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| 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. Oracle designates this | ||
| 7 | * particular file as subject to the "Classpath" exception as provided | ||
| 8 | * by Oracle 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 Oracle, 500 Oracle Parkway, Redwood Shores, CA 94065 USA | ||
| 21 | * or visit www.oracle.com if you need additional information or have any | ||
| 22 | * 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 and Martin Buchholz with assistance from members of | ||
| 32 | * JCP JSR-166 Expert Group and released to the public domain, as explained | ||
| 33 | * at http://creativecommons.org/publicdomain/zero/1.0/ | ||
| 34 | */ | ||
| 35 | |||
| 36 | package java.util.concurrent; | ||
| 37 | |||
| 38 | import java.util.AbstractQueue; | ||
| 39 | import java.util.ArrayList; | ||
| 40 | import java.util.Collection; | ||
| 41 | import java.util.Iterator; | ||
| 42 | import java.util.NoSuchElementException; | ||
| 43 | import java.util.Queue; | ||
| 44 | |||
| 45 | /** | ||
| 46 | * An unbounded thread-safe {@linkplain Queue queue} based on linked nodes. | ||
| 47 | * This queue orders elements FIFO (first-in-first-out). | ||
| 48 | * The <em>head</em> of the queue is that element that has been on the | ||
| 49 | * queue the longest time. | ||
| 50 | * The <em>tail</em> of the queue is that element that has been on the | ||
| 51 | * queue the shortest time. New elements | ||
| 52 | * are inserted at the tail of the queue, and the queue retrieval | ||
| 53 | * operations obtain elements at the head of the queue. | ||
| 54 | * A {@code ConcurrentLinkedQueue} is an appropriate choice when | ||
| 55 | * many threads will share access to a common collection. | ||
| 56 | * Like most other concurrent collection implementations, this class | ||
| 57 | * does not permit the use of {@code null} elements. | ||
| 58 | * | ||
| 59 | * <p>This implementation employs an efficient "wait-free" | ||
| 60 | * algorithm based on one described in <a | ||
| 61 | * href="http://www.cs.rochester.edu/u/michael/PODC96.html"> Simple, | ||
| 62 | * Fast, and Practical Non-Blocking and Blocking Concurrent Queue | ||
| 63 | * Algorithms</a> by Maged M. Michael and Michael L. Scott. | ||
| 64 | * | ||
| 65 | * <p>Iterators are <i>weakly consistent</i>, returning elements | ||
| 66 | * reflecting the state of the queue at some point at or since the | ||
| 67 | * creation of the iterator. They do <em>not</em> throw {@link | ||
| 68 | * java.util.ConcurrentModificationException}, and may proceed concurrently | ||
| 69 | * with other operations. Elements contained in the queue since the creation | ||
| 70 | * of the iterator will be returned exactly once. | ||
| 71 | * | ||
| 72 | * <p>Beware that, unlike in most collections, the {@code size} method | ||
| 73 | * is <em>NOT</em> a constant-time operation. Because of the | ||
| 74 | * asynchronous nature of these queues, determining the current number | ||
| 75 | * of elements requires a traversal of the elements, and so may report | ||
| 76 | * inaccurate results if this collection is modified during traversal. | ||
| 77 | * Additionally, the bulk operations {@code addAll}, | ||
| 78 | * {@code removeAll}, {@code retainAll}, {@code containsAll}, | ||
| 79 | * {@code equals}, and {@code toArray} are <em>not</em> guaranteed | ||
| 80 | * to be performed atomically. For example, an iterator operating | ||
| 81 | * concurrently with an {@code addAll} operation might view only some | ||
| 82 | * of the added elements. | ||
| 83 | * | ||
| 84 | * <p>This class and its iterator implement all of the <em>optional</em> | ||
| 85 | * methods of the {@link Queue} and {@link Iterator} interfaces. | ||
| 86 | * | ||
| 87 | * <p>Memory consistency effects: As with other concurrent | ||
| 88 | * collections, actions in a thread prior to placing an object into a | ||
| 89 | * {@code ConcurrentLinkedQueue} | ||
| 90 | * <a href="package-summary.html#MemoryVisibility"><i>happen-before</i></a> | ||
| 91 | * actions subsequent to the access or removal of that element from | ||
| 92 | * the {@code ConcurrentLinkedQueue} in another thread. | ||
| 93 | * | ||
| 94 | * <p>This class is a member of the | ||
| 95 | * <a href="{@docRoot}/../technotes/guides/collections/index.html"> | ||
