J. Duke | 319a3b9 | 2007-12-01 00:00:00 +0000 | [diff] [blame^] | 1 | /* |
| 2 | * DO NOT ALTER OR REMOVE COPYRIGHT NOTICES OR THIS FILE HEADER. |
| 3 | * |
| 4 | * This code is free software; you can redistribute it and/or modify it |
| 5 | * under the terms of the GNU General Public License version 2 only, as |
| 6 | * published by the Free Software Foundation. Sun designates this |
| 7 | * particular file as subject to the "Classpath" exception as provided |
| 8 | * by Sun in the LICENSE file that accompanied this code. |
| 9 | * |
| 10 | * This code is distributed in the hope that it will be useful, but WITHOUT |
| 11 | * ANY WARRANTY; without even the implied warranty of MERCHANTABILITY or |
| 12 | * FITNESS FOR A PARTICULAR PURPOSE. See the GNU General Public License |
| 13 | * version 2 for more details (a copy is included in the LICENSE file that |
| 14 | * accompanied this code). |
| 15 | * |
| 16 | * You should have received a copy of the GNU General Public License version |
| 17 | * 2 along with this work; if not, write to the Free Software Foundation, |
| 18 | * Inc., 51 Franklin St, Fifth Floor, Boston, MA 02110-1301 USA. |
| 19 | * |
| 20 | * Please contact Sun Microsystems, Inc., 4150 Network Circle, Santa Clara, |
| 21 | * CA 95054 USA or visit www.sun.com if you need additional information or |
| 22 | * have any questions. |
| 23 | */ |
| 24 | |
| 25 | /* |
| 26 | * This file is available under and governed by the GNU General Public |
| 27 | * License version 2 only, as published by the Free Software Foundation. |
| 28 | * However, the following notice accompanied the original version of this |
| 29 | * file: |
| 30 | * |
| 31 | * Written by Doug Lea with assistance from members of JCP JSR-166 |
| 32 | * Expert Group and released to the public domain, as explained at |
| 33 | * http://creativecommons.org/licenses/publicdomain |
| 34 | */ |
| 35 | |
| 36 | package java.util.concurrent; |
| 37 | import java.util.*; |
| 38 | import java.util.concurrent.atomic.*; |
| 39 | |
| 40 | |
| 41 | /** |
| 42 | * An unbounded thread-safe {@linkplain Queue queue} based on linked nodes. |
| 43 | * This queue orders elements FIFO (first-in-first-out). |
| 44 | * The <em>head</em> of the queue is that element that has been on the |
| 45 | * queue the longest time. |
| 46 | * The <em>tail</em> of the queue is that element that has been on the |
| 47 | * queue the shortest time. New elements |
| 48 | * are inserted at the tail of the queue, and the queue retrieval |
| 49 | * operations obtain elements at the head of the queue. |
| 50 | * A <tt>ConcurrentLinkedQueue</tt> is an appropriate choice when |
| 51 | * many threads will share access to a common collection. |
| 52 | * This queue does not permit <tt>null</tt> elements. |
| 53 | * |
| 54 | * <p>This implementation employs an efficient "wait-free" |
| 55 | * algorithm based on one described in <a |
| 56 | * href="http://www.cs.rochester.edu/u/michael/PODC96.html"> Simple, |
| 57 | * Fast, and Practical Non-Blocking and Blocking Concurrent Queue |
| 58 | * Algorithms</a> by Maged M. Michael and Michael L. Scott. |
| 59 | * |
| 60 | * <p>Beware that, unlike in most collections, the <tt>size</tt> method |
| 61 | * is <em>NOT</em> a constant-time operation. Because of the |
| 62 | * asynchronous nature of these queues, determining the current number |
| 63 | * of elements requires a traversal of the elements. |
| 64 | * |
| 65 | * <p>This class and its iterator implement all of the |
| 66 | * <em>optional</em> methods of the {@link Collection} and {@link |
| 67 | * Iterator} interfaces. |
| 68 | * |
| 69 | * <p>Memory consistency effects: As with other concurrent |
