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 36package java.util.concurrent; 37 38import java.lang.invoke.MethodHandles; 39import java.lang.invoke.VarHandle; 40import java.util.AbstractCollection; 41import java.util.Arrays; 42import java.util.Collection; 43import java.util.Deque; 44import java.util.Iterator; 45import java.util.NoSuchElementException; 46import java.util.Objects; 47import java.util.Queue; 48import java.util.Spliterator; 49import java.util.Spliterators; 50import java.util.function.Consumer; 51import java.util.function.Predicate; 52 53/** 54 * An unbounded concurrent {@linkplain Deque deque} based on linked nodes. 55 * Concurrent insertion, removal, and access operations execute safely 56 * across multiple threads. 57 * A {@code ConcurrentLinkedDeque} is an appropriate choice when 58 * many threads will share access to a common collection. 59 * Like most other concurrent collection implementations, this class 60 * does not permit the use of {@code null} elements. 61 * 62 * <p>Iterators and spliterators are 63 * <a href="package-summary.html#Weakly"><i>weakly consistent</i></a>. 64 * 65 * <p>Beware that, unlike in most collections, the {@code size} method 66 * is <em>NOT</em> a constant-time operation. Because of the 67 * asynchronous nature of these deques, determining the current number 68 * of elements requires a traversal of the elements, and so may report 69 * inaccurate results if this collection is modified during traversal. 70 * 71 * <p>Bulk operations that add, remove, or examine multiple elements, 72 * such as {@link #addAll}, {@link #removeIf} or {@link #forEach}, 73 * are <em>not</em> guaranteed to be performed atomically. 74 * For example, a {@code forEach} traversal concurrent with an {@code 75 * addAll} operation might observe only some of the added elements. 76 * 77 * <p>This class and its iterator implement all of the <em>optional</em> 78 * methods of the {@link Deque} and {@link Iterator} interfaces. 79 * 80 * <p>Memory consistency effects: As with other concurrent collections, 81 * actions in a thread prior to placing an object into a 82 * {@code ConcurrentLinkedDeque} 83 * <a href="package-summary.html#MemoryVisibility"><i>happen-before</i></a> 84 * actions subsequent to the access or removal of that element from 85 * the {@code ConcurrentLinkedDeque} in another thread. 86 * 87 * <p>This class is a member of the 88 * <a href="{@docRoot}/java/util/package-summary.html#CollectionsFramework"> 89 * Java Collections Framework</a>. 90 * 91 * @since 1.7 92 * @author Doug Lea 93 * @author Martin Buchholz 94 * @param <E> the type of elements held in this deque 95 */ 96public class ConcurrentLinkedDeque<E> 97 extends AbstractCollection<E> 98 implements Deque<E>, java.io.Serializable { 99 100 /* 101 * This is an implementation of a concurrent lock-free deque 102 * supporting interior removes but not interior insertions, as 103 * required to support the entire Deque interface. 104 * 105 * We extend the techniques developed for ConcurrentLinkedQueue and 106 * LinkedTransferQueue (see the internal docs for those classes). 107 * Understanding the ConcurrentLinkedQueue implementation is a 108 * prerequisite for understanding the implementation of this class. 109 * 110 * The data structure is a symmetrical doubly-linked "GC-robust" 111 * linked list of nodes. We minimize the number of volatile writes 112 * using two techniques: advancing multiple hops with a single CAS 113 * and mixing volatile and non-volatile writes of the same memory 114 * locations. 115 * 116 * A node contains the expected E ("item") and links to predecessor 117 * ("prev") and successor ("next") nodes: 118 * 119 * class Node<E> { volatile Node<E> prev, next; volatile E item; } 120 * 121 * A node p is considered "live" if it contains a non-null item 122 * (p.item != null). When an item is CASed to null, the item is 123 * atomically logically deleted from the collection. 124 * 125 * At any time, there is precisely one "first" node with a null 126 * prev reference that terminates any chain of prev references 127 * starting at a live node. Similarly there is precisely one 128 * "last" node terminating any chain of next references starting at 129 * a live node. The "first" and "last" nodes may or may not be live. 130 * The "first" and "last" nodes are always mutually reachable. 131 * 132 * A new element is added atomically by CASing the null prev or 133 * next reference in the first or last node to a fresh node 134 * containing the element. The element's node atomically becomes 135 * "live" at that point. 136 * 137 * A node is considered "active" if it is a live node, or the 138 * first or last node. Active nodes cannot be unlinked. 139 * 140 * A "self-link" is a next or prev reference that is the same node: 141 * p.prev == p or p.next == p 142 * Self-links are used in the node unlinking process. Active nodes 143 * never have self-links. 144 * 145 * A node p is active if and only if: 146 * 147 * p.item != null || 148 * (p.prev == null && p.next != p) || 149 * (p.next == null && p.prev != p) 150 * 151 * The deque object has two node references, "head" and "tail". 152 * The head and tail are only approximations to the first and last 153 * nodes of the deque. The first node can always be found by 154 * following prev pointers from head; likewise for tail. However, 155 * it is permissible for head and tail to be referring to deleted 156 * nodes that have been unlinked and so may not be reachable from 157 * any live node. 158 * 159 * There are 3 stages of node deletion; 160 * "logical deletion", "unlinking", and "gc-unlinking". 161 * 162 * 1. "logical deletion" by CASing item to null atomically removes 163 * the element from the collection, and makes the containing node 164 * eligible for unlinking. 165 * 166 * 2. "unlinking" makes a deleted node unreachable from active 167 * nodes, and thus eventually reclaimable by GC. Unlinked nodes 168 * may remain reachable indefinitely from an iterator. 169 * 170 * Physical node unlinking is merely an optimization (albeit a 171 * critical one), and so can be performed at our convenience. At 172 * any time, the set of live nodes maintained by prev and next 173 * links are identical, that is, the live nodes found via next 174 * links from the first node is equal to the elements found via 175 * prev links from the last node. However, this is not true for 176 * nodes that have already been logically deleted - such nodes may 177 * be reachable in one direction only. 178 * 179 * 3. "gc-unlinking" takes unlinking further by making active 180 * nodes unreachable from deleted nodes, making it easier for the 181 * GC to reclaim future deleted nodes. This step makes the data 182 * structure "gc-robust", as first described in detail by Boehm 183 * (http://portal.acm.org/citation.cfm?doid=503272.503282). 