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 with assistance from members of JCP JSR-166
32 * Expert Group and released to the public domain, as explained at
33 * http://creativecommons.org/licenses/publicdomain
34 */
35
36 package java.util.concurrent;
37
38 import java.util.AbstractQueue;
39 import java.util.Collection;
40 import java.util.Iterator;
41 import java.util.NoSuchElementException;
42 import java.util.concurrent.locks.Condition;
43 import java.util.concurrent.locks.ReentrantLock;
44
45 /**
46 * An optionally-bounded {@linkplain BlockingDeque blocking deque} based on
47 * linked nodes.
48 *
49 * <p> The optional capacity bound constructor argument serves as a
50 * way to prevent excessive expansion. The capacity, if unspecified,
51 * is equal to {@link Integer#MAX_VALUE}. Linked nodes are
52 * dynamically created upon each insertion unless this would bring the
53 * deque above capacity.
54 *
55 * <p>Most operations run in constant time (ignoring time spent
56 * blocking). Exceptions include {@link #remove(Object) remove},
57 * {@link #removeFirstOccurrence removeFirstOccurrence}, {@link
58 * #removeLastOccurrence removeLastOccurrence}, {@link #contains
59 * contains}, {@link #iterator iterator.remove()}, and the bulk
60 * operations, all of which run in linear time.
61 *
62 * <p>This class and its iterator implement all of the
63 * <em>optional</em> methods of the {@link Collection} and {@link
64 * Iterator} interfaces.
65 *
66 * <p>This class is a member of the
67 * <a href="{@docRoot}/../technotes/guides/collections/index.html">
68 * Java Collections Framework</a>.
69 *
70 * @since 1.6
71 * @author Doug Lea
72 * @param <E> the type of elements held in this collection
73 */
74 public class LinkedBlockingDeque<E>
75 extends AbstractQueue<E>
76 implements BlockingDeque<E>, java.io.Serializable {
77
78 /*
79 * Implemented as a simple doubly-linked list protected by a
80 * single lock and using conditions to manage blocking.
81 *
82 * To implement weakly consistent iterators, it appears we need to
83 * keep all Nodes GC-reachable from a predecessor dequeued Node.
84 * That would cause two problems:
85 * - allow a rogue Iterator to cause unbounded memory retention
86 * - cause cross-generational linking of old Nodes to new Nodes if
87 * a Node was tenured while live, which generational GCs have a
88 * hard time dealing with, causing repeated major collections.
89 * However, only non-deleted Nodes need to be reachable from
90 * dequeued Nodes, and reachability does not necessarily have to
91 * be of the kind understood by the GC. We use the trick of
92 * linking a Node that has just been dequeued to itself. Such a
93 * self-link implicitly means to jump to "first" (for next links)
94 * or "last" (for prev links).
95 */
96
97 /*
98 * We have "diamond" multiple interface/abstract class inheritance
99 * here, and that introduces ambiguities. Often we want the
100 * BlockingDeque javadoc combined with the AbstractQueue
101 * implementation, so a lot of method specs are duplicated here.
102 */
103
104 private static final long serialVersionUID = -387911632671998426L;
105
106 /** Doubly-linked list node class */
107 static final class Node<E> {
108 /**
109 * The item, or null if this node has been removed.
110 */
111 E item;
112
113 /**
114 * One of:
115 * - the real predecessor Node
116 * - this Node, meaning the predecessor is tail
117 * - null, meaning there is no predecessor
118 */
119 Node<E> prev;
120
121 /**
122 * One of:
123 * - the real successor Node
124 * - this Node, meaning the successor is head
125 * - null, meaning there is no successor
126 */
127 Node<E> next;
128
129 Node(E x, Node<E> p, Node<E> n) {
130 item = x;
131 prev = p;
132 next = n;
133 }
134 }
135
136 /**
137 * Pointer to first node.
138 * Invariant: (first == null && last == null) ||
139 * (first.prev == null && first.item != null)
140 */
141 transient Node<E> first;
142
143 /**
144 * Pointer to last node.
