e = e.next;
404 }
405 return null;
406 }
407
408 /**
409 * Returns {@code true} if this map contains a mapping for the
410 * specified key.
411 *
412 * @param key The key whose presence in this map is to be tested
413 * @return {@code true} if there is a mapping for {@code key};
414 * {@code false} otherwise
415 */
416 public boolean containsKey(Object key) {
417 return getEntry(key) != null;
418 }
419
420 /**
421 * Returns the entry associated with the specified key in this map.
422 * Returns null if the map contains no mapping for this key.
423 */
424 Entry<K,V> getEntry(Object key) {
425 Object k = maskNull(key);
426 int h = hash(k);
427 Entry<K,V>[] tab = getTable();
428 int index = indexFor(h, tab.length);
429 Entry<K,V> e = tab[index];
430 while (e != null && !(e.hash == h && eq(k, e.get())))
431 e = e.next;
432 return e;
433 }
434
435 /**
436 * Associates the specified value with the specified key in this map.
437 * If the map previously contained a mapping for this key, the old
438 * value is replaced.
439 *
440 * @param key key with which the specified value is to be associated.
441 * @param value value to be associated with the specified key.
442 * @return the previous value associated with {@code key}, or
443 * {@code null} if there was no mapping for {@code key}.
444 * (A {@code null} return can also indicate that the map
445 * previously associated {@code null} with {@code key}.)
446 */
447 public V put(K key, V value) {
448 Object k = maskNull(key);
449 int h = hash(k);
450 Entry<K,V>[] tab = getTable();
451 int i = indexFor(h, tab.length);
452
453 for (Entry<K,V> e = tab[i]; e != null; e = e.next) {
454 if (h == e.hash && eq(k, e.get())) {
455 V oldValue = e.value;
456 if (value != oldValue)
457 e.value = value;
458 return oldValue;
459 }
460 }
461
462 modCount++;
463 Entry<K,V> e = tab[i];
464 tab[i] = new Entry<>(k, value, queue, h, e);
465 if (++size >= threshold)
551 * By using the conservative calculation, we subject ourself
552 * to at most one extra resize.
553 */
554 if (numKeysToBeAdded > threshold) {
555 int targetCapacity = (int)(numKeysToBeAdded / loadFactor + 1);
556 if (targetCapacity > MAXIMUM_CAPACITY)
557 targetCapacity = MAXIMUM_CAPACITY;
558 int newCapacity = table.length;
559 while (newCapacity < targetCapacity)
560 newCapacity <<= 1;
561 if (newCapacity > table.length)
562 resize(newCapacity);
563 }
564
565 for (Map.Entry<? extends K, ? extends V> e : m.entrySet())
566 put(e.getKey(), e.getValue());
567 }
568
569 /**
570 * Removes the mapping for a key from this weak hash map if it is present.
571 * More formally, if this map contains a mapping from key {@code k} to
572 * value {@code v} such that <code>(key==null ? k==null :
573 * key.equals(k))</code>, that mapping is removed. (The map can contain
574 * at most one such mapping.)
575 *
576 * <p>Returns the value to which this map previously associated the key,
577 * or {@code null} if the map contained no mapping for the key. A
578 * return value of {@code null} does not <i>necessarily</i> indicate
579 * that the map contained no mapping for the key; it's also possible
580 * that the map explicitly mapped the key to {@code null}.
581 *
582 * <p>The map will not contain a mapping for the specified key once the
583 * call returns.
584 *
585 * @param key key whose mapping is to be removed from the map
586 * @return the previous value associated with {@code key}, or
587 * {@code null} if there was no mapping for {@code key}
588 */
589 public V remove(Object key) {
590 Object k = maskNull(key);
591 int h = hash(k);
592 Entry<K,V>[] tab = getTable();
593 int i = indexFor(h, tab.length);
594 Entry<K,V> prev = tab[i];
595 Entry<K,V> e = prev;
596
597 while (e != null) {
598 Entry<K,V> next = e.next;
599 if (h == e.hash && eq(k, e.get())) {
600 modCount++;
601 size--;
602 if (prev == e)
603 tab[i] = next;
604 else
605 prev.next = next;
606 return e.value;
607 }
647 * The map will be empty after this call returns.
648 */
649 public void clear() {
650 // clear out ref queue. We don't need to expunge entries
651 // since table is getting cleared.
652 while (queue.poll() != null)
653 ;
654
655 modCount++;
656 Arrays.fill(table, null);
657 size = 0;
658
659 // Allocation of array may have caused GC, which may have caused
660 // additional entries to go stale. Removing these entries from the
661 // reference queue will make them eligible for reclamation.
662 while (queue.poll() != null)
663 ;
664 }
665
666 /**
667 * Returns {@code true} if this map maps one or more keys to the
668 * specified value.
