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