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/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.Comparator;
44 import java.util.Iterator;
45 import java.util.NoSuchElementException;
46 import java.util.Objects;
47 import java.util.PriorityQueue;
48 import java.util.Queue;
49 import java.util.SortedSet;
50 import java.util.Spliterator;
51 import java.util.concurrent.locks.Condition;
52 import java.util.concurrent.locks.ReentrantLock;
53 import java.util.function.Consumer;
54 import java.util.function.Predicate;
55 import jdk.internal.misc.SharedSecrets;
56
57 /**
58 * An unbounded {@linkplain BlockingQueue blocking queue} that uses
59 * the same ordering rules as class {@link PriorityQueue} and supplies
60 * blocking retrieval operations. While this queue is logically
61 * unbounded, attempted additions may fail due to resource exhaustion
62 * (causing {@code OutOfMemoryError}). This class does not permit
63 * {@code null} elements. A priority queue relying on {@linkplain
64 * Comparable natural ordering} also does not permit insertion of
65 * non-comparable objects (doing so results in
66 * {@code ClassCastException}).
67 *
68 * <p>This class and its iterator implement all of the <em>optional</em>
69 * methods of the {@link Collection} and {@link Iterator} interfaces.
70 * The Iterator provided in method {@link #iterator()} and the
71 * Spliterator provided in method {@link #spliterator()} are <em>not</em>
72 * guaranteed to traverse the elements of the PriorityBlockingQueue in
73 * any particular order. If you need ordered traversal, consider using
74 * {@code Arrays.sort(pq.toArray())}. Also, method {@code drainTo} can
75 * be used to <em>remove</em> some or all elements in priority order and
76 * place them in another collection.
77 *
78 * <p>Operations on this class make no guarantees about the ordering
79 * of elements with equal priority. If you need to enforce an
80 * ordering, you can define custom classes or comparators that use a
81 * secondary key to break ties in primary priority values. For
82 * example, here is a class that applies first-in-first-out
83 * tie-breaking to comparable elements. To use it, you would insert a
84 * {@code new FIFOEntry(anEntry)} instead of a plain entry object.
85 *
86 * <pre> {@code
87 * class FIFOEntry<E extends Comparable<? super E>>
88 * implements Comparable<FIFOEntry<E>> {
89 * static final AtomicLong seq = new AtomicLong(0);
90 * final long seqNum;
91 * final E entry;
92 * public FIFOEntry(E entry) {
93 * seqNum = seq.getAndIncrement();
94 * this.entry = entry;
95 * }
96 * public E getEntry() { return entry; }
97 * public int compareTo(FIFOEntry<E> other) {
98 * int res = entry.compareTo(other.entry);
99 * if (res == 0 && other.entry != this.entry)
100 * res = (seqNum < other.seqNum ? -1 : 1);
101 * return res;
102 * }
103 * }}</pre>
104 *
105 * <p>This class is a member of the
106 * <a href="{@docRoot}/java.base/java/util/package-summary.html#CollectionsFramework">
107 * Java Collections Framework</a>.
108 *
109 * @since 1.5
110 * @author Doug Lea
111 * @param <E> the type of elements held in this queue
112 */
113 @SuppressWarnings("unchecked")
114 public class PriorityBlockingQueue<E> extends AbstractQueue<E>
115 implements BlockingQueue<E>, java.io.Serializable {
116 private static final long serialVersionUID = 5595510919245408276L;
117
118 /*
119 * The implementation uses an array-based binary heap, with public
120 * operations protected with a single lock. However, allocation
121 * during resizing uses a simple spinlock (used only while not
122 * holding main lock) in order to allow takes to operate
123 * concurrently with allocation. This avoids repeated
124 * postponement of waiting consumers and consequent element
125 * build-up. The need to back away from lock during allocation
126 * makes it impossible to simply wrap delegated
127 * java.util.PriorityQueue operations within a lock, as was done
128 * in a previous version of this class. To maintain
129 * interoperability, a plain PriorityQueue is still used during
130 * serialization, which maintains compatibility at the expense of
131 * transiently doubling overhead.
132 */
133
134 /**
135 * Default array capacity.
136 */
137 private static final int DEFAULT_INITIAL_CAPACITY = 11;
138
139 /**
140 * The maximum size of array to allocate.
141 * Some VMs reserve some header words in an array.
142 * Attempts to allocate larger arrays may result in
143 * OutOfMemoryError: Requested array size exceeds VM limit
144 */
145 private static final int MAX_ARRAY_SIZE = Integer.MAX_VALUE - 8;
146
147 /**
148 * Priority queue represented as a balanced binary heap: the two
149 * children of queue[n] are queue[2*n+1] and queue[2*(n+1)]. The
150 * priority queue is ordered by comparator, or by the elements'
151 * natural ordering, if comparator is null: For each node n in the
152 * heap and each descendant d of n, n <= d. The element with the
153 * lowest value is in queue[0], assuming the queue is nonempty.