| 70 | * collections, actions in a thread prior to placing an object into a |
| 71 | * {@code ConcurrentLinkedQueue} |
| 72 | * <a href="package-summary.html#MemoryVisibility"><i>happen-before</i></a> |
| 73 | * actions subsequent to the access or removal of that element from |
| 74 | * the {@code ConcurrentLinkedQueue} in another thread. |
| 75 | * |
| 76 | * <p>This class is a member of the |
| 77 | * <a href="{@docRoot}/../technotes/guides/collections/index.html"> |
| 78 | * Java Collections Framework</a>. |
| 79 | * |
| 80 | * @since 1.5 |
| 81 | * @author Doug Lea |
| 82 | * @param <E> the type of elements held in this collection |
| 83 | * |
| 84 | */ |
| 85 | public class ConcurrentLinkedQueue<E> extends AbstractQueue<E> |
| 86 | implements Queue<E>, java.io.Serializable { |
| 87 | private static final long serialVersionUID = 196745693267521676L; |
| 88 | |
| 89 | /* |
| 90 | * This is a straight adaptation of Michael & Scott algorithm. |
| 91 | * For explanation, read the paper. The only (minor) algorithmic |
| 92 | * difference is that this version supports lazy deletion of |
| 93 | * internal nodes (method remove(Object)) -- remove CAS'es item |
| 94 | * fields to null. The normal queue operations unlink but then |
| 95 | * pass over nodes with null item fields. Similarly, iteration |
| 96 | * methods ignore those with nulls. |
| 97 | * |
| 98 | * Also note that like most non-blocking algorithms in this |
| 99 | * package, this implementation relies on the fact that in garbage |
| 100 | * collected systems, there is no possibility of ABA problems due |
| 101 | * to recycled nodes, so there is no need to use "counted |
| 102 | * pointers" or related techniques seen in versions used in |
| 103 | * non-GC'ed settings. |
| 104 | */ |
| 105 | |
| 106 | private static class Node<E> { |
| 107 | private volatile E item; |
| 108 | private volatile Node<E> next; |
| 109 | |
| 110 | private static final |
| 111 | AtomicReferenceFieldUpdater<Node, Node> |
| 112 | nextUpdater = |
| 113 | AtomicReferenceFieldUpdater.newUpdater |
| 114 | (Node.class, Node.class, "next"); |
| 115 | private static final |
| 116 | AtomicReferenceFieldUpdater<Node, Object> |
| 117 | itemUpdater = |
| 118 | AtomicReferenceFieldUpdater.newUpdater |
| 119 | (Node.class, Object.class, "item"); |
| 120 | |
| 121 | Node(E x) { item = x; } |
| 122 | |
| 123 | Node(E x, Node<E> n) { item = x; next = n; } |
| 124 | |
| 125 | E getItem() { |
| 126 | return item; |
| 127 | } |
| 128 | |
| 129 | boolean casItem(E cmp, E val) { |
| 130 | return itemUpdater.compareAndSet(this, cmp, val); |
| 131 | } |
| 132 | |
| 133 | void setItem(E val) { |
| 134 | itemUpdater.set(this, val); |
| 135 | } |
| 136 | |
| 137 | Node<E> getNext() { |
| 138 | return next; |
| 139 | } |
| 140 | |
| 141 | boolean casNext(Node<E> cmp, Node<E> val) { |
| 142 | return nextUpdater.compareAndSet(this, cmp, val); |
| 143 | } |
| 144 | |
| 145 | void setNext(Node<E> val) { |
| 146 | nextUpdater.set(this, val); |
| 147 | } |
| 148 | |
| 149 | } |
| 150 | |
| 151 | private static final |
| 152 | AtomicReferenceFieldUpdater<ConcurrentLinkedQueue, Node> |
| 153 | tailUpdater = |
| 154 | AtomicReferenceFieldUpdater.newUpdater |
| 155 | (ConcurrentLinkedQueue.class, Node.class, "tail"); |
| 156 | private static final |
| 157 | AtomicReferenceFieldUpdater<ConcurrentLinkedQueue, Node> |
| 158 | headUpdater = |
| 159 | AtomicReferenceFieldUpdater.newUpdater |
| 160 | (ConcurrentLinkedQueue.class, Node.class, "head"); |
| 161 | |
| 162 | private boolean casTail(Node<E> cmp, Node<E> val) { |
| 163 | return tailUpdater.compareAndSet(this, cmp, val); |
| 164 | } |
| 165 | |
| 166 | private boolean casHead(Node<E> cmp, Node<E> val) { |
