WeakHashMap.java revision 0976dc2e109a3ca2bd977d18eee74e4b7c9ada30
1/* 2 * Copyright (C) 2014 The Android Open Source Project 3 * Copyright (c) 1998, 2013, Oracle and/or its affiliates. All rights reserved. 4 * DO NOT ALTER OR REMOVE COPYRIGHT NOTICES OR THIS FILE HEADER. 5 * 6 * This code is free software; you can redistribute it and/or modify it 7 * under the terms of the GNU General Public License version 2 only, as 8 * published by the Free Software Foundation. Oracle designates this 9 * particular file as subject to the "Classpath" exception as provided 10 * by Oracle in the LICENSE file that accompanied this code. 11 * 12 * This code is distributed in the hope that it will be useful, but WITHOUT 13 * ANY WARRANTY; without even the implied warranty of MERCHANTABILITY or 14 * FITNESS FOR A PARTICULAR PURPOSE. See the GNU General Public License 15 * version 2 for more details (a copy is included in the LICENSE file that 16 * accompanied this code). 17 * 18 * You should have received a copy of the GNU General Public License version 19 * 2 along with this work; if not, write to the Free Software Foundation, 20 * Inc., 51 Franklin St, Fifth Floor, Boston, MA 02110-1301 USA. 21 * 22 * Please contact Oracle, 500 Oracle Parkway, Redwood Shores, CA 94065 USA 23 * or visit www.oracle.com if you need additional information or have any 24 * questions. 25 */ 26 27package java.util; 28import java.lang.ref.WeakReference; 29import java.lang.ref.ReferenceQueue; 30import java.util.function.BiConsumer; 31import java.util.function.BiFunction; 32import java.util.function.Consumer; 33 34 35/** 36 * Hash table based implementation of the <tt>Map</tt> interface, with 37 * <em>weak keys</em>. 38 * An entry in a <tt>WeakHashMap</tt> will automatically be removed when 39 * its key is no longer in ordinary use. More precisely, the presence of a 40 * mapping for a given key will not prevent the key from being discarded by the 41 * garbage collector, that is, made finalizable, finalized, and then reclaimed. 42 * When a key has been discarded its entry is effectively removed from the map, 43 * so this class behaves somewhat differently from other <tt>Map</tt> 44 * implementations. 45 * 46 * <p> Both null values and the null key are supported. This class has 47 * performance characteristics similar to those of the <tt>HashMap</tt> 48 * class, and has the same efficiency parameters of <em>initial capacity</em> 49 * and <em>load factor</em>. 50 * 51 * <p> Like most collection classes, this class is not synchronized. 52 * A synchronized <tt>WeakHashMap</tt> may be constructed using the 53 * {@link Collections#synchronizedMap Collections.synchronizedMap} 54 * method. 55 * 56 * <p> This class is intended primarily for use with key objects whose 57 * <tt>equals</tt> methods test for object identity using the 58 * <tt>==</tt> operator. Once such a key is discarded it can never be 59 * recreated, so it is impossible to do a lookup of that key in a 60 * <tt>WeakHashMap</tt> at some later time and be surprised that its entry 61 * has been removed. This class will work perfectly well with key objects 62 * whose <tt>equals</tt> methods are not based upon object identity, such 63 * as <tt>String</tt> instances. With such recreatable key objects, 64 * however, the automatic removal of <tt>WeakHashMap</tt> entries whose 65 * keys have been discarded may prove to be confusing. 66 * 67 * <p> The behavior of the <tt>WeakHashMap</tt> class depends in part upon 68 * the actions of the garbage collector, so several familiar (though not 69 * required) <tt>Map</tt> invariants do not hold for this class. Because 70 * the garbage collector may discard keys at any time, a 71 * <tt>WeakHashMap</tt> may behave as though an unknown thread is silently 72 * removing entries. In particular, even if you synchronize on a 73 * <tt>WeakHashMap</tt> instance and invoke none of its mutator methods, it 74 * is possible for the <tt>size</tt> method to return smaller values over 75 * time, for the <tt>isEmpty</tt> method to return <tt>false</tt> and 76 * then <tt>true</tt>, for the <tt>containsKey</tt> method to return 77 * <tt>true</tt> and later <tt>false</tt> for a given key, for the 78 * <tt>get</tt> method to return a value for a given key but later return 79 * <tt>null</tt>, for the <tt>put</tt> method to return 80 * <tt>null</tt> and the <tt>remove</tt> method to return 81 * <tt>false</tt> for a key that previously appeared to be in the map, and 82 * for successive examinations of the key set, the value collection, and 83 * the entry set to yield successively smaller numbers of elements. 84 * 85 * <p> Each key object in a <tt>WeakHashMap</tt> is stored indirectly as 86 * the referent of a weak reference. Therefore a key will automatically be 87 * removed only after the weak references to it, both inside and outside of the 88 * map, have been cleared by the garbage collector. 89 * 90 * <p> <strong>Implementation note:</strong> The value objects in a 91 * <tt>WeakHashMap</tt> are held by ordinary strong references. Thus care 92 * should be taken to ensure that value objects do not strongly refer to their 93 * own keys, either directly or indirectly, since that will prevent the keys 94 * from being discarded. Note that a value object may refer indirectly to its 95 * key via the <tt>WeakHashMap</tt> itself; that is, a value object may 96 * strongly refer to some other key object whose associated value object, in 97 * turn, strongly refers to the key of the first value object. If the values 98 * in the map do not rely on the map holding strong references to them, one way 99 * to deal with this is to wrap values themselves within 100 * <tt>WeakReferences</tt> before 101 * inserting, as in: <tt>m.put(key, new WeakReference(value))</tt>, 102 * and then unwrapping upon each <tt>get</tt>. 