184 * 185 * GC-unlinked nodes may remain reachable indefinitely from an 186 * iterator, but unlike unlinked nodes, are never reachable from 187 * head or tail. 188 * 189 * Making the data structure GC-robust will eliminate the risk of 190 * unbounded memory retention with conservative GCs and is likely 191 * to improve performance with generational GCs. 192 * 193 * When a node is dequeued at either end, e.g. via poll(), we would 194 * like to break any references from the node to active nodes. We 195 * develop further the use of self-links that was very effective in 196 * other concurrent collection classes. The idea is to replace 197 * prev and next pointers with special values that are interpreted 198 * to mean off-the-list-at-one-end. These are approximations, but 199 * good enough to preserve the properties we want in our 200 * traversals, e.g. we guarantee that a traversal will never visit 201 * the same element twice, but we don't guarantee whether a 202 * traversal that runs out of elements will be able to see more 203 * elements later after enqueues at that end. Doing gc-unlinking 204 * safely is particularly tricky, since any node can be in use 205 * indefinitely (for example by an iterator). We must ensure that 206 * the nodes pointed at by head/tail never get gc-unlinked, since 207 * head/tail are needed to get "back on track" by other nodes that 208 * are gc-unlinked. gc-unlinking accounts for much of the 209 * implementation complexity. 210 * 211 * Since neither unlinking nor gc-unlinking are necessary for 212 * correctness, there are many implementation choices regarding 213 * frequency (eagerness) of these operations. Since volatile 214 * reads are likely to be much cheaper than CASes, saving CASes by 215 * unlinking multiple adjacent nodes at a time may be a win. 216 * gc-unlinking can be performed rarely and still be effective, 217 * since it is most important that long chains of deleted nodes 218 * are occasionally broken. 219 * 220 * The actual representation we use is that p.next == p means to 221 * goto the first node (which in turn is reached by following prev 222 * pointers from head), and p.next == null && p.prev == p means 223 * that the iteration is at an end and that p is a (static final) 224 * dummy node, NEXT_TERMINATOR, and not the last active node. 225 * Finishing the iteration when encountering such a TERMINATOR is 226 * good enough for read-only traversals, so such traversals can use 227 * p.next == null as the termination condition. When we need to 228 * find the last (active) node, for enqueueing a new node, we need 229 * to check whether we have reached a TERMINATOR node; if so, 230 * restart traversal from tail. 231 * 232 * The implementation is completely directionally symmetrical, 233 * except that most public methods that iterate through the list 234 * follow next pointers ("forward" direction). 235 * 236 * We believe (without full proof) that all single-element deque 237 * operations (e.g., addFirst, peekLast, pollLast) are linearizable 238 * (see Herlihy and Shavit's book). However, some combinations of 239 * operations are known not to be linearizable. In particular, 240 * when an addFirst(A) is racing with pollFirst() removing B, it is 241 * possible for an observer iterating over the elements to observe 242 * A B C and subsequently observe A C, even though no interior 243 * removes are ever performed. Nevertheless, iterators behave 244 * reasonably, providing the "weakly consistent" guarantees. 245 * 246 * Empirically, microbenchmarks suggest that this class adds about 247 * 40% overhead relative to ConcurrentLinkedQueue, which feels as 248 * good as we can hope for. 249 */ 250 251 private static final long serialVersionUID = 876323262645176354L; 252 253 /** 254 * A node from which the first node on list (that is, the unique node p 255 * with p.prev == null && p.next != p) can be reached in O(1) time. 256 * Invariants: 257 * - the first node is always O(1) reachable from head via prev links 258 * - all live nodes are reachable from the first node via succ() 259 * - head != null 260 * - (tmp = head).next != tmp || tmp != head 261 * - head is never gc-unlinked (but may be unlinked) 262 * Non-invariants: 263 * - head.item may or may not be null 264 * - head may not be reachable from the first or last node, or from tail 265 */ 266 private transient volatile Node<E> head; 267 268 /** 269 * A node from which the last node on list (that is, the unique node p 270 * with p.next == null && p.prev != p) can be reached in O(1) time. 271 * Invariants: 272 * - the last node is always O(1) reachable from tail via next links 273 * - all live nodes are reachable from the last node via pred() 274 * - tail != null 275 * - tail is never gc-unlinked (but may be unlinked) 276 * Non-invariants: 277 * - tail.item may or may not be null 278 * - tail may not be reachable from the first or last node, or from head 279 */ 280 private transient volatile Node<E> tail; 281 282 private static final Node<Object> PREV_TERMINATOR, NEXT_TERMINATOR; 283 284 @SuppressWarnings("unchecked") 285 Node<E> prevTerminator() { 286 return (Node<E>) PREV_TERMINATOR; 287 } 288 289 @SuppressWarnings("unchecked") 290 Node<E> nextTerminator() { 291 return (Node<E>) NEXT_TERMINATOR; 292 } 293 294 static final class Node<E> { 295 volatile Node<E> prev; 296 volatile E item; 297 volatile Node<E> next; 298 } 299 300 /** 301 * Returns a new node holding item. Uses relaxed write because item 302 * can only be seen after piggy-backing publication via CAS. 303 */ 304 static <E> Node<E> newNode(E item) { 305 Node<E> node = new Node<E>(); 306 ITEM.set(node, item); 307 return node; 308 } 309 310 /** 311 * Links e as first element. 312 */ 313 private void linkFirst(E e) { 314 final Node<E> newNode = newNode(Objects.requireNonNull(e)); 315 316 restartFromHead: 317 for (;;) 318 for (Node<E> h = head, p = h, q;;) { 319 if ((q = p.prev) != null && 320 (q = (p = q).prev) != null) 321 // Check for head updates every other hop. 322 // If p == q, we are sure to follow head instead. 