| 167 | return headUpdater.compareAndSet(this, cmp, val); |
| 168 | } |
| 169 | |
| 170 | |
| 171 | /** |
| 172 | * Pointer to header node, initialized to a dummy node. The first |
| 173 | * actual node is at head.getNext(). |
| 174 | */ |
| 175 | private transient volatile Node<E> head = new Node<E>(null, null); |
| 176 | |
| 177 | /** Pointer to last node on list **/ |
| 178 | private transient volatile Node<E> tail = head; |
| 179 | |
| 180 | |
| 181 | /** |
| 182 | * Creates a <tt>ConcurrentLinkedQueue</tt> that is initially empty. |
| 183 | */ |
| 184 | public ConcurrentLinkedQueue() {} |
| 185 | |
| 186 | /** |
| 187 | * Creates a <tt>ConcurrentLinkedQueue</tt> |
| 188 | * initially containing the elements of the given collection, |
| 189 | * added in traversal order of the collection's iterator. |
| 190 | * @param c the collection of elements to initially contain |
| 191 | * @throws NullPointerException if the specified collection or any |
| 192 | * of its elements are null |
| 193 | */ |
| 194 | public ConcurrentLinkedQueue(Collection<? extends E> c) { |
| 195 | for (Iterator<? extends E> it = c.iterator(); it.hasNext();) |
| 196 | add(it.next()); |
| 197 | } |
| 198 | |
| 199 | // Have to override just to update the javadoc |
| 200 | |
| 201 | /** |
| 202 | * Inserts the specified element at the tail of this queue. |
| 203 | * |
| 204 | * @return <tt>true</tt> (as specified by {@link Collection#add}) |
| 205 | * @throws NullPointerException if the specified element is null |
| 206 | */ |
| 207 | public boolean add(E e) { |
| 208 | return offer(e); |
| 209 | } |
| 210 | |
| 211 | /** |
| 212 | * Inserts the specified element at the tail of this queue. |
| 213 | * |
| 214 | * @return <tt>true</tt> (as specified by {@link Queue#offer}) |
| 215 | * @throws NullPointerException if the specified element is null |
| 216 | */ |
| 217 | public boolean offer(E e) { |
| 218 | if (e == null) throw new NullPointerException(); |
| 219 | Node<E> n = new Node<E>(e, null); |
| 220 | for (;;) { |
| 221 | Node<E> t = tail; |
| 222 | Node<E> s = t.getNext(); |
| 223 | if (t == tail) { |
| 224 | if (s == null) { |
| 225 | if (t.casNext(s, n)) { |
| 226 | casTail(t, n); |
| 227 | return true; |
| 228 | } |
| 229 | } else { |
| 230 | casTail(t, s); |
| 231 | } |
| 232 | } |
| 233 | } |
| 234 | } |
| 235 | |
| 236 | public E poll() { |
| 237 | for (;;) { |
| 238 | Node<E> h = head; |
| 239 | Node<E> t = tail; |
| 240 | Node<E> first = h.getNext(); |
| 241 | if (h == head) { |
| 242 | if (h == t) { |
| 243 | if (first == null) |
| 244 | return null; |
| 245 | else |
| 246 | casTail(t, first); |
| 247 | } else if (casHead(h, first)) { |
| 248 | E item = first.getItem(); |
| 249 | if (item != null) { |
| 250 | first.setItem(null); |
| 251 | return item; |
| 252 | } |
| 253 | // else skip over deleted item, continue loop, |
| 254 | } |
| 255 | } |
| 256 | } |
| 257 | } |
| 258 | |
| 259 | public E peek() { // same as poll except don't remove item |
| 260 | for (;;) { |
| 261 | Node<E> h = head; |
| 262 | Node<E> t = tail; |
| 263 | Node<E> first = h.getNext(); |
| 264 | if (h == head) { |
| 265 | if (h == t) { |
| 266 | if (first == null) |
| 267 | return null; |
| 268 | else |
| 269 | casTail(t, first); |
| 270 | } else { |
| 271 | E item = first.getItem(); |
| 272 | if (item != null) |
| 273 | return item; |
| 274 | else // remove deleted node and continue |
| 275 | casHead(h, first); |
| 276 | } |
| 277 | } |
| 278 | } |
| 279 | } |
| 280 | |
| 281 | /** |
| 282 | * Returns the first actual (non-header) node on list. This is yet |
| 283 | * another variant of poll/peek; here returning out the first |
| 284 | * node, not element (so we cannot collapse with peek() without |