| 96 | * Java Collections Framework</a>. | ||
| 97 | * | ||
| 98 | * @since 1.5 | ||
| 99 | * @author Doug Lea | ||
| 100 | * @param <E> the type of elements held in this collection | ||
| 101 | * | ||
| 102 | */ | ||
| 103 | public class ConcurrentLinkedQueue<E> extends AbstractQueue<E> | ||
| 104 | implements Queue<E>, java.io.Serializable { | ||
| 105 | private static final long serialVersionUID = 196745693267521676L; | ||
| 106 | |||
| 107 | /* | ||
| 108 | * This is a modification of the Michael & Scott algorithm, | ||
| 109 | * adapted for a garbage-collected environment, with support for | ||
| 110 | * interior node deletion (to support remove(Object)). For | ||
| 111 | * explanation, read the paper. | ||
| 112 | * | ||
| 113 | * Note that like most non-blocking algorithms in this package, | ||
| 114 | * this implementation relies on the fact that in garbage | ||
| 115 | * collected systems, there is no possibility of ABA problems due | ||
| 116 | * to recycled nodes, so there is no need to use "counted | ||
| 117 | * pointers" or related techniques seen in versions used in | ||
| 118 | * non-GC'ed settings. | ||
| 119 | * | ||
| 120 | * The fundamental invariants are: | ||
| 121 | * - There is exactly one (last) Node with a null next reference, | ||
| 122 | * which is CASed when enqueueing. This last Node can be | ||
| 123 | * reached in O(1) time from tail, but tail is merely an | ||
| 124 | * optimization - it can always be reached in O(N) time from | ||
| 125 | * head as well. | ||
| 126 | * - The elements contained in the queue are the non-null items in | ||
| 127 | * Nodes that are reachable from head. CASing the item | ||
| 128 | * reference of a Node to null atomically removes it from the | ||
| 129 | * queue. Reachability of all elements from head must remain | ||
| 130 | * true even in the case of concurrent modifications that cause | ||
| 131 | * head to advance. A dequeued Node may remain in use | ||
| 132 | * indefinitely due to creation of an Iterator or simply a | ||
| 133 | * poll() that has lost its time slice. | ||
| 134 | * | ||
| 135 | * The above might appear to imply that all Nodes are GC-reachable | ||
| 136 | * from a predecessor dequeued Node. That would cause two problems: | ||
| 137 | * - allow a rogue Iterator to cause unbounded memory retention | ||
| 138 | * - cause cross-generational linking of old Nodes to new Nodes if | ||
| 139 | * a Node was tenured while live, which generational GCs have a | ||
| 140 | * hard time dealing with, causing repeated major collections. | ||
| 141 | * However, only non-deleted Nodes need to be reachable from | ||
| 142 | * dequeued Nodes, and reachability does not necessarily have to | ||
| 143 | * be of the kind understood by the GC. We use the trick of | ||
| 144 | * linking a Node that has just been dequeued to itself. Such a | ||
| 145 | * self-link implicitly means to advance to head. | ||
| 146 | * | ||
| 147 | * Both head and tail are permitted to lag. In fact, failing to | ||
| 148 | * update them every time one could is a significant optimization | ||
| 149 | * (fewer CASes). As with LinkedTransferQueue (see the internal | ||
| 150 | * documentation for that class), we use a slack threshold of two; | ||
| 151 | * that is, we update head/tail when the current pointer appears | ||
| 152 | * to be two or more steps away from the first/last node. | ||
| 153 | * | ||
| 154 | * Since head and tail are updated concurrently and independently, | ||
| 155 | * it is possible for tail to lag behind head (why not)? | ||
| 156 | * | ||
| 157 | * CASing a Node's item reference to null atomically removes the | ||
| 158 | * element from the queue. Iterators skip over Nodes with null | ||
| 159 | * items. Prior implementations of this class had a race between | ||
| 160 | * poll() and remove(Object) where the same element would appear | ||
| 161 | * to be successfully removed by two concurrent operations. The | ||
| 162 | * method remove(Object) also lazily unlinks deleted Nodes, but | ||
| 163 | * this is merely an optimization. | ||
| 164 | * | ||
| 165 | * When constructing a Node (before enqueuing it) we avoid paying | ||
| 166 | * for a volatile write to item by using Unsafe.putObject instead | ||
| 167 | * of a normal write. This allows the cost of enqueue to be | ||