103 * 104 * <p>The iterators returned by the <tt>iterator</tt> method of the collections 105 * returned by all of this class's "collection view methods" are 106 * <i>fail-fast</i>: if the map is structurally modified at any time after the 107 * iterator is created, in any way except through the iterator's own 108 * <tt>remove</tt> method, the iterator will throw a {@link 109 * ConcurrentModificationException}. Thus, in the face of concurrent 110 * modification, the iterator fails quickly and cleanly, rather than risking 111 * arbitrary, non-deterministic behavior at an undetermined time in the future. 112 * 113 * <p>Note that the fail-fast behavior of an iterator cannot be guaranteed 114 * as it is, generally speaking, impossible to make any hard guarantees in the 115 * presence of unsynchronized concurrent modification. Fail-fast iterators 116 * throw <tt>ConcurrentModificationException</tt> on a best-effort basis. 117 * Therefore, it would be wrong to write a program that depended on this 118 * exception for its correctness: <i>the fail-fast behavior of iterators 119 * should be used only to detect bugs.</i> 120 * 121 * <p>This class is a member of the 122 * <a href="{@docRoot}/../technotes/guides/collections/index.html"> 123 * Java Collections Framework</a>. 124 * 125 * @param <K> the type of keys maintained by this map 126 * @param <V> the type of mapped values 127 * 128 * @author Doug Lea 129 * @author Josh Bloch 130 * @author Mark Reinhold 131 * @since 1.2 132 * @see java.util.HashMap 133 * @see java.lang.ref.WeakReference 134 */ 135public class WeakHashMap<K,V> 136 extends AbstractMap<K,V> 137 implements Map<K,V> { 138 139 /** 140 * The default initial capacity -- MUST be a power of two. 141 */ 142 private static final int DEFAULT_INITIAL_CAPACITY = 16; 143 144 /** 145 * The maximum capacity, used if a higher value is implicitly specified 146 * by either of the constructors with arguments. 147 * MUST be a power of two <= 1<<30. 148 */ 149 private static final int MAXIMUM_CAPACITY = 1 << 30; 150 151 /** 152 * The load factor used when none specified in constructor. 153 */ 154 private static final float DEFAULT_LOAD_FACTOR = 0.75f; 155 156 /** 157 * The table, resized as necessary. Length MUST Always be a power of two. 158 */ 159 Entry<K,V>[] table; 160 161 /** 162 * The number of key-value mappings contained in this weak hash map. 163 */ 164 private int size; 165 166 /** 167 * The next size value at which to resize (capacity * load factor). 168 */ 169 private int threshold; 170 171 /** 172 * The load factor for the hash table. 173 */ 174 private final float loadFactor; 175 176 /** 177 * Reference queue for cleared WeakEntries 178 */ 179 private final ReferenceQueue<Object> queue = new ReferenceQueue<>(); 180 181 /** 182 * The number of times this WeakHashMap has been structurally modified. 183 * Structural modifications are those that change the number of 184 * mappings in the map or otherwise modify its internal structure 185 * (e.g., rehash). This field is used to make iterators on 186 * Collection-views of the map fail-fast. 187 * 188 * @see ConcurrentModificationException 189 */ 190 int modCount; 191 192 /** 193 * The default threshold of map capacity above which alternative hashing is 194 * used for String keys. Alternative hashing reduces the incidence of 195 * collisions due to weak hash code calculation for String keys. 196 * <p/> 197 * This value may be overridden by defining the system property 198 * {@code jdk.map.althashing.threshold}. A property value of {@code 1} 199 * forces alternative hashing to be used at all times whereas 200 * {@code -1} value ensures that alternative hashing is never used. 201 */ 202 static final int ALTERNATIVE_HASHING_THRESHOLD_DEFAULT = Integer.MAX_VALUE; 203 204 /** 205 * holds values which can't be initialized until after VM is booted. 206 */ 207 private static class Holder { 208 209 /** 210 * Table capacity above which to switch to use alternative hashing. 211 */ 212 static final int ALTERNATIVE_HASHING_THRESHOLD; 213 214 static { 215 String altThreshold = java.security.AccessController.doPrivileged( 216 new sun.security.action.GetPropertyAction( 217 "jdk.map.althashing.threshold")); 218 219 int threshold; 220 try { 221 threshold = (null != altThreshold) 222 ? Integer.parseInt(altThreshold) 223 : ALTERNATIVE_HASHING_THRESHOLD_DEFAULT; 224 225 // disable alternative hashing if -1 226 if (threshold == -1) { 227 threshold = Integer.MAX_VALUE; 228 } 229 230 if (threshold < 0) { 231 throw new IllegalArgumentException("value must be positive integer."); 232 } 233 } catch(IllegalArgumentException failed) { 234 throw new Error("Illegal value for 'jdk.map.althashing.threshold'", failed); 235 } 236 ALTERNATIVE_HASHING_THRESHOLD = threshold; 237 } 238 } 239 240 /** 241 * If {@code true} then perform alternate hashing to reduce the incidence of 242 * collisions due to weak hash code calculation. 243 */ 244 transient boolean useAltHashing; 245 246 /** 247 * A randomizing value associated with this instance that is applied to 248 * hash code of keys to make hash collisions harder to find. 249 * 250 * This hash seed is only used if {@code useAltHashing} is true. 251 */ 252 transient int hashSeed; 253 254 @SuppressWarnings("unchecked") 255 private Entry<K,V>[] newTable(int n) { 256 return (Entry<K,V>[]) new Entry[n]; 257 } 258 259 /** 260 * Constructs a new, empty <tt>WeakHashMap</tt> with the given initial 261 * capacity and the given load factor. 