1 /*
2 * DO NOT ALTER OR REMOVE COPYRIGHT NOTICES OR THIS FILE HEADER.
3 *
4 * This code is free software; you can redistribute it and/or modify it
5 * under the terms of the GNU General Public License version 2 only, as
6 * published by the Free Software Foundation. Oracle designates this
7 * particular file as subject to the "Classpath" exception as provided
8 * by Oracle in the LICENSE file that accompanied this code.
9 *
10 * This code is distributed in the hope that it will be useful, but WITHOUT
11 * ANY WARRANTY; without even the implied warranty of MERCHANTABILITY or
12 * FITNESS FOR A PARTICULAR PURPOSE. See the GNU General Public License
13 * version 2 for more details (a copy is included in the LICENSE file that
14 * accompanied this code).
15 *
16 * You should have received a copy of the GNU General Public License version
17 * 2 along with this work; if not, write to the Free Software Foundation,
18 * Inc., 51 Franklin St, Fifth Floor, Boston, MA 02110-1301 USA.
19 *
20 * Please contact Oracle, 500 Oracle Parkway, Redwood Shores, CA 94065 USA
21 * or visit www.oracle.com if you need additional information or have any
22 * questions.
23 */
24
25 /*
26 * This file is available under and governed by the GNU General Public
27 * License version 2 only, as published by the Free Software Foundation.
28 * However, the following notice accompanied the original version of this
29 * file:
30 *
31 * Written by Doug Lea and Martin Buchholz with assistance from members of
32 * JCP JSR-166 Expert Group and released to the public domain, as explained
33 * at http://creativecommons.org/publicdomain/zero/1.0/
34 */
35
36 package java.util.concurrent;
37
38 import java.lang.invoke.MethodHandles;
39 import java.lang.invoke.VarHandle;
40 import java.util.AbstractQueue;
41 import java.util.Arrays;
42 import java.util.Collection;
43 import java.util.Iterator;
44 import java.util.NoSuchElementException;
45 import java.util.Objects;
46 import java.util.Queue;
47 import java.util.Spliterator;
48 import java.util.Spliterators;
49 import java.util.function.Consumer;
50 import java.util.function.Predicate;
51
52 /**
53 * An unbounded thread-safe {@linkplain Queue queue} based on linked nodes.
54 * This queue orders elements FIFO (first-in-first-out).
55 * The <em>head</em> of the queue is that element that has been on the
56 * queue the longest time.
57 * The <em>tail</em> of the queue is that element that has been on the
58 * queue the shortest time. New elements
59 * are inserted at the tail of the queue, and the queue retrieval
60 * operations obtain elements at the head of the queue.
61 * A {@code ConcurrentLinkedQueue} is an appropriate choice when
62 * many threads will share access to a common collection.
63 * Like most other concurrent collection implementations, this class
64 * does not permit the use of {@code null} elements.
65 *
66 * <p>This implementation employs an efficient <em>non-blocking</em>
67 * algorithm based on one described in
68 * <a href="http://www.cs.rochester.edu/~scott/papers/1996_PODC_queues.pdf">
69 * Simple, Fast, and Practical Non-Blocking and Blocking Concurrent Queue
70 * Algorithms</a> by Maged M. Michael and Michael L. Scott.
71 *
72 * <p>Iterators are <i>weakly consistent</i>, returning elements
73 * reflecting the state of the queue at some point at or since the
74 * creation of the iterator. They do <em>not</em> throw {@link
75 * java.util.ConcurrentModificationException}, and may proceed concurrently
76 * with other operations. Elements contained in the queue since the creation
77 * of the iterator will be returned exactly once.
78 *
79 * <p>Beware that, unlike in most collections, the {@code size} method
80 * is <em>NOT</em> a constant-time operation. Because of the
81 * asynchronous nature of these queues, determining the current number
82 * of elements requires a traversal of the elements, and so may report
83 * inaccurate results if this collection is modified during traversal.
84 *
85 * <p>Bulk operations that add, remove, or examine multiple elements,
86 * such as {@link #addAll}, {@link #removeIf} or {@link #forEach},
87 * are <em>not</em> guaranteed to be performed atomically.
88 * For example, a {@code forEach} traversal concurrent with an {@code
89 * addAll} operation might observe only some of the added elements.
90 *
91 * <p>This class and its iterator implement all of the <em>optional</em>
92 * methods of the {@link Queue} and {@link Iterator} interfaces.
93 *
94 * <p>Memory consistency effects: As with other concurrent
95 * collections, actions in a thread prior to placing an object into a
96 * {@code ConcurrentLinkedQueue}
97 * <a href="package-summary.html#MemoryVisibility"><i>happen-before</i></a>
98 * actions subsequent to the access or removal of that element from
99 * the {@code ConcurrentLinkedQueue} in another thread.