145 * Invariant: (first == null && last == null) ||
146 * (last.next == null && last.item != null)
147 */
148 transient Node<E> last;
149
150 /** Number of items in the deque */
151 private transient int count;
152
153 /** Maximum number of items in the deque */
154 private final int capacity;
155
156 /** Main lock guarding all access */
157 final ReentrantLock lock = new ReentrantLock();
158
159 /** Condition for waiting takes */
160 private final Condition notEmpty = lock.newCondition();
161
162 /** Condition for waiting puts */
163 private final Condition notFull = lock.newCondition();
164
165 /**
166 * Creates a {@code LinkedBlockingDeque} with a capacity of
167 * {@link Integer#MAX_VALUE}.
168 */
169 public LinkedBlockingDeque() {
170 this(Integer.MAX_VALUE);
171 }
172
173 /**
174 * Creates a {@code LinkedBlockingDeque} with the given (fixed) capacity.
175 *
176 * @param capacity the capacity of this deque
177 * @throws IllegalArgumentException if {@code capacity} is less than 1
178 */
179 public LinkedBlockingDeque(int capacity) {
180 if (capacity <= 0) throw new IllegalArgumentException();
181 this.capacity = capacity;
182 }
183
184 /**
185 * Creates a {@code LinkedBlockingDeque} with a capacity of
186 * {@link Integer#MAX_VALUE}, initially containing the elements of
187 * the given collection, added in traversal order of the
188 * collection's iterator.
189 *
190 * @param c the collection of elements to initially contain
191 * @throws NullPointerException if the specified collection or any
192 * of its elements are null
193 */
194 public LinkedBlockingDeque(Collection<? extends E> c) {
195 this(Integer.MAX_VALUE);
196 final ReentrantLock lock = this.lock;
197 lock.lock(); // Never contended, but necessary for visibility
198 try {
199 for (E e : c) {
200 if (e == null)
201 throw new NullPointerException();
202 if (!linkLast(e))
203 throw new IllegalStateException("Deque full");
204 }
205 } finally {
206 lock.unlock();
207 }
208 }
209
210
211 // Basic linking and unlinking operations, called only while holding lock
212
213 /**
214 * Links e as first element, or returns false if full.
215 */
216 private boolean linkFirst(E e) {
217 // assert lock.isHeldByCurrentThread();
218 if (count >= capacity)
219 return false;
220 Node<E> f = first;
221 Node<E> x = new Node<E>(e, null, f);
222 first = x;
223 if (last == null)
224 last = x;
225 else
226 f.prev = x;
227 ++count;
228 notEmpty.signal();
229 return true;
230 }
231
232 /**
233 * Links e as last element, or returns false if full.
234 */
235 private boolean linkLast(E e) {
236 // assert lock.isHeldByCurrentThread();
237 if (count >= capacity)
238 return false;
239 Node<E> l = last;
240 Node<E> x = new Node<E>(e, l, null);
241 last = x;
242 if (first == null)
243 first = x;
244 else
245 l.next = x;
246 ++count;
247 notEmpty.signal();
248 return true;
249 }
250
251 /**
252 * Removes and returns first element, or null if empty.
253 */
254 private E unlinkFirst() {
255 // assert lock.isHeldByCurrentThread();
256 Node<E> f = first;
257 if (f == null)
258 return null;
259 Node<E> n = f.next;
260 E item = f.item;
261 f.item = null;
262 f.next = f; // help GC
263 first = n;
264 if (n == null)
265 last = null;
266 else
267 n.prev = null;
268 --count;
269 notFull.signal();
270 return item;
271 }
272
273 /**
274 * Removes and returns last element, or null if empty.
275 */
276 private E unlinkLast() {
277 // assert lock.isHeldByCurrentThread();
278 Node<E> l = last;
279 if (l == null)
280 return null;
281 Node<E> p = l.prev;
282 E item = l.item;
283 l.item = null;
284 l.prev = l; // help GC
285 last = p;
286 if (p == null)
287 first = null;
288 else
289 p.next = null;
290 --count;
291 notFull.signal();
292 return item;
293 }
294
295 /**
296 * Unlinks x.
297 */
298 void unlink(Node<E> x) {
299 // assert lock.isHeldByCurrentThread();
300 Node<E> p = x.prev;
301 Node<E> n = x.next;
302 if (p == null) {
303 unlinkFirst();
304 } else if (n == null) {
305 unlinkLast();
306 } else {
307 p.next = n;
308 n.prev = p;
309 x.item = null;
310 // Don't mess with x's links. They may still be in use by
311 // an iterator.
