WeakHashMap.java revision 42
0N/A * Copyright 1998-2007 Sun Microsystems, Inc. All Rights Reserved. 0N/A * DO NOT ALTER OR REMOVE COPYRIGHT NOTICES OR THIS FILE HEADER. 0N/A * This code is free software; you can redistribute it and/or modify it 0N/A * under the terms of the GNU General Public License version 2 only, as 0N/A * published by the Free Software Foundation. Sun designates this 0N/A * particular file as subject to the "Classpath" exception as provided 0N/A * by Sun in the LICENSE file that accompanied this code. 0N/A * This code is distributed in the hope that it will be useful, but WITHOUT 0N/A * ANY WARRANTY; without even the implied warranty of MERCHANTABILITY or 0N/A * FITNESS FOR A PARTICULAR PURPOSE. See the GNU General Public License 0N/A * version 2 for more details (a copy is included in the LICENSE file that 0N/A * accompanied this code). 0N/A * You should have received a copy of the GNU General Public License version 0N/A * 2 along with this work; if not, write to the Free Software Foundation, 0N/A * Inc., 51 Franklin St, Fifth Floor, Boston, MA 02110-1301 USA. 0N/A * Please contact Sun Microsystems, Inc., 4150 Network Circle, Santa Clara, 0N/A * CA 95054 USA or visit www.sun.com if you need additional information or 0N/A * have any questions. * Hash table based implementation of the <tt>Map</tt> interface, with * An entry in a <tt>WeakHashMap</tt> will automatically be removed when * its key is no longer in ordinary use. More precisely, the presence of a * mapping for a given key will not prevent the key from being discarded by the * garbage collector, that is, made finalizable, finalized, and then reclaimed. * When a key has been discarded its entry is effectively removed from the map, * so this class behaves somewhat differently from other <tt>Map</tt> * <p> Both null values and the null key are supported. This class has * performance characteristics similar to those of the <tt>HashMap</tt> * class, and has the same efficiency parameters of <em>initial capacity</em> * and <em>load factor</em>. * <p> Like most collection classes, this class is not synchronized. * A synchronized <tt>WeakHashMap</tt> may be constructed using the * {@link Collections#synchronizedMap Collections.synchronizedMap} * <p> This class is intended primarily for use with key objects whose * <tt>equals</tt> methods test for object identity using the * <tt>==</tt> operator. Once such a key is discarded it can never be * recreated, so it is impossible to do a lookup of that key in a * <tt>WeakHashMap</tt> at some later time and be surprised that its entry * has been removed. This class will work perfectly well with key objects * whose <tt>equals</tt> methods are not based upon object identity, such * as <tt>String</tt> instances. With such recreatable key objects, * however, the automatic removal of <tt>WeakHashMap</tt> entries whose * keys have been discarded may prove to be confusing. * <p> The behavior of the <tt>WeakHashMap</tt> class depends in part upon * the actions of the garbage collector, so several familiar (though not * required) <tt>Map</tt> invariants do not hold for this class. Because * the garbage collector may discard keys at any time, a * <tt>WeakHashMap</tt> may behave as though an unknown thread is silently * removing entries. In particular, even if you synchronize on a * <tt>WeakHashMap</tt> instance and invoke none of its mutator methods, it * is possible for the <tt>size</tt> method to return smaller values over * time, for the <tt>isEmpty</tt> method to return <tt>false</tt> and * then <tt>true</tt>, for the <tt>containsKey</tt> method to return * <tt>true</tt> and later <tt>false</tt> for a given key, for the * <tt>get</tt> method to return a value for a given key but later return * <tt>null</tt>, for the <tt>put</tt> method to return * <tt>null</tt> and the <tt>remove</tt> method to return * <tt>false</tt> for a key that previously appeared to be in the map, and * for successive examinations of the key set, the value collection, and * the entry set to yield successively smaller numbers of elements. * <p> Each key object in a <tt>WeakHashMap</tt> is stored indirectly as * the referent of a weak reference. Therefore a key will automatically be * removed only after the weak references to it, both inside and outside of the * map, have been cleared by the garbage collector. * <p> <strong>Implementation note:</strong> The value objects in a * <tt>WeakHashMap</tt> are held by ordinary strong references. Thus care * should be taken to ensure that value objects do not strongly refer to their * own keys, either directly or indirectly, since that will prevent the keys * from being discarded. Note that a value object may refer indirectly to its * key via the <tt>WeakHashMap</tt> itself; that is, a value object may * strongly refer to some other key object whose associated value object, in * turn, strongly refers to the key of the first value object. One way * to deal with this is to wrap values themselves within * <tt>WeakReferences</tt> before * inserting, as in: <tt>m.put(key, new WeakReference(value))</tt>, * and then unwrapping upon each <tt>get</tt>. * <p>The iterators returned by the <tt>iterator</tt> method of the collections * returned by all of this class's "collection view methods" are * <i>fail-fast</i>: if the map is structurally modified at any time after the * iterator is created, in any way except through the iterator's own * <tt>remove</tt> method, the iterator will throw a {@link * ConcurrentModificationException}. Thus, in the face of concurrent * modification, the iterator fails quickly and cleanly, rather than risking * arbitrary, non-deterministic behavior at an undetermined time in the future. * <p>Note that the fail-fast behavior of an iterator cannot be guaranteed * as it is, generally speaking, impossible to make any hard guarantees in the * presence of unsynchronized concurrent modification. Fail-fast iterators * throw <tt>ConcurrentModificationException</tt> on a best-effort basis. * Therefore, it would be wrong to write a program that depended on this * exception for its correctness: <i>the fail-fast behavior of iterators * should be used only to detect bugs.