| 168 | * "one-and-a-half" CASes. | ||
| 169 | * | ||
| 170 | * Both head and tail may or may not point to a Node with a | ||
| 171 | * non-null item. If the queue is empty, all items must of course | ||
| 172 | * be null. Upon creation, both head and tail refer to a dummy | ||
| 173 | * Node with null item. Both head and tail are only updated using | ||
| 174 | * CAS, so they never regress, although again this is merely an | ||
| 175 | * optimization. | ||
| 176 | */ | ||
| 177 | |||
| 178 | private static class Node<E> { | ||
| 179 | volatile E item; | ||
| 180 | volatile Node<E> next; | ||
| 181 | |||
| 182 | /** | ||
| 183 | * Constructs a new node. Uses relaxed write because item can | ||
| 184 | * only be seen after publication via casNext. | ||
| 185 | */ | ||
| 186 | Node(E item) { | ||
| 187 | UNSAFE.putObject(this, itemOffset, item); | ||
| 188 | } | ||
| 189 | |||
| 190 | boolean casItem(E cmp, E val) { | ||
| 191 | return UNSAFE.compareAndSwapObject(this, itemOffset, cmp, val); | ||
| 192 | } | ||
| 193 | |||
| 194 | void lazySetNext(Node<E> val) { | ||
| 195 | UNSAFE.putOrderedObject(this, nextOffset, val); | ||
| 196 | } | ||
| 197 | |||
| 198 | boolean casNext(Node<E> cmp, Node<E> val) { | ||
| 199 | return UNSAFE.compareAndSwapObject(this, nextOffset, cmp, val); | ||
| 200 | } | ||
| 201 | |||
| 202 | // Unsafe mechanics | ||
| 203 | |||
| 204 | private static final sun.misc.Unsafe UNSAFE; | ||
| 205 | private static final long itemOffset; | ||
| 206 | private static final long nextOffset; | ||
| 207 | |||
| 208 | static { | ||
| 209 | try { | ||
| 210 | UNSAFE = sun.misc.Unsafe.getUnsafe(); | ||
| 211 | Class k = Node.class; | ||
| 212 | itemOffset = UNSAFE.objectFieldOffset | ||
| 213 | (k.getDeclaredField("item")); | ||
| 214 | nextOffset = UNSAFE.objectFieldOffset | ||
| 215 | (k.getDeclaredField("next")); | ||
| 216 | } catch (Exception e) { | ||
| 217 | throw new Error(e); | ||
| 218 | } | ||
| 219 | } | ||
| 220 | } | ||
| 221 | |||
| 222 | /** | ||
| 223 | * A node from which the first live (non-deleted) node (if any) | ||
| 224 | * can be reached in O(1) time. | ||
| 225 | * Invariants: | ||
| 226 | * - all live nodes are reachable from head via succ() | ||
| 227 | * - head != null | ||
| 228 | * - (tmp = head).next != tmp || tmp != head | ||
| 229 | * Non-invariants: | ||
| 230 | * - head.item may or may not be null. | ||
| 231 | * - it is permitted for tail to lag behind head, that is, for tail | ||
| 232 | * to not be reachable from head! | ||
| 233 | */ | ||
| 234 | private transient volatile Node<E> head; | ||
| 235 | |||
| 236 | /** | ||
| 237 | * A node from which the last node on list (that is, the unique | ||
| 238 | * node with node.next == null) can be reached in O(1) time. | ||
| 239 | * Invariants: | ||
| 240 | * - the last node is always reachable from tail via succ() | ||
| 241 | * - tail != null | ||
| 242 | * Non-invariants: | ||
| 243 | * - tail.item may or may not be null. | ||
| 244 | * - it is permitted for tail to lag behind head, that is, for tail | ||
| 245 | * to not be reachable from head! | ||
| 246 | * - tail.next may or may not be self-pointing to tail. | ||
| 247 | */ | ||
| 248 | private transient volatile Node<E> tail; | ||
| 249 | |||
| 250 | |||
| 251 | /** | ||
| 252 | * Creates a {@code ConcurrentLinkedQueue} that is initially empty. | ||
| 253 | */ | ||
| 254 | public ConcurrentLinkedQueue() { | ||
| 255 | head = tail = new Node<E>(null); | ||
| 256 | } | ||
| 257 | |||
| 258 | /** | ||
| 259 | * Creates a {@code ConcurrentLinkedQueue} | ||
| 260 | * initially containing the elements of the given collection, | ||
| 261 | * added in traversal order of the collection's iterator. | ||
| 262 | * | ||
| 263 | * @param c the collection of elements to initially contain | ||
| 264 | * @throws NullPointerException if the specified collection or any | ||
| 265 | * of its elements are null | ||
| 266 | */ | ||
| 267 | public ConcurrentLinkedQueue(Collection<? extends E> c) { | ||
| 268 | Node<E> h = null, t = null; | ||
| 269 | for (E e : c) { | ||
| 270 | checkNotNull(e); | ||
| 271 | Node<E> newNode = new Node<E>(e); | ||
| 272 | if (h == null) | ||
| 273 | h = t = newNode; | ||
| 274 | else { | ||
| 275 | t.lazySetNext(newNode); | ||
| 276 | t = newNode; | ||
| 277 | } | ||