323 p = (h != (h = head)) ? h : q; 324 else if (p.next == p) // PREV_TERMINATOR 325 continue restartFromHead; 326 else { 327 // p is first node 328 NEXT.set(newNode, p); // CAS piggyback 329 if (PREV.compareAndSet(p, null, newNode)) { 330 // Successful CAS is the linearization point 331 // for e to become an element of this deque, 332 // and for newNode to become "live". 333 if (p != h) // hop two nodes at a time; failure is OK 334 HEAD.weakCompareAndSet(this, h, newNode); 335 return; 336 } 337 // Lost CAS race to another thread; re-read prev 338 } 339 } 340 } 341 342 /** 343 * Links e as last element. 344 */ 345 private void linkLast(E e) { 346 final Node<E> newNode = newNode(Objects.requireNonNull(e)); 347 348 restartFromTail: 349 for (;;) 350 for (Node<E> t = tail, p = t, q;;) { 351 if ((q = p.next) != null && 352 (q = (p = q).next) != null) 353 // Check for tail updates every other hop. 354 // If p == q, we are sure to follow tail instead. 355 p = (t != (t = tail)) ? t : q; 356 else if (p.prev == p) // NEXT_TERMINATOR 357 continue restartFromTail; 358 else { 359 // p is last node 360 PREV.set(newNode, p); // CAS piggyback 361 if (NEXT.compareAndSet(p, null, newNode)) { 362 // Successful CAS is the linearization point 363 // for e to become an element of this deque, 364 // and for newNode to become "live". 365 if (p != t) // hop two nodes at a time; failure is OK 366 TAIL.weakCompareAndSet(this, t, newNode); 367 return; 368 } 369 // Lost CAS race to another thread; re-read next 370 } 371 } 372 } 373 374 private static final int HOPS = 2; 375 376 /** 377 * Unlinks non-null node x. 378 */ 379 void unlink(Node<E> x) { 380 // assert x != null; 381 // assert x.item == null; 382 // assert x != PREV_TERMINATOR; 383 // assert x != NEXT_TERMINATOR; 384 385 final Node<E> prev = x.prev; 386 final Node<E> next = x.next; 387 if (prev == null) { 388 unlinkFirst(x, next); 389 } else if (next == null) { 390 unlinkLast(x, prev); 391 } else { 392 // Unlink interior node. 393 // 394 // This is the common case, since a series of polls at the 395 // same end will be "interior" removes, except perhaps for 396 // the first one, since end nodes cannot be unlinked. 397 // 398 // At any time, all active nodes are mutually reachable by 399 // following a sequence of either next or prev pointers. 400 // 401 // Our strategy is to find the unique active predecessor 402 // and successor of x. Try to fix up their links so that 403 // they point to each other, leaving x unreachable from 404 // active nodes. If successful, and if x has no live 405 // predecessor/successor, we additionally try to gc-unlink, 406 // leaving active nodes unreachable from x, by rechecking 407 // that the status of predecessor and successor are 408 // unchanged and ensuring that x is not reachable from 409 // tail/head, before setting x's prev/next links to their 410 // logical approximate replacements, self/TERMINATOR. 411 Node<E> activePred, activeSucc; 412 boolean isFirst, isLast; 413 int hops = 1; 414 415 // Find active predecessor 416 for (Node<E> p = prev; ; ++hops) { 417 if (p.item != null) { 418 activePred = p; 419 isFirst = false; 420 break; 421 } 422 Node<E> q = p.prev; 423 if (q == null) { 424 if (p.next == p) 425 return; 426 activePred = p; 427 isFirst = true; 428 break; 429 } 430 else if (p == q) 431 return; 432 else 433 p = q; 434 } 435 436 // Find active successor 437 for (Node<E> p = next; ; ++hops) { 438 if (p.item != null) { 439 activeSucc = p; 440 isLast = false; 441 break; 442 } 443 Node<E> q = p.next; 444 if (q == null) { 445 if (p.prev == p) 446 return; 447 activeSucc = p; 448 isLast = true; 449 break; 450 } 451 else if (p == q) 452 return; 453 else 454 p = q; 455 } 456 457 // TODO: better HOP heuristics 458 if (hops < HOPS 459 // always squeeze out interior deleted nodes 460 && (isFirst | isLast)) 461 return; 462 463 // Squeeze out deleted nodes between activePred and 464 // activeSucc, including x. 465 skipDeletedSuccessors(activePred); 466 skipDeletedPredecessors(activeSucc); 467 468 // Try to gc-unlink, if possible 469 if ((isFirst | isLast) && 470 471 // Recheck expected state of predecessor and successor 472 (activePred.next == activeSucc) && 473 (activeSucc.prev == activePred) && 474 (isFirst ? activePred.prev == null : activePred.item != null) && 475 (isLast ? activeSucc.next == null : activeSucc.item != null)) { 476 477 updateHead(); // Ensure x is not reachable from head 478 updateTail(); // Ensure x is not reachable from tail 479 480 // Finally, actually gc-unlink 481 PREV.setRelease(x, isFirst ? prevTerminator() : x); 482 NEXT.setRelease(x, isLast ? nextTerminator() : x); 483 } 484 } 485 } 486 487 /** 488 * Unlinks non-null first node. 489 */ 490 private void unlinkFirst(Node<E> first, Node<E> next) { 491 // assert first != null; 492 // assert next != null; 493 // assert first.item == null; 494 for (Node<E> o = null, p = next, q;;) { 495 if (p.item != null || (q = p.next) == null) { 496 if (o != null && p.prev != p && 497 NEXT.compareAndSet(first, next, p)) { 498 skipDeletedPredecessors(p); 499 if (first.prev == null && 500 (p.next == null || p.item != null) && 501 p.prev == first) { 502 503 updateHead(); // Ensure o is not reachable from head 504 updateTail(); // Ensure o is not reachable from tail 505 506 // Finally, actually gc-unlink 507 NEXT.setRelease(o, o); 508 PREV.setRelease(o, prevTerminator()); 509 } 510 } 511 return; 512 } 513 else if (p == q) 514 return; 515 else { 516 o = p; 517 p = q; 518 } 519 } 520 } 521 522 /** 523 * Unlinks non-null last node. 524 */ 525 private void unlinkLast(Node<E> last, Node<E> prev) { 526 // assert last != null; 527 // assert prev != null; 528 // assert last.item == null; 529 for (Node<E> o = null, p = prev, q;;) { 530 if (p.item != null || (q = p.prev) == null) { 531 if (o != null && p.next != p && 532 PREV.compareAndSet(last, prev, p)) { 533 skipDeletedSuccessors(p); 534 if (last.next == null && 535 (p.prev == null || p.item != null) && 536 p.next == last) { 537 538 updateHead(); // Ensure o is not reachable from head 539 updateTail(); // Ensure o is not reachable from tail 540 541 // Finally, actually gc-unlink 542 PREV.setRelease(o, o); 543 NEXT.setRelease(o, nextTerminator()); 544 } 545 } 546 return; 547 } 548 else if (p == q) 549 return; 550 else { 551 o = p; 552 p = q; 553 } 554 } 555 } 556 557 /** 558 * Guarantees that any node which was unlinked before a call to 559 * this method will be unreachable from head after it returns. 