100 *
101 * <p>This class is a member of the
102 * <a href="{@docRoot}/../technotes/guides/collections/index.html">
103 * Java Collections Framework</a>.
104 *
105 * @since 1.5
106 * @author Doug Lea
107 * @param <E> the type of elements held in this queue
108 */
109 public class ConcurrentLinkedQueue<E> extends AbstractQueue<E>
110 implements Queue<E>, java.io.Serializable {
111 private static final long serialVersionUID = 196745693267521676L;
112
113 /*
114 * This is a modification of the Michael & Scott algorithm,
115 * adapted for a garbage-collected environment, with support for
116 * interior node deletion (to support e.g. remove(Object)). For
117 * explanation, read the paper.
118 *
119 * Note that like most non-blocking algorithms in this package,
120 * this implementation relies on the fact that in garbage
121 * collected systems, there is no possibility of ABA problems due
122 * to recycled nodes, so there is no need to use "counted
123 * pointers" or related techniques seen in versions used in
124 * non-GC'ed settings.
125 *
126 * The fundamental invariants are:
127 * - There is exactly one (last) Node with a null next reference,
128 * which is CASed when enqueueing. This last Node can be
129 * reached in O(1) time from tail, but tail is merely an
130 * optimization - it can always be reached in O(N) time from
131 * head as well.
132 * - The elements contained in the queue are the non-null items in
133 * Nodes that are reachable from head. CASing the item
134 * reference of a Node to null atomically removes it from the
135 * queue. Reachability of all elements from head must remain
136 * true even in the case of concurrent modifications that cause
137 * head to advance. A dequeued Node may remain in use
138 * indefinitely due to creation of an Iterator or simply a
139 * poll() that has lost its time slice.
140 *
141 * The above might appear to imply that all Nodes are GC-reachable
142 * from a predecessor dequeued Node. That would cause two problems:
143 * - allow a rogue Iterator to cause unbounded memory retention
144 * - cause cross-generational linking of old Nodes to new Nodes if
145 * a Node was tenured while live, which generational GCs have a
146 * hard time dealing with, causing repeated major collections.
147 * However, only non-deleted Nodes need to be reachable from
148 * dequeued Nodes, and reachability does not necessarily have to
149 * be of the kind understood by the GC. We use the trick of
150 * linking a Node that has just been dequeued to itself. Such a
151 * self-link implicitly means to advance to head.
152 *
153 * Both head and tail are permitted to lag. In fact, failing to
154 * update them every time one could is a significant optimization
155 * (fewer CASes). As with LinkedTransferQueue (see the internal
156 * documentation for that class), we use a slack threshold of two;
157 * that is, we update head/tail when the current pointer appears
158 * to be two or more steps away from the first/last node.
159 *
160 * Since head and tail are updated concurrently and independently,
161 * it is possible for tail to lag behind head (why not)?
162 *
163 * CASing a Node's item reference to null atomically removes the
164 * element from the queue, leaving a "dead" node that should later
165 * be unlinked (but unlinking is merely an optimization).
166 * Interior element removal methods (other than Iterator.remove())
167 * keep track of the predecessor node during traversal so that the
168 * node can be CAS-unlinked. Some traversal methods try to unlink
169 * any deleted nodes encountered during traversal. See comments
170 * in bulkRemove.
171 *
172 * When constructing a Node (before enqueuing it) we avoid paying
173 * for a volatile write to item. This allows the cost of enqueue
174 * to be "one-and-a-half" CASes.
175 *
176 * Both head and tail may or may not point to a Node with a
177 * non-null item. If the queue is empty, all items must of course
178 * be null. Upon creation, both head and tail refer to a dummy
179 * Node with null item. Both head and tail are only updated using
180 * CAS, so they never regress, although again this is merely an
181 * optimization.
182 */
183
184 static final class Node<E> {
185 volatile E item;
186 volatile Node<E> next;
187
188 /**
189 * Constructs a node holding item. Uses relaxed write because
190 * item can only be seen after piggy-backing publication via CAS.
191 */
192 Node(E item) {
193 ITEM.set(this, item);
194 }
195
196 /** Constructs a dead dummy node. */
197 Node() {}
198
199 void appendRelaxed(Node<E> next) {
200 // assert next != null;
201 // assert this.next == null;
202 NEXT.set(this, next);
203 }
204
205 boolean casItem(E cmp, E val) {
206 // assert item == cmp || item == null;
207 // assert cmp != null;
208 // assert val == null;
209 return ITEM.compareAndSet(this, cmp, val);
210 }
211 }
212
213 /**
214 * A node from which the first live (non-deleted) node (if any)
215 * can be reached in O(1) time.