</i> * <p>This class is a member of the * Java Collections Framework</a>. * @param <K> the type of keys maintained by this map * @param <V> the type of mapped values * @see java.lang.ref.WeakReference * The default initial capacity -- MUST be a power of two. * The maximum capacity, used if a higher value is implicitly specified * by either of the constructors with arguments. * MUST be a power of two <= 1<<30. * The load factor used when none specified in constructor. * The table, resized as necessary. Length MUST Always be a power of two. * The number of key-value mappings contained in this weak hash map. * The next size value at which to resize (capacity * load factor). * The load factor for the hash table. * Reference queue for cleared WeakEntries * The number of times this WeakHashMap has been structurally modified. * Structural modifications are those that change the number of * mappings in the map or otherwise modify its internal structure * (e.g., rehash). This field is used to make iterators on * Collection-views of the map fail-fast. * @see ConcurrentModificationException * Constructs a new, empty <tt>WeakHashMap</tt> with the given initial * capacity and the given load factor. * @param initialCapacity The initial capacity of the <tt>WeakHashMap</tt> * @param loadFactor The load factor of the <tt>WeakHashMap</tt> * @throws IllegalArgumentException if the initial capacity is negative, * or if the load factor is nonpositive. * Constructs a new, empty <tt>WeakHashMap</tt> with the given initial * capacity and the default load factor (0.75). * @param initialCapacity The initial capacity of the <tt>WeakHashMap</tt> * @throws IllegalArgumentException if the initial capacity is negative * Constructs a new, empty <tt>WeakHashMap</tt> with the default initial * capacity (16) and load factor (0.75). * Constructs a new <tt>WeakHashMap</tt> with the same mappings as the * specified map. The <tt>WeakHashMap</tt> is created with the default * load factor (0.75) and an initial capacity sufficient to hold the * mappings in the specified map. * @param m the map whose mappings are to be placed in this map * @throws NullPointerException if the specified map is null * Value representing null keys inside tables. * Use NULL_KEY for key if it is null. * Returns internal representation of null key back to caller as null. * Checks for equality of non-null reference x and possibly-null y. By * default uses Object.equals. * Returns index for hash code h. * Expunges stale entries from the table. // Must not null out e.next; // stale entries may be in use by a HashIterator * Returns the table after first expunging stale entries. * Returns the number of key-value mappings in this map. * This result is a snapshot, and may not reflect unprocessed * entries that will be removed before next attempted access * because they are no longer referenced. * Returns <tt>true</tt> if this map contains no key-value mappings. * This result is a snapshot, and may not reflect unprocessed * entries that will be removed before next attempted access * because they are no longer referenced. * Returns the value to which the specified key is mapped, * or {@code null} if this map contains no mapping for the key. * <p>More formally, if this map contains a mapping from a key * {@code k} to a value {@code v} such that {@code (key==null ? k==null : * key.equals(k))}, then this method returns {@code v}; otherwise * it returns {@code null}. (There can be at most one such mapping.) * <p>A return value of {@code null} does not <i>necessarily</i> * indicate that the map contains no mapping for the key; it's also * possible that the map explicitly maps the key to {@code null}. * The {@link #containsKey containsKey} operation may be used to * distinguish these two cases. * @see #put(Object, Object) * Returns <tt>true</tt> if this map contains a mapping for the * @param key The key whose presence in this map is to be tested * @return <tt>true</tt> if there is a mapping for <tt>key</tt>; * <tt>false</tt> otherwise * Returns the entry associated with the specified key in this map. * Returns null if the map contains no mapping for this key. * Associates the specified value with the specified key in this map. * If the map previously contained a mapping for this key, the old * @param key key with which the specified value is to be associated. * @param value value to be associated with the specified key. * @return the previous value associated with <tt>key</tt>, or * <tt>null</tt> if there was no mapping for <tt>key</tt>. * (A <tt>null</tt> return can also indicate that the map * previously associated <tt>null</tt> with <tt>key</tt>.) * Rehashes the contents of this map into a new array with a * larger capacity. This method is called automatically when the * number of keys in this map reaches its threshold. * If current capacity is MAXIMUM_CAPACITY, this method does not * resize the map, but sets threshold to Integer.MAX_VALUE. * This has the effect of preventing future calls. * @param newCapacity the new capacity, MUST be a power of two; * must be greater than current capacity unless current * capacity is MAXIMUM_CAPACITY (in which case value * If ignoring null elements and processing ref queue caused massive * shrinkage, then restore old table. This should be rare, but avoids * unbounded expansion of garbage-filled tables. /** Transfers all entries from src to dest tables */ * Copies all of the mappings from the specified map to this map. * These mappings will replace any mappings that this map had for any * of the keys currently in the specified map. * @param m mappings to be stored in this map. * @throws NullPointerException if the specified map is null. public void putAll(
Map<?