312 --count;
313 notFull.signal();
314 }
315 }
316
317 // BlockingDeque methods
318
319 /**
320 * @throws IllegalStateException {@inheritDoc}
321 * @throws NullPointerException {@inheritDoc}
322 */
323 public void addFirst(E e) {
324 if (!offerFirst(e))
325 throw new IllegalStateException("Deque full");
326 }
327
328 /**
329 * @throws IllegalStateException {@inheritDoc}
330 * @throws NullPointerException {@inheritDoc}
331 */
332 public void addLast(E e) {
333 if (!offerLast(e))
334 throw new IllegalStateException("Deque full");
335 }
336
337 /**
338 * @throws NullPointerException {@inheritDoc}
339 */
340 public boolean offerFirst(E e) {
341 if (e == null) throw new NullPointerException();
342 final ReentrantLock lock = this.lock;
343 lock.lock();
344 try {
345 return linkFirst(e);
346 } finally {
347 lock.unlock();
348 }
349 }
350
351 /**
352 * @throws NullPointerException {@inheritDoc}
353 */
354 public boolean offerLast(E e) {
355 if (e == null) throw new NullPointerException();
356 final ReentrantLock lock = this.lock;
357 lock.lock();
358 try {
359 return linkLast(e);
360 } finally {
361 lock.unlock();
362 }
363 }
364
365 /**
366 * @throws NullPointerException {@inheritDoc}
367 * @throws InterruptedException {@inheritDoc}
368 */
369 public void putFirst(E e) throws InterruptedException {
370 if (e == null) throw new NullPointerException();
371 final ReentrantLock lock = this.lock;
372 lock.lock();
373 try {
374 while (!linkFirst(e))
375 notFull.await();
376 } finally {
377 lock.unlock();
378 }
379 }
380
381 /**
382 * @throws NullPointerException {@inheritDoc}
383 * @throws InterruptedException {@inheritDoc}
384 */
385 public void putLast(E e) throws InterruptedException {
386 if (e == null) throw new NullPointerException();
387 final ReentrantLock lock = this.lock;
388 lock.lock();
389 try {
390 while (!linkLast(e))
391 notFull.await();
392 } finally {
393 lock.unlock();
394 }
395 }
396
397 /**
398 * @throws NullPointerException {@inheritDoc}
399 * @throws InterruptedException {@inheritDoc}
400 */
401 public boolean offerFirst(E e, long timeout, TimeUnit unit)
402 throws InterruptedException {
403 if (e == null) throw new NullPointerException();
404 long nanos = unit.toNanos(timeout);
405 final ReentrantLock lock = this.lock;
406 lock.lockInterruptibly();
407 try {
408 while (!linkFirst(e)) {
409 if (nanos <= 0)
410 return false;
411 nanos = notFull.awaitNanos(nanos);
412 }
413 return true;
414 } finally {
415 lock.unlock();
416 }
417 }
418
419 /**
420 * @throws NullPointerException {@inheritDoc}
421 * @throws InterruptedException {@inheritDoc}
422 */
423 public boolean offerLast(E e, long timeout, TimeUnit unit)
424 throws InterruptedException {
425 if (e == null) throw new NullPointerException();
426 long nanos = unit.toNanos(timeout);
427 final ReentrantLock lock = this.lock;
428 lock.lockInterruptibly();
429 try {
430 while (!linkLast(e)) {
431 if (nanos <= 0)
432 return false;
433 nanos = notFull.awaitNanos(nanos);
434 }
435 return true;
436 } finally {
437 lock.unlock();
438 }
439 }
440
441 /**
442 * @throws NoSuchElementException {@inheritDoc}
443 */
444 public E removeFirst() {
445 E x = pollFirst();
446 if (x == null) throw new NoSuchElementException();
447 return x;
448 }
449
450 /**
451 * @throws NoSuchElementException {@inheritDoc}
452 */
453 public E removeLast() {
454 E x = pollLast();
455 if (x == null) throw new NoSuchElementException();
456 return x;
457 }
458
459 public E pollFirst() {