216 * Invariants:
217 * - all live nodes are reachable from head via succ()
218 * - head != null
219 * - (tmp = head).next != tmp || tmp != head
220 * Non-invariants:
221 * - head.item may or may not be null.
222 * - it is permitted for tail to lag behind head, that is, for tail
223 * to not be reachable from head!
224 */
225 transient volatile Node<E> head;
226
227 /**
228 * A node from which the last node on list (that is, the unique
229 * node with node.next == null) can be reached in O(1) time.
230 * Invariants:
231 * - the last node is always reachable from tail via succ()
232 * - tail != null
233 * Non-invariants:
234 * - tail.item may or may not be null.
235 * - it is permitted for tail to lag behind head, that is, for tail
236 * to not be reachable from head!
237 * - tail.next may or may not be self-linked.
238 */
239 private transient volatile Node<E> tail;
240
241 /**
242 * Creates a {@code ConcurrentLinkedQueue} that is initially empty.
243 */
244 public ConcurrentLinkedQueue() {
245 head = tail = new Node<E>();
246 }
247
248 /**
249 * Creates a {@code ConcurrentLinkedQueue}
250 * initially containing the elements of the given collection,
251 * added in traversal order of the collection's iterator.
252 *
253 * @param c the collection of elements to initially contain
254 * @throws NullPointerException if the specified collection or any
255 * of its elements are null
256 */
257 public ConcurrentLinkedQueue(Collection<? extends E> c) {
258 Node<E> h = null, t = null;
259 for (E e : c) {
260 Node<E> newNode = new Node<E>(Objects.requireNonNull(e));
261 if (h == null)
262 h = t = newNode;
263 else
264 t.appendRelaxed(t = newNode);
265 }
266 if (h == null)
267 h = t = new Node<E>();
268 head = h;
269 tail = t;
270 }
271
272 // Have to override just to update the javadoc
273
274 /**
275 * Inserts the specified element at the tail of this queue.
276 * As the queue is unbounded, this method will never throw
277 * {@link IllegalStateException} or return {@code false}.
278 *
279 * @return {@code true} (as specified by {@link Collection#add})
280 * @throws NullPointerException if the specified element is null
281 */
282 public boolean add(E e) {
283 return offer(e);
284 }
285
286 /**
287 * Tries to CAS head to p. If successful, repoint old head to itself
288 * as sentinel for succ(), below.
289 */
290 final void updateHead(Node<E> h, Node<E> p) {
291 // assert h != null && p != null && (h == p || h.item == null);
292 if (h != p && HEAD.compareAndSet(this, h, p))
293 NEXT.setRelease(h, h);
294 }
295
296 /**
297 * Returns the successor of p, or the head node if p.next has been
298 * linked to self, which will only be true if traversing with a
299 * stale pointer that is now off the list.
300 */
301 final Node<E> succ(Node<E> p) {
302 if (p == (p = p.next))
303 p = head;
304 return p;
305 }
306
307 /**
308 * Tries to CAS pred.next (or head, if pred is null) from c to p.
309 * Caller must ensure that we're not unlinking the trailing node.
310 */
311 private boolean tryCasSuccessor(Node<E> pred, Node<E> c, Node<E> p) {
312 // assert p != null;
313 // assert c.item == null;
314 // assert c != p;
315 if (pred != null)
316 return NEXT.compareAndSet(pred, c, p);
317 if (HEAD.compareAndSet(this, c, p)) {
318 NEXT.setRelease(c, c);
319 return true;
320 }
321 return false;
322 }
323
324 /**
325 * Collapse dead nodes between pred and q.
326 * @param pred the last known live node, or null if none
327 * @param c the first dead node
328 * @param p the last dead node
329 * @param q p.next: the next live node, or null if at end
330 * @return either old pred or p if pred dead or CAS failed
331 */
332 private Node<E> skipDeadNodes(Node<E> pred, Node<E> c, Node<E> p, Node<E> q) {
333 // assert pred != c;
334 // assert p != q;
335 // assert c.item == null;
336 // assert p.item == null;
337 if (q == null) {
338 // Never unlink trailing node.
339 if (c == p) return pred;
340 q = p;
341 }
342 return (tryCasSuccessor(pred, c, q)
343 && (pred == null || ITEM.get(pred) != null))
344 ? pred : p;
345 }
346
347 /**
348 * Inserts the specified element at the tail of this queue.
349 * As the queue is unbounded, this method will never return {@code false}.