| 278 | } | ||
| 279 | if (h == null) | ||
| 280 | h = t = new Node<E>(null); | ||
| 281 | head = h; | ||
| 282 | tail = t; | ||
| 283 | } | ||
| 284 | |||
| 285 | // Have to override just to update the javadoc | ||
| 286 | |||
| 287 | /** | ||
| 288 | * Inserts the specified element at the tail of this queue. | ||
| 289 | * As the queue is unbounded, this method will never throw | ||
| 290 | * {@link IllegalStateException} or return {@code false}. | ||
| 291 | * | ||
| 292 | * @return {@code true} (as specified by {@link Collection#add}) | ||
| 293 | * @throws NullPointerException if the specified element is null | ||
| 294 | */ | ||
| 295 | public boolean add(E e) { | ||
| 296 | return offer(e); | ||
| 297 | } | ||
| 298 | |||
| 299 | /** | ||
| 300 | * Try to CAS head to p. If successful, repoint old head to itself | ||
| 301 | * as sentinel for succ(), below. | ||
| 302 | */ | ||
| 303 | final void updateHead(Node<E> h, Node<E> p) { | ||
| 304 | if (h != p && casHead(h, p)) | ||
| 305 | h.lazySetNext(h); | ||
| 306 | } | ||
| 307 | |||
| 308 | /** | ||
| 309 | * Returns the successor of p, or the head node if p.next has been | ||
| 310 | * linked to self, which will only be true if traversing with a | ||
| 311 | * stale pointer that is now off the list. | ||
| 312 | */ | ||
| 313 | final Node<E> succ(Node<E> p) { | ||
| 314 | Node<E> next = p.next; | ||
| 315 | return (p == next) ? head : next; | ||
| 316 | } | ||
| 317 | |||
| 318 | /** | ||
| 319 | * Inserts the specified element at the tail of this queue. | ||
| 320 | * As the queue is unbounded, this method will never return {@code false}. | ||
| 321 | * | ||
| 322 | * @return {@code true} (as specified by {@link Queue#offer}) | ||
| 323 | * @throws NullPointerException if the specified element is null | ||
| 324 | */ | ||
| 325 | public boolean offer(E e) { | ||
| 326 | checkNotNull(e); | ||
| 327 | final Node<E> newNode = new Node<E>(e); | ||
| 328 | |||
| 329 | for (Node<E> t = tail, p = t;;) { | ||
| 330 | Node<E> q = p.next; | ||
| 331 | if (q == null) { | ||
| 332 | // p is last node | ||
| 333 | if (p.casNext(null, newNode)) { | ||
| 334 | // Successful CAS is the linearization point | ||
| 335 | // for e to become an element of this queue, | ||
| 336 | // and for newNode to become "live". | ||
| 337 | if (p != t) // hop two nodes at a time | ||
| 338 | casTail(t, newNode); // Failure is OK. | ||
| 339 | return true; | ||
| 340 | } | ||
| 341 | // Lost CAS race to another thread; re-read next | ||
| 342 | } | ||
| 343 | else if (p == q) | ||
| 344 | // We have fallen off list. If tail is unchanged, it | ||
| 345 | // will also be off-list, in which case we need to | ||
| 346 | // jump to head, from which all live nodes are always | ||
| 347 | // reachable. Else the new tail is a better bet. | ||
| 348 | p = (t != (t = tail)) ? t : head; | ||
| 349 | else | ||
| 350 | // Check for tail updates after two hops. | ||
| 351 | p = (p != t && t != (t = tail)) ? t : q; | ||
| 352 | } | ||
| 353 | } | ||
| 354 | |||
| 355 | public E poll() { | ||
| 356 | restartFromHead: | ||
| 357 | for (;;) { | ||
| 358 | for (Node<E> h = head, p = h, q;;) { | ||
| 359 | E item = p.item; | ||
| 360 | |||
| 361 | if (item != null && p.casItem(item, null)) { | ||
| 362 | // Successful CAS is the linearization point | ||
| 363 | // for item to be removed from this queue. | ||
| 364 | if (p != h) // hop two nodes at a time | ||
| 365 | updateHead(h, ((q = p.next) != null) ? q : p); | ||
| 366 | return item; | ||
| 367 | } | ||
| 368 | else if ((q = p.next) == null) { | ||
| 369 | updateHead(h, p); | ||
| 370 | return null; | ||
| 371 | } | ||
| 372 | else if (p == q) | ||
| 373 | continue restartFromHead; | ||
| 374 | else | ||
| 375 | p = q; | ||
| 376 | } | ||
| 377 | } | ||
| 378 | } | ||
| 379 | |||
| 380 | public E peek() { | ||
| 381 | restartFromHead: | ||
| 382 | for (;;) { | ||
| 383 | for (Node<E> h = head, p = h, q;;) { | ||
| 384 | E item = p.item; | ||
| 385 | if (item != null || (q = p.next) == null) { | ||
| 386 | updateHead(h, p); | ||
| 387 | return item; | ||
| 388 | } | ||
| 389 | else if (p == q) | ||
| 390 | continue restartFromHead; | ||
| 391 | else | ||
| 392 | p = q; | ||
| 393 | } | ||
| 394 | } | ||
| 395 | } | ||
| 396 | |||
| 397 | /** | ||
