1 /* 2 * Written by Doug Lea and Martin Buchholz with assistance from members of 3 * JCP JSR-166 Expert Group and released to the public domain, as explained 4 * at http://creativecommons.org/publicdomain/zero/1.0/ 5 */ 6 7 package java.util.concurrent; 8 9 import java.util.AbstractCollection; 10 import java.util.ArrayList; 11 import java.util.Collection; 12 import java.util.Deque; 13 import java.util.Iterator; 14 import java.util.NoSuchElementException; 15 import java.util.Queue; 16 17 // BEGIN android-note 18 // removed link to collections framework docs 19 // END android-note 20 21 /** 22 * An unbounded concurrent {@linkplain Deque deque} based on linked nodes. 23 * Concurrent insertion, removal, and access operations execute safely 24 * across multiple threads. 25 * A {@code ConcurrentLinkedDeque} is an appropriate choice when 26 * many threads will share access to a common collection. 27 * Like most other concurrent collection implementations, this class 28 * does not permit the use of {@code null} elements. 29 * 30 * <p>Iterators are <i>weakly consistent</i>, returning elements 31 * reflecting the state of the deque at some point at or since the 32 * creation of the iterator. They do <em>not</em> throw {@link 33 * java.util.ConcurrentModificationException 34 * ConcurrentModificationException}, and may proceed concurrently with 35 * other operations. 36 * 37 * <p>Beware that, unlike in most collections, the {@code size} method 38 * is <em>NOT</em> a constant-time operation. Because of the 39 * asynchronous nature of these deques, determining the current number 40 * of elements requires a traversal of the elements, and so may report 41 * inaccurate results if this collection is modified during traversal. 42 * Additionally, the bulk operations {@code addAll}, 43 * {@code removeAll}, {@code retainAll}, {@code containsAll}, 44 * {@code equals}, and {@code toArray} are <em>not</em> guaranteed 45 * to be performed atomically. For example, an iterator operating 46 * concurrently with an {@code addAll} operation might view only some 47 * of the added elements. 48 * 49 * <p>This class and its iterator implement all of the <em>optional</em> 50 * methods of the {@link Deque} and {@link Iterator} interfaces. 51 * 52 * <p>Memory consistency effects: As with other concurrent collections, 53 * actions in a thread prior to placing an object into a 54 * {@code ConcurrentLinkedDeque} 55 * <a href="package-summary.html#MemoryVisibility"><i>happen-before</i></a> 56 * actions subsequent to the access or removal of that element from 57 * the {@code ConcurrentLinkedDeque} in another thread. 58 * 59 * @since 1.7 60 * @author Doug Lea 61 * @author Martin Buchholz 62 * @param <E> the type of elements held in this collection 63 */ 64 public class ConcurrentLinkedDeque<E> 65 extends AbstractCollection<E> 66 implements Deque<E>, java.io.Serializable { 67 68 /* 69 * This is an implementation of a concurrent lock-free deque 70 * supporting interior removes but not interior insertions, as 71 * required to support the entire Deque interface. 72 * 73 * We extend the techniques developed for ConcurrentLinkedQueue and 74 * LinkedTransferQueue (see the internal docs for those classes). 75 * Understanding the ConcurrentLinkedQueue implementation is a 76 * prerequisite for understanding the implementation of this class. 77 * 78 * The data structure is a symmetrical doubly-linked "GC-robust" 79 * linked list of nodes. We minimize the number of volatile writes 80 * using two techniques: advancing multiple hops with a single CAS 81 * and mixing volatile and non-volatile writes of the same memory 82 * locations. 83 * 84 * A node contains the expected E ("item") and links to predecessor 85 * ("prev") and successor ("next") nodes: 86 * 87 * class Node<E> { volatile Node<E> prev, next; volatile E item; } 88 * 89 * A node p is considered "live" if it contains a non-null item 90 * (p.item != null). When an item is CASed to null, the item is 91 * atomically logically deleted from the collection. 92 * 93 * At any time, there is precisely one "first" node with a null 94 * prev reference that terminates any chain of prev references 95 * starting at a live node. Similarly there is precisely one 96 * "last" node terminating any chain of next references starting at 97 * a live node. The "first" and "last" nodes may or may not be live. 98 * The "first" and "last" nodes are always mutually reachable. 99 * 100 * A new element is added atomically by CASing the null prev or 101 * next reference in the first or last node to a fresh node 102 * containing the element. The element's node atomically becomes 103 * "live" at that point. 104 * 105 * A node is considered "active" if it is a live node, or the 106 * first or last node. Active nodes cannot be unlinked. 107 * 108 * A "self-link" is a next or prev reference that is the same node: 109 * p.prev == p or p.next == p 110 * Self-links are used in the node unlinking process. Active nodes 111 * never have self-links. 112 * 113 * A node p is active if and only if: 114 * 115 * p.item != null || 116 * (p.prev == null && p.next != p) || 117 * (p.next == null && p.prev != p) 118 * 119 * The deque object has two node references, "head" and "tail". 120 * The head and tail are only approximations to the first and last 121 * nodes of the deque. The first node can always be found by 122 * following prev pointers from head; likewise for tail. However, 123 * it is permissible for head and tail to be referring to deleted 124 * nodes that have been unlinked and so may not be reachable from 125 * any live node. 126 * 127 * There are 3 stages of node deletion; 128 * "logical deletion", "unlinking", and "gc-unlinking". 