350 *
351 * @return {@code true} (as specified by {@link Queue#offer})
352 * @throws NullPointerException if the specified element is null
353 */
354 public boolean offer(E e) {
355 final Node<E> newNode = new Node<E>(Objects.requireNonNull(e));
356
357 for (Node<E> t = tail, p = t;;) {
358 Node<E> q = p.next;
359 if (q == null) {
360 // p is last node
361 if (NEXT.compareAndSet(p, null, newNode)) {
362 // Successful CAS is the linearization point
363 // for e to become an element of this queue,
364 // and for newNode to become "live".
365 if (p != t) // hop two nodes at a time; failure is OK
366 TAIL.weakCompareAndSet(this, t, newNode);
367 return true;
368 }
369 // Lost CAS race to another thread; re-read next
370 }
371 else if (p == q)
372 // We have fallen off list. If tail is unchanged, it
373 // will also be off-list, in which case we need to
374 // jump to head, from which all live nodes are always
375 // reachable. Else the new tail is a better bet.
376 p = (t != (t = tail)) ? t : head;
377 else
378 // Check for tail updates after two hops.
379 p = (p != t && t != (t = tail)) ? t : q;
380 }
381 }
382
383 public E poll() {
384 restartFromHead: for (;;) {
385 for (Node<E> h = head, p = h, q;; p = q) {
386 final E item;
387 if ((item = p.item) != null && p.casItem(item, null)) {
388 // Successful CAS is the linearization point
389 // for item to be removed from this queue.
390 if (p != h) // hop two nodes at a time
391 updateHead(h, ((q = p.next) != null) ? q : p);
392 return item;
393 }
394 else if ((q = p.next) == null) {
395 updateHead(h, p);
396 return null;
397 }
398 else if (p == q)
399 continue restartFromHead;
400 }
401 }
402 }
403
404 public E peek() {
405 restartFromHead: for (;;) {
406 for (Node<E> h = head, p = h, q;; p = q) {
407 final E item;
408 if ((item = p.item) != null
409 || (q = p.next) == null) {
410 updateHead(h, p);
411 return item;
412 }
413 else if (p == q)
414 continue restartFromHead;
415 }
416 }
417 }
418
419 /**
420 * Returns the first live (non-deleted) node on list, or null if none.
421 * This is yet another variant of poll/peek; here returning the
422 * first node, not element. We could make peek() a wrapper around
423 * first(), but that would cost an extra volatile read of item,
424 * and the need to add a retry loop to deal with the possibility
425 * of losing a race to a concurrent poll().
426 */
427 Node<E> first() {
428 restartFromHead: for (;;) {
429 for (Node<E> h = head, p = h, q;; p = q) {
430 boolean hasItem = (p.item != null);
431 if (hasItem || (q = p.next) == null) {
432 updateHead(h, p);
433 return hasItem ? p : null;
434 }
435 else if (p == q)
436 continue restartFromHead;
437 }
438 }
439 }
440
441 /**
442 * Returns {@code true} if this queue contains no elements.
443 *
444 * @return {@code true} if this queue contains no elements
445 */
446 public boolean isEmpty() {
447 return first() == null;
448 }
449
450 /**
451 * Returns the number of elements in this queue. If this queue
452 * contains more than {@code Integer.MAX_VALUE} elements, returns
453 * {@code Integer.MAX_VALUE}.
454 *
455 * <p>Beware that, unlike in most collections, this method is
456 * <em>NOT</em> a constant-time operation. Because of the
457 * asynchronous nature of these queues, determining the current
458 * number of elements requires an O(n) traversal.
459 * Additionally, if elements are added or removed during execution
460 * of this method, the returned result may be inaccurate. Thus,
461 * this method is typically not very useful in concurrent
462 * applications.
463 *
464 * @return the number of elements in this queue
465 */
466 public int size() {
467 restartFromHead: for (;;) {
468 int count = 0;
469 for (Node<E> p = first(); p != null;) {
470 if (p.item != null)
471 if (++count == Integer.MAX_VALUE)
472 break; // @see Collection.size()
473 if (p == (p = p.next))
474 continue restartFromHead;
475 }
476 return count;
477 }
478 }
479
480 /**
481 * Returns {@code true} if this queue contains the specified element.
482 * More formally, returns {@code true} if and only if this queue contains
483 * at least one element {@code e} such that {@code o.equals(e)}.