262 * 263 * @param initialCapacity The initial capacity of the <tt>WeakHashMap</tt> 264 * @param loadFactor The load factor of the <tt>WeakHashMap</tt> 265 * @throws IllegalArgumentException if the initial capacity is negative, 266 * or if the load factor is nonpositive. 267 */ 268 public WeakHashMap(int initialCapacity, float loadFactor) { 269 if (initialCapacity < 0) 270 throw new IllegalArgumentException("Illegal Initial Capacity: "+ 271 initialCapacity); 272 if (initialCapacity > MAXIMUM_CAPACITY) 273 initialCapacity = MAXIMUM_CAPACITY; 274 275 if (loadFactor <= 0 || Float.isNaN(loadFactor)) 276 throw new IllegalArgumentException("Illegal Load factor: "+ 277 loadFactor); 278 int capacity = 1; 279 while (capacity < initialCapacity) 280 capacity <<= 1; 281 table = newTable(capacity); 282 this.loadFactor = loadFactor; 283 threshold = (int)(capacity * loadFactor); 284 useAltHashing = sun.misc.VM.isBooted() && 285 (capacity >= Holder.ALTERNATIVE_HASHING_THRESHOLD); 286 if (useAltHashing) { 287 hashSeed = sun.misc.Hashing.randomHashSeed(this); 288 } else { 289 hashSeed = 0; 290 } 291 } 292 293 /** 294 * Constructs a new, empty <tt>WeakHashMap</tt> with the given initial 295 * capacity and the default load factor (0.75). 296 * 297 * @param initialCapacity The initial capacity of the <tt>WeakHashMap</tt> 298 * @throws IllegalArgumentException if the initial capacity is negative 299 */ 300 public WeakHashMap(int initialCapacity) { 301 this(initialCapacity, DEFAULT_LOAD_FACTOR); 302 } 303 304 /** 305 * Constructs a new, empty <tt>WeakHashMap</tt> with the default initial 306 * capacity (16) and load factor (0.75). 307 */ 308 public WeakHashMap() { 309 this(DEFAULT_INITIAL_CAPACITY, DEFAULT_LOAD_FACTOR); 310 } 311 312 /** 313 * Constructs a new <tt>WeakHashMap</tt> with the same mappings as the 314 * specified map. The <tt>WeakHashMap</tt> is created with the default 315 * load factor (0.75) and an initial capacity sufficient to hold the 316 * mappings in the specified map. 317 * 318 * @param m the map whose mappings are to be placed in this map 319 * @throws NullPointerException if the specified map is null 320 * @since 1.3 321 */ 322 public WeakHashMap(Map<? extends K, ? extends V> m) { 323 this(Math.max((int) (m.size() / DEFAULT_LOAD_FACTOR) + 1, 324 DEFAULT_INITIAL_CAPACITY), 325 DEFAULT_LOAD_FACTOR); 326 putAll(m); 327 } 328 329 // internal utilities 330 331 /** 332 * Value representing null keys inside tables. 333 */ 334 private static final Object NULL_KEY = new Object(); 335 336 /** 337 * Use NULL_KEY for key if it is null. 338 */ 339 private static Object maskNull(Object key) { 340 return (key == null) ? NULL_KEY : key; 341 } 342 343 /** 344 * Returns internal representation of null key back to caller as null. 345 */ 346 static Object unmaskNull(Object key) { 347 return (key == NULL_KEY) ? null : key; 348 } 349 350 /** 351 * Checks for equality of non-null reference x and possibly-null y. By 352 * default uses Object.equals. 353 */ 354 private static boolean eq(Object x, Object y) { 355 return x == y || x.equals(y); 356 } 357 358 /** 359 * Retrieve object hash code and applies a supplemental hash function to the 360 * result hash, which defends against poor quality hash functions. This is 361 * critical because HashMap uses power-of-two length hash tables, that 362 * otherwise encounter collisions for hashCodes that do not differ 363 * in lower bits. 364 */ 365 int hash(Object k) { 366 367 int h; 368 if (useAltHashing) { 369 h = hashSeed; 370 if (k instanceof String) { 371 return sun.misc.Hashing.stringHash32((String) k); 372 } else { 373 h ^= k.hashCode(); 374 } 375 } else { 376 h = k.hashCode(); 377 } 378 379 // This function ensures that hashCodes that differ only by 380 // constant multiples at each bit position have a bounded 381 // number of collisions (approximately 8 at default load factor). 382 h ^= (h >>> 20) ^ (h >>> 12); 383 return h ^ (h >>> 7) ^ (h >>> 4); 384 } 385 386 /** 387 * Returns index for hash code h. 388 */ 389 private static int indexFor(int h, int length) { 390 return h & (length-1); 391 } 392 393 /** 394 * Expunges stale entries from the table. 395 */ 396 private void expungeStaleEntries() { 397 for (Object x; (x = queue.poll()) != null; ) { 398 synchronized (queue) { 399 @SuppressWarnings("unchecked") 400 Entry<K,V> e = (Entry<K,V>) x; 401 int i = indexFor(e.hash, table.length); 402 403 Entry<K,V> prev = table[i]; 404 Entry<K,V> p = prev; 405 while (p != null) { 406 Entry<K,V> next = p.next; 407 if (p == e) { 408 if (prev == e) 409 table[i] = next; 410 else 411 prev.next = next; 412 // Must not null out e.next; 413 // stale entries may be in use by a HashIterator 414 e.value = null; // Help GC 415 size--; 416 break; 417 } 418 prev = p; 419 p = next; 420 } 421 } 422 } 423 } 424 425 /** 426 * Returns the table after first expunging stale entries. 427 */ 428 private Entry<K,V>[] getTable() { 429 expungeStaleEntries(); 430 return table; 431 } 432 433 /** 434 * Returns the number of key-value mappings in this map. 435 * This result is a snapshot, and may not reflect unprocessed 436 * entries that will be removed before next attempted access 437 * because they are no longer referenced. 438 */ 439 public int size() { 440 if (size == 0) 441 return 0; 442 expungeStaleEntries(); 443 return size; 444 } 445 446 /** 447 * Returns <tt>true</tt> if this map contains no key-value mappings. 448 * This result is a snapshot, and may not reflect unprocessed 449 * entries that will be removed before next attempted access 450 * because they are no longer referenced. 