129 * 130 * 1. "logical deletion" by CASing item to null atomically removes 131 * the element from the collection, and makes the containing node 132 * eligible for unlinking. 133 * 134 * 2. "unlinking" makes a deleted node unreachable from active 135 * nodes, and thus eventually reclaimable by GC. Unlinked nodes 136 * may remain reachable indefinitely from an iterator. 137 * 138 * Physical node unlinking is merely an optimization (albeit a 139 * critical one), and so can be performed at our convenience. At 140 * any time, the set of live nodes maintained by prev and next 141 * links are identical, that is, the live nodes found via next 142 * links from the first node is equal to the elements found via 143 * prev links from the last node. However, this is not true for 144 * nodes that have already been logically deleted - such nodes may 145 * be reachable in one direction only. 146 * 147 * 3. "gc-unlinking" takes unlinking further by making active 148 * nodes unreachable from deleted nodes, making it easier for the 149 * GC to reclaim future deleted nodes. This step makes the data 150 * structure "gc-robust", as first described in detail by Boehm 151 * (http://portal.acm.org/citation.cfm?doid=503272.503282). 152 * 153 * GC-unlinked nodes may remain reachable indefinitely from an 154 * iterator, but unlike unlinked nodes, are never reachable from 155 * head or tail. 156 * 157 * Making the data structure GC-robust will eliminate the risk of 158 * unbounded memory retention with conservative GCs and is likely 159 * to improve performance with generational GCs. 160 * 161 * When a node is dequeued at either end, e.g. via poll(), we would 162 * like to break any references from the node to active nodes. We 163 * develop further the use of self-links that was very effective in 164 * other concurrent collection classes. The idea is to replace 165 * prev and next pointers with special values that are interpreted 166 * to mean off-the-list-at-one-end. These are approximations, but 167 * good enough to preserve the properties we want in our 168 * traversals, e.g. we guarantee that a traversal will never visit 169 * the same element twice, but we don't guarantee whether a 170 * traversal that runs out of elements will be able to see more 171 * elements later after enqueues at that end. Doing gc-unlinking 172 * safely is particularly tricky, since any node can be in use 173 * indefinitely (for example by an iterator). We must ensure that 174 * the nodes pointed at by head/tail never get gc-unlinked, since 175 * head/tail are needed to get "back on track" by other nodes that 176 * are gc-unlinked. gc-unlinking accounts for much of the 177 * implementation complexity. 178 * 179 * Since neither unlinking nor gc-unlinking are necessary for 180 * correctness, there are many implementation choices regarding 181 * frequency (eagerness) of these operations. Since volatile 182 * reads are likely to be much cheaper than CASes, saving CASes by 183 * unlinking multiple adjacent nodes at a time may be a win. 184 * gc-unlinking can be performed rarely and still be effective, 185 * since it is most important that long chains of deleted nodes 186 * are occasionally broken. 187 * 188 * The actual representation we use is that p.next == p means to 189 * goto the first node (which in turn is reached by following prev 190 * pointers from head), and p.next == null && p.prev == p means 191 * that the iteration is at an end and that p is a (static final) 192 * dummy node, NEXT_TERMINATOR, and not the last active node. 193 * Finishing the iteration when encountering such a TERMINATOR is 194 * good enough for read-only traversals, so such traversals can use 195 * p.next == null as the termination condition. When we need to 196 * find the last (active) node, for enqueueing a new node, we need 197 * to check whether we have reached a TERMINATOR node; if so, 198 * restart traversal from tail. 199 * 200 * The implementation is completely directionally symmetrical, 201 * except that most public methods that iterate through the list 202 * follow next pointers ("forward" direction). 203 * 204 * We believe (without full proof) that all single-element deque 205 * operations (e.g., addFirst, peekLast, pollLast) are linearizable 206 * (see Herlihy and Shavit's book). However, some combinations of 207 * operations are known not to be linearizable. In particular, 208 * when an addFirst(A) is racing with pollFirst() removing B, it is 209 * possible for an observer iterating over the elements to observe 210 * A B C and subsequently observe A C, even though no interior 211 * removes are ever performed. Nevertheless, iterators behave 212 * reasonably, providing the "weakly consistent" guarantees. 213 * 214 * Empirically, microbenchmarks suggest that this class adds about 215 * 40% overhead relative to ConcurrentLinkedQueue, which feels as 216 * good as we can hope for. 217 */ 218 219 private static final long serialVersionUID = 876323262645176354L; 220 221 /** 222 * A node from which the first node on list (that is, the unique node p 223 * with p.prev == null && p.next != p) can be reached in O(1) time. 224 * Invariants: 225 * - the first node is always O(1) reachable from head via prev links 226 * - all live nodes are reachable from the first node via succ() 227 * - head != null 228 * - (tmp = head).next != tmp || tmp != head 229 * - head is never gc-unlinked (but may be unlinked) 230 * Non-invariants: 231 * - head.item may or may not be null 232 * - head may not be reachable from the first or last node, or from tail 233 */ 234 private transient volatile Node<E> head; 235 236 /** 237 * A node from which the last node on list (that is, the unique node p 238 * with p.next == null && p.prev != p) can be reached in O(1) time. 239 * Invariants: 240 * - the last node is always O(1) reachable from tail via next