484 *
485 * @param o object to be checked for containment in this queue
486 * @return {@code true} if this queue contains the specified element
487 */
488 public boolean contains(Object o) {
489 if (o == null) return false;
490 restartFromHead: for (;;) {
491 for (Node<E> p = head, pred = null; p != null; ) {
492 Node<E> q = p.next;
493 final E item;
494 if ((item = p.item) != null) {
495 if (o.equals(item))
496 return true;
497 pred = p; p = q; continue;
498 }
499 for (Node<E> c = p;; q = p.next) {
500 if (q == null || q.item != null) {
501 pred = skipDeadNodes(pred, c, p, q); p = q; break;
502 }
503 if (p == (p = q)) continue restartFromHead;
504 }
505 }
506 return false;
507 }
508 }
509
510 /**
511 * Removes a single instance of the specified element from this queue,
512 * if it is present. More formally, removes an element {@code e} such
513 * that {@code o.equals(e)}, if this queue contains one or more such
514 * elements.
515 * Returns {@code true} if this queue contained the specified element
516 * (or equivalently, if this queue changed as a result of the call).
517 *
518 * @param o element to be removed from this queue, if present
519 * @return {@code true} if this queue changed as a result of the call
520 */
521 public boolean remove(Object o) {
522 if (o == null) return false;
523 restartFromHead: for (;;) {
524 for (Node<E> p = head, pred = null; p != null; ) {
525 Node<E> q = p.next;
526 final E item;
527 if ((item = p.item) != null) {
528 if (o.equals(item) && p.casItem(item, null)) {
529 skipDeadNodes(pred, p, p, q);
530 return true;
531 }
532 pred = p; p = q; continue;
533 }
534 for (Node<E> c = p;; q = p.next) {
535 if (q == null || q.item != null) {
536 pred = skipDeadNodes(pred, c, p, q); p = q; break;
537 }
538 if (p == (p = q)) continue restartFromHead;
539 }
540 }
541 return false;
542 }
543 }
544
545 /**
546 * Appends all of the elements in the specified collection to the end of
547 * this queue, in the order that they are returned by the specified
548 * collection's iterator. Attempts to {@code addAll} of a queue to
549 * itself result in {@code IllegalArgumentException}.
550 *
551 * @param c the elements to be inserted into this queue
552 * @return {@code true} if this queue changed as a result of the call
553 * @throws NullPointerException if the specified collection or any
554 * of its elements are null
555 * @throws IllegalArgumentException if the collection is this queue
556 */
557 public boolean addAll(Collection<? extends E> c) {
558 if (c == this)
559 // As historically specified in AbstractQueue#addAll
560 throw new IllegalArgumentException();
561
562 // Copy c into a private chain of Nodes
563 Node<E> beginningOfTheEnd = null, last = null;
564 for (E e : c) {
565 Node<E> newNode = new Node<E>(Objects.requireNonNull(e));
566 if (beginningOfTheEnd == null)
567 beginningOfTheEnd = last = newNode;
568 else
569 last.appendRelaxed(last = newNode);
570 }
571 if (beginningOfTheEnd == null)
572 return false;
573
574 // Atomically append the chain at the tail of this collection
575 for (Node<E> t = tail, p = t;;) {
576 Node<E> q = p.next;
577 if (q == null) {
578 // p is last node
579 if (NEXT.compareAndSet(p, null, beginningOfTheEnd)) {
580 // Successful CAS is the linearization point
581 // for all elements to be added to this queue.
582 if (!TAIL.weakCompareAndSet(this, t, last)) {
583 // Try a little harder to update tail,
584 // since we may be adding many elements.
585 t = tail;
586 if (last.next == null)
587 TAIL.weakCompareAndSet(this, t, last);
588 }
589 return true;
590 }
591 // Lost CAS race to another thread; re-read next
592 }
593 else if (p == q)
594 // We have fallen off list. If tail is unchanged, it
595 // will also be off-list, in which case we need to
596 // jump to head, from which all live nodes are always
597 // reachable. Else the new tail is a better bet.
598 p = (t != (t = tail)) ? t : head;
599 else
600 // Check for tail updates after two hops.
601 p = (p != t && t != (t = tail)) ? t : q;
602 }
603 }
604
605 public String toString() {
606 String[] a = null;
607 restartFromHead: for (;;) {
608 int charLength = 0;
609 int size = 0;
610 for (Node<E> p = first(); p != null;) {
611 final E item;
612 if ((item = p.item) != null) {
613 if (a == null)
614 a = new String[4];
615 else if (size == a.length)
616 a = Arrays.copyOf(a, 2 * size);
617 String s = item.toString();
618 a[size++] = s;
619 charLength += s.length();
620 }
621 if (p == (p = p.next))
622 continue restartFromHead;
623 }
624
625 if (size == 0)
626 return "[]";
627
628 return Helpers.toString(a, size, charLength);
629 }
630 }
631
632 private Object[] toArrayInternal(Object[] a) {
633 Object[] x = a;
634 restartFromHead: for (;;) {
635 int size = 0;
636 for (Node<E> p = first(); p != null;) {
637 final E item;
638 if ((item = p.item) != null) {
639 if (x == null)
640 x = new Object[4];
641 else if (size == x.length)
642 x = Arrays.copyOf(x, 2 * (size + 4));
643 x[size++] = item;
644 }
645 if (p == (p = p.next))
646 continue restartFromHead;
647 }
648 if (x == null)
649 return new Object[0];
650 else if (a != null && size <= a.length) {
651 if (a != x)
652 System.arraycopy(x, 0, a, 0, size);
653 if (size < a.length)
654 a[size] = null;
655 return a;
656 }
657 return (size == x.length) ? x : Arrays.copyOf(x, size);
658 }
659 }
660
661 /**
662 * Returns an array containing all of the elements in this queue, in
663 * proper sequence.