451 */ 452 public boolean isEmpty() { 453 return size() == 0; 454 } 455 456 /** 457 * Returns the value to which the specified key is mapped, 458 * or {@code null} if this map contains no mapping for the key. 459 * 460 * <p>More formally, if this map contains a mapping from a key 461 * {@code k} to a value {@code v} such that {@code (key==null ? k==null : 462 * key.equals(k))}, then this method returns {@code v}; otherwise 463 * it returns {@code null}. (There can be at most one such mapping.) 464 * 465 * <p>A return value of {@code null} does not <i>necessarily</i> 466 * indicate that the map contains no mapping for the key; it's also 467 * possible that the map explicitly maps the key to {@code null}. 468 * The {@link #containsKey containsKey} operation may be used to 469 * distinguish these two cases. 470 * 471 * @see #put(Object, Object) 472 */ 473 public V get(Object key) { 474 Object k = maskNull(key); 475 int h = hash(k); 476 Entry<K,V>[] tab = getTable(); 477 int index = indexFor(h, tab.length); 478 Entry<K,V> e = tab[index]; 479 while (e != null) { 480 if (e.hash == h && eq(k, e.get())) 481 return e.value; 482 e = e.next; 483 } 484 return null; 485 } 486 487 /** 488 * Returns <tt>true</tt> if this map contains a mapping for the 489 * specified key. 490 * 491 * @param key The key whose presence in this map is to be tested 492 * @return <tt>true</tt> if there is a mapping for <tt>key</tt>; 493 * <tt>false</tt> otherwise 494 */ 495 public boolean containsKey(Object key) { 496 return getEntry(key) != null; 497 } 498 499 /** 500 * Returns the entry associated with the specified key in this map. 501 * Returns null if the map contains no mapping for this key. 502 */ 503 Entry<K,V> getEntry(Object key) { 504 Object k = maskNull(key); 505 int h = hash(k); 506 Entry<K,V>[] tab = getTable(); 507 int index = indexFor(h, tab.length); 508 Entry<K,V> e = tab[index]; 509 while (e != null && !(e.hash == h && eq(k, e.get()))) 510 e = e.next; 511 return e; 512 } 513 514 /** 515 * Associates the specified value with the specified key in this map. 516 * If the map previously contained a mapping for this key, the old 517 * value is replaced. 518 * 519 * @param key key with which the specified value is to be associated. 520 * @param value value to be associated with the specified key. 521 * @return the previous value associated with <tt>key</tt>, or 522 * <tt>null</tt> if there was no mapping for <tt>key</tt>. 523 * (A <tt>null</tt> return can also indicate that the map 524 * previously associated <tt>null</tt> with <tt>key</tt>.) 525 */ 526 public V put(K key, V value) { 527 Object k = maskNull(key); 528 int h = hash(k); 529 Entry<K,V>[] tab = getTable(); 530 int i = indexFor(h, tab.length); 531 532 for (Entry<K,V> e = tab[i]; e != null; e = e.next) { 533 if (h == e.hash && eq(k, e.get())) { 534 V oldValue = e.value; 535 if (value != oldValue) 536 e.value = value; 537 return oldValue; 538 } 539 } 540 541 modCount++; 542 Entry<K,V> e = tab[i]; 543 tab[i] = new Entry<>(k, value, queue, h, e); 544 if (++size >= threshold) 545 resize(tab.length * 2); 546 return null; 547 } 548 549 /** 550 * Rehashes the contents of this map into a new array with a 551 * larger capacity. This method is called automatically when the 552 * number of keys in this map reaches its threshold. 553 * 554 * If current capacity is MAXIMUM_CAPACITY, this method does not 555 * resize the map, but sets threshold to Integer.MAX_VALUE. 556 * This has the effect of preventing future calls. 557 * 558 * @param newCapacity the new capacity, MUST be a power of two; 559 * must be greater than current capacity unless current 560 * capacity is MAXIMUM_CAPACITY (in which case value 561 * is irrelevant). 562 */ 563 void resize(int newCapacity) { 564 Entry<K,V>[] oldTable = getTable(); 565 int oldCapacity = oldTable.length; 566 if (oldCapacity == MAXIMUM_CAPACITY) { 567 threshold = Integer.MAX_VALUE; 568 return; 569 } 570 571 Entry<K,V>[] newTable = newTable(newCapacity); 572 boolean oldAltHashing = useAltHashing; 573 useAltHashing |= sun.misc.VM.isBooted() && 574 (newCapacity >= Holder.ALTERNATIVE_HASHING_THRESHOLD); 575 boolean rehash = oldAltHashing ^ useAltHashing; 576 if (rehash) { 577 hashSeed = sun.misc.Hashing.randomHashSeed(this); 578 } 579 transfer(oldTable, newTable, rehash); 580 table = newTable; 581 582 /* 583 * If ignoring null elements and processing ref queue caused massive 584 * shrinkage, then restore old table. This should be rare, but avoids 585 * unbounded expansion of garbage-filled tables. 586 */ 587 if (size >= threshold / 2) { 588 threshold = (int)(newCapacity * loadFactor); 589 } else { 590 expungeStaleEntries(); 591 transfer(newTable, oldTable, false); 592 table = oldTable; 593 } 594 } 595 596 /** Transfers all entries from src to dest tables */ 597 private void transfer(Entry<K,V>[] src, Entry<K,V>[] dest, boolean rehash) { 598 for (int j = 0; j < src.length; ++j) { 599 Entry<K,V> e = src[j]; 600 src[j] = null; 601 while (e != null) { 602 Entry<K,V> next = e.next; 603 Object key = e.get(); 604 if (key == null) { 605 e.next = null; // Help GC 606 e.value = null; // " " 607 size--; 608 } else { 609 if (rehash) { 610 e.hash = hash(key); 611 } 612 int i = indexFor(e.hash, dest.length); 613 e.next = dest[i]; 614 dest[i] = e; 615 } 616 e = next; 617 } 618 } 619 } 620 621 /** 622 * Copies all of the mappings from the specified map to this map. 623 * These mappings will replace any mappings that this map had for any 624 * of the keys currently in the specified map. 625 * 626 * @param m mappings to be stored in this map. 627 * @throws NullPointerException if the specified map is null. 628 */ 629 public void putAll(Map<? extends K, ? extends V> m) { 630 int numKeysToBeAdded = m.size(); 631 if (numKeysToBeAdded == 0) 632 return; 633 634 /* 635 * Expand the map if the map if the number of mappings to be added 636 * is greater than or equal to threshold. This is conservative; the 637 * obvious condition is (m.size() + size) >= threshold, but this 638 * condition could result in a map with twice the appropriate capacity, 639 * if the keys to be added overlap with the keys already in this map. 640 * By using the conservative calculation, we subject ourself 641 * to at most one extra resize. 