460 final ReentrantLock lock = this.lock;
461 lock.lock();
462 try {
463 return unlinkFirst();
464 } finally {
465 lock.unlock();
466 }
467 }
468
469 public E pollLast() {
470 final ReentrantLock lock = this.lock;
471 lock.lock();
472 try {
473 return unlinkLast();
474 } finally {
475 lock.unlock();
476 }
477 }
478
479 public E takeFirst() throws InterruptedException {
480 final ReentrantLock lock = this.lock;
481 lock.lock();
482 try {
483 E x;
484 while ( (x = unlinkFirst()) == null)
485 notEmpty.await();
486 return x;
487 } finally {
488 lock.unlock();
489 }
490 }
491
492 public E takeLast() throws InterruptedException {
493 final ReentrantLock lock = this.lock;
494 lock.lock();
495 try {
496 E x;
497 while ( (x = unlinkLast()) == null)
498 notEmpty.await();
499 return x;
500 } finally {
501 lock.unlock();
502 }
503 }
504
505 public E pollFirst(long timeout, TimeUnit unit)
506 throws InterruptedException {
507 long nanos = unit.toNanos(timeout);
508 final ReentrantLock lock = this.lock;
509 lock.lockInterruptibly();
510 try {
511 E x;
512 while ( (x = unlinkFirst()) == null) {
513 if (nanos <= 0)
514 return null;
515 nanos = notEmpty.awaitNanos(nanos);
516 }
517 return x;
518 } finally {
519 lock.unlock();
520 }
521 }
522
523 public E pollLast(long timeout, TimeUnit unit)
524 throws InterruptedException {
525 long nanos = unit.toNanos(timeout);
526 final ReentrantLock lock = this.lock;
527 lock.lockInterruptibly();
528 try {
529 E x;
530 while ( (x = unlinkLast()) == null) {
531 if (nanos <= 0)
532 return null;
533 nanos = notEmpty.awaitNanos(nanos);
534 }
535 return x;
536 } finally {
537 lock.unlock();
538 }
539 }
540
541 /**
542 * @throws NoSuchElementException {@inheritDoc}
543 */
544 public E getFirst() {
545 E x = peekFirst();
546 if (x == null) throw new NoSuchElementException();
547 return x;
548 }
549
550 /**
551 * @throws NoSuchElementException {@inheritDoc}
552 */
553 public E getLast() {
554 E x = peekLast();
555 if (x == null) throw new NoSuchElementException();
556 return x;
557 }
558
559 public E peekFirst() {
560 final ReentrantLock lock = this.lock;
561 lock.lock();
562 try {
563 return (first == null) ? null : first.item;
564 } finally {
565 lock.unlock();
566 }
567 }
568
569 public E peekLast() {
570 final ReentrantLock lock = this.lock;
571 lock.lock();
572 try {
573 return (last == null) ? null : last.item;
574 } finally {
575 lock.unlock();
576 }
577 }
578
579 public boolean removeFirstOccurrence(Object o) {
580 if (o == null) return false;
581 final ReentrantLock lock = this.lock;
582 lock.lock();
583 try {
584 for (Node<E> p = first; p != null; p = p.next) {
585 if (o.equals(p.item)) {
586 unlink(p);
587 return true;
588 }
589 }
590 return false;
591 } finally {
592 lock.unlock();
593 }
594 }
595
596 public boolean removeLastOccurrence(Object o) {
597 if (o == null) return false;
598 final ReentrantLock lock = this.lock;
599 lock.lock();
600 try {
601 for (Node<E> p = last; p != null; p = p.prev) {
602 if (o.equals(p.item)) {
603 unlink(p);
604 return true;
605 }
606 }
607 return false;
608 } finally {
609 lock.unlock();
610 }
611 }
612
613 // BlockingQueue methods
614
615 /**
616 * Inserts the specified element at the end of this deque unless it would
617 * violate capacity restrictions. When using a capacity-restricted deque,
618 * it is generally preferable to use method {@link #offer(Object) offer}.
619 *
620 * <p>This method is equivalent to {@link #addLast}.