154 */
155 private transient Object[] queue;
156
157 /**
158 * The number of elements in the priority queue.
159 */
160 private transient int size;
161
162 /**
163 * The comparator, or null if priority queue uses elements'
164 * natural ordering.
165 */
166 private transient Comparator<? super E> comparator;
167
168 /**
169 * Lock used for all public operations.
170 */
171 private final ReentrantLock lock = new ReentrantLock();
172
173 /**
174 * Condition for blocking when empty.
175 */
176 private final Condition notEmpty = lock.newCondition();
177
178 /**
179 * Spinlock for allocation, acquired via CAS.
180 */
181 private transient volatile int allocationSpinLock;
182
183 /**
184 * A plain PriorityQueue used only for serialization,
185 * to maintain compatibility with previous versions
186 * of this class. Non-null only during serialization/deserialization.
187 */
188 private PriorityQueue<E> q;
189
190 /**
191 * Creates a {@code PriorityBlockingQueue} with the default
192 * initial capacity (11) that orders its elements according to
193 * their {@linkplain Comparable natural ordering}.
194 */
195 public PriorityBlockingQueue() {
196 this(DEFAULT_INITIAL_CAPACITY, null);
197 }
198
199 /**
200 * Creates a {@code PriorityBlockingQueue} with the specified
201 * initial capacity that orders its elements according to their
202 * {@linkplain Comparable natural ordering}.
203 *
204 * @param initialCapacity the initial capacity for this priority queue
205 * @throws IllegalArgumentException if {@code initialCapacity} is less
206 * than 1
207 */
208 public PriorityBlockingQueue(int initialCapacity) {
209 this(initialCapacity, null);
210 }
211
212 /**
213 * Creates a {@code PriorityBlockingQueue} with the specified initial
214 * capacity that orders its elements according to the specified
215 * comparator.
216 *
217 * @param initialCapacity the initial capacity for this priority queue
218 * @param comparator the comparator that will be used to order this
219 * priority queue. If {@code null}, the {@linkplain Comparable
220 * natural ordering} of the elements will be used.
221 * @throws IllegalArgumentException if {@code initialCapacity} is less
222 * than 1
223 */
224 public PriorityBlockingQueue(int initialCapacity,
225 Comparator<? super E> comparator) {
226 if (initialCapacity < 1)
227 throw new IllegalArgumentException();
228 this.comparator = comparator;
229 this.queue = new Object[Math.max(1, initialCapacity)];
230 }
231
232 /**
233 * Creates a {@code PriorityBlockingQueue} containing the elements
234 * in the specified collection. If the specified collection is a
235 * {@link SortedSet} or a {@link PriorityQueue}, this
236 * priority queue will be ordered according to the same ordering.
237 * Otherwise, this priority queue will be ordered according to the
238 * {@linkplain Comparable natural ordering} of its elements.
239 *
240 * @param c the collection whose elements are to be placed
241 * into this priority queue
242 * @throws ClassCastException if elements of the specified collection
243 * cannot be compared to one another according to the priority
244 * queue's ordering
245 * @throws NullPointerException if the specified collection or any
246 * of its elements are null
247 */
248 public PriorityBlockingQueue(Collection<? extends E> c) {
249 boolean heapify = true; // true if not known to be in heap order
250 boolean screen = true; // true if must screen for nulls
251 if (c instanceof SortedSet<?>) {
252 SortedSet<? extends E> ss = (SortedSet<? extends E>) c;
253 this.comparator = (Comparator<? super E>) ss.comparator();
254 heapify = false;
255 }
256 else if (c instanceof PriorityBlockingQueue<?>) {
257 PriorityBlockingQueue<? extends E> pq =
258 (PriorityBlockingQueue<? extends E>) c;
259 this.comparator = (Comparator<? super E>) pq.comparator();
260 screen = false;
261 if (pq.getClass() == PriorityBlockingQueue.class) // exact match
262 heapify = false;
263 }
264 Object[] es = c.toArray();
265 int n = es.length;
266 // If c.toArray incorrectly doesn't return Object[], copy it.
267 if (es.getClass() != Object[].class)
268 es = Arrays.copyOf(es, n, Object[].class);
269 if (screen && (n == 1 || this.comparator != null)) {
270 for (Object e : es)
271 if (e == null)
272 throw new NullPointerException();
273 }
274 this.queue = ensureNonEmpty(es);
275 this.size = n;
276 if (heapify)
277 heapify();
278 }
279
280 /** Ensures that queue[0] exists, helping peek() and poll(). */
281 private static Object[] ensureNonEmpty(Object[] es) {
282 return (es.length > 0) ? es : new Object[1];
283 }
284
285 /**
286 * Tries to grow array to accommodate at least one more element
287 * (but normally expand by about 50%), giving up (allowing retry)
288 * on contention (which we expect to be rare). Call only while
289 * holding lock.