| 285 | * introducing race.) |
| 286 | */ |
| 287 | Node<E> first() { |
| 288 | for (;;) { |
| 289 | Node<E> h = head; |
| 290 | Node<E> t = tail; |
| 291 | Node<E> first = h.getNext(); |
| 292 | if (h == head) { |
| 293 | if (h == t) { |
| 294 | if (first == null) |
| 295 | return null; |
| 296 | else |
| 297 | casTail(t, first); |
| 298 | } else { |
| 299 | if (first.getItem() != null) |
| 300 | return first; |
| 301 | else // remove deleted node and continue |
| 302 | casHead(h, first); |
| 303 | } |
| 304 | } |
| 305 | } |
| 306 | } |
| 307 | |
| 308 | |
| 309 | /** |
| 310 | * Returns <tt>true</tt> if this queue contains no elements. |
| 311 | * |
| 312 | * @return <tt>true</tt> if this queue contains no elements |
| 313 | */ |
| 314 | public boolean isEmpty() { |
| 315 | return first() == null; |
| 316 | } |
| 317 | |
| 318 | /** |
| 319 | * Returns the number of elements in this queue. If this queue |
| 320 | * contains more than <tt>Integer.MAX_VALUE</tt> elements, returns |
| 321 | * <tt>Integer.MAX_VALUE</tt>. |
| 322 | * |
| 323 | * <p>Beware that, unlike in most collections, this method is |
| 324 | * <em>NOT</em> a constant-time operation. Because of the |
| 325 | * asynchronous nature of these queues, determining the current |
| 326 | * number of elements requires an O(n) traversal. |
| 327 | * |
| 328 | * @return the number of elements in this queue |
| 329 | */ |
| 330 | public int size() { |
| 331 | int count = 0; |
| 332 | for (Node<E> p = first(); p != null; p = p.getNext()) { |
| 333 | if (p.getItem() != null) { |
| 334 | // Collections.size() spec says to max out |
| 335 | if (++count == Integer.MAX_VALUE) |
| 336 | break; |
| 337 | } |
| 338 | } |
| 339 | return count; |
| 340 | } |
| 341 | |
| 342 | /** |
| 343 | * Returns <tt>true</tt> if this queue contains the specified element. |
| 344 | * More formally, returns <tt>true</tt> if and only if this queue contains |
| 345 | * at least one element <tt>e</tt> such that <tt>o.equals(e)</tt>. |
| 346 | * |
| 347 | * @param o object to be checked for containment in this queue |
| 348 | * @return <tt>true</tt> if this queue contains the specified element |
| 349 | */ |
| 350 | public boolean contains(Object o) { |
| 351 | if (o == null) return false; |
| 352 | for (Node<E> p = first(); p != null; p = p.getNext()) { |
| 353 | E item = p.getItem(); |
| 354 | if (item != null && |
| 355 | o.equals(item)) |
| 356 | return true; |
| 357 | } |
| 358 | return false; |
| 359 | } |
| 360 | |
| 361 | /** |
| 362 | * Removes a single instance of the specified element from this queue, |
| 363 | * if it is present. More formally, removes an element <tt>e</tt> such |
| 364 | * that <tt>o.equals(e)</tt>, if this queue contains one or more such |
| 365 | * elements. |
| 366 | * Returns <tt>true</tt> if this queue contained the specified element |
| 367 | * (or equivalently, if this queue changed as a result of the call). |
| 368 | * |
| 369 | * @param o element to be removed from this queue, if present |
| 370 | * @return <tt>true</tt> if this queue changed as a result of the call |
| 371 | */ |
| 372 | public boolean remove(Object o) { |
| 373 | if (o == null) return false; |
| 374 | for (Node<E> p = first(); p != null; p = p.getNext()) { |
| 375 | E item = p.getItem(); |
| 376 | if (item != null && |
| 377 | o.equals(item) && |
| 378 | p.casItem(item, null)) |
| 379 | return true; |
| 380 | } |
| 381 | return false; |
| 382 | } |
| 383 | |
| 384 | /** |
| 385 | * Returns an array containing all of the elements in this queue, in |
| 386 | * proper sequence. |
| 387 | * |
| 388 | * <p>The returned array will be "safe" in that no references to it are |