664 *
665 * <p>The returned array will be "safe" in that no references to it are
666 * maintained by this queue. (In other words, this method must allocate
667 * a new array). The caller is thus free to modify the returned array.
668 *
669 * <p>This method acts as bridge between array-based and collection-based
670 * APIs.
671 *
672 * @return an array containing all of the elements in this queue
673 */
674 public Object[] toArray() {
675 return toArrayInternal(null);
676 }
677
678 /**
679 * Returns an array containing all of the elements in this queue, in
680 * proper sequence; the runtime type of the returned array is that of
681 * the specified array. If the queue fits in the specified array, it
682 * is returned therein. Otherwise, a new array is allocated with the
683 * runtime type of the specified array and the size of this queue.
684 *
685 * <p>If this queue fits in the specified array with room to spare
686 * (i.e., the array has more elements than this queue), the element in
687 * the array immediately following the end of the queue is set to
688 * {@code null}.
689 *
690 * <p>Like the {@link #toArray()} method, this method acts as bridge between
691 * array-based and collection-based APIs. Further, this method allows
692 * precise control over the runtime type of the output array, and may,
693 * under certain circumstances, be used to save allocation costs.
694 *
695 * <p>Suppose {@code x} is a queue known to contain only strings.
696 * The following code can be used to dump the queue into a newly
697 * allocated array of {@code String}:
698 *
699 * <pre> {@code String[] y = x.toArray(new String[0]);}</pre>
700 *
701 * Note that {@code toArray(new Object[0])} is identical in function to
702 * {@code toArray()}.
703 *
704 * @param a the array into which the elements of the queue are to
705 * be stored, if it is big enough; otherwise, a new array of the
706 * same runtime type is allocated for this purpose
707 * @return an array containing all of the elements in this queue
708 * @throws ArrayStoreException if the runtime type of the specified array
709 * is not a supertype of the runtime type of every element in
710 * this queue
711 * @throws NullPointerException if the specified array is null
712 */
713 @SuppressWarnings("unchecked")
714 public <T> T[] toArray(T[] a) {
715 Objects.requireNonNull(a);
716 return (T[]) toArrayInternal(a);
717 }
718
719 /**
720 * Returns an iterator over the elements in this queue in proper sequence.
721 * The elements will be returned in order from first (head) to last (tail).
722 *
723 * <p>The returned iterator is
724 * <a href="package-summary.html#Weakly"><i>weakly consistent</i></a>.
725 *
726 * @return an iterator over the elements in this queue in proper sequence
727 */
728 public Iterator<E> iterator() {
729 return new Itr();
730 }
731
732 private class Itr implements Iterator<E> {
733 /**
734 * Next node to return item for.
735 */
736 private Node<E> nextNode;
737
738 /**
739 * nextItem holds on to item fields because once we claim
740 * that an element exists in hasNext(), we must return it in
741 * the following next() call even if it was in the process of
742 * being removed when hasNext() was called.
743 */
744 private E nextItem;
745
746 /**
747 * Node of the last returned item, to support remove.
748 */
749 private Node<E> lastRet;
750
751 Itr() {
752 restartFromHead: for (;;) {
753 Node<E> h, p, q;
754 for (p = h = head;; p = q) {
755 final E item;
756 if ((item = p.item) != null) {
757 nextNode = p;
758 nextItem = item;
759 break;
760 }
761 else if ((q = p.next) == null)
762 break;
763 else if (p == q)
764 continue restartFromHead;
765 }
766 updateHead(h, p);
767 return;
768 }
769 }
770
771 public boolean hasNext() {
772 return nextItem != null;
773 }
774
775 public E next() {
776 final Node<E> pred = nextNode;
777 if (pred == null) throw new NoSuchElementException();
778 // assert nextItem != null;
779 lastRet = pred;
780 E item = null;
781
782 for (Node<E> p = succ(pred), q;; p = q) {
783 if (p == null || (item = p.item) != null) {
784 nextNode = p;
785 E x = nextItem;
786 nextItem = item;
787 return x;
788 }
789 // unlink deleted nodes
790 if ((q = succ(p)) != null)
791 NEXT.compareAndSet(pred, p, q);
792 }
793 }
794
795 // Default implementation of forEachRemaining is "good enough".