642 */ 643 if (numKeysToBeAdded > threshold) { 644 int targetCapacity = (int)(numKeysToBeAdded / loadFactor + 1); 645 if (targetCapacity > MAXIMUM_CAPACITY) 646 targetCapacity = MAXIMUM_CAPACITY; 647 int newCapacity = table.length; 648 while (newCapacity < targetCapacity) 649 newCapacity <<= 1; 650 if (newCapacity > table.length) 651 resize(newCapacity); 652 } 653 654 for (Map.Entry<? extends K, ? extends V> e : m.entrySet()) 655 put(e.getKey(), e.getValue()); 656 } 657 658 /** 659 * Removes the mapping for a key from this weak hash map if it is present. 660 * More formally, if this map contains a mapping from key <tt>k</tt> to 661 * value <tt>v</tt> such that <code>(key==null ? k==null : 662 * key.equals(k))</code>, that mapping is removed. (The map can contain 663 * at most one such mapping.) 664 * 665 * <p>Returns the value to which this map previously associated the key, 666 * or <tt>null</tt> if the map contained no mapping for the key. A 667 * return value of <tt>null</tt> does not <i>necessarily</i> indicate 668 * that the map contained no mapping for the key; it's also possible 669 * that the map explicitly mapped the key to <tt>null</tt>. 670 * 671 * <p>The map will not contain a mapping for the specified key once the 672 * call returns. 673 * 674 * @param key key whose mapping is to be removed from the map 675 * @return the previous value associated with <tt>key</tt>, or 676 * <tt>null</tt> if there was no mapping for <tt>key</tt> 677 */ 678 public V remove(Object key) { 679 Object k = maskNull(key); 680 int h = hash(k); 681 Entry<K,V>[] tab = getTable(); 682 int i = indexFor(h, tab.length); 683 Entry<K,V> prev = tab[i]; 684 Entry<K,V> e = prev; 685 686 while (e != null) { 687 Entry<K,V> next = e.next; 688 if (h == e.hash && eq(k, e.get())) { 689 modCount++; 690 size--; 691 if (prev == e) 692 tab[i] = next; 693 else 694 prev.next = next; 695 return e.value; 696 } 697 prev = e; 698 e = next; 699 } 700 701 return null; 702 } 703 704 /** Special version of remove needed by Entry set */ 705 boolean removeMapping(Object o) { 706 if (!(o instanceof Map.Entry)) 707 return false; 708 Entry<K,V>[] tab = getTable(); 709 Map.Entry<?,?> entry = (Map.Entry<?,?>)o; 710 Object k = maskNull(entry.getKey()); 711 int h = hash(k); 712 int i = indexFor(h, tab.length); 713 Entry<K,V> prev = tab[i]; 714 Entry<K,V> e = prev; 715 716 while (e != null) { 717 Entry<K,V> next = e.next; 718 if (h == e.hash && e.equals(entry)) { 719 modCount++; 720 size--; 721 if (prev == e) 722 tab[i] = next; 723 else 724 prev.next = next; 725 return true; 726 } 727 prev = e; 728 e = next; 729 } 730 731 return false; 732 } 733 734 /** 735 * Removes all of the mappings from this map. 736 * The map will be empty after this call returns. 737 */ 738 public void clear() { 739 // clear out ref queue. We don't need to expunge entries 740 // since table is getting cleared. 741 while (queue.poll() != null) 742 ; 743 744 modCount++; 745 Arrays.fill(table, null); 746 size = 0; 747 748 // Allocation of array may have caused GC, which may have caused 749 // additional entries to go stale. Removing these entries from the 750 // reference queue will make them eligible for reclamation. 751 while (queue.poll() != null) 752 ; 753 } 754 755 /** 756 * Returns <tt>true</tt> if this map maps one or more keys to the 757 * specified value. 758 * 759 * @param value value whose presence in this map is to be tested 760 * @return <tt>true</tt> if this map maps one or more keys to the 761 * specified value 762 */ 763 public boolean containsValue(Object value) { 764 if (value==null) 765 return containsNullValue(); 766 767 Entry<K,V>[] tab = getTable(); 768 for (int i = tab.length; i-- > 0;) 769 for (Entry<K,V> e = tab[i]; e != null; e = e.next) 770 if (value.equals(e.value)) 771 return true; 772 return false; 773 } 774 775 /** 776 * Special-case code for containsValue with null argument 777 */ 778 private boolean containsNullValue() { 779 Entry<K,V>[] tab = getTable(); 780 for (int i = tab.length; i-- > 0;) 781 for (Entry<K,V> e = tab[i]; e != null; e = e.next) 782 if (e.value==null) 783 return true; 784 return false; 785 } 786 787 /** 788 * The entries in this hash table extend WeakReference, using its main ref 789 * field as the key. 790 */ 791 private static class Entry<K,V> extends WeakReference<Object> implements Map.Entry<K,V> { 792 V value; 793 int hash; 794 Entry<K,V> next; 795 796 /** 797 * Creates new entry. 798 */ 799 Entry(Object key, V value, 800 ReferenceQueue<Object> queue, 801 int hash, Entry<K,V> next) { 802 super(key, queue); 803 this.value = value; 804 this.hash = hash; 805 this.next = next; 806 } 807 808 @SuppressWarnings("unchecked") 809 public K getKey() { 810 return (K) WeakHashMap.unmaskNull(get()); 811 } 812 813 public V getValue() { 814 return value; 815 } 816 817 public V setValue(V newValue) { 818 V oldValue = value; 819 value = newValue; 820 return oldValue; 821 } 822 823 public boolean equals(Object o) { 824 if (!