links 241 * - all live nodes are reachable from the last node via pred() 242 * - tail != null 243 * - tail is never gc-unlinked (but may be unlinked) 244 * Non-invariants: 245 * - tail.item may or may not be null 246 * - tail may not be reachable from the first or last node, or from head 247 */ 248 private transient volatile Node<E> tail; 249 250 private static final Node<Object> PREV_TERMINATOR, NEXT_TERMINATOR; 251 252 @SuppressWarnings("unchecked") 253 Node<E> prevTerminator() { 254 return (Node<E>) PREV_TERMINATOR; 255 } 256 257 @SuppressWarnings("unchecked") 258 Node<E> nextTerminator() { 259 return (Node<E>) NEXT_TERMINATOR; 260 } 261 262 static final class Node<E> { 263 volatile Node<E> prev; 264 volatile E item; 265 volatile Node<E> next; 266 267 Node() { // default constructor for NEXT_TERMINATOR, PREV_TERMINATOR 268 } 269 270 /** 271 * Constructs a new node. Uses relaxed write because item can 272 * only be seen after publication via casNext or casPrev. 273 */ 274 Node(E item) { 275 UNSAFE.putObject(this, itemOffset, item); 276 } 277 278 boolean casItem(E cmp, E val) { 279 return UNSAFE.compareAndSwapObject(this, itemOffset, cmp, val); 280 } 281 282 void lazySetNext(Node<E> val) { 283 UNSAFE.putOrderedObject(this, nextOffset, val); 284 } 285 286 boolean casNext(Node<E> cmp, Node<E> val) { 287 return UNSAFE.compareAndSwapObject(this, nextOffset, cmp, val); 288 } 289 290 void lazySetPrev(Node<E> val) { 291 UNSAFE.putOrderedObject(this, prevOffset, val); 292 } 293 294 boolean casPrev(Node<E> cmp, Node<E> val) { 295 return UNSAFE.compareAndSwapObject(this, prevOffset, cmp, val); 296 } 297 298 // Unsafe mechanics 299 300 private static final sun.misc.Unsafe UNSAFE; 301 private static final long prevOffset; 302 private static final long itemOffset; 303 private static final long nextOffset; 304 305 static { 306 try { 307 UNSAFE = sun.misc.Unsafe.getUnsafe(); 308 Class<?> k = Node.class; 309 prevOffset = UNSAFE.objectFieldOffset 310 (k.getDeclaredField("prev")); 311 itemOffset = UNSAFE.objectFieldOffset 312 (k.getDeclaredField("item")); 313 nextOffset = UNSAFE.objectFieldOffset 314 (k.getDeclaredField("next")); 315 } catch (Exception e) { 316 throw new Error(e); 317 } 318 } 319 } 320 321 /** 322 * Links e as first element. 323 */ 324 private void linkFirst(E e) { 325 checkNotNull(e); 326 final Node<E> newNode = new Node<E>(e); 327 328 restartFromHead: 329 for (;;) 330 for (Node<E> h = head, p = h, q;;) { 331 if ((q = p.prev) != null && 332 (q = (p = q).prev) != null) 333 // Check for head updates every other hop. 334 // If p == q, we are sure to follow head instead. 335 p = (h != (h = head)) ? h : q; 336 else if (p.next == p) // PREV_TERMINATOR 337 continue restartFromHead; 338 else { 339 // p is first node 340 newNode.lazySetNext(p); // CAS piggyback 341 if (p.casPrev(null, newNode)) { 342 // Successful CAS is the linearization point 343 // for e to become an element of this deque, 344 // and for newNode to become "live". 345 if (p != h) // hop two nodes at a time 346 casHead(h, newNode); // Failure is OK. 347 return; 348 } 349 // Lost CAS race to another thread; re-read prev 350 } 351 } 352 } 353 354 /** 355 * Links e as last element. 356 */ 357 private void linkLast(E e) { 358 checkNotNull(e); 359 final Node<E> newNode = new Node<E>(e); 360 361 restartFromTail: 362 for (;;) 363 for (Node<E> t = tail, p = t, q;;) { 364 if ((q = p.next) != null && 365 (q = (p = q).next) != null) 366 // Check for tail updates every other hop. 367 // If p == q, we are sure to follow tail instead. 368 p = (t != (t = tail)) ? t : q; 369 else if (p.prev == p) // NEXT_TERMINATOR 370 continue restartFromTail; 371 else { 372 // p is last node 373 newNode.lazySetPrev(p); // CAS piggyback 374 if (p.casNext(null, newNode)) { 375 // Successful CAS is the linearization point 376 // for e to become an element of this deque, 377 // and for newNode to become "live". 378 if (p != t) // hop two nodes at a time 379 casTail(t, newNode); // Failure is OK. 380 return; 381 } 382 // Lost CAS race to another thread; re-read next 383 } 384 } 385 } 386 387 private static final int HOPS = 2; 388 389 /** 390 * Unlinks non-null node x. 391 */ 392 void unlink(Node<E> x) { 393 // assert x != null; 394 // assert x.item == null; 395 // assert x != PREV_TERMINATOR; 396 // assert x != NEXT_TERMINATOR; 397 398 final Node<E> prev = x.prev; 399 final Node<E> next = x.next; 400 if (prev == null) { 401 unlinkFirst(x, next); 402 } else if (next == null) { 403 unlinkLast(x, prev); 404 } else { 405 // Unlink interior node. 406 // 407 // This is the common case, since a series of polls at the 408 // same end will be "interior" removes, except perhaps for 409 // the first one, since end nodes cannot be unlinked. 410 // 411 // At any time, all active nodes are mutually reachable by 412 // following a sequence of either next or prev pointers. 413 // 414 // Our strategy is to find the unique active predecessor 415 // and successor of x. Try to fix up their links so that 416 // they point to each other, leaving x unreachable from 417 // active nodes. If successful, and if x has no live 418 // predecessor/successor, we additionally try to gc-unlink, 419 // leaving active nodes unreachable from x, by rechecking 420 // that the status of predecessor and successor are 421 // unchanged and ensuring that x is not reachable from 422 // tail/head, before setting x's prev/next links to their 423 // logical approximate replacements, self/TERMINATOR. 