621 *
622 * @throws IllegalStateException if the element cannot be added at this
623 * time due to capacity restrictions
624 * @throws NullPointerException if the specified element is null
625 */
626 public boolean add(E e) {
627 addLast(e);
628 return true;
629 }
630
631 /**
632 * @throws NullPointerException if the specified element is null
633 */
634 public boolean offer(E e) {
635 return offerLast(e);
636 }
637
638 /**
639 * @throws NullPointerException {@inheritDoc}
640 * @throws InterruptedException {@inheritDoc}
641 */
642 public void put(E e) throws InterruptedException {
643 putLast(e);
644 }
645
646 /**
647 * @throws NullPointerException {@inheritDoc}
648 * @throws InterruptedException {@inheritDoc}
649 */
650 public boolean offer(E e, long timeout, TimeUnit unit)
651 throws InterruptedException {
652 return offerLast(e, timeout, unit);
653 }
654
655 /**
656 * Retrieves and removes the head of the queue represented by this deque.
657 * This method differs from {@link #poll poll} only in that it throws an
658 * exception if this deque is empty.
659 *
660 * <p>This method is equivalent to {@link #removeFirst() removeFirst}.
661 *
662 * @return the head of the queue represented by this deque
663 * @throws NoSuchElementException if this deque is empty
664 */
665 public E remove() {
666 return removeFirst();
667 }
668
669 public E poll() {
670 return pollFirst();
671 }
672
673 public E take() throws InterruptedException {
674 return takeFirst();
675 }
676
677 public E poll(long timeout, TimeUnit unit) throws InterruptedException {
678 return pollFirst(timeout, unit);
679 }
680
681 /**
682 * Retrieves, but does not remove, the head of the queue represented by
683 * this deque. This method differs from {@link #peek peek} only in that
684 * it throws an exception if this deque is empty.
685 *
686 * <p>This method is equivalent to {@link #getFirst() getFirst}.
687 *
688 * @return the head of the queue represented by this deque
689 * @throws NoSuchElementException if this deque is empty
690 */
691 public E element() {
692 return getFirst();
693 }
694
695 public E peek() {
696 return peekFirst();
697 }
698
699 /**
700 * Returns the number of additional elements that this deque can ideally
701 * (in the absence of memory or resource constraints) accept without
702 * blocking. This is always equal to the initial capacity of this deque
703 * less the current {@code size} of this deque.
704 *
705 * <p>Note that you <em>cannot</em> always tell if an attempt to insert
706 * an element will succeed by inspecting {@code remainingCapacity}
707 * because it may be the case that another thread is about to
708 * insert or remove an element.
709 */
710 public int remainingCapacity() {
711 final ReentrantLock lock = this.lock;
712 lock.lock();
713 try {
714 return capacity - count;
715 } finally {
716 lock.unlock();
717 }
718 }
719
720 /**
721 * @throws UnsupportedOperationException {@inheritDoc}
722 * @throws ClassCastException {@inheritDoc}
723 * @throws NullPointerException {@inheritDoc}
724 * @throws IllegalArgumentException {@inheritDoc}
725 */
726 public int drainTo(Collection<? super E> c) {
727 return drainTo(c, Integer.MAX_VALUE);
728 }
729
730 /**
731 * @throws UnsupportedOperationException {@inheritDoc}
732 * @throws ClassCastException {@inheritDoc}
733 * @throws NullPointerException {@inheritDoc}
734 * @throws IllegalArgumentException {@inheritDoc}
735 */
736 public int drainTo(Collection<? super E> c, int maxElements) {
737 if (c == null)
738 throw new NullPointerException();
739 if (c == this)
740 throw new IllegalArgumentException();
741 final ReentrantLock lock = this.lock;
742 lock.lock();
743 try {
744 int n = Math.min(maxElements, count);
745 for (int i = 0; i < n; i++) {
746 c.add(first.item); // In this order, in case add() throws.
747 unlinkFirst();
748 }
749 return n;
750 } finally {
751 lock.unlock();
752 }
753 }
754
755 // Stack methods
756
757 /**
758 * @throws IllegalStateException {@inheritDoc}
759 * @throws NullPointerException {@inheritDoc}
760 */
761 public void push(E e) {
762 addFirst(e);
763 }
764
765 /**
766 * @throws NoSuchElementException {@inheritDoc}
767 */
768 public E pop() {
769 return removeFirst();
770 }
771
772 // Collection methods
773
774 /**
775 * Removes the first occurrence of the specified element from this deque.