(o instanceof Map.Entry)) 825 return false; 826 Map.Entry<?,?> e = (Map.Entry<?,?>)o; 827 K k1 = getKey(); 828 Object k2 = e.getKey(); 829 if (k1 == k2 || (k1 != null && k1.equals(k2))) { 830 V v1 = getValue(); 831 Object v2 = e.getValue(); 832 if (v1 == v2 || (v1 != null && v1.equals(v2))) 833 return true; 834 } 835 return false; 836 } 837 838 public int hashCode() { 839 K k = getKey(); 840 V v = getValue(); 841 return ((k==null ? 0 : k.hashCode()) ^ 842 (v==null ? 0 : v.hashCode())); 843 } 844 845 public String toString() { 846 return getKey() + "=" + getValue(); 847 } 848 } 849 850 private abstract class HashIterator<T> implements Iterator<T> { 851 private int index; 852 private Entry<K,V> entry = null; 853 private Entry<K,V> lastReturned = null; 854 private int expectedModCount = modCount; 855 856 /** 857 * Strong reference needed to avoid disappearance of key 858 * between hasNext and next 859 */ 860 private Object nextKey = null; 861 862 /** 863 * Strong reference needed to avoid disappearance of key 864 * between nextEntry() and any use of the entry 865 */ 866 private Object currentKey = null; 867 868 HashIterator() { 869 index = isEmpty() ? 0 : table.length; 870 } 871 872 public boolean hasNext() { 873 Entry<K,V>[] t = table; 874 875 while (nextKey == null) { 876 Entry<K,V> e = entry; 877 int i = index; 878 while (e == null && i > 0) 879 e = t[--i]; 880 entry = e; 881 index = i; 882 if (e == null) { 883 currentKey = null; 884 return false; 885 } 886 nextKey = e.get(); // hold on to key in strong ref 887 if (nextKey == null) 888 entry = entry.next; 889 } 890 return true; 891 } 892 893 /** The common parts of next() across different types of iterators */ 894 protected Entry<K,V> nextEntry() { 895 if (modCount != expectedModCount) 896 throw new ConcurrentModificationException(); 897 if (nextKey == null && !hasNext()) 898 throw new NoSuchElementException(); 899 900 lastReturned = entry; 901 entry = entry.next; 902 currentKey = nextKey; 903 nextKey = null; 904 return lastReturned; 905 } 906 907 public void remove() { 908 if (lastReturned == null) 909 throw new IllegalStateException(); 910 if (modCount != expectedModCount) 911 throw new ConcurrentModificationException(); 912 913 WeakHashMap.this.remove(currentKey); 914 expectedModCount = modCount; 915 lastReturned = null; 916 currentKey = null; 917 } 918 919 } 920 921 private class ValueIterator extends HashIterator<V> { 922 public V next() { 923 return nextEntry().value; 924 } 925 } 926 927 private class KeyIterator extends HashIterator<K> { 928 public K next() { 929 return nextEntry().getKey(); 930 } 931 } 932 933 private class EntryIterator extends HashIterator<Map.Entry<K,V>> { 934 public Map.Entry<K,V> next() { 935 return nextEntry(); 936 } 937 } 938 939 // Views 940 941 private transient Set<Map.Entry<K,V>> entrySet = null; 942 943 /** 944 * Returns a {@link Set} view of the keys contained in this map. 945 * The set is backed by the map, so changes to the map are 946 * reflected in the set, and vice-versa. If the map is modified 947 * while an iteration over the set is in progress (except through 948 * the iterator's own <tt>remove</tt> operation), the results of 949 * the iteration are undefined. The set supports element removal, 950 * which removes the corresponding mapping from the map, via the 951 * <tt>Iterator.remove</tt>, <tt>Set.remove</tt>, 952 * <tt>removeAll</tt>, <tt>retainAll</tt>, and <tt>clear</tt> 953 * operations. It does not support the <tt>add</tt> or <tt>addAll</tt> 954 * operations. 955 */ 956 public Set<K> keySet() { 957 Set<K> ks = keySet; 958 return (ks != null ? ks : (keySet = new KeySet())); 959 } 960 961 private class KeySet extends AbstractSet<K> { 962 public Iterator<K> iterator() { 963 return new KeyIterator(); 964 } 965 966 public int size() { 967 return WeakHashMap.this.size(); 968 } 969 970 public boolean contains(Object o) { 971 return containsKey(o); 972 } 973 974 public boolean remove(Object o) { 975 if (containsKey(o)) { 976 WeakHashMap.this.remove(o); 977 return true; 978 } 979 else 980 return false; 981 } 982 983 public void clear() { 984 WeakHashMap.this.clear(); 985 } 986 987 public Spliterator<K> spliterator() { 988 return new KeySpliterator<>(WeakHashMap.this, 0, -1, 0, 0); 989 } 990 } 991 992 /** 993 * Returns a {@link Collection} view of the values contained in this map. 994 * The collection is backed by the map, so changes to the map are 995 * reflected in the collection, and vice-versa. If the map is 996 * modified while an iteration over the collection is in progress 997 * (except through the iterator's own <tt>remove</tt> operation), 998 * the results of the iteration are undefined. The collection 999 * supports element removal, which removes the corresponding 1000 * mapping from the map, via the <tt>Iterator.remove</tt>, 1001 * <tt>Collection.remove</tt>, <tt>removeAll</tt>, 1002 * <tt>retainAll</tt> and <tt>clear</tt> operations. It does not 1003 * support the <tt>add</tt> or <tt>addAll</tt> operations. 