424 Node<E> activePred, activeSucc; 425 boolean isFirst, isLast; 426 int hops = 1; 427 428 // Find active predecessor 429 for (Node<E> p = prev; ; ++hops) { 430 if (p.item != null) { 431 activePred = p; 432 isFirst = false; 433 break; 434 } 435 Node<E> q = p.prev; 436 if (q == null) { 437 if (p.next == p) 438 return; 439 activePred = p; 440 isFirst = true; 441 break; 442 } 443 else if (p == q) 444 return; 445 else 446 p = q; 447 } 448 449 // Find active successor 450 for (Node<E> p = next; ; ++hops) { 451 if (p.item != null) { 452 activeSucc = p; 453 isLast = false; 454 break; 455 } 456 Node<E> q = p.next; 457 if (q == null) { 458 if (p.prev == p) 459 return; 460 activeSucc = p; 461 isLast = true; 462 break; 463 } 464 else if (p == q) 465 return; 466 else 467 p = q; 468 } 469 470 // TODO: better HOP heuristics 471 if (hops < HOPS 472 // always squeeze out interior deleted nodes 473 && (isFirst | isLast)) 474 return; 475 476 // Squeeze out deleted nodes between activePred and 477 // activeSucc, including x. 478 skipDeletedSuccessors(activePred); 479 skipDeletedPredecessors(activeSucc); 480 481 // Try to gc-unlink, if possible 482 if ((isFirst | isLast) && 483 484 // Recheck expected state of predecessor and successor 485 (activePred.next == activeSucc) && 486 (activeSucc.prev == activePred) && 487 (isFirst ? activePred.prev == null : activePred.item != null) && 488 (isLast ? activeSucc.next == null : activeSucc.item != null)) { 489 490 updateHead(); // Ensure x is not reachable from head 491 updateTail(); // Ensure x is not reachable from tail 492 493 // Finally, actually gc-unlink 494 x.lazySetPrev(isFirst ? prevTerminator() : x); 495 x.lazySetNext(isLast ? nextTerminator() : x); 496 } 497 } 498 } 499 500 /** 501 * Unlinks non-null first node. 502 */ 503 private void unlinkFirst(Node<E> first, Node<E> next) { 504 // assert first != null; 505 // assert next != null; 506 // assert first.item == null; 507 for (Node<E> o = null, p = next, q;;) { 508 if (p.item != null || (q = p.next) == null) { 509 if (o != null && p.prev != p && first.casNext(next, p)) { 510 skipDeletedPredecessors(p); 511 if (first.prev == null && 512 (p.next == null || p.item != null) && 513 p.prev == first) { 514 515 updateHead(); // Ensure o is not reachable from head 516 updateTail(); // Ensure o is not reachable from tail 517 518 // Finally, actually gc-unlink 519 o.lazySetNext(o); 520 o.lazySetPrev(prevTerminator()); 521 } 522 } 523 return; 524 } 525 else if (p == q) 526 return; 527 else { 528 o = p; 529 p = q; 530 } 531 } 532 } 533 534 /** 535 * Unlinks non-null last node. 536 */ 537 private void unlinkLast(Node<E> last, Node<E> prev) { 538 // assert last != null; 539 // assert prev != null; 540 // assert last.item == null; 541 for (Node<E> o = null, p = prev, q;;) { 542 if (p.item != null || (q = p.prev) == null) { 543 if (o != null && p.next != p && last.casPrev(prev, p)) { 544 skipDeletedSuccessors(p); 545 if (last.next == null && 546 (p.prev == null || p.item != null) && 547 p.next == last) { 548 549 updateHead(); // Ensure o is not reachable from head 550 updateTail(); // Ensure o is not reachable from tail 551 552 // Finally, actually gc-unlink 553 o.lazySetPrev(o); 554 o.lazySetNext(nextTerminator()); 555 } 556 } 557 return; 558 } 559 else if (p == q) 560 return; 561 else { 562 o = p; 563 p = q; 564 } 565 } 566 } 567 568 /** 569 * Guarantees that any node which was unlinked before a call to 570 * this method will be unreachable from head after it returns. 571 * Does not guarantee to eliminate slack, only that head will 572 * point to a node that was active while this method was running. 573 */ 574 private final void updateHead() { 575 // Either head already points to an active node, or we keep 576 // trying to cas it to the first node until it does. 577 Node<E> h, p, q; 578 restartFromHead: 579 while ((h = head).item == null && (p = h.prev) != null) { 580 for (;;) { 581 if ((q = p.prev) == null || 582 (q = (p = q).prev) == null) { 583 // It is possible that p is PREV_TERMINATOR, 584 // but if so, the CAS is guaranteed to fail. 585 if (casHead(h, p)) 586 return; 587 else 588 continue restartFromHead; 589 } 590 else if (h != head) 591 continue restartFromHead; 592 else 593 p = q; 594 } 595 } 596 } 597 598 /** 599 * Guarantees that any node which was unlinked before a call to 600 * this method will be unreachable from tail after it returns. 601 * Does not guarantee to eliminate slack, only that tail will 602 * point to a node that was active while this method was running. 603 */ 604 private final void updateTail() { 605 // Either tail already points to an active node, or we keep 606 // trying to cas it to the last node until it does. 607 Node<E> t, p, q; 608 restartFromTail: 609 while ((t = tail).item == null && (p = t.next) != null) { 610 for (;;) { 611 if ((q = p.next) == null || 612 (q = (p = q).next) == null) { 613 // It is possible that p is NEXT_TERMINATOR, 614 // but if so, the CAS is guaranteed to fail. 615 if (casTail(t, p)) 616 return; 617 else 618 continue restartFromTail; 619 } 620 else if (t != tail) 621 continue restartFromTail; 622 else 623 p = q; 624 } 625 } 626 } 627 628 private void skipDeletedPredecessors(Node<E> x) { 629 whileActive: 630 do { 631 Node<E> prev = x.prev; 632 // assert prev != null; 633 // assert x != NEXT_TERMINATOR; 634 // assert x != PREV_TERMINATOR; 635 Node<E> p = prev; 636 findActive: 637 for (;;) { 638 if (p.item != null) 639 break findActive; 640 Node<E> q = p.prev; 641 if (q == null) { 642 if (p.next == p) 643 continue whileActive; 644 break findActive; 645 } 646 else if (p == q) 647 continue whileActive; 648 else 649 p = q; 650 } 651 652 // found active CAS target 653 if (prev == p || x.casPrev(prev, p)) 654 return; 655 656 } while (x.item != null || x.next == null); 657 } 658 659 private void skipDeletedSuccessors(Node<E> x) { 660 whileActive: 661 do { 662 Node<E> next = x.next; 663 // assert next != null; 664 // assert x != NEXT_TERMINATOR; 665 // assert x != PREV_TERMINATOR; 666 Node<E> p = next; 667 findActive: 668 for (;;) { 669 if (p.item != null) 