560 * Does not guarantee to eliminate slack, only that head will 561 * point to a node that was active while this method was running. 562 */ 563 private final void updateHead() { 564 // Either head already points to an active node, or we keep 565 // trying to cas it to the first node until it does. 566 Node<E> h, p, q; 567 restartFromHead: 568 while ((h = head).item == null && (p = h.prev) != null) { 569 for (;;) { 570 if ((q = p.prev) == null || 571 (q = (p = q).prev) == null) { 572 // It is possible that p is PREV_TERMINATOR, 573 // but if so, the CAS is guaranteed to fail. 574 if (HEAD.compareAndSet(this, h, p)) 575 return; 576 else 577 continue restartFromHead; 578 } 579 else if (h != head) 580 continue restartFromHead; 581 else 582 p = q; 583 } 584 } 585 } 586 587 /** 588 * Guarantees that any node which was unlinked before a call to 589 * this method will be unreachable from tail after it returns. 590 * Does not guarantee to eliminate slack, only that tail will 591 * point to a node that was active while this method was running. 592 */ 593 private final void updateTail() { 594 // Either tail already points to an active node, or we keep 595 // trying to cas it to the last node until it does. 596 Node<E> t, p, q; 597 restartFromTail: 598 while ((t = tail).item == null && (p = t.next) != null) { 599 for (;;) { 600 if ((q = p.next) == null || 601 (q = (p = q).next) == null) { 602 // It is possible that p is NEXT_TERMINATOR, 603 // but if so, the CAS is guaranteed to fail. 604 if (TAIL.compareAndSet(this, t, p)) 605 return; 606 else 607 continue restartFromTail; 608 } 609 else if (t != tail) 610 continue restartFromTail; 611 else 612 p = q; 613 } 614 } 615 } 616 617 private void skipDeletedPredecessors(Node<E> x) { 618 whileActive: 619 do { 620 Node<E> prev = x.prev; 621 // assert prev != null; 622 // assert x != NEXT_TERMINATOR; 623 // assert x != PREV_TERMINATOR; 624 Node<E> p = prev; 625 findActive: 626 for (;;) { 627 if (p.item != null) 628 break findActive; 629 Node<E> q = p.prev; 630 if (q == null) { 631 if (p.next == p) 632 continue whileActive; 633 break findActive; 634 } 635 else if (p == q) 636 continue whileActive; 637 else 638 p = q; 639 } 640 641 // found active CAS target 642 if (prev == p || PREV.compareAndSet(x, prev, p)) 643 return; 644 645 } while (x.item != null || x.next == null); 646 } 647 648 private void skipDeletedSuccessors(Node<E> x) { 649 whileActive: 650 do { 651 Node<E> next = x.next; 652 // assert next != null; 653 // assert x != NEXT_TERMINATOR; 654 // assert x != PREV_TERMINATOR; 655 Node<E> p = next; 656 findActive: 657 for (;;) { 658 if (p.item != null) 659 break findActive; 660 Node<E> q = p.next; 661 if (q == null) { 662 if (p.prev == p) 663 continue whileActive; 664 break findActive; 665 } 666 else if (p == q) 667 continue whileActive; 668 else 669 p = q; 670 } 671 672 // found active CAS target 673 if (next == p || NEXT.compareAndSet(x, next, p)) 674 return; 675 676 } while (x.item != null || x.prev == null); 677 } 678 679 /** 680 * Returns the successor of p, or the first node if p.next has been 681 * linked to self, which will only be true if traversing with a 682 * stale pointer that is now off the list. 683 */ 684 final Node<E> succ(Node<E> p) { 685 // TODO: should we skip deleted nodes here? 686 if (p == (p = p.next)) 687 p = first(); 688 return p; 689 } 690 691 /** 692 * Returns the predecessor of p, or the last node if p.prev has been 693 * linked to self, which will only be true if traversing with a 694 * stale pointer that is now off the list. 695 */ 696 final Node<E> pred(Node<E> p) { 697 Node<E> q = p.prev; 698 return (p == q) ? last() : q; 699 } 700 701 /** 702 * Returns the first node, the unique node p for which: 703 * p.prev == null && p.next != p 704 * The returned node may or may not be logically deleted. 705 * Guarantees that head is set to the returned node. 706 */ 707 Node<E> first() { 708 restartFromHead: 709 for (;;) 710 for (Node<E> h = head, p = h, q;;) { 711 if ((q = p.prev) != null && 712 (q = (p = q).prev) != null) 713 // Check for head updates every other hop. 714 // If p == q, we are sure to follow head instead. 715 p = (h != (h = head)) ? h : q; 716 else if (p == h 717 // It is possible that p is PREV_TERMINATOR, 718 // but if so, the CAS is guaranteed to fail. 719 || HEAD.compareAndSet(this, h, p)) 720 return p; 721 else 722 continue restartFromHead; 723 } 724 } 725 726 /** 727 * Returns the last node, the unique node p for which: 728 * p.next == null && p.prev != p 729 * The returned node may or may not be logically deleted. 730 * Guarantees that tail is set to the returned node. 731 */ 732 Node<E> last() { 733 restartFromTail: 734 for (;;) 735 for (Node<E> t = tail, p = t, q;;) { 736 if ((q = p.next) != null && 737 (q = (p = q).next) != null) 738 // Check for tail updates every other hop. 739 // If p == q, we are sure to follow tail instead. 740 p = (t != (t = tail)) ? t : q; 741 else if (p == t 742 // It is possible that p is NEXT_TERMINATOR, 743 // but if so, the CAS is guaranteed to fail. 744 || TAIL.compareAndSet(this, t, p)) 745 return p; 746 else 747 continue restartFromTail; 748 } 749 } 750 751 // Minor convenience utilities 752 753 /** 754 * Returns element unless it is null, in which case throws 755 * NoSuchElementException. 756 * 757 * @param v the element 758 * @return the element 759 */ 760 private E screenNullResult(E v) { 761 if (v == null) 762 throw new NoSuchElementException(); 763 return v; 764 } 765 766 /** 767 * Constructs an empty deque. 768 */ 769 public ConcurrentLinkedDeque() { 770 head = tail = new Node<E>(); 771 } 772 773 /** 774 * Constructs a deque initially containing the elements of 775 * the given collection, added in traversal order of the 776 * collection's iterator. 777 * 778 * @param c the collection of elements to initially contain 779 * @throws NullPointerException if the specified collection or any 780 * of its elements are null 781 */ 782 public ConcurrentLinkedDeque(Collection<? extends E> c) { 783 // Copy c into a private chain of Nodes 784 Node<E> h = null, t = null; 785 for (E e : c) { 786 Node<E> newNode = newNode(Objects.requireNonNull(e)); 787 if (h == null) 788 h = t = newNode; 789 else { 790 NEXT.set(t, newNode); 791 PREV.set(newNode, t); 792 t = newNode; 793 } 794 } 795 initHeadTail(h, t); 796 } 797 798 /** 799 * Initializes head and tail, ensuring invariants hold. 800 */ 801 private void initHeadTail(Node<E> h, Node<E> t) { 802 if (h == t) { 803 if (h == null) 804 h = t = new Node<E>(); 805 else { 806 // Avoid edge case of a single Node with non-null item. 807 Node<E> newNode = new Node<E>(); 808 NEXT.set(t, newNode); 809 PREV.set(newNode, t); 810 t = newNode; 811 } 812 } 813 head = h; 814 tail = t; 815 } 816 817 /** 818 * Inserts the specified element at the front of this deque. 