776 * If the deque does not contain the element, it is unchanged.
777 * More formally, removes the first element {@code e} such that
778 * {@code o.equals(e)} (if such an element exists).
779 * Returns {@code true} if this deque contained the specified element
780 * (or equivalently, if this deque changed as a result of the call).
781 *
782 * <p>This method is equivalent to
783 * {@link #removeFirstOccurrence(Object) removeFirstOccurrence}.
784 *
785 * @param o element to be removed from this deque, if present
786 * @return {@code true} if this deque changed as a result of the call
787 */
788 public boolean remove(Object o) {
789 return removeFirstOccurrence(o);
790 }
791
792 /**
793 * Returns the number of elements in this deque.
794 *
795 * @return the number of elements in this deque
796 */
797 public int size() {
798 final ReentrantLock lock = this.lock;
799 lock.lock();
800 try {
801 return count;
802 } finally {
803 lock.unlock();
804 }
805 }
806
807 /**
808 * Returns {@code true} if this deque contains the specified element.
809 * More formally, returns {@code true} if and only if this deque contains
810 * at least one element {@code e} such that {@code o.equals(e)}.
811 *
812 * @param o object to be checked for containment in this deque
813 * @return {@code true} if this deque contains the specified element
814 */
815 public boolean contains(Object o) {
816 if (o == null) return false;
817 final ReentrantLock lock = this.lock;
818 lock.lock();
819 try {
820 for (Node<E> p = first; p != null; p = p.next)
821 if (o.equals(p.item))
822 return true;
823 return false;
824 } finally {
825 lock.unlock();
826 }
827 }
828
829 /*
830 * TODO: Add support for more efficient bulk operations.
831 *
832 * We don't want to acquire the lock for every iteration, but we
833 * also want other threads a chance to interact with the
834 * collection, especially when count is close to capacity.
835 */
836
837 // /**
838 // * Adds all of the elements in the specified collection to this
839 // * queue. Attempts to addAll of a queue to itself result in
840 // * {@code IllegalArgumentException}. Further, the behavior of
841 // * this operation is undefined if the specified collection is
842 // * modified while the operation is in progress.
843 // *
844 // * @param c collection containing elements to be added to this queue
845 // * @return {@code true} if this queue changed as a result of the call
846 // * @throws ClassCastException {@inheritDoc}
847 // * @throws NullPointerException {@inheritDoc}
848 // * @throws IllegalArgumentException {@inheritDoc}
849 // * @throws IllegalStateException {@inheritDoc}
850 // * @see #add(Object)
851 // */
852 // public boolean addAll(Collection<? extends E> c) {
853 // if (c == null)
854 // throw new NullPointerException();
855 // if (c == this)
856 // throw new IllegalArgumentException();
857 // final ReentrantLock lock = this.lock;
858 // lock.lock();
859 // try {
860 // boolean modified = false;
861 // for (E e : c)
862 // if (linkLast(e))
863 // modified = true;
864 // return modified;
865 // } finally {
866 // lock.unlock();
867 // }
868 // }
869
870 /**
871 * Returns an array containing all of the elements in this deque, in
872 * proper sequence (from first to last element).
873 *
874 * <p>The returned array will be "safe" in that no references to it are
875 * maintained by this deque. (In other words, this method must allocate
876 * a new array). The caller is thus free to modify the returned array.
877 *
878 * <p>This method acts as bridge between array-based and collection-based
879 * APIs.
880 *
881 * @return an array containing all of the elements in this deque
882 */
883 @SuppressWarnings("unchecked")
884 public Object[] toArray() {
885 final ReentrantLock lock = this.lock;
886 lock.lock();
887 try {
888 Object[] a = new Object[count];
889 int k = 0;
890 for (Node<E> p = first; p != null; p = p.next)
891 a[k++] = p.item;
892 return a;
893 } finally {
894 lock.unlock();
895 }
896 }
897
898 /**
899 * Returns an array containing all of the elements in this deque, in
900 * proper sequence; the runtime type of the returned array is that of
901 * the specified array. If the deque fits in the specified array, it
902 * is returned therein. Otherwise, a new array is allocated with the
903 * runtime type of the specified array and the size of this deque.