670 break findActive; 671 Node<E> q = p.next; 672 if (q == null) { 673 if (p.prev == p) 674 continue whileActive; 675 break findActive; 676 } 677 else if (p == q) 678 continue whileActive; 679 else 680 p = q; 681 } 682 683 // found active CAS target 684 if (next == p || x.casNext(next, p)) 685 return; 686 687 } while (x.item != null || x.prev == null); 688 } 689 690 /** 691 * Returns the successor of p, or the first node if p.next has been 692 * linked to self, which will only be true if traversing with a 693 * stale pointer that is now off the list. 694 */ 695 final Node<E> succ(Node<E> p) { 696 // TODO: should we skip deleted nodes here? 697 Node<E> q = p.next; 698 return (p == q) ? first() : q; 699 } 700 701 /** 702 * Returns the predecessor of p, or the last node if p.prev has been 703 * linked to self, which will only be true if traversing with a 704 * stale pointer that is now off the list. 705 */ 706 final Node<E> pred(Node<E> p) { 707 Node<E> q = p.prev; 708 return (p == q) ? last() : q; 709 } 710 711 /** 712 * Returns the first node, the unique node p for which: 713 * p.prev == null && p.next != p 714 * The returned node may or may not be logically deleted. 715 * Guarantees that head is set to the returned node. 716 */ 717 Node<E> first() { 718 restartFromHead: 719 for (;;) 720 for (Node<E> h = head, p = h, q;;) { 721 if ((q = p.prev) != null && 722 (q = (p = q).prev) != null) 723 // Check for head updates every other hop. 724 // If p == q, we are sure to follow head instead. 725 p = (h != (h = head)) ? h : q; 726 else if (p == h 727 // It is possible that p is PREV_TERMINATOR, 728 // but if so, the CAS is guaranteed to fail. 729 || casHead(h, p)) 730 return p; 731 else 732 continue restartFromHead; 733 } 734 } 735 736 /** 737 * Returns the last node, the unique node p for which: 738 * p.next == null && p.prev != p 739 * The returned node may or may not be logically deleted. 740 * Guarantees that tail is set to the returned node. 741 */ 742 Node<E> last() { 743 restartFromTail: 744 for (;;) 745 for (Node<E> t = tail, p = t, q;;) { 746 if ((q = p.next) != null && 747 (q = (p = q).next) != null) 748 // Check for tail updates every other hop. 749 // If p == q, we are sure to follow tail instead. 750 p = (t != (t = tail)) ? t : q; 751 else if (p == t 752 // It is possible that p is NEXT_TERMINATOR, 753 // but if so, the CAS is guaranteed to fail. 754 || casTail(t, p)) 755 return p; 756 else 757 continue restartFromTail; 758 } 759 } 760 761 // Minor convenience utilities 762 763 /** 764 * Throws NullPointerException if argument is null. 765 * 766 * @param v the element 767 */ 768 private static void checkNotNull(Object v) { 769 if (v == null) 770 throw new NullPointerException(); 771 } 772 773 /** 774 * Returns element unless it is null, in which case throws 775 * NoSuchElementException. 776 * 777 * @param v the element 778 * @return the element 779 */ 780 private E screenNullResult(E v) { 781 if (v == null) 782 throw new NoSuchElementException(); 783 return v; 784 } 785 786 /** 787 * Creates an array list and fills it with elements of this list. 788 * Used by toArray. 789 * 790 * @return the array list 791 */ 792 private ArrayList<E> toArrayList() { 793 ArrayList<E> list = new ArrayList<E>(); 794 for (Node<E> p = first(); p != null; p = succ(p)) { 795 E item = p.item; 796 if (item != null) 797 list.add(item); 798 } 799 return list; 800 } 801 802 /** 803 * Constructs an empty deque. 804 */ 805 public ConcurrentLinkedDeque() { 806 head = tail = new Node<E>(null); 807 } 808 809 /** 810 * Constructs a deque initially containing the elements of 811 * the given collection, added in traversal order of the 812 * collection's iterator. 813 * 814 * @param c the collection of elements to initially contain 815 * @throws NullPointerException if the specified collection or any 816 * of its elements are null 817 */ 818 public ConcurrentLinkedDeque(Collection<? extends E> c) { 819 // Copy c into a private chain of Nodes 820 Node<E> h = null, t = null; 821 for (E e : c) { 822 checkNotNull(e); 823 Node<E> newNode = new Node<E>(e); 824 if (h == null) 825 h = t = newNode; 826 else { 827 t.lazySetNext(newNode); 828 newNode.lazySetPrev(t); 829 t = newNode; 830 } 831 } 832 initHeadTail(h, t); 833 } 834 835 /** 836 * Initializes head and tail, ensuring invariants hold. 837 */ 838 private void initHeadTail(Node<E> h, Node<E> t) { 839 if (h == t) { 840 if (h == null) 841 h = t = new Node<E>(null); 842 else { 843 // Avoid edge case of a single Node with non-null item. 844 Node<E> newNode = new Node<E>(null); 845 t.lazySetNext(newNode); 846 newNode.lazySetPrev(t); 847 t = newNode; 848 } 849 } 850 head = h; 851 tail = t; 852 } 853 854 /** 855 * Inserts the specified element at the front of this deque. 856 * As the deque is unbounded, this method will never throw 857 * {@link IllegalStateException}. 858 * 859 * @throws NullPointerException if the specified element is null 860 */ 861 public void addFirst(E e) { 862 linkFirst(e); 863 } 864 865 /** 866 * Inserts the specified element at the end of this deque. 867 * As the deque is unbounded, this method will never throw 868 * {@link IllegalStateException}. 869 * 870 * <p>This method is equivalent to {@link #add}. 871 * 872 * @throws NullPointerException if the specified element is null 873 */ 874 public void addLast(E e) { 875 linkLast(e); 876 } 877 878 /** 879 * Inserts the specified element at the front of this deque. 880 * As the deque is unbounded, this method will never return {@code false}. 881 * 882 * @return {@code true} (as specified by {@link Deque#offerFirst}) 883 * @throws NullPointerException if the specified element is null 884 */ 885 public boolean offerFirst(E e) { 886 linkFirst(e); 887 return true; 888 } 889 890 /** 891 * Inserts the specified element at the end of this deque. 