819 * As the deque is unbounded, this method will never throw 820 * {@link IllegalStateException}. 821 * 822 * @throws NullPointerException if the specified element is null 823 */ 824 public void addFirst(E e) { 825 linkFirst(e); 826 } 827 828 /** 829 * Inserts the specified element at the end of this deque. 830 * As the deque is unbounded, this method will never throw 831 * {@link IllegalStateException}. 832 * 833 * <p>This method is equivalent to {@link #add}. 834 * 835 * @throws NullPointerException if the specified element is null 836 */ 837 public void addLast(E e) { 838 linkLast(e); 839 } 840 841 /** 842 * Inserts the specified element at the front of this deque. 843 * As the deque is unbounded, this method will never return {@code false}. 844 * 845 * @return {@code true} (as specified by {@link Deque#offerFirst}) 846 * @throws NullPointerException if the specified element is null 847 */ 848 public boolean offerFirst(E e) { 849 linkFirst(e); 850 return true; 851 } 852 853 /** 854 * Inserts the specified element at the end of this deque. 855 * As the deque is unbounded, this method will never return {@code false}. 856 * 857 * <p>This method is equivalent to {@link #add}. 858 * 859 * @return {@code true} (as specified by {@link Deque#offerLast}) 860 * @throws NullPointerException if the specified element is null 861 */ 862 public boolean offerLast(E e) { 863 linkLast(e); 864 return true; 865 } 866 867 public E peekFirst() { 868 for (Node<E> p = first(); p != null; p = succ(p)) { 869 final E item; 870 if ((item = p.item) != null) 871 return item; 872 } 873 return null; 874 } 875 876 public E peekLast() { 877 for (Node<E> p = last(); p != null; p = pred(p)) { 878 final E item; 879 if ((item = p.item) != null) 880 return item; 881 } 882 return null; 883 } 884 885 /** 886 * @throws NoSuchElementException {@inheritDoc} 887 */ 888 public E getFirst() { 889 return screenNullResult(peekFirst()); 890 } 891 892 /** 893 * @throws NoSuchElementException {@inheritDoc} 894 */ 895 public E getLast() { 896 return screenNullResult(peekLast()); 897 } 898 899 public E pollFirst() { 900 for (Node<E> p = first(); p != null; p = succ(p)) { 901 final E item; 902 if ((item = p.item) != null 903 && ITEM.compareAndSet(p, item, null)) { 904 unlink(p); 905 return item; 906 } 907 } 908 return null; 909 } 910 911 public E pollLast() { 912 for (Node<E> p = last(); p != null; p = pred(p)) { 913 final E item; 914 if ((item = p.item) != null 915 && ITEM.compareAndSet(p, item, null)) { 916 unlink(p); 917 return item; 918 } 919 } 920 return null; 921 } 922 923 /** 924 * @throws NoSuchElementException {@inheritDoc} 925 */ 926 public E removeFirst() { 927 return screenNullResult(pollFirst()); 928 } 929 930 /** 931 * @throws NoSuchElementException {@inheritDoc} 932 */ 933 public E removeLast() { 934 return screenNullResult(pollLast()); 935 } 936 937 // *** Queue and stack methods *** 938 939 /** 940 * Inserts the specified element at the tail of this deque. 941 * As the deque is unbounded, this method will never return {@code false}. 942 * 943 * @return {@code true} (as specified by {@link Queue#offer}) 944 * @throws NullPointerException if the specified element is null 945 */ 946 public boolean offer(E e) { 947 return offerLast(e); 948 } 949 950 /** 951 * Inserts the specified element at the tail of this deque. 952 * As the deque is unbounded, this method will never throw 953 * {@link IllegalStateException} or return {@code false}. 954 * 955 * @return {@code true} (as specified by {@link Collection#add}) 956 * @throws NullPointerException if the specified element is null 957 */ 958 public boolean add(E e) { 959 return offerLast(e); 960 } 961 962 public E poll() { return pollFirst(); } 963 public E peek() { return peekFirst(); } 964 965 /** 966 * @throws NoSuchElementException {@inheritDoc} 967 */ 968 public E remove() { return removeFirst(); } 969 970 /** 971 * @throws NoSuchElementException {@inheritDoc} 972 */ 973 public E pop() { return removeFirst(); } 974 975 /** 976 * @throws NoSuchElementException {@inheritDoc} 977 */ 978 public E element() { return getFirst(); } 979 980 /** 981 * @throws NullPointerException {@inheritDoc} 982 */ 983 public void push(E e) { addFirst(e); } 984 985 /** 986 * Removes the first occurrence of the specified element from this deque. 987 * If the deque does not contain the element, it is unchanged. 988 * More formally, removes the first element {@code e} such that 989 * {@code o.equals(e)} (if such an element exists). 990 * Returns {@code true} if this deque contained the specified element 991 * (or equivalently, if this deque changed as a result of the call). 992 * 993 * @param o element to be removed from this deque, if present 994 * @return {@code true} if the deque contained the specified element 995 * @throws NullPointerException if the specified element is null 996 */ 997 public boolean removeFirstOccurrence(Object o) { 998 Objects.requireNonNull(o); 999 for (Node<E> p = first(); p != null; p = succ(p)) { 1000 final E item; 1001 if ((item = p.item) != null 1002 && o.equals(item) 1003 && ITEM.compareAndSet(p, item, null)) { 1004 unlink(p); 1005 return true; 1006 } 1007 } 1008 return false; 1009 } 1010 1011 /** 1012 * Removes the last occurrence of the specified element from this deque. 1013 * If the deque does not contain the element, it is unchanged. 1014 * More formally, removes the last element {@code e} such that 1015 * {@code o.equals(e)} (if such an element exists). 1016 * Returns {@code true} if this deque contained the specified element 1017 * (or equivalently, if this deque changed as a result of the call). 1018 * 1019 * @param o element to be removed from this deque, if present 1020 * @return {@code true} if the deque contained the specified element 1021 * @throws NullPointerException if the specified element is null 1022 */ 1023 public boolean removeLastOccurrence(Object o) { 1024 Objects.requireNonNull(o); 1025 for (Node<E> p = last(); p != null; p = pred(p)) { 1026 final E item; 1027 if ((item = p.item) != null 1028 && o.equals(item) 1029 && ITEM.compareAndSet(p, item, null)) { 1030 unlink(p); 1031 return true; 1032 } 1033 } 1034 return false; 1035 } 1036 1037 /** 1038 * Returns {@code true} if this deque contains the specified element. 