| 398 | * Returns the first live (non-deleted) node on list, or null if none. | ||
| 399 | * This is yet another variant of poll/peek; here returning the | ||
| 400 | * first node, not element. We could make peek() a wrapper around | ||
| 401 | * first(), but that would cost an extra volatile read of item, | ||
| 402 | * and the need to add a retry loop to deal with the possibility | ||
| 403 | * of losing a race to a concurrent poll(). | ||
| 404 | */ | ||
| 405 | Node<E> first() { | ||
| 406 | restartFromHead: | ||
| 407 | for (;;) { | ||
| 408 | for (Node<E> h = head, p = h, q;;) { | ||
| 409 | boolean hasItem = (p.item != null); | ||
| 410 | if (hasItem || (q = p.next) == null) { | ||
| 411 | updateHead(h, p); | ||
| 412 | return hasItem ? p : null; | ||
| 413 | } | ||
| 414 | else if (p == q) | ||
| 415 | continue restartFromHead; | ||
| 416 | else | ||
| 417 | p = q; | ||
| 418 | } | ||
| 419 | } | ||
| 420 | } | ||
| 421 | |||
| 422 | /** | ||
| 423 | * Returns {@code true} if this queue contains no elements. | ||
| 424 | * | ||
| 425 | * @return {@code true} if this queue contains no elements | ||
| 426 | */ | ||
| 427 | public boolean isEmpty() { | ||
| 428 | return first() == null; | ||
| 429 | } | ||
| 430 | |||
| 431 | /** | ||
| 432 | * Returns the number of elements in this queue. If this queue | ||
| 433 | * contains more than {@code Integer.MAX_VALUE} elements, returns | ||
| 434 | * {@code Integer.MAX_VALUE}. | ||
| 435 | * | ||
| 436 | * <p>Beware that, unlike in most collections, this method is | ||
| 437 | * <em>NOT</em> a constant-time operation. Because of the | ||
| 438 | * asynchronous nature of these queues, determining the current | ||
| 439 | * number of elements requires an O(n) traversal. | ||
| 440 | * Additionally, if elements are added or removed during execution | ||
| 441 | * of this method, the returned result may be inaccurate. Thus, | ||
| 442 | * this method is typically not very useful in concurrent | ||
| 443 | * applications. | ||
| 444 | * | ||
| 445 | * @return the number of elements in this queue | ||
| 446 | */ | ||
| 447 | public int size() { | ||
| 448 | int count = 0; | ||
| 449 | for (Node<E> p = first(); p != null; p = succ(p)) | ||
| 450 | if (p.item != null) | ||
| 451 | // Collection.size() spec says to max out | ||
| 452 | if (++count == Integer.MAX_VALUE) | ||
| 453 | break; | ||
| 454 | return count; | ||
| 455 | } | ||
| 456 | |||
| 457 | /** | ||
| 458 | * Returns {@code true} if this queue contains the specified element. | ||
| 459 | * More formally, returns {@code true} if and only if this queue contains | ||
| 460 | * at least one element {@code e} such that {@code o.equals(e)}. | ||
| 461 | * | ||
| 462 | * @param o object to be checked for containment in this queue | ||
| 463 | * @return {@code true} if this queue contains the specified element | ||
| 464 | */ | ||
| 465 | public boolean contains(Object o) { | ||
| 466 | if (o == null) return false; | ||
| 467 | for (Node<E> p = first(); p != null; p = succ(p)) { | ||
| 468 | E item = p.item; | ||
| 469 | if (item != null && o.equals(item)) | ||
| 470 | return true; | ||
| 471 | } | ||
| 472 | return false; | ||
| 473 | } | ||
| 474 | |||
| 475 | /** | ||
| 476 | * Removes a single instance of the specified element from this queue, | ||
| 477 | * if it is present. More formally, removes an element {@code e} such | ||
| 478 | * that {@code o.equals(e)}, if this queue contains one or more such | ||
| 479 | * elements. | ||
| 480 | * Returns {@code true} if this queue contained the specified element | ||
| 481 | * (or equivalently, if this queue changed as a result of the call). | ||
| 482 | * | ||
| 483 | * @param o element to be removed from this queue, if present | ||
| 484 | * @return {@code true} if this queue changed as a result of the call | ||
| 485 | */ | ||
| 486 | public boolean remove(Object o) { | ||
| 487 | if (o == null) return false; | ||
| 488 | Node<E> pred = null; | ||
| 489 | for (Node<E> p = first(); p != null; p = succ(p)) { | ||
| 490 | E item = p.item; | ||
| 491 | if (item != null && | ||
| 492 | o.equals(item) && | ||
| 493 | p.casItem(item, null)) { | ||
| 494 | Node<E> next = succ(p); | ||
| 495 | if (pred != null && next != null) | ||
| 496 | pred.casNext(p, next); | ||
| 497 | return true; | ||
| 498 | } | ||
| 499 | pred = p; | ||