904 *
905 * <p>If this deque fits in the specified array with room to spare
906 * (i.e., the array has more elements than this deque), the element in
907 * the array immediately following the end of the deque is set to
908 * {@code null}.
909 *
910 * <p>Like the {@link #toArray()} method, this method acts as bridge between
911 * array-based and collection-based APIs. Further, this method allows
912 * precise control over the runtime type of the output array, and may,
913 * under certain circumstances, be used to save allocation costs.
914 *
915 * <p>Suppose {@code x} is a deque known to contain only strings.
916 * The following code can be used to dump the deque into a newly
917 * allocated array of {@code String}:
918 *
919 * <pre>
920 * String[] y = x.toArray(new String[0]);</pre>
921 *
922 * Note that {@code toArray(new Object[0])} is identical in function to
923 * {@code toArray()}.
924 *
925 * @param a the array into which the elements of the deque are to
926 * be stored, if it is big enough; otherwise, a new array of the
927 * same runtime type is allocated for this purpose
928 * @return an array containing all of the elements in this deque
929 * @throws ArrayStoreException if the runtime type of the specified array
930 * is not a supertype of the runtime type of every element in
931 * this deque
932 * @throws NullPointerException if the specified array is null
933 */
934 @SuppressWarnings("unchecked")
935 public <T> T[] toArray(T[] a) {
936 final ReentrantLock lock = this.lock;
937 lock.lock();
938 try {
939 if (a.length < count)
940 a = (T[])java.lang.reflect.Array.newInstance
941 (a.getClass().getComponentType(), count);
942
943 int k = 0;
944 for (Node<E> p = first; p != null; p = p.next)
945 a[k++] = (T)p.item;
946 if (a.length > k)
947 a[k] = null;
948 return a;
949 } finally {
950 lock.unlock();
951 }
952 }
953
954 public String toString() {
955 final ReentrantLock lock = this.lock;
956 lock.lock();
957 try {
958 return super.toString();
959 } finally {
960 lock.unlock();
961 }
962 }
963
964 /**
965 * Atomically removes all of the elements from this deque.
966 * The deque will be empty after this call returns.
967 */
968 public void clear() {
969 final ReentrantLock lock = this.lock;
970 lock.lock();
971 try {
972 for (Node<E> f = first; f != null; ) {
973 f.item = null;
974 Node<E> n = f.next;
975 f.prev = null;
976 f.next = null;
977 f = n;
978 }
979 first = last = null;
980 count = 0;
981 notFull.signalAll();
982 } finally {
983 lock.unlock();
984 }
985 }
986
987 /**
988 * Returns an iterator over the elements in this deque in proper sequence.
989 * The elements will be returned in order from first (head) to last (tail).
990 * The returned {@code Iterator} is a "weakly consistent" iterator that
991 * will never throw {@link java.util.ConcurrentModificationException
992 * ConcurrentModificationException},
993 * and guarantees to traverse elements as they existed upon
994 * construction of the iterator, and may (but is not guaranteed to)
995 * reflect any modifications subsequent to construction.
996 *
997 * @return an iterator over the elements in this deque in proper sequence
998 */
999 public Iterator<E> iterator() {
1000 return new Itr();
1001 }
1002
1003 /**
1004 * Returns an iterator over the elements in this deque in reverse
1005 * sequential order. The elements will be returned in order from
1006 * last (tail) to first (head).
1007 * The returned {@code Iterator} is a "weakly consistent" iterator that
1008 * will never throw {@link java.util.ConcurrentModificationException
1009 * ConcurrentModificationException},
1010 * and guarantees to traverse elements as they existed upon
1011 * construction of the iterator, and may (but is not guaranteed to)
1012 * reflect any modifications subsequent to construction.
1013 */
1014 public Iterator<E> descendingIterator() {
1015 return new DescendingItr();
1016 }
1017
1018 /**
1019 * Base class for Iterators for LinkedBlockingDeque
1020 */
1021 private abstract class AbstractItr implements Iterator<E> {
1022 /**
1023 * The next node to return in next()
1024 */
1025 Node<E> next;
1026
1027 /**
1028 * nextItem holds on to item fields because once we claim that
1029 * an element exists in hasNext(), we must return item read
1030 * under lock (in advance()) even if it was in the process of
1031 * being removed when hasNext() was called.