1039 * More formally, returns {@code true} if and only if this deque contains 1040 * at least one element {@code e} such that {@code o.equals(e)}. 1041 * 1042 * @param o element whose presence in this deque is to be tested 1043 * @return {@code true} if this deque contains the specified element 1044 */ 1045 public boolean contains(Object o) { 1046 if (o != null) { 1047 for (Node<E> p = first(); p != null; p = succ(p)) { 1048 final E item; 1049 if ((item = p.item) != null && o.equals(item)) 1050 return true; 1051 } 1052 } 1053 return false; 1054 } 1055 1056 /** 1057 * Returns {@code true} if this collection contains no elements. 1058 * 1059 * @return {@code true} if this collection contains no elements 1060 */ 1061 public boolean isEmpty() { 1062 return peekFirst() == null; 1063 } 1064 1065 /** 1066 * Returns the number of elements in this deque. If this deque 1067 * contains more than {@code Integer.MAX_VALUE} elements, it 1068 * returns {@code Integer.MAX_VALUE}. 1069 * 1070 * <p>Beware that, unlike in most collections, this method is 1071 * <em>NOT</em> a constant-time operation. Because of the 1072 * asynchronous nature of these deques, determining the current 1073 * number of elements requires traversing them all to count them. 1074 * Additionally, it is possible for the size to change during 1075 * execution of this method, in which case the returned result 1076 * will be inaccurate. Thus, this method is typically not very 1077 * useful in concurrent applications. 1078 * 1079 * @return the number of elements in this deque 1080 */ 1081 public int size() { 1082 restartFromHead: for (;;) { 1083 int count = 0; 1084 for (Node<E> p = first(); p != null;) { 1085 if (p.item != null) 1086 if (++count == Integer.MAX_VALUE) 1087 break; // @see Collection.size() 1088 if (p == (p = p.next)) 1089 continue restartFromHead; 1090 } 1091 return count; 1092 } 1093 } 1094 1095 /** 1096 * Removes the first occurrence of the specified element from this deque. 1097 * If the deque does not contain the element, it is unchanged. 1098 * More formally, removes the first element {@code e} such that 1099 * {@code o.equals(e)} (if such an element exists). 1100 * Returns {@code true} if this deque contained the specified element 1101 * (or equivalently, if this deque changed as a result of the call). 1102 * 1103 * <p>This method is equivalent to {@link #removeFirstOccurrence(Object)}. 1104 * 1105 * @param o element to be removed from this deque, if present 1106 * @return {@code true} if the deque contained the specified element 1107 * @throws NullPointerException if the specified element is null 1108 */ 1109 public boolean remove(Object o) { 1110 return removeFirstOccurrence(o); 1111 } 1112 1113 /** 1114 * Appends all of the elements in the specified collection to the end of 1115 * this deque, in the order that they are returned by the specified 1116 * collection's iterator. Attempts to {@code addAll} of a deque to 1117 * itself result in {@code IllegalArgumentException}. 1118 * 1119 * @param c the elements to be inserted into this deque 1120 * @return {@code true} if this deque changed as a result of the call 1121 * @throws NullPointerException if the specified collection or any 1122 * of its elements are null 1123 * @throws IllegalArgumentException if the collection is this deque 1124 */ 1125 public boolean addAll(Collection<? extends E> c) { 1126 if (c == this) 1127 // As historically specified in AbstractQueue#addAll 1128 throw new IllegalArgumentException(); 1129 1130 // Copy c into a private chain of Nodes 1131 Node<E> beginningOfTheEnd = null, last = null; 1132 for (E e : c) { 1133 Node<E> newNode = newNode(Objects.requireNonNull(e)); 1134 if (beginningOfTheEnd == null) 1135 beginningOfTheEnd = last = newNode; 1136 else { 1137 NEXT.set(last, newNode); 1138 PREV.set(newNode, last); 1139 last = newNode; 1140 } 1141 } 1142 if (beginningOfTheEnd == null) 1143 return false; 1144 1145 // Atomically append the chain at the tail of this collection 1146 restartFromTail: 1147 for (;;) 1148 for (Node<E> t = tail, p = t, q;;) { 1149 if ((q = p.next) != null && 1150 (q = (p = q).next) != null) 1151 // Check for tail updates every other hop. 1152 // If p == q, we are sure to follow tail instead. 1153 p = (t != (t = tail)) ? t : q; 1154 else if (p.prev == p) // NEXT_TERMINATOR 1155 continue restartFromTail; 1156 else { 1157 // p is last node 1158 PREV.set(beginningOfTheEnd, p); // CAS piggyback 1159 if (NEXT.compareAndSet(p, null, beginningOfTheEnd)) { 1160 // Successful CAS is the linearization point 1161 // for all elements to be added to this deque. 1162 if (!TAIL.weakCompareAndSet(this, t, last)) { 1163 // Try a little harder to update tail, 1164 // since we may be adding many elements. 1165 t = tail; 1166 if (last.next == null) 1167 TAIL.weakCompareAndSet(this, t, last); 1168 } 1169 return true; 1170 } 1171 // Lost CAS race to another thread; re-read next 1172 } 1173 } 1174 } 1175 1176 /** 1177 * Removes all of the elements from this deque. 1178 */ 1179 public void clear() { 1180 while (pollFirst() != null) 1181 ; 1182 } 1183 1184 public String toString() { 1185 String[] a = null; 1186 restartFromHead: for (;;) { 1187 int charLength = 0; 1188 int size = 0; 1189 for (Node<E> p = first(); p != null;) { 1190 final E item; 1191 if ((item = p.item) != null) { 1192 if (a == null) 1193 a = new String[4]; 1194 else if (size == a.length) 1195 a = Arrays.copyOf(a, 2 * size); 1196 String s = item.toString(); 1197 a[size++] = s; 1198 charLength += s.length(); 1199 } 1200 if (p == (p = p.next)) 1201 continue restartFromHead; 1202 } 1203 1204 if (size == 0) 1205 return "[]"; 1206 1207 return Helpers.toString(a, size, charLength); 1208 } 1209 } 1210 1211 private Object[] toArrayInternal(Object[] a) { 1212 Object[] x = a; 1213 restartFromHead: for (;;) { 1214 int size = 0; 1215 for (Node<E> p = first(); p != null;) { 1216 final E item; 1217 if ((item = p.item) != null) { 1218 if (x == null) 1219 x = new Object[4]; 1220 else if (size == x.length) 1221 x = Arrays.copyOf(x, 2 * (size + 4)); 1222 x[size++] = item; 1223 } 1224 if (p == (p = p.next)) 1225 continue restartFromHead; 1226 } 1227 if (x == null) 1228 return new Object[0]; 1229 else if (a != null && size <= a.length) { 1230 if (a != x) 1231 System.arraycopy(x, 0, a, 0, size); 1232 if (size < a.length) 1233 a[size] = null; 1234 return a; 1235 } 1236 return (size == x.length) ? x : Arrays.copyOf(x, size); 1237 } 1238 } 1239 1240 /** 1241 * Returns an array containing all of the elements in this deque, in 1242 * proper sequence (from first to last element). 