796
797 public void remove() {
798 Node<E> l = lastRet;
799 if (l == null) throw new IllegalStateException();
800 // rely on a future traversal to relink.
801 l.item = null;
802 lastRet = null;
803 }
804 }
805
806 /**
807 * Saves this queue to a stream (that is, serializes it).
808 *
809 * @param s the stream
810 * @throws java.io.IOException if an I/O error occurs
811 * @serialData All of the elements (each an {@code E}) in
812 * the proper order, followed by a null
813 */
814 private void writeObject(java.io.ObjectOutputStream s)
815 throws java.io.IOException {
816
817 // Write out any hidden stuff
818 s.defaultWriteObject();
819
820 // Write out all elements in the proper order.
821 for (Node<E> p = first(); p != null; p = succ(p)) {
822 final E item;
823 if ((item = p.item) != null)
824 s.writeObject(item);
825 }
826
827 // Use trailing null as sentinel
828 s.writeObject(null);
829 }
830
831 /**
832 * Reconstitutes this queue from a stream (that is, deserializes it).
833 * @param s the stream
834 * @throws ClassNotFoundException if the class of a serialized object
835 * could not be found
836 * @throws java.io.IOException if an I/O error occurs
837 */
838 private void readObject(java.io.ObjectInputStream s)
839 throws java.io.IOException, ClassNotFoundException {
840 s.defaultReadObject();
841
842 // Read in elements until trailing null sentinel found
843 Node<E> h = null, t = null;
844 for (Object item; (item = s.readObject()) != null; ) {
845 @SuppressWarnings("unchecked")
846 Node<E> newNode = new Node<E>((E) item);
847 if (h == null)
848 h = t = newNode;
849 else
850 t.appendRelaxed(t = newNode);
851 }
852 if (h == null)
853 h = t = new Node<E>();
854 head = h;
855 tail = t;
856 }
857
858 /** A customized variant of Spliterators.IteratorSpliterator */
859 final class CLQSpliterator implements Spliterator<E> {
860 static final int MAX_BATCH = 1 << 25; // max batch array size;
861 Node<E> current; // current node; null until initialized
862 int batch; // batch size for splits
863 boolean exhausted; // true when no more nodes
864
865 public Spliterator<E> trySplit() {
866 Node<E> p, q;
867 if ((p = current()) == null || (q = p.next) == null)
868 return null;
869 int i = 0, n = batch = Math.min(batch + 1, MAX_BATCH);
870 Object[] a = null;
871 do {
872 final E e;
873 if ((e = p.item) != null) {
874 if (a == null)
875 a = new Object[n];
876 a[i++] = e;
877 }
878 if (p == (p = q))
879 p = first();
880 } while (p != null && (q = p.next) != null && i < n);
881 setCurrent(p);
882 return (i == 0) ? null :
883 Spliterators.spliterator(a, 0, i, (Spliterator.ORDERED |
884 Spliterator.NONNULL |
885 Spliterator.CONCURRENT));
886 }
887
888 public void forEachRemaining(Consumer<? super E> action) {
889 Objects.requireNonNull(action);
890 final Node<E> p;
891 if ((p = current()) != null) {
892 current = null;
893 exhausted = true;
894 forEachFrom(action, p);
895 }
896 }
897
898 public boolean tryAdvance(Consumer<? super E> action) {
899 Objects.requireNonNull(action);
900 Node<E> p;
901 if ((p = current()) != null) {
902 E e;
903 do {
904 e = p.item;
905 if (p == (p = p.next))
906 p = first();
907 } while (e == null && p != null);
908 setCurrent(p);
909 if (e != null) {
910 action.accept(e);
911 return true;
912 }
913 }
914 return false;
915 }
916
917 private void setCurrent(Node<E> p) {
918 if ((current = p) == null)
919 exhausted = true;
920 }
921
922 private Node<E> current() {
923 Node<E> p;
924 if ((p = current) == null && !exhausted)
925 setCurrent(p = first());
926 return p;
927 }
928
929 public long estimateSize() { return Long.MAX_VALUE; }
930
931 public int characteristics() {
932 return (Spliterator.ORDERED |
933 Spliterator.NONNULL |
934 Spliterator.CONCURRENT);
935 }
936 }
937
938 /**
939 * Returns a {@link Spliterator} over the elements in this queue.
940 *
941 * <p>The returned spliterator is
942 * <a href="package-summary.html#Weakly"><i>weakly consistent</i></a>.