| 500 | } | ||
| 501 | return false; | ||
| 502 | } | ||
| 503 | |||
| 504 | /** | ||
| 505 | * Appends all of the elements in the specified collection to the end of | ||
| 506 | * this queue, in the order that they are returned by the specified | ||
| 507 | * collection's iterator. Attempts to {@code addAll} of a queue to | ||
| 508 | * itself result in {@code IllegalArgumentException}. | ||
| 509 | * | ||
| 510 | * @param c the elements to be inserted into this queue | ||
| 511 | * @return {@code true} if this queue changed as a result of the call | ||
| 512 | * @throws NullPointerException if the specified collection or any | ||
| 513 | * of its elements are null | ||
| 514 | * @throws IllegalArgumentException if the collection is this queue | ||
| 515 | */ | ||
| 516 | public boolean addAll(Collection<? extends E> c) { | ||
| 517 | if (c == this) | ||
| 518 | // As historically specified in AbstractQueue#addAll | ||
| 519 | throw new IllegalArgumentException(); | ||
| 520 | |||
| 521 | // Copy c into a private chain of Nodes | ||
| 522 | Node<E> beginningOfTheEnd = null, last = null; | ||
| 523 | for (E e : c) { | ||
| 524 | checkNotNull(e); | ||
| 525 | Node<E> newNode = new Node<E>(e); | ||
| 526 | if (beginningOfTheEnd == null) | ||
| 527 | beginningOfTheEnd = last = newNode; | ||
| 528 | else { | ||
| 529 | last.lazySetNext(newNode); | ||
| 530 | last = newNode; | ||
| 531 | } | ||
| 532 | } | ||
| 533 | if (beginningOfTheEnd == null) | ||
| 534 | return false; | ||
| 535 | |||
| 536 | // Atomically append the chain at the tail of this collection | ||
| 537 | for (Node<E> t = tail, p = t;;) { | ||
| 538 | Node<E> q = p.next; | ||
| 539 | if (q == null) { | ||
| 540 | // p is last node | ||
| 541 | if (p.casNext(null, beginningOfTheEnd)) { | ||
| 542 | // Successful CAS is the linearization point | ||
| 543 | // for all elements to be added to this queue. | ||
| 544 | if (!casTail(t, last)) { | ||
| 545 | // Try a little harder to update tail, | ||
| 546 | // since we may be adding many elements. | ||
| 547 | t = tail; | ||
| 548 | if (last.next == null) | ||
| 549 | casTail(t, last); | ||
| 550 | } | ||
| 551 | return true; | ||
| 552 | } | ||
| 553 | // Lost CAS race to another thread; re-read next | ||
| 554 | } | ||
| 555 | else if (p == q) | ||
| 556 | // We have fallen off list. If tail is unchanged, it | ||
| 557 | // will also be off-list, in which case we need to | ||
| 558 | // jump to head, from which all live nodes are always | ||
| 559 | // reachable. Else the new tail is a better bet. | ||
| 560 | p = (t != (t = tail)) ? t : head; | ||
| 561 | else | ||
| 562 | // Check for tail updates after two hops. | ||
| 563 | p = (p != t && t != (t = tail)) ? t : q; | ||
| 564 | } | ||
| 565 | } | ||
| 566 | |||
| 567 | /** | ||
| 568 | * Returns an array containing all of the elements in this queue, in | ||
| 569 | * proper sequence. | ||
| 570 | * | ||
| 571 | * <p>The returned array will be "safe" in that no references to it are | ||
| 572 | * maintained by this queue. (In other words, this method must allocate | ||
| 573 | * a new array). The caller is thus free to modify the returned array. | ||
| 574 | * | ||
| 575 | * <p>This method acts as bridge between array-based and collection-based | ||
| 576 | * APIs. | ||
| 577 | * | ||
| 578 | * @return an array containing all of the elements in this queue | ||
| 579 | */ | ||
| 580 | public Object[] toArray() { | ||
| 581 | // Use ArrayList to deal with resizing. | ||
| 582 | ArrayList<E> al = new ArrayList<E>(); | ||
| 583 | for (Node<E> p = first(); p != null; p = succ(p)) { | ||
| 584 | E item = p.item; | ||
| 585 | if (item != null) | ||
| 586 | al.add(item); | ||
| 587 | } | ||
| 588 | return al.toArray(); | ||
| 589 | } | ||
| 590 | |||
| 591 | /** | ||
| 592 | * Returns an array containing all of the elements in this queue, in | ||
| 593 | * proper sequence; the runtime type of the returned array is that of | ||
| 594 | * the specified array. If the queue fits in the specified array, it | ||
| 595 | * is returned therein. Otherwise, a new array is allocated with the | ||
| 596 | * runtime type of the specified array and the size of this queue. | ||
| 597 | * | ||
| 598 | * <p>If this queue fits in the specified array with room to spare | ||