1243 * 1244 * <p>The returned array will be "safe" in that no references to it are 1245 * maintained by this deque. (In other words, this method must allocate 1246 * a new array). The caller is thus free to modify the returned array. 1247 * 1248 * <p>This method acts as bridge between array-based and collection-based 1249 * APIs. 1250 * 1251 * @return an array containing all of the elements in this deque 1252 */ 1253 public Object[] toArray() { 1254 return toArrayInternal(null); 1255 } 1256 1257 /** 1258 * Returns an array containing all of the elements in this deque, 1259 * in proper sequence (from first to last element); the runtime 1260 * type of the returned array is that of the specified array. If 1261 * the deque fits in the specified array, it is returned therein. 1262 * Otherwise, a new array is allocated with the runtime type of 1263 * the specified array and the size of this deque. 1264 * 1265 * <p>If this deque fits in the specified array with room to spare 1266 * (i.e., the array has more elements than this deque), the element in 1267 * the array immediately following the end of the deque is set to 1268 * {@code null}. 1269 * 1270 * <p>Like the {@link #toArray()} method, this method acts as 1271 * bridge between array-based and collection-based APIs. Further, 1272 * this method allows precise control over the runtime type of the 1273 * output array, and may, under certain circumstances, be used to 1274 * save allocation costs. 1275 * 1276 * <p>Suppose {@code x} is a deque known to contain only strings. 1277 * The following code can be used to dump the deque into a newly 1278 * allocated array of {@code String}: 1279 * 1280 * <pre> {@code String[] y = x.toArray(new String[0]);}</pre> 1281 * 1282 * Note that {@code toArray(new Object[0])} is identical in function to 1283 * {@code toArray()}. 1284 * 1285 * @param a the array into which the elements of the deque are to 1286 * be stored, if it is big enough; otherwise, a new array of the 1287 * same runtime type is allocated for this purpose 1288 * @return an array containing all of the elements in this deque 1289 * @throws ArrayStoreException if the runtime type of the specified array 1290 * is not a supertype of the runtime type of every element in 1291 * this deque 1292 * @throws NullPointerException if the specified array is null 1293 */ 1294 @SuppressWarnings("unchecked") 1295 public <T> T[] toArray(T[] a) { 1296 if (a == null) throw new NullPointerException(); 1297 return (T[]) toArrayInternal(a); 1298 } 1299 1300 /** 1301 * Returns an iterator over the elements in this deque in proper sequence. 1302 * The elements will be returned in order from first (head) to last (tail). 1303 * 1304 * <p>The returned iterator is 1305 * <a href="package-summary.html#Weakly"><i>weakly consistent</i></a>. 1306 * 1307 * @return an iterator over the elements in this deque in proper sequence 1308 */ 1309 public Iterator<E> iterator() { 1310 return new Itr(); 1311 } 1312 1313 /** 1314 * Returns an iterator over the elements in this deque in reverse 1315 * sequential order. The elements will be returned in order from 1316 * last (tail) to first (head). 1317 * 1318 * <p>The returned iterator is 1319 * <a href="package-summary.html#Weakly"><i>weakly consistent</i></a>. 1320 * 1321 * @return an iterator over the elements in this deque in reverse order 1322 */ 1323 public Iterator<E> descendingIterator() { 1324 return new DescendingItr(); 1325 } 1326 1327 private abstract class AbstractItr implements Iterator<E> { 1328 /** 1329 * Next node to return item for. 1330 */ 1331 private Node<E> nextNode; 1332 1333 /** 1334 * nextItem holds on to item fields because once we claim 1335 * that an element exists in hasNext(), we must return it in 1336 * the following next() call even if it was in the process of 1337 * being removed when hasNext() was called. 1338 */ 1339 private E nextItem; 1340 1341 /** 1342 * Node returned by most recent call to next. Needed by remove. 1343 * Reset to null if this element is deleted by a call to remove. 1344 */ 1345 private Node<E> lastRet; 1346 1347 abstract Node<E> startNode(); 1348 abstract Node<E> nextNode(Node<E> p); 1349 1350 AbstractItr() { 1351 advance(); 1352 } 1353 1354 /** 1355 * Sets nextNode and nextItem to next valid node, or to null 1356 * if no such. 1357 */ 1358 private void advance() { 1359 lastRet = nextNode; 1360 1361 Node<E> p = (nextNode == null) ? startNode() : nextNode(nextNode); 1362 for (;; p = nextNode(p)) { 1363 if (p == null) { 1364 // might be at active end or TERMINATOR node; both are OK 1365 nextNode = null; 1366 nextItem = null; 1367 break; 1368 } 1369 final E item; 1370 if ((item = p.item) != null) { 1371 nextNode = p; 1372 nextItem = item; 1373 break; 1374 } 1375 } 1376 } 1377 1378 public boolean hasNext() { 1379 return nextItem != null; 1380 } 1381 1382 public E next() { 1383 E item = nextItem; 1384 if (item == null) throw new NoSuchElementException(); 1385 advance(); 1386 return item; 1387 } 1388 1389 public void remove() { 1390 Node<E> l = lastRet; 1391 if (l == null) throw new IllegalStateException(); 1392 l.item = null; 1393 unlink(l); 1394 lastRet = null; 1395 } 1396 } 1397 1398 /** Forward iterator */ 1399 private class Itr extends AbstractItr { 1400 Itr() {} // prevent access constructor creation 1401 Node<E> startNode() { return first(); } 1402 Node<E> nextNode(Node<E> p) { return succ(p); } 1403 } 1404 1405 /** Descending iterator */ 1406 private class DescendingItr extends AbstractItr { 1407 DescendingItr() {} // prevent access constructor creation 1408 Node<E> startNode() { return last(); } 1409 Node<E> nextNode(Node<E> p) { return pred(p); } 1410 } 1411 1412 /** A customized variant of Spliterators.IteratorSpliterator */ 1413 final class CLDSpliterator implements Spliterator<E> { 1414 static final int MAX_BATCH = 1 << 25; // max batch array size; 1415 Node<E> current; // current node; null until initialized 1416 int batch; // batch size for