892 * As the deque is unbounded, this method will never return {@code false}. 893 * 894 * <p>This method is equivalent to {@link #add}. 895 * 896 * @return {@code true} (as specified by {@link Deque#offerLast}) 897 * @throws NullPointerException if the specified element is null 898 */ 899 public boolean offerLast(E e) { 900 linkLast(e); 901 return true; 902 } 903 904 public E peekFirst() { 905 for (Node<E> p = first(); p != null; p = succ(p)) { 906 E item = p.item; 907 if (item != null) 908 return item; 909 } 910 return null; 911 } 912 913 public E peekLast() { 914 for (Node<E> p = last(); p != null; p = pred(p)) { 915 E item = p.item; 916 if (item != null) 917 return item; 918 } 919 return null; 920 } 921 922 /** 923 * @throws NoSuchElementException {@inheritDoc} 924 */ 925 public E getFirst() { 926 return screenNullResult(peekFirst()); 927 } 928 929 /** 930 * @throws NoSuchElementException {@inheritDoc} 931 */ 932 public E getLast() { 933 return screenNullResult(peekLast()); 934 } 935 936 public E pollFirst() { 937 for (Node<E> p = first(); p != null; p = succ(p)) { 938 E item = p.item; 939 if (item != null && p.casItem(item, null)) { 940 unlink(p); 941 return item; 942 } 943 } 944 return null; 945 } 946 947 public E pollLast() { 948 for (Node<E> p = last(); p != null; p = pred(p)) { 949 E item = p.item; 950 if (item != null && p.casItem(item, null)) { 951 unlink(p); 952 return item; 953 } 954 } 955 return null; 956 } 957 958 /** 959 * @throws NoSuchElementException {@inheritDoc} 960 */ 961 public E removeFirst() { 962 return screenNullResult(pollFirst()); 963 } 964 965 /** 966 * @throws NoSuchElementException {@inheritDoc} 967 */ 968 public E removeLast() { 969 return screenNullResult(pollLast()); 970 } 971 972 // *** Queue and stack methods *** 973 974 /** 975 * Inserts the specified element at the tail of this deque. 976 * As the deque is unbounded, this method will never return {@code false}. 977 * 978 * @return {@code true} (as specified by {@link Queue#offer}) 979 * @throws NullPointerException if the specified element is null 980 */ 981 public boolean offer(E e) { 982 return offerLast(e); 983 } 984 985 /** 986 * Inserts the specified element at the tail of this deque. 987 * As the deque is unbounded, this method will never throw 988 * {@link IllegalStateException} or return {@code false}. 989 * 990 * @return {@code true} (as specified by {@link Collection#add}) 991 * @throws NullPointerException if the specified element is null 992 */ 993 public boolean add(E e) { 994 return offerLast(e); 995 } 996 997 public E poll() { return pollFirst(); } 998 public E remove() { return removeFirst(); } 999 public E peek() { return peekFirst(); } 1000 public E element() { return getFirst(); } 1001 public void push(E e) { addFirst(e); } 1002 public E pop() { return removeFirst(); } 1003 1004 /** 1005 * Removes the first element {@code e} such that 1006 * {@code o.equals(e)}, if such an element exists in this deque. 1007 * If the deque does not contain the element, it is unchanged. 1008 * 1009 * @param o element to be removed from this deque, if present 1010 * @return {@code true} if the deque contained the specified element 1011 * @throws NullPointerException if the specified element is null 1012 */ 1013 public boolean removeFirstOccurrence(Object o) { 1014 checkNotNull(o); 1015 for (Node<E> p = first(); p != null; p = succ(p)) { 1016 E item = p.item; 1017 if (item != null && o.equals(item) && p.casItem(item, null)) { 1018 unlink(p); 1019 return true; 1020 } 1021 } 1022 return false; 1023 } 1024 1025 /** 1026 * Removes the last element {@code e} such that 1027 * {@code o.equals(e)}, if such an element exists in this deque. 1028 * If the deque does not contain the element, it is unchanged. 1029 * 1030 * @param o element to be removed from this deque, if present 1031 * @return {@code true} if the deque contained the specified element 1032 * @throws NullPointerException if the specified element is null 1033 */ 1034 public boolean removeLastOccurrence(Object o) { 1035 checkNotNull(o); 1036 for (Node<E> p = last(); p != null; p = pred(p)) { 1037 E item = p.item; 1038 if (item != null && o.equals(item) && p.casItem(item, null)) { 1039 unlink(p); 1040 return true; 1041 } 1042 } 1043 return false; 1044 } 1045 1046 /** 1047 * Returns {@code true} if this deque contains at least one 1048 * element {@code e} such that {@code o.equals(e)}. 1049 * 1050 * @param o element whose presence in this deque is to be tested 1051 * @return {@code true} if this deque contains the specified element 1052 */ 1053 public boolean contains(Object o) { 1054 if (o == null) return false; 1055 for (Node<E> p = first(); p != null; p = succ(p)) { 1056 E item = p.item; 1057 if (item != null && o.equals(item)) 1058 return true; 1059 } 1060 return false; 1061 } 1062 1063 /** 1064 * Returns {@code true} if this collection contains no elements. 1065 * 1066 * @return {@code true} if this collection contains no elements 1067 */ 1068 public boolean isEmpty() { 1069 return peekFirst() == null; 1070 } 1071 1072 /** 1073 * Returns the number of elements in this deque. If this deque 1074 * contains more than {@code Integer.MAX_VALUE} elements, it 1075 * returns {@code Integer.MAX_VALUE}. 1076 * 1077 * <p>Beware that, unlike in most collections, this method is 1078 * <em>NOT</em> a constant-time operation. Because of the 1079 * asynchronous nature of these deques, determining the current 1080 * number of elements requires traversing them all to count them. 1081 * Additionally, it is possible for the size to change during 1082 * execution of this method, in which case the returned result 1083 * will be inaccurate. Thus, this method is typically not very 1084 * useful in concurrent applications. 