1032 */
1033 E nextItem;
1034
1035 /**
1036 * Node returned by most recent call to next. Needed by remove.
1037 * Reset to null if this element is deleted by a call to remove.
1038 */
1039 private Node<E> lastRet;
1040
1041 abstract Node<E> firstNode();
1042 abstract Node<E> nextNode(Node<E> n);
1043
1044 AbstractItr() {
1045 // set to initial position
1046 final ReentrantLock lock = LinkedBlockingDeque.this.lock;
1047 lock.lock();
1048 try {
1049 next = firstNode();
1050 nextItem = (next == null) ? null : next.item;
1051 } finally {
1052 lock.unlock();
1053 }
1054 }
1055
1056 /**
1057 * Advances next.
1058 */
1059 void advance() {
1060 final ReentrantLock lock = LinkedBlockingDeque.this.lock;
1061 lock.lock();
1062 try {
1063 // assert next != null;
1064 Node<E> s = nextNode(next);
1065 if (s == next) {
1066 next = firstNode();
1067 } else {
1068 // Skip over removed nodes.
1069 // May be necessary if multiple interior Nodes are removed.
1070 while (s != null && s.item == null)
1071 s = nextNode(s);
1072 next = s;
1073 }
1074 nextItem = (next == null) ? null : next.item;
1075 } finally {
1076 lock.unlock();
1077 }
1078 }
1079
1080 public boolean hasNext() {
1081 return next != null;
1082 }
1083
1084 public E next() {
1085 if (next == null)
1086 throw new NoSuchElementException();
1087 lastRet = next;
1088 E x = nextItem;
1089 advance();
1090 return x;
1091 }
1092
1093 public void remove() {
1094 Node<E> n = lastRet;
1095 if (n == null)
1096 throw new IllegalStateException();
1097 lastRet = null;
1098 final ReentrantLock lock = LinkedBlockingDeque.this.lock;
1099 lock.lock();
1100 try {
1101 if (n.item != null)
1102 unlink(n);
1103 } finally {
1104 lock.unlock();
1105 }
1106 }
1107 }
1108
1109 /** Forward iterator */
1110 private class Itr extends AbstractItr {
1111 Node<E> firstNode() { return first; }
1112 Node<E> nextNode(Node<E> n) { return n.next; }
1113 }
1114
1115 /** Descending iterator */
1116 private class DescendingItr extends AbstractItr {
1117 Node<E> firstNode() { return last; }
1118 Node<E> nextNode(Node<E> n) { return n.prev; }
1119 }
1120
1121 /**
1122 * Save the state of this deque to a stream (that is, serialize it).
1123 *
1124 * @serialData The capacity (int), followed by elements (each an
1125 * {@code Object}) in the proper order, followed by a null
1126 * @param s the stream
1127 */
1128 private void writeObject(java.io.ObjectOutputStream s)
1129 throws java.io.IOException {
1130 final ReentrantLock lock = this.lock;
1131 lock.lock();
1132 try {
1133 // Write out capacity and any hidden stuff
1134 s.defaultWriteObject();
1135 // Write out all elements in the proper order.
1136 for (Node<E> p = first; p != null; p = p.next)
1137 s.writeObject(p.item);
1138 // Use trailing null as sentinel
1139 s.writeObject(null);
1140 } finally {
1141 lock.unlock();
1142 }
1143 }
1144
1145 /**
1146 * Reconstitute this deque from a stream (that is,
1147 * deserialize it).
1148 * @param s the stream
1149 */
1150 private void readObject(java.io.ObjectInputStream s)
1151 throws java.io.IOException, ClassNotFoundException {
1152 s.defaultReadObject();
1153 count = 0;
1154 first = null;
1155 last = null;
1156 // Read in all elements and place in queue
1157 for (;;) {
1158 @SuppressWarnings("unchecked")
1159 E item = (E)s.readObject();
1160 if (item == null)
1161 break;
1162 add(item);
1163 }
1164 }
1165
1166 }