669 *
670 * @param value value whose presence in this map is to be tested
671 * @return {@code true} if this map maps one or more keys to the
672 * specified value
673 */
674 public boolean containsValue(Object value) {
675 if (value==null)
676 return containsNullValue();
677
678 Entry<K,V>[] tab = getTable();
679 for (int i = tab.length; i-- > 0;)
680 for (Entry<K,V> e = tab[i]; e != null; e = e.next)
681 if (value.equals(e.value))
682 return true;
683 return false;
684 }
685
686 /**
687 * Special-case code for containsValue with null argument
688 */
689 private boolean containsNullValue() {
690 Entry<K,V>[] tab = getTable();
691 for (int i = tab.length; i-- > 0;)
838 public K next() {
839 return nextEntry().getKey();
840 }
841 }
842
843 private class EntryIterator extends HashIterator<Map.Entry<K,V>> {
844 public Map.Entry<K,V> next() {
845 return nextEntry();
846 }
847 }
848
849 // Views
850
851 private transient Set<Map.Entry<K,V>> entrySet;
852
853 /**
854 * Returns a {@link Set} view of the keys contained in this map.
855 * The set is backed by the map, so changes to the map are
856 * reflected in the set, and vice-versa. If the map is modified
857 * while an iteration over the set is in progress (except through
858 * the iterator's own {@code remove} operation), the results of
859 * the iteration are undefined. The set supports element removal,
860 * which removes the corresponding mapping from the map, via the
861 * {@code Iterator.remove}, {@code Set.remove},
862 * {@code removeAll}, {@code retainAll}, and {@code clear}
863 * operations. It does not support the {@code add} or {@code addAll}
864 * operations.
865 */
866 public Set<K> keySet() {
867 Set<K> ks = keySet;
868 return (ks != null ? ks : (keySet = new KeySet()));
869 }
870
871 private class KeySet extends AbstractSet<K> {
872 public Iterator<K> iterator() {
873 return new KeyIterator();
874 }
875
876 public int size() {
877 return WeakHashMap.this.size();
878 }
879
880 public boolean contains(Object o) {
881 return containsKey(o);
882 }
883
887 return true;
888 }
889 else
890 return false;
891 }
892
893 public void clear() {
894 WeakHashMap.this.clear();
895 }
896
897 public Spliterator<K> spliterator() {
898 return new KeySpliterator<>(WeakHashMap.this, 0, -1, 0, 0);
899 }
900 }
901
902 /**
903 * Returns a {@link Collection} view of the values contained in this map.
904 * The collection is backed by the map, so changes to the map are
905 * reflected in the collection, and vice-versa. If the map is
906 * modified while an iteration over the collection is in progress
907 * (except through the iterator's own {@code remove} operation),
908 * the results of the iteration are undefined. The collection
909 * supports element removal, which removes the corresponding
910 * mapping from the map, via the {@code Iterator.remove},
911 * {@code Collection.remove}, {@code removeAll},
912 * {@code retainAll} and {@code clear} operations. It does not
913 * support the {@code add} or {@code addAll} operations.
914 */
915 public Collection<V> values() {
916 Collection<V> vs = values;
917 return (vs != null) ? vs : (values = new Values());
918 }
919
920 private class Values extends AbstractCollection<V> {
921 public Iterator<V> iterator() {
922 return new ValueIterator();
923 }
924
925 public int size() {
926 return WeakHashMap.this.size();
927 }
928
929 public boolean contains(Object o) {
930 return containsValue(o);
931 }
932
933 public void clear() {
934 WeakHashMap.this.clear();
935 }
936
937 public Spliterator<V> spliterator() {
938 return new ValueSpliterator<>(WeakHashMap.this, 0, -1, 0, 0);
939 }
940 }
941
942 /**
943 * Returns a {@link Set} view of the mappings contained in this map.
944 * The set is backed by the map, so changes to the map are
945 * reflected in the set, and vice-versa. If the map is modified
946 * while an iteration over the set is in progress (except through
947 * the iterator's own {@code remove} operation, or through the
948 * {@code setValue} operation on a map entry returned by the
949 * iterator) the results of the iteration are undefined. The set
950 * supports element removal, which removes the corresponding
951 * mapping from the map, via the {@code Iterator.remove},
952 * {@code Set.remove}, {@code removeAll}, {@code retainAll} and
953 * {@code clear} operations. It does not support the
954 * {@code add} or {@code addAll} operations.
955 */
956 public Set<Map.Entry<K,V>> entrySet() {
957 Set<Map.Entry<K,V>> es = entrySet;
958 return es != null ? es : (entrySet = new EntrySet());
959 }
960
961 private class EntrySet extends AbstractSet<Map.Entry<K,V>> {
962 public Iterator<Map.Entry<K,V>> iterator() {
963 return new EntryIterator();
964 }
965
966 public boolean contains(Object o) {
967 if (!(o instanceof Map.Entry))
968 return false;
969 Map.Entry<?,?> e = (Map.Entry<?,?>)o;
970 Entry<K,V> candidate = getEntry(e.getKey());
971 return candidate != null && candidate.equals(e);
972 }
973
974 public boolean remove(Object o) {
|