| 599 | * (i.e., the array has more elements than this queue), the element in | ||
| 600 | * the array immediately following the end of the queue is set to | ||
| 601 | * {@code null}. | ||
| 602 | * | ||
| 603 | * <p>Like the {@link #toArray()} method, this method acts as bridge between | ||
| 604 | * array-based and collection-based APIs. Further, this method allows | ||
| 605 | * precise control over the runtime type of the output array, and may, | ||
| 606 | * under certain circumstances, be used to save allocation costs. | ||
| 607 | * | ||
| 608 | * <p>Suppose {@code x} is a queue known to contain only strings. | ||
| 609 | * The following code can be used to dump the queue into a newly | ||
| 610 | * allocated array of {@code String}: | ||
| 611 | * | ||
| 612 | * <pre> | ||
| 613 | * String[] y = x.toArray(new String[0]);</pre> | ||
| 614 | * | ||
| 615 | * Note that {@code toArray(new Object[0])} is identical in function to | ||
| 616 | * {@code toArray()}. | ||
| 617 | * | ||
| 618 | * @param a the array into which the elements of the queue are to | ||
| 619 | * be stored, if it is big enough; otherwise, a new array of the | ||
| 620 | * same runtime type is allocated for this purpose | ||
| 621 | * @return an array containing all of the elements in this queue | ||
| 622 | * @throws ArrayStoreException if the runtime type of the specified array | ||
| 623 | * is not a supertype of the runtime type of every element in | ||
| 624 | * this queue | ||
| 625 | * @throws NullPointerException if the specified array is null | ||
| 626 | */ | ||
| 627 | @SuppressWarnings("unchecked") | ||
| 628 | public <T> T[] toArray(T[] a) { | ||
| 629 | // try to use sent-in array | ||
| 630 | int k = 0; | ||
| 631 | Node<E> p; | ||
| 632 | for (p = first(); p != null && k < a.length; p = succ(p)) { | ||
| 633 | E item = p.item; | ||
| 634 | if (item != null) | ||
| 635 | a[k++] = (T)item; | ||
| 636 | } | ||
| 637 | if (p == null) { | ||
| 638 | if (k < a.length) | ||
| 639 | a[k] = null; | ||
| 640 | return a; | ||
| 641 | } | ||
| 642 | |||
| 643 | // If won't fit, use ArrayList version | ||
| 644 | ArrayList<E> al = new ArrayList<E>(); | ||
| 645 | for (Node<E> q = first(); q != null; q = succ(q)) { | ||
| 646 | E item = q.item; | ||
| 647 | if (item != null) | ||
| 648 | al.add(item); | ||
| 649 | } | ||
| 650 | return al.toArray(a); | ||
| 651 | } | ||
| 652 | |||
| 653 | /** | ||
| 654 | * Returns an iterator over the elements in this queue in proper sequence. | ||
| 655 | * The elements will be returned in order from first (head) to last (tail). | ||
| 656 | * | ||
| 657 | * <p>The returned iterator is a "weakly consistent" iterator that | ||
| 658 | * will never throw {@link java.util.ConcurrentModificationException | ||
| 659 | * ConcurrentModificationException}, and guarantees to traverse | ||
| 660 | * elements as they existed upon construction of the iterator, and | ||
| 661 | * may (but is not guaranteed to) reflect any modifications | ||
| 662 | * subsequent to construction. | ||
| 663 | * | ||
| 664 | * @return an iterator over the elements in this queue in proper sequence | ||
| 665 | */ | ||
| 666 | public Iterator<E> iterator() { | ||
| 667 | return new Itr(); | ||
| 668 | } | ||
| 669 | |||
| 670 | private class Itr implements Iterator<E> { | ||
| 671 | /** | ||
| 672 | * Next node to return item for. | ||
| 673 | */ | ||
| 674 | private Node<E> nextNode; | ||
| 675 | |||
| 676 | /** | ||
| 677 | * nextItem holds on to item fields because once we claim | ||
| 678 | * that an element exists in hasNext(), we must return it in | ||
| 679 | * the following next() call even if it was in the process of | ||
| 680 | * being removed when hasNext() was called. | ||
| 681 | */ | ||
| 682 | private E nextItem; | ||
| 683 | |||
| 684 | /** | ||
| 685 | * Node of the last returned item, to support remove. | ||
| 686 | */ | ||
| 687 | private Node<E> lastRet; | ||
| 688 | |||
| 689 | Itr() { | ||
| 690 | advance(); | ||
| 691 | } | ||
| 692 | |||
| 693 | /** | ||
| 694 | * Moves to next valid node and returns item to return for | ||
| 695 | * next(), or null if no such. | ||
| 696 | */ | ||
| 697 | private E advance() { | ||
| 698 | lastRet = nextNode; | ||
| 699 | E x = nextItem; | ||
| 700 | |||
| 701 | Node<E> pred, p; | ||
| 702 | if (nextNode == null) { | ||
| 703 | p = first(); | ||