943 *
944 * <p>The {@code Spliterator} reports {@link Spliterator#CONCURRENT},
945 * {@link Spliterator#ORDERED}, and {@link Spliterator#NONNULL}.
946 *
947 * @implNote
948 * The {@code Spliterator} implements {@code trySplit} to permit limited
949 * parallelism.
950 *
951 * @return a {@code Spliterator} over the elements in this queue
952 * @since 1.8
953 */
954 @Override
955 public Spliterator<E> spliterator() {
956 return new CLQSpliterator();
957 }
958
959 /**
960 * @throws NullPointerException {@inheritDoc}
961 */
962 public boolean removeIf(Predicate<? super E> filter) {
963 Objects.requireNonNull(filter);
964 return bulkRemove(filter);
965 }
966
967 /**
968 * @throws NullPointerException {@inheritDoc}
969 */
970 public boolean removeAll(Collection<?> c) {
971 Objects.requireNonNull(c);
972 return bulkRemove(e -> c.contains(e));
973 }
974
975 /**
976 * @throws NullPointerException {@inheritDoc}
977 */
978 public boolean retainAll(Collection<?> c) {
979 Objects.requireNonNull(c);
980 return bulkRemove(e -> !c.contains(e));
981 }
982
983 public void clear() {
984 bulkRemove(e -> true);
985 }
986
987 /**
988 * Tolerate this many consecutive dead nodes before CAS-collapsing.
989 * Amortized cost of clear() is (1 + 1/MAX_HOPS) CASes per element.
990 */
991 private static final int MAX_HOPS = 8;
992
993 /** Implementation of bulk remove methods. */
994 private boolean bulkRemove(Predicate<? super E> filter) {
995 boolean removed = false;
996 restartFromHead: for (;;) {
997 int hops = MAX_HOPS;
998 // c will be CASed to collapse intervening dead nodes between
999 // pred (or head if null) and p.
1000 for (Node<E> p = head, c = p, pred = null, q; p != null; p = q) {
1001 q = p.next;
1002 final E item; boolean pAlive;
1003 if (pAlive = ((item = p.item) != null)) {
1004 if (filter.test(item)) {
1005 if (p.casItem(item, null))
1006 removed = true;
1007 pAlive = false;
1008 }
1009 }
1010 if (pAlive || q == null || --hops == 0) {
1011 // p might already be self-linked here, but if so:
1012 // - CASing head will surely fail
1013 // - CASing pred's next will be useless but harmless.
1014 if ((c != p && !tryCasSuccessor(pred, c, c = p))
1015 || pAlive) {
1016 // if CAS failed or alive, abandon old pred
1017 hops = MAX_HOPS;
1018 pred = p;
1019 c = q;
1020 }
1021 } else if (p == q)
1022 continue restartFromHead;
1023 }
1024 return removed;
1025 }
1026 }
1027
1028 /**
1029 * Runs action on each element found during a traversal starting at p.
1030 * If p is null, the action is not run.
1031 */
1032 void forEachFrom(Consumer<? super E> action, Node<E> p) {
1033 for (Node<E> pred = null; p != null; ) {
1034 Node<E> q = p.next;
1035 final E item;
1036 if ((item = p.item) != null) {
1037 action.accept(item);
1038 pred = p; p = q; continue;
1039 }
1040 for (Node<E> c = p;; q = p.next) {
1041 if (q == null || q.item != null) {
1042 pred = skipDeadNodes(pred, c, p, q); p = q; break;
1043 }
1044 if (p == (p = q)) { pred = null; p = head; break; }
1045 }
1046 }
1047 }
1048
1049 /**
1050 * @throws NullPointerException {@inheritDoc}
1051 */
1052 public void forEach(Consumer<? super E> action) {
1053 Objects.requireNonNull(action);
1054 forEachFrom(action, head);
1055 }
1056
1057 // VarHandle mechanics
1058 private static final VarHandle HEAD;
1059 private static final VarHandle TAIL;
1060 static final VarHandle ITEM;
1061 static final VarHandle NEXT;
1062 static {
1063 try {
1064 MethodHandles.Lookup l = MethodHandles.lookup();
1065 HEAD = l.findVarHandle(ConcurrentLinkedQueue.class, "head",
1066 Node.class);
1067 TAIL = l.findVarHandle(ConcurrentLinkedQueue.class, "tail",
1068 Node.class);
1069 ITEM = l.findVarHandle(Node.class, "item", Object.class);
1070 NEXT = l.findVarHandle(Node.class, "next", Node.class);
1071 } catch (ReflectiveOperationException e) {
1072 throw new Error(e);
1073 }
1074 }
1075 }