1085 * 1086 * @return the number of elements in this deque 1087 */ 1088 public int size() { 1089 int count = 0; 1090 for (Node<E> p = first(); p != null; p = succ(p)) 1091 if (p.item != null) 1092 // Collection.size() spec says to max out 1093 if (++count == Integer.MAX_VALUE) 1094 break; 1095 return count; 1096 } 1097 1098 /** 1099 * Removes the first element {@code e} such that 1100 * {@code o.equals(e)}, if such an element exists in this deque. 1101 * If the deque does not contain the element, it is unchanged. 1102 * 1103 * @param o element to be removed from this deque, if present 1104 * @return {@code true} if the deque contained the specified element 1105 * @throws NullPointerException if the specified element is null 1106 */ 1107 public boolean remove(Object o) { 1108 return removeFirstOccurrence(o); 1109 } 1110 1111 /** 1112 * Appends all of the elements in the specified collection to the end of 1113 * this deque, in the order that they are returned by the specified 1114 * collection's iterator. Attempts to {@code addAll} of a deque to 1115 * itself result in {@code IllegalArgumentException}. 1116 * 1117 * @param c the elements to be inserted into this deque 1118 * @return {@code true} if this deque changed as a result of the call 1119 * @throws NullPointerException if the specified collection or any 1120 * of its elements are null 1121 * @throws IllegalArgumentException if the collection is this deque 1122 */ 1123 public boolean addAll(Collection<? extends E> c) { 1124 if (c == this) 1125 // As historically specified in AbstractQueue#addAll 1126 throw new IllegalArgumentException(); 1127 1128 // Copy c into a private chain of Nodes 1129 Node<E> beginningOfTheEnd = null, last = null; 1130 for (E e : c) { 1131 checkNotNull(e); 1132 Node<E> newNode = new Node<E>(e); 1133 if (beginningOfTheEnd == null) 1134 beginningOfTheEnd = last = newNode; 1135 else { 1136 last.lazySetNext(newNode); 1137 newNode.lazySetPrev(last); 1138 last = newNode; 1139 } 1140 } 1141 if (beginningOfTheEnd == null) 1142 return false; 1143 1144 // Atomically append the chain at the tail of this collection 1145 restartFromTail: 1146 for (;;) 1147 for (Node<E> t = tail, p = t, q;;) { 1148 if ((q = p.next) != null && 1149 (q = (p = q).next) != null) 1150 // Check for tail updates every other hop. 1151 // If p == q, we are sure to follow tail instead. 1152 p = (t != (t = tail)) ? t : q; 1153 else if (p.prev == p) // NEXT_TERMINATOR 1154 continue restartFromTail; 1155 else { 1156 // p is last node 1157 beginningOfTheEnd.lazySetPrev(p); // CAS piggyback 1158 if (p.casNext(null, beginningOfTheEnd)) { 1159 // Successful CAS is the linearization point 1160 // for all elements to be added to this deque. 1161 if (!casTail(t, last)) { 1162 // Try a little harder to update tail, 1163 // since we may be adding many elements. 1164 t = tail; 1165 if (last.next == null) 1166 casTail(t, last); 1167 } 1168 return true; 1169 } 1170 // Lost CAS race to another thread; re-read next 1171 } 1172 } 1173 } 1174 1175 /** 1176 * Removes all of the elements from this deque. 1177 */ 1178 public void clear() { 1179 while (pollFirst() != null) 1180 ; 1181 } 1182 1183 /** 1184 * Returns an array containing all of the elements in this deque, in 1185 * proper sequence (from first to last element). 1186 * 1187 * <p>The returned array will be "safe" in that no references to it are 1188 * maintained by this deque. (In other words, this method must allocate 1189 * a new array). The caller is thus free to modify the returned array. 1190 * 1191 * <p>This method acts as bridge between array-based and collection-based 1192 * APIs. 1193 * 1194 * @return an array containing all of the elements in this deque 1195 */ 1196 public Object[] toArray() { 1197 return toArrayList().toArray(); 1198 } 1199 1200 /** 1201 * Returns an array containing all of the elements in this deque, 1202 * in proper sequence (from first to last element); the runtime 1203 * type of the returned array is that of the specified array. If 1204 * the deque fits in the specified array, it is returned therein. 1205 * Otherwise, a new array is allocated with the runtime type of 1206 * the specified array and the size of this deque. 1207 * 1208 * <p>If this deque fits in the specified array with room to spare 1209 * (i.e., the array has more elements than this deque), the element in 1210 * the array immediately following the end of the deque is set to 1211 * {@code null}. 1212 * 1213 * <p>Like the {@link #toArray()} method, this method acts as 1214 * bridge between array-based and collection-based APIs. Further, 1215 * this method allows precise control over the runtime type of the 1216 * output array, and may, under certain circumstances, be used to 1217 * save allocation costs. 1218 * 1219 * <p>Suppose {@code x} is a deque known to contain only strings. 1220 * The following code can be used to dump the deque into a newly 1221 * allocated array of {@code String}: 1222 * 1223 * <pre> {@code String[] y = x.toArray(new String[0]);}</pre> 1224 * 1225 * Note that {@code toArray(new Object[0])} is identical in function to 1226 * {@code toArray()}. 1227 * 1228 * @param a the array into which the elements of the deque are to 1229 * be stored, if it is big enough; otherwise, a new array of the 1230 * same runtime type is allocated for this purpose 1231 * @return an array containing all of the elements in this deque 1232 * @throws ArrayStoreException if the runtime type of the specified array 1233 * is not a supertype of the runtime type of every element in 1234 * this deque 1235 * @throws NullPointerException if the specified array is null 1236 */ 1237 public <T> T[] toArray(T[] a) { 1238 return toArrayList().toArray(a); 1239 } 1240 1241 /** 1242 * Returns an iterator over the elements in this deque in proper sequence. 