extends K, ?
extends V> m) {
* Expand the map if the map if the number of mappings to be added * is greater than or equal to threshold. This is conservative; the * obvious condition is (m.size() + size) >= threshold, but this * condition could result in a map with twice the appropriate capacity, * if the keys to be added overlap with the keys already in this map. * By using the conservative calculation, we subject ourself * to at most one extra resize. * Removes the mapping for a key from this weak hash map if it is present. * More formally, if this map contains a mapping from key <tt>k</tt> to * value <tt>v</tt> such that <code>(key==null ? k==null : * key.equals(k))</code>, that mapping is removed. (The map can contain * at most one such mapping.) * <p>Returns the value to which this map previously associated the key, * or <tt>null</tt> if the map contained no mapping for the key. A * return value of <tt>null</tt> does not <i>necessarily</i> indicate * that the map contained no mapping for the key; it's also possible * that the map explicitly mapped the key to <tt>null</tt>. * <p>The map will not contain a mapping for the specified key once the * @param key key whose mapping is to be removed from the map * @return the previous value associated with <tt>key</tt>, or * <tt>null</tt> if there was no mapping for <tt>key</tt> /** Special version of remove needed by Entry set */ * Removes all of the mappings from this map. * The map will be empty after this call returns. // clear out ref queue. We don't need to expunge entries // since table is getting cleared. // Allocation of array may have caused GC, which may have caused // additional entries to go stale. Removing these entries from the // reference queue will make them eligible for reclamation. * Returns <tt>true</tt> if this map maps one or more keys to the * @param value value whose presence in this map is to be tested * @return <tt>true</tt> if this map maps one or more keys to the * Special-case code for containsValue with null argument * The entries in this hash table extend WeakReference, using its main ref * Strong reference needed to avoid disappearance of key * between hasNext and next * Strong reference needed to avoid disappearance of key * between nextEntry() and any use of the entry while (e ==
null && i >
0)
nextKey = e.
get();
// hold on to key in strong ref /** The common parts of next() across different types of iterators */ * Returns a {@link Set} view of the keys contained in this map. * The set is backed by the map, so changes to the map are * reflected in the set, and vice-versa. If the map is modified * while an iteration over the set is in progress (except through * the iterator's own <tt>remove</tt> operation), the results of * the iteration are undefined. The set supports element removal, * which removes the corresponding mapping from the map, via the * <tt>Iterator.remove</tt>, <tt>Set.remove</tt>, * <tt>removeAll</tt>, <tt>retainAll</tt>, and <tt>clear</tt> * operations. It does not support the <tt>add</tt> or <tt>addAll</tt> * Returns a {@link Collection} view of the values contained in this map. * The collection is backed by the map, so changes to the map are * reflected in the collection, and vice-versa. If the map is * modified while an iteration over the collection is in progress * (except through the iterator's own <tt>remove</tt> operation), * the results of the iteration are undefined. The collection * supports element removal, which removes the corresponding * mapping from the map, via the <tt>Iterator.remove</tt>, * <tt>Collection.remove</tt>, <tt>removeAll</tt>, * <tt>retainAll</tt> and <tt>clear</tt> operations. It does not * support the <tt>add</tt> or <tt>addAll</tt> operations. * Returns a {@link Set} view of the mappings contained in this map. * The set is backed by the map, so changes to the map are * reflected in the set, and vice-versa. If the map is modified * while an iteration over the set is in progress (except through * the iterator's own <tt>remove</tt> operation, or through the * <tt>setValue</tt> operation on a map entry returned by the * iterator) the results of the iteration are undefined. The set * supports element removal, which removes the corresponding * mapping from the map, via the <tt>Iterator.remove</tt>, * <tt>Set.remove</tt>, <tt>removeAll</tt>, <tt>retainAll</tt> and * <tt>clear</tt> operations. It does not support the * <tt>add</tt> or <tt>addAll</tt> operations.