| 389 | * maintained by this queue. (In other words, this method must allocate |
| 390 | * a new array). The caller is thus free to modify the returned array. |
| 391 | * |
| 392 | * <p>This method acts as bridge between array-based and collection-based |
| 393 | * APIs. |
| 394 | * |
| 395 | * @return an array containing all of the elements in this queue |
| 396 | */ |
| 397 | public Object[] toArray() { |
| 398 | // Use ArrayList to deal with resizing. |
| 399 | ArrayList<E> al = new ArrayList<E>(); |
| 400 | for (Node<E> p = first(); p != null; p = p.getNext()) { |
| 401 | E item = p.getItem(); |
| 402 | if (item != null) |
| 403 | al.add(item); |
| 404 | } |
| 405 | return al.toArray(); |
| 406 | } |
| 407 | |
| 408 | /** |
| 409 | * Returns an array containing all of the elements in this queue, in |
| 410 | * proper sequence; the runtime type of the returned array is that of |
| 411 | * the specified array. If the queue fits in the specified array, it |
| 412 | * is returned therein. Otherwise, a new array is allocated with the |
| 413 | * runtime type of the specified array and the size of this queue. |
| 414 | * |
| 415 | * <p>If this queue fits in the specified array with room to spare |
| 416 | * (i.e., the array has more elements than this queue), the element in |
| 417 | * the array immediately following the end of the queue is set to |
| 418 | * <tt>null</tt>. |
| 419 | * |
| 420 | * <p>Like the {@link #toArray()} method, this method acts as bridge between |
| 421 | * array-based and collection-based APIs. Further, this method allows |
| 422 | * precise control over the runtime type of the output array, and may, |
| 423 | * under certain circumstances, be used to save allocation costs. |
| 424 | * |
| 425 | * <p>Suppose <tt>x</tt> is a queue known to contain only strings. |
| 426 | * The following code can be used to dump the queue into a newly |
| 427 | * allocated array of <tt>String</tt>: |
| 428 | * |
| 429 | * <pre> |
| 430 | * String[] y = x.toArray(new String[0]);</pre> |
| 431 | * |
| 432 | * Note that <tt>toArray(new Object[0])</tt> is identical in function to |
| 433 | * <tt>toArray()</tt>. |
| 434 | * |
| 435 | * @param a the array into which the elements of the queue are to |
| 436 | * be stored, if it is big enough; otherwise, a new array of the |
| 437 | * same runtime type is allocated for this purpose |
| 438 | * @return an array containing all of the elements in this queue |
| 439 | * @throws ArrayStoreException if the runtime type of the specified array |
| 440 | * is not a supertype of the runtime type of every element in |
| 441 | * this queue |
| 442 | * @throws NullPointerException if the specified array is null |
| 443 | */ |
| 444 | public <T> T[] toArray(T[] a) { |
| 445 | // try to use sent-in array |
| 446 | int k = 0; |
| 447 | Node<E> p; |
| 448 | for (p = first(); p != null && k < a.length; p = p.getNext()) { |
| 449 | E item = p.getItem(); |
| 450 | if (item != null) |
| 451 | a[k++] = (T)item; |
| 452 | } |
| 453 | if (p == null) { |
| 454 | if (k < a.length) |
| 455 | a[k] = null; |
| 456 | return a; |
| 457 | } |
| 458 | |
| 459 | // If won't fit, use ArrayList version |
| 460 | ArrayList<E> al = new ArrayList<E>(); |
| 461 | for (Node<E> q = first(); q != null; q = q.getNext()) { |
| 462 | E item = q.getItem(); |
| 463 | if (item != null) |
| 464 | al.add(item); |
| 465 | } |
| 466 | return al.toArray(a); |
| 467 | } |
| 468 | |
| 469 | /** |
| 470 | * Returns an iterator over the elements in this queue in proper sequence. |
| 471 | * The returned iterator is a "weakly consistent" iterator that |
| 472 | * will never throw {@link ConcurrentModificationException}, |