splits 1417 boolean exhausted; // true when no more nodes 1418 1419 public Spliterator<E> trySplit() { 1420 Node<E> p, q; 1421 if ((p = current()) == null || (q = p.next) == null) 1422 return null; 1423 int i = 0, n = batch = Math.min(batch + 1, MAX_BATCH); 1424 Object[] a = null; 1425 do { 1426 final E e; 1427 if ((e = p.item) != null) { 1428 if (a == null) 1429 a = new Object[n]; 1430 a[i++] = e; 1431 } 1432 if (p == (p = q)) 1433 p = first(); 1434 } while (p != null && (q = p.next) != null && i < n); 1435 setCurrent(p); 1436 return (i == 0) ? null : 1437 Spliterators.spliterator(a, 0, i, (Spliterator.ORDERED | 1438 Spliterator.NONNULL | 1439 Spliterator.CONCURRENT)); 1440 } 1441 1442 public void forEachRemaining(Consumer<? super E> action) { 1443 Objects.requireNonNull(action); 1444 Node<E> p; 1445 if ((p = current()) != null) { 1446 current = null; 1447 exhausted = true; 1448 do { 1449 final E e; 1450 if ((e = p.item) != null) 1451 action.accept(e); 1452 if (p == (p = p.next)) 1453 p = first(); 1454 } while (p != null); 1455 } 1456 } 1457 1458 public boolean tryAdvance(Consumer<? super E> action) { 1459 Objects.requireNonNull(action); 1460 Node<E> p; 1461 if ((p = current()) != null) { 1462 E e; 1463 do { 1464 e = p.item; 1465 if (p == (p = p.next)) 1466 p = first(); 1467 } while (e == null && p != null); 1468 setCurrent(p); 1469 if (e != null) { 1470 action.accept(e); 1471 return true; 1472 } 1473 } 1474 return false; 1475 } 1476 1477 private void setCurrent(Node<E> p) { 1478 if ((current = p) == null) 1479 exhausted = true; 1480 } 1481 1482 private Node<E> current() { 1483 Node<E> p; 1484 if ((p = current) == null && !exhausted) 1485 setCurrent(p = first()); 1486 return p; 1487 } 1488 1489 public long estimateSize() { return Long.MAX_VALUE; } 1490 1491 public int characteristics() { 1492 return (Spliterator.ORDERED | 1493 Spliterator.NONNULL | 1494 Spliterator.CONCURRENT); 1495 } 1496 } 1497 1498 /** 1499 * Returns a {@link Spliterator} over the elements in this deque. 1500 * 1501 * <p>The returned spliterator is 1502 * <a href="package-summary.html#Weakly"><i>weakly consistent</i></a>. 1503 * 1504 * <p>The {@code Spliterator} reports {@link Spliterator#CONCURRENT}, 1505 * {@link Spliterator#ORDERED}, and {@link Spliterator#NONNULL}. 1506 * 1507 * @implNote 1508 * The {@code Spliterator} implements {@code trySplit} to permit limited 1509 * parallelism. 1510 * 1511 * @return a {@code Spliterator} over the elements in this deque 1512 * @since 1.8 1513 */ 1514 public Spliterator<E> spliterator() { 1515 return new CLDSpliterator(); 1516 } 1517 1518 /** 1519 * Saves this deque to a stream (that is, serializes it). 1520 * 1521 * @param s the stream 1522 * @throws java.io.IOException if an I/O error occurs 1523 * @serialData All of the elements (each an {@code E}) in 1524 * the proper order, followed by a null 1525 */ 1526 private void writeObject(java.io.ObjectOutputStream s) 1527 throws java.io.IOException { 1528 1529 // Write out any hidden stuff 1530 s.defaultWriteObject(); 1531 1532 // Write out all elements in the proper order. 1533 for (Node<E> p = first(); p != null; p = succ(p)) { 1534 final E item; 1535 if ((item = p.item) != null) 1536 s.writeObject(item); 1537 } 1538 1539 // Use trailing null as sentinel 1540 s.writeObject(null); 1541 } 1542 1543 /** 1544 * Reconstitutes this deque from a stream (that is, deserializes it). 1545 * @param s the stream 1546 * @throws ClassNotFoundException if the class of a serialized object 1547 * could not be found 1548 * @throws java.io.IOException if an I/O error occurs 1549 */ 1550 private void readObject(java.io.ObjectInputStream s) 1551 throws java.io.IOException, ClassNotFoundException { 1552 s.defaultReadObject(); 1553 1554 // Read in elements until trailing null sentinel found 1555 Node<E> h = null, t = null; 1556 for (Object item; (item = s.readObject()) != null; ) { 1557 @SuppressWarnings("unchecked") 1558 Node<E> newNode = newNode((E) item); 1559 if (h == null) 1560 h = t = newNode; 1561 else { 1562 NEXT.set(t, newNode); 1563 PREV.set(newNode, t); 1564 t = newNode; 1565 } 1566 } 1567 initHeadTail(h, t); 1568 } 1569 1570 /** 1571 * @throws NullPointerException {@inheritDoc} 1572 */ 1573 public boolean removeIf(Predicate<? super E> filter) { 1574 Objects.requireNonNull(filter); 1575 return bulkRemove(filter); 1576 } 1577 1578 /** 1579 * @throws NullPointerException {@inheritDoc} 1580 */ 1581 public boolean removeAll(Collection<?> c) { 1582 Objects.requireNonNull(c); 1583 return bulkRemove(e -> c.contains(e)); 1584 } 1585 1586 /** 1587 * @throws NullPointerException {@inheritDoc} 1588 */ 1589 public boolean retainAll(Collection<?> c) { 1590 Objects.requireNonNull(c); 1591 return bulkRemove(e -> !c.contains(e)); 1592 } 1593 1594 /** Implementation of bulk remove methods. */ 1595 private boolean bulkRemove(Predicate<? super E> filter) { 1596 boolean removed = false; 1597 for (Node<E> p = first(), succ; p != null; p = succ) { 1598 succ = succ(p); 1599 final E item; 1600 if ((item = p.item) != null 1601 && filter.test(item) 1602 && ITEM.compareAndSet(p, item, null)) { 1603 unlink(p); 1604 removed = true; 1605 } 1606 } 1607 return removed; 1608 } 1609 1610 /** 1611 * @throws NullPointerException {@inheritDoc} 1612 */ 1613 public void forEach(Consumer<? super E> action) { 1614 Objects.requireNonNull(action); 1615 E item; 1616 for (Node<E> p = first(); p != null; p = succ(p)) 1617 if ((item = p.item) != null) 1618 action.accept(item); 1619 } 1620 1621 // VarHandle mechanics 1622 private static final VarHandle HEAD; 1623 private static final VarHandle TAIL; 1624 private static final VarHandle PREV; 1625 private static final VarHandle NEXT; 1626 private static final VarHandle ITEM; 1627 static { 1628 PREV_TERMINATOR = new Node<Object>(); 1629 PREV_TERMINATOR.next = PREV_TERMINATOR; 1630 NEXT_TERMINATOR = new Node<Object>(); 1631 NEXT_TERMINATOR.prev = NEXT_TERMINATOR; 1632 try { 1633 MethodHandles.Lookup l = MethodHandles.lookup(); 1634 HEAD = l.findVarHandle(ConcurrentLinkedDeque.class, "head", 1635 Node.class); 1636 TAIL = l.findVarHandle(ConcurrentLinkedDeque.class, "tail", 1637 Node.class); 1638 PREV = l.findVarHandle(Node.class, "prev", Node.class); 1639 NEXT = l.findVarHandle(Node.class, "next", Node.class); 1640 ITEM = l.findVarHandle(Node.class, "item", Object.class); 1641 } catch (ReflectiveOperationException e) { 1642 throw new Error(e); 1643 } 1644 } 1645} 1646