1004 */ 1005 public Collection<V> values() { 1006 Collection<V> vs = values; 1007 return (vs != null) ? vs : (values = new Values()); 1008 } 1009 1010 private class Values extends AbstractCollection<V> { 1011 public Iterator<V> iterator() { 1012 return new ValueIterator(); 1013 } 1014 1015 public int size() { 1016 return WeakHashMap.this.size(); 1017 } 1018 1019 public boolean contains(Object o) { 1020 return containsValue(o); 1021 } 1022 1023 public void clear() { 1024 WeakHashMap.this.clear(); 1025 } 1026 1027 public Spliterator<V> spliterator() { 1028 return new ValueSpliterator<>(WeakHashMap.this, 0, -1, 0, 0); 1029 } 1030 } 1031 1032 /** 1033 * Returns a {@link Set} view of the mappings contained in this map. 1034 * The set is backed by the map, so changes to the map are 1035 * reflected in the set, and vice-versa. If the map is modified 1036 * while an iteration over the set is in progress (except through 1037 * the iterator's own <tt>remove</tt> operation, or through the 1038 * <tt>setValue</tt> operation on a map entry returned by the 1039 * iterator) the results of the iteration are undefined. The set 1040 * supports element removal, which removes the corresponding 1041 * mapping from the map, via the <tt>Iterator.remove</tt>, 1042 * <tt>Set.remove</tt>, <tt>removeAll</tt>, <tt>retainAll</tt> and 1043 * <tt>clear</tt> operations. It does not support the 1044 * <tt>add</tt> or <tt>addAll</tt> operations. 1045 */ 1046 public Set<Map.Entry<K,V>> entrySet() { 1047 Set<Map.Entry<K,V>> es = entrySet; 1048 return es != null ? es : (entrySet = new EntrySet()); 1049 } 1050 1051 private class EntrySet extends AbstractSet<Map.Entry<K,V>> { 1052 public Iterator<Map.Entry<K,V>> iterator() { 1053 return new EntryIterator(); 1054 } 1055 1056 public boolean contains(Object o) { 1057 if (!(o instanceof Map.Entry)) 1058 return false; 1059 Map.Entry<?,?> e = (Map.Entry<?,?>)o; 1060 Entry<K,V> candidate = getEntry(e.getKey()); 1061 return candidate != null && candidate.equals(e); 1062 } 1063 1064 public boolean remove(Object o) { 1065 return removeMapping(o); 1066 } 1067 1068 public int size() { 1069 return WeakHashMap.this.size(); 1070 } 1071 1072 public void clear() { 1073 WeakHashMap.this.clear(); 1074 } 1075 1076 private List<Map.Entry<K,V>> deepCopy() { 1077 List<Map.Entry<K,V>> list = new ArrayList<>(size()); 1078 for (Map.Entry<K,V> e : this) 1079 list.add(new AbstractMap.SimpleEntry<>(e)); 1080 return list; 1081 } 1082 1083 public Object[] toArray() { 1084 return deepCopy().toArray(); 1085 } 1086 1087 public <T> T[] toArray(T[] a) { 1088 return deepCopy().toArray(a); 1089 } 1090 1091 public Spliterator<Map.Entry<K,V>> spliterator() { 1092 return new EntrySpliterator<>(WeakHashMap.this, 0, -1, 0, 0); 1093 } 1094 } 1095 1096 @SuppressWarnings("unchecked") 1097 @Override 1098 public void forEach(BiConsumer<? super K, ? super V> action) { 1099 Objects.requireNonNull(action); 1100 int expectedModCount = modCount; 1101 1102 Entry<K, V>[] tab = getTable(); 1103 for (Entry<K, V> entry : tab) { 1104 while (entry != null) { 1105 Object key = entry.get(); 1106 if (key != null) { 1107 action.accept((K)WeakHashMap.unmaskNull(key), entry.value); 1108 } 1109 entry = entry.next; 1110 1111 if (expectedModCount != modCount) { 1112 throw new ConcurrentModificationException(); 1113 } 1114 } 1115 } 1116 } 1117 1118 @SuppressWarnings("unchecked") 1119 @Override 1120 public void replaceAll(BiFunction<? super K, ? super V, ? extends V> function) { 1121 Objects.requireNonNull(function); 1122 int expectedModCount = modCount; 1123 1124 Entry<K, V>[] tab = getTable(); 1125 for (Entry<K, V> entry : tab) { 1126 while (entry != null) { 1127 Object key = entry.get(); 1128 if (key != null) { 1129 entry.value = function.apply((K)WeakHashMap.unmaskNull(key), entry.value); 1130 } 1131 entry = entry.next; 1132 1133 if (expectedModCount != modCount) { 1134 throw new ConcurrentModificationException(); 1135 } 1136 } 1137 } 1138 } 1139 1140 1141 /** 1142 * Similar form as other hash Spliterators, but skips dead 1143 * elements. 1144 */ 1145 static class WeakHashMapSpliterator<K,V> { 1146 final WeakHashMap<K,V> map; 1147 WeakHashMap.Entry<K,V> current; // current node 1148 int index; // current index, modified on advance/split 1149 int fence; // -1 until first use; then one past last index 1150 int est; // size estimate 1151 int expectedModCount; // for comodification checks 1152 1153 WeakHashMapSpliterator(WeakHashMap<K,V> m, int origin, 1154 int fence, int est, 1155 int expectedModCount) { 1156 this.map = m; 1157 this.index = origin; 1158 this.fence = fence; 1159 this.est = est; 1160 this.expectedModCount = expectedModCount; 1161 } 1162 1163 final int getFence() { // initialize fence and size on first use 1164 int hi; 1165 if ((hi = fence) < 0) { 1166 WeakHashMap<K,V> m = map; 1167 est = m.size(); 1168 expectedModCount = m.modCount; 1169 hi = fence = m.table.length; 1170 } 1171 return hi; 1172 } 1173 1174 public final long estimateSize() { 1175 getFence(); // force init 1176 return (long) est; 1177 } 1178 } 1179 1180 static final class KeySpliterator<K,V> 1181 extends WeakHashMapSpliterator<K,V> 1182 implements Spliterator<K> { 1183 KeySpliterator(WeakHashMap<K,V> m, int origin, int fence, int est, 1184 int expectedModCount) { 1185 super(m, origin, fence, est, expectedModCount); 1186 } 1187 1188 public KeySpliterator<K,V> trySplit() { 1189 int hi = getFence(), lo = index, mid = (lo + hi) >>> 1; 1190 return (lo >= mid) ? null : 1191 new KeySpliterator<K,V>(map, lo, index = mid, est >>>= 1, 1192 expectedModCount); 1193 } 1194 1195 public void forEachRemaining(Consumer<? super K> action) { 1196 int i, hi, mc; 1197 if (action == null) 1198 throw new NullPointerException(); 1199 WeakHashMap<K,V> m = map; 1200 WeakHashMap.Entry<K,V>[] tab = m.table; 1201 if ((hi = fence) < 0) { 1202 mc = expectedModCount = m.modCount; 1203 hi = fence = tab.length; 1204 } 1205 else 1206 mc = expectedModCount; 1207 if (tab.length >= hi && (i = index) >= 0 && 1208 (i < (index = hi) || current != null)) { 1209 WeakHashMap.Entry<K,V> p = current; 1210 current = null; // exhaust 1211 do { 1212 if (p == null) 1213 p = tab[i++]; 1214 else { 1215 Object x = p.get(); 