| 473 | * and guarantees to traverse elements as they existed upon |
| 474 | * construction of the iterator, and may (but is not guaranteed to) |
| 475 | * reflect any modifications subsequent to construction. |
| 476 | * |
| 477 | * @return an iterator over the elements in this queue in proper sequence |
| 478 | */ |
| 479 | public Iterator<E> iterator() { |
| 480 | return new Itr(); |
| 481 | } |
| 482 | |
| 483 | private class Itr implements Iterator<E> { |
| 484 | /** |
| 485 | * Next node to return item for. |
| 486 | */ |
| 487 | private Node<E> nextNode; |
| 488 | |
| 489 | /** |
| 490 | * nextItem holds on to item fields because once we claim |
| 491 | * that an element exists in hasNext(), we must return it in |
| 492 | * the following next() call even if it was in the process of |
| 493 | * being removed when hasNext() was called. |
| 494 | */ |
| 495 | private E nextItem; |
| 496 | |
| 497 | /** |
| 498 | * Node of the last returned item, to support remove. |
| 499 | */ |
| 500 | private Node<E> lastRet; |
| 501 | |
| 502 | Itr() { |
| 503 | advance(); |
| 504 | } |
| 505 | |
| 506 | /** |
| 507 | * Moves to next valid node and returns item to return for |
| 508 | * next(), or null if no such. |
| 509 | */ |
| 510 | private E advance() { |
| 511 | lastRet = nextNode; |
| 512 | E x = nextItem; |
| 513 | |
| 514 | Node<E> p = (nextNode == null)? first() : nextNode.getNext(); |
| 515 | for (;;) { |
| 516 | if (p == null) { |
| 517 | nextNode = null; |
| 518 | nextItem = null; |
| 519 | return x; |
| 520 | } |
| 521 | E item = p.getItem(); |
| 522 | if (item != null) { |
| 523 | nextNode = p; |
| 524 | nextItem = item; |
| 525 | return x; |
| 526 | } else // skip over nulls |
| 527 | p = p.getNext(); |
| 528 | } |
| 529 | } |
| 530 | |
| 531 | public boolean hasNext() { |
| 532 | return nextNode != null; |
| 533 | } |
| 534 | |
| 535 | public E next() { |
| 536 | if (nextNode == null) throw new NoSuchElementException(); |
| 537 | return advance(); |
| 538 | } |
| 539 | |
| 540 | public void remove() { |
| 541 | Node<E> l = lastRet; |
| 542 | if (l == null) throw new IllegalStateException(); |
| 543 | // rely on a future traversal to relink. |
| 544 | l.setItem(null); |
| 545 | lastRet = null; |
| 546 | } |
| 547 | } |
| 548 | |
| 549 | /** |
| 550 | * Save the state to a stream (that is, serialize it). |
| 551 | * |
| 552 | * @serialData All of the elements (each an <tt>E</tt>) in |
| 553 | * the proper order, followed by a null |
| 554 | * @param s the stream |
| 555 | */ |
| 556 | private void writeObject(java.io.ObjectOutputStream s) |
| 557 | throws java.io.IOException { |
| 558 | |
| 559 | // Write out any hidden stuff |
| 560 | s.defaultWriteObject(); |
| 561 | |
| 562 | // Write out all elements in the proper order. |
| 563 | for (Node<E> p = first(); p != null; p = p.getNext()) { |
| 564 | Object item = p.getItem(); |
| 565 | if (item != null) |
| 566 | s.writeObject(item); |
| 567 | } |
| 568 | |
| 569 | // Use trailing null as sentinel |
| 570 | s.writeObject(null); |
| 571 | } |
| 572 | |
| 573 | /** |
| 574 | * Reconstitute the Queue instance from a stream (that is, |
| 575 | * deserialize it). |
| 576 | * @param s the stream |
| 577 | */ |
| 578 | private void readObject(java.io.ObjectInputStream s) |
| 579 | throws java.io.IOException, ClassNotFoundException { |
| 580 | // Read in capacity, and any hidden stuff |
| 581 | s.defaultReadObject(); |
| 582 | head = new Node<E>(null, null); |
| 583 | tail = head; |
| 584 | // Read in all elements and place in queue |
| 585 | for (;;) { |
| 586 | E item = (E)s.readObject(); |
| 587 | if (item == null) |
| 588 | break; |
| 589 | else |
| 590 | offer(item); |
| 591 | } |
| 592 | } |
| 593 | |
| 594 | } |