1243 * The elements will be returned in order from first (head) to last (tail). 1244 * 1245 * <p>The returned iterator is a "weakly consistent" iterator that 1246 * will never throw {@link java.util.ConcurrentModificationException 1247 * ConcurrentModificationException}, and guarantees to traverse 1248 * elements as they existed upon construction of the iterator, and 1249 * may (but is not guaranteed to) reflect any modifications 1250 * subsequent to construction. 1251 * 1252 * @return an iterator over the elements in this deque in proper sequence 1253 */ 1254 public Iterator<E> iterator() { 1255 return new Itr(); 1256 } 1257 1258 /** 1259 * Returns an iterator over the elements in this deque in reverse 1260 * sequential order. The elements will be returned in order from 1261 * last (tail) to first (head). 1262 * 1263 * <p>The returned iterator is a "weakly consistent" iterator that 1264 * will never throw {@link java.util.ConcurrentModificationException 1265 * ConcurrentModificationException}, and guarantees to traverse 1266 * elements as they existed upon construction of the iterator, and 1267 * may (but is not guaranteed to) reflect any modifications 1268 * subsequent to construction. 1269 * 1270 * @return an iterator over the elements in this deque in reverse order 1271 */ 1272 public Iterator<E> descendingIterator() { 1273 return new DescendingItr(); 1274 } 1275 1276 private abstract class AbstractItr implements Iterator<E> { 1277 /** 1278 * Next node to return item for. 1279 */ 1280 private Node<E> nextNode; 1281 1282 /** 1283 * nextItem holds on to item fields because once we claim 1284 * that an element exists in hasNext(), we must return it in 1285 * the following next() call even if it was in the process of 1286 * being removed when hasNext() was called. 1287 */ 1288 private E nextItem; 1289 1290 /** 1291 * Node returned by most recent call to next. Needed by remove. 1292 * Reset to null if this element is deleted by a call to remove. 1293 */ 1294 private Node<E> lastRet; 1295 1296 abstract Node<E> startNode(); 1297 abstract Node<E> nextNode(Node<E> p); 1298 1299 AbstractItr() { 1300 advance(); 1301 } 1302 1303 /** 1304 * Sets nextNode and nextItem to next valid node, or to null 1305 * if no such. 1306 */ 1307 private void advance() { 1308 lastRet = nextNode; 1309 1310 Node<E> p = (nextNode == null) ? startNode() : nextNode(nextNode); 1311 for (;; p = nextNode(p)) { 1312 if (p == null) { 1313 // p might be active end or TERMINATOR node; both are OK 1314 nextNode = null; 1315 nextItem = null; 1316 break; 1317 } 1318 E item = p.item; 1319 if (item != null) { 1320 nextNode = p; 1321 nextItem = item; 1322 break; 1323 } 1324 } 1325 } 1326 1327 public boolean hasNext() { 1328 return nextItem != null; 1329 } 1330 1331 public E next() { 1332 E item = nextItem; 1333 if (item == null) throw new NoSuchElementException(); 1334 advance(); 1335 return item; 1336 } 1337 1338 public void remove() { 1339 Node<E> l = lastRet; 1340 if (l == null) throw new IllegalStateException(); 1341 l.item = null; 1342 unlink(l); 1343 lastRet = null; 1344 } 1345 } 1346 1347 /** Forward iterator */ 1348 private class Itr extends AbstractItr { 1349 Node<E> startNode() { return first(); } 1350 Node<E> nextNode(Node<E> p) { return succ(p); } 1351 } 1352 1353 /** Descending iterator */ 1354 private class DescendingItr extends AbstractItr { 1355 Node<E> startNode() { return last(); } 1356 Node<E> nextNode(Node<E> p) { return pred(p); } 1357 } 1358 1359 /** 1360 * Saves this deque to a stream (that is, serializes it). 1361 * 1362 * @serialData All of the elements (each an {@code E}) in 1363 * the proper order, followed by a null 1364 */ 1365 private void writeObject(java.io.ObjectOutputStream s) 1366 throws java.io.IOException { 1367 1368 // Write out any hidden stuff 1369 s.defaultWriteObject(); 1370 1371 // Write out all elements in the proper order. 1372 for (Node<E> p = first(); p != null; p = succ(p)) { 1373 E item = p.item; 1374 if (item != null) 1375 s.writeObject(item); 1376 } 1377 1378 // Use trailing null as sentinel 1379 s.writeObject(null); 1380 } 1381 1382 /** 1383 * Reconstitutes this deque from a stream (that is, deserializes it). 1384 */ 1385 private void readObject(java.io.ObjectInputStream s) 1386 throws java.io.IOException, ClassNotFoundException { 1387 s.defaultReadObject(); 1388 1389 // Read in elements until trailing null sentinel found 1390 Node<E> h = null, t = null; 1391 Object item; 1392 while ((item = s.readObject()) != null) { 1393 @SuppressWarnings("unchecked") 1394 Node<E> newNode = new Node<E>((E) item); 1395 if (h == null) 1396 h = t = newNode; 1397 else { 1398 t.lazySetNext(newNode); 1399 newNode.lazySetPrev(t); 1400 t = newNode; 1401 } 1402 } 1403 initHeadTail(h, t); 1404 } 1405 1406 private boolean casHead(Node<E> cmp, Node<E> val) { 1407 return UNSAFE.compareAndSwapObject(this, headOffset, cmp, val); 1408 } 1409 1410 private boolean casTail(Node<E> cmp, Node<E> val) { 1411 return UNSAFE.compareAndSwapObject(this, tailOffset, cmp, val); 1412 } 1413 1414 // Unsafe mechanics 1415 1416 private static final sun.misc.Unsafe UNSAFE; 1417 private static final long headOffset; 1418 private static final long tailOffset; 1419 static { 1420 PREV_TERMINATOR = new Node<Object>(); 1421 PREV_TERMINATOR.next = PREV_TERMINATOR; 1422 NEXT_TERMINATOR = new Node<Object>(); 1423 NEXT_TERMINATOR.prev = NEXT_TERMINATOR; 1424 try { 1425 UNSAFE = sun.misc.Unsafe.getUnsafe(); 1426 Class<?> k = ConcurrentLinkedDeque.class; 1427 headOffset = UNSAFE.objectFieldOffset 1428 (k.getDeclaredField("head")); 1429 tailOffset = UNSAFE.objectFieldOffset 1430 (k.getDeclaredField("tail")); 1431 } catch (Exception e) { 1432 throw new Error(e); 1433 } 1434 } 1435 } 1436