| 704 | pred = null; | ||
| 705 | } else { | ||
| 706 | pred = nextNode; | ||
| 707 | p = succ(nextNode); | ||
| 708 | } | ||
| 709 | |||
| 710 | for (;;) { | ||
| 711 | if (p == null) { | ||
| 712 | nextNode = null; | ||
| 713 | nextItem = null; | ||
| 714 | return x; | ||
| 715 | } | ||
| 716 | E item = p.item; | ||
| 717 | if (item != null) { | ||
| 718 | nextNode = p; | ||
| 719 | nextItem = item; | ||
| 720 | return x; | ||
| 721 | } else { | ||
| 722 | // skip over nulls | ||
| 723 | Node<E> next = succ(p); | ||
| 724 | if (pred != null && next != null) | ||
| 725 | pred.casNext(p, next); | ||
| 726 | p = next; | ||
| 727 | } | ||
| 728 | } | ||
| 729 | } | ||
| 730 | |||
| 731 | public boolean hasNext() { | ||
| 732 | return nextNode != null; | ||
| 733 | } | ||
| 734 | |||
| 735 | public E next() { | ||
| 736 | if (nextNode == null) throw new NoSuchElementException(); | ||
| 737 | return advance(); | ||
| 738 | } | ||
| 739 | |||
| 740 | public void remove() { | ||
| 741 | Node<E> l = lastRet; | ||
| 742 | if (l == null) throw new IllegalStateException(); | ||
| 743 | // rely on a future traversal to relink. | ||
| 744 | l.item = null; | ||
| 745 | lastRet = null; | ||
| 746 | } | ||
| 747 | } | ||
| 748 | |||
| 749 | /** | ||
| 750 | * Saves the state to a stream (that is, serializes it). | ||
| 751 | * | ||
| 752 | * @serialData All of the elements (each an {@code E}) in | ||
| 753 | * the proper order, followed by a null | ||
| 754 | * @param s the stream | ||
| 755 | */ | ||
| 756 | private void writeObject(java.io.ObjectOutputStream s) | ||
| 757 | throws java.io.IOException { | ||
| 758 | |||
| 759 | // Write out any hidden stuff | ||
| 760 | s.defaultWriteObject(); | ||
| 761 | |||
| 762 | // Write out all elements in the proper order. | ||
| 763 | for (Node<E> p = first(); p != null; p = succ(p)) { | ||
| 764 | Object item = p.item; | ||
| 765 | if (item != null) | ||
| 766 | s.writeObject(item); | ||
| 767 | } | ||
| 768 | |||
| 769 | // Use trailing null as sentinel | ||
| 770 | s.writeObject(null); | ||
| 771 | } | ||
| 772 | |||
| 773 | /** | ||
| 774 | * Reconstitutes the instance from a stream (that is, deserializes it). | ||
| 775 | * @param s the stream | ||
| 776 | */ | ||
| 777 | private void readObject(java.io.ObjectInputStream s) | ||
| 778 | throws java.io.IOException, ClassNotFoundException { | ||
| 779 | s.defaultReadObject(); | ||
| 780 | |||
| 781 | // Read in elements until trailing null sentinel found | ||
| 782 | Node<E> h = null, t = null; | ||
| 783 | Object item; | ||
| 784 | while ((item = s.readObject()) != null) { | ||
| 785 | @SuppressWarnings("unchecked") | ||
| 786 | Node<E> newNode = new Node<E>((E) item); | ||
| 787 | if (h == null) | ||
| 788 | h = t = newNode; | ||
| 789 | else { | ||
| 790 | t.lazySetNext(newNode); | ||
| 791 | t = newNode; | ||
| 792 | } | ||
| 793 | } | ||
| 794 | if (h == null) | ||
| 795 | h = t = new Node<E>(null); | ||
| 796 | head = h; | ||
| 797 | tail = t; | ||
| 798 | } | ||
| 799 | |||
| 800 | /** | ||
| 801 | * Throws NullPointerException if argument is null. | ||
| 802 | * | ||
| 803 | * @param v the element | ||
| 804 | */ | ||
| 805 | private static void checkNotNull(Object v) { | ||
| 806 | if (v == null) | ||
| 807 | throw new NullPointerException(); | ||
| 808 | } | ||
| 809 | |||
| 810 | private boolean casTail(Node<E> cmp, Node<E> val) { | ||
| 811 | return UNSAFE.compareAndSwapObject(this, tailOffset, cmp, val); | ||
| 812 | } | ||
| 813 | |||
| 814 | private boolean casHead(Node<E> cmp, Node<E> val) { | ||
| 815 | return UNSAFE.compareAndSwapObject(this, headOffset, cmp, val); | ||
| 816 | } | ||
| 817 | |||
| 818 | // Unsafe mechanics | ||
| 819 | |||
| 820 | private static final sun.misc.Unsafe UNSAFE; | ||
| 821 | private static final long headOffset; | ||
| 822 | private static final long tailOffset; | ||
| 823 | static { | ||
| 824 | try { | ||
| 825 | UNSAFE = sun.misc.Unsafe.getUnsafe(); | ||
| 826 | Class k = ConcurrentLinkedQueue.class; | ||
| 827 | headOffset = UNSAFE.objectFieldOffset | ||
| 828 | (k.getDeclaredField("head")); | ||
| 829 | tailOffset = UNSAFE.objectFieldOffset | ||
| 830 | (k.getDeclaredField("tail")); | ||
| 831 | } catch (Exception e) { | ||
| 832 | throw new Error(e); | ||
| 833 | } | ||
| 834 | } | ||
| 835 | } | ||