1216 p = p.next; 1217 if (x != null) { 1218 @SuppressWarnings("unchecked") K k = 1219 (K) WeakHashMap.unmaskNull(x); 1220 action.accept(k); 1221 } 1222 } 1223 } while (p != null || i < hi); 1224 } 1225 if (m.modCount != mc) 1226 throw new ConcurrentModificationException(); 1227 } 1228 1229 public boolean tryAdvance(Consumer<? super K> action) { 1230 int hi; 1231 if (action == null) 1232 throw new NullPointerException(); 1233 WeakHashMap.Entry<K,V>[] tab = map.table; 1234 if (tab.length >= (hi = getFence()) && index >= 0) { 1235 while (current != null || index < hi) { 1236 if (current == null) 1237 current = tab[index++]; 1238 else { 1239 Object x = current.get(); 1240 current = current.next; 1241 if (x != null) { 1242 @SuppressWarnings("unchecked") K k = 1243 (K) WeakHashMap.unmaskNull(x); 1244 action.accept(k); 1245 if (map.modCount != expectedModCount) 1246 throw new ConcurrentModificationException(); 1247 return true; 1248 } 1249 } 1250 } 1251 } 1252 return false; 1253 } 1254 1255 public int characteristics() { 1256 return Spliterator.DISTINCT; 1257 } 1258 } 1259 1260 static final class ValueSpliterator<K,V> 1261 extends WeakHashMapSpliterator<K,V> 1262 implements Spliterator<V> { 1263 ValueSpliterator(WeakHashMap<K,V> m, int origin, int fence, int est, 1264 int expectedModCount) { 1265 super(m, origin, fence, est, expectedModCount); 1266 } 1267 1268 public ValueSpliterator<K,V> trySplit() { 1269 int hi = getFence(), lo = index, mid = (lo + hi) >>> 1; 1270 return (lo >= mid) ? null : 1271 new ValueSpliterator<K,V>(map, lo, index = mid, est >>>= 1, 1272 expectedModCount); 1273 } 1274 1275 public void forEachRemaining(Consumer<? super V> action) { 1276 int i, hi, mc; 1277 if (action == null) 1278 throw new NullPointerException(); 1279 WeakHashMap<K,V> m = map; 1280 WeakHashMap.Entry<K,V>[] tab = m.table; 1281 if ((hi = fence) < 0) { 1282 mc = expectedModCount = m.modCount; 1283 hi = fence = tab.length; 1284 } 1285 else 1286 mc = expectedModCount; 1287 if (tab.length >= hi && (i = index) >= 0 && 1288 (i < (index = hi) || current != null)) { 1289 WeakHashMap.Entry<K,V> p = current; 1290 current = null; // exhaust 1291 do { 1292 if (p == null) 1293 p = tab[i++]; 1294 else { 1295 Object x = p.get(); 1296 V v = p.value; 1297 p = p.next; 1298 if (x != null) 1299 action.accept(v); 1300 } 1301 } while (p != null || i < hi); 1302 } 1303 if (m.modCount != mc) 1304 throw new ConcurrentModificationException(); 1305 } 1306 1307 public boolean tryAdvance(Consumer<? super V> action) { 1308 int hi; 1309 if (action == null) 1310 throw new NullPointerException(); 1311 WeakHashMap.Entry<K,V>[] tab = map.table; 1312 if (tab.length >= (hi = getFence()) && index >= 0) { 1313 while (current != null || index < hi) { 1314 if (current == null) 1315 current = tab[index++]; 1316 else { 1317 Object x = current.get(); 1318 V v = current.value; 1319 current = current.next; 1320 if (x != null) { 1321 action.accept(v); 1322 if (map.modCount != expectedModCount) 1323 throw new ConcurrentModificationException(); 1324 return true; 1325 } 1326 } 1327 } 1328 } 1329 return false; 1330 } 1331 1332 public int characteristics() { 1333 return 0; 1334 } 1335 } 1336 1337 static final class EntrySpliterator<K,V> 1338 extends WeakHashMapSpliterator<K,V> 1339 implements Spliterator<Map.Entry<K,V>> { 1340 EntrySpliterator(WeakHashMap<K,V> m, int origin, int fence, int est, 1341 int expectedModCount) { 1342 super(m, origin, fence, est, expectedModCount); 1343 } 1344 1345 public EntrySpliterator<K,V> trySplit() { 1346 int hi = getFence(), lo = index, mid = (lo + hi) >>> 1; 1347 return (lo >= mid) ? null : 1348 new EntrySpliterator<K,V>(map, lo, index = mid, est >>>= 1, 1349 expectedModCount); 1350 } 1351 1352 1353 public void forEachRemaining(Consumer<? super Map.Entry<K, V>> action) { 1354 int i, hi, mc; 1355 if (action == null) 1356 throw new NullPointerException(); 1357 WeakHashMap<K,V> m = map; 1358 WeakHashMap.Entry<K,V>[] tab = m.table; 1359 if ((hi = fence) < 0) { 1360 mc = expectedModCount = m.modCount; 1361 hi = fence = tab.length; 1362 } 1363 else 1364 mc = expectedModCount; 1365 if (tab.length >= hi && (i = index) >= 0 && 1366 (i < (index = hi) || current != null)) { 1367 WeakHashMap.Entry<K,V> p = current; 1368 current = null; // exhaust 1369 do { 1370 if (p == null) 1371 p = tab[i++]; 1372 else { 1373 Object x = p.get(); 1374 V v = p.value; 1375 p = p.next; 1376 if (x != null) { 1377 @SuppressWarnings("unchecked") K k = 1378 (K) WeakHashMap.unmaskNull(x); 1379 action.accept 1380 (new AbstractMap.SimpleImmutableEntry<K,V>(k, v)); 1381 } 1382 } 1383 } while (p != null || i < hi); 1384 } 1385 if (m.modCount != mc) 1386 throw new ConcurrentModificationException(); 1387 } 1388 1389 public boolean tryAdvance(Consumer<? super Map.Entry<K,V>> action) { 1390 int hi; 1391 if (action == null) 1392 throw new NullPointerException(); 1393 WeakHashMap.Entry<K,V>[] tab = map.table; 1394 if (tab.length >= (hi = getFence()) && index >= 0) { 1395 while (current != null || index < hi) { 1396 if (current == null) 1397 current = tab[index++]; 1398 else { 1399 Object x = current.get(); 1400 V v = current.value; 1401 current = current.next; 1402 if (x != null) { 1403 @SuppressWarnings("unchecked") K k = 1404 (K) WeakHashMap.unmaskNull(x); 1405 action.accept 1406 (new AbstractMap.SimpleImmutableEntry<K,V>(k, v)); 1407 if (map.modCount != expectedModCount) 1408 throw new ConcurrentModificationException(); 1409 return true; 1410 } 1411 } 1412 } 1413 } 1414 return false; 1415 } 1416 1417 public int characteristics() { 1418 return Spliterator.DISTINCT; 1419 } 1420 } 1421 1422} 1423