290 *
291 * @param array the heap array
292 * @param oldCap the length of the array
293 */
294 private void tryGrow(Object[] array, int oldCap) {
295 lock.unlock(); // must release and then re-acquire main lock
296 Object[] newArray = null;
297 if (allocationSpinLock == 0 &&
298 ALLOCATIONSPINLOCK.compareAndSet(this, 0, 1)) {
299 try {
300 int newCap = oldCap + ((oldCap < 64) ?
301 (oldCap + 2) : // grow faster if small
302 (oldCap >> 1));
303 if (newCap - MAX_ARRAY_SIZE > 0) { // possible overflow
304 int minCap = oldCap + 1;
305 if (minCap < 0 || minCap > MAX_ARRAY_SIZE)
306 throw new OutOfMemoryError();
307 newCap = MAX_ARRAY_SIZE;
308 }
309 if (newCap > oldCap && queue == array)
310 newArray = new Object[newCap];
311 } finally {
312 allocationSpinLock = 0;
313 }
314 }
315 if (newArray == null) // back off if another thread is allocating
316 Thread.yield();
317 lock.lock();
318 if (newArray != null && queue == array) {
319 queue = newArray;
320 System.arraycopy(array, 0, newArray, 0, oldCap);
321 }
322 }
323
324 /**
325 * Mechanics for poll(). Call only while holding lock.
326 */
327 private E dequeue() {
328 // assert lock.isHeldByCurrentThread();
329 final Object[] es;
330 final E result;
331
332 if ((result = (E) ((es = queue)[0])) != null) {
333 final int n;
334 final E x = (E) es[(n = --size)];
335 es[n] = null;
336 if (n > 0) {
337 final Comparator<? super E> cmp;
338 if ((cmp = comparator) == null)
339 siftDownComparable(0, x, es, n);
340 else
341 siftDownUsingComparator(0, x, es, n, cmp);
342 }
343 }
344 return result;
345 }
346
347 /**
348 * Inserts item x at position k, maintaining heap invariant by
349 * promoting x up the tree until it is greater than or equal to
350 * its parent, or is the root.
351 *
352 * To simplify and speed up coercions and comparisons, the
353 * Comparable and Comparator versions are separated into different
354 * methods that are otherwise identical. (Similarly for siftDown.)
355 *
356 * @param k the position to fill
357 * @param x the item to insert
358 * @param es the heap array
359 */
360 private static <T> void siftUpComparable(int k, T x, Object[] es) {
361 Comparable<? super T> key = (Comparable<? super T>) x;
362 while (k > 0) {
363 int parent = (k - 1) >>> 1;
364 Object e = es[parent];
365 if (key.compareTo((T) e) >= 0)
366 break;
367 es[k] = e;
368 k = parent;
369 }
370 es[k] = key;
371 }
372
373 private static <T> void siftUpUsingComparator(
374 int k, T x, Object[] es, Comparator<? super T> cmp) {
375 while (k > 0) {
376 int parent = (k - 1) >>> 1;
377 Object e = es[parent];
378 if (cmp.compare(x, (T) e) >= 0)
379 break;
380 es[k] = e;
381 k = parent;
382 }
383 es[k] = x;
384 }
385
386 /**
387 * Inserts item x at position k, maintaining heap invariant by
388 * demoting x down the tree repeatedly until it is less than or
389 * equal to its children or is a leaf.
390 *
391 * @param k the position to fill
392 * @param x the item to insert
393 * @param es the heap array
394 * @param n heap size
395 */
396 private static <T> void siftDownComparable(int k, T x, Object[] es, int n) {
397 // assert n > 0;
398 Comparable<? super T> key = (Comparable<? super T>)x;
399 int half = n >>> 1; // loop while a non-leaf
400 while (k < half) {
401 int child = (k << 1) + 1; // assume left child is least
402 Object c = es[child];
403 int right = child + 1;
404 if (right < n &&
405 ((Comparable<? super T>) c).compareTo((T) es[right]) > 0)
406 c = es[child = right];
407 if (key.compareTo((T) c) <= 0)
408 break;
409 es[k] = c;
410 k = child;
411 }
412 es[k] = key;
413 }
414
415 private static <T> void siftDownUsingComparator(
416 int k, T x, Object[] es, int n, Comparator<? super T> cmp) {
417 // assert n > 0;
418 int half = n >>> 1;
419 while (k < half) {
420 int child = (k << 1) + 1;
421 Object c = es[child];
422 int right = child + 1;
423 if (right < n && cmp.compare((T) c, (T) es[right]) > 0)
424 c = es[child = right];
425 if (cmp.compare(x, (T) c) <= 0)
426 break;
427 es[k] = c;
428 k = child;
429 }
430 es[k] = x;
431 }
432
433 /**
434 * Establishes the heap invariant (described above) in the entire tree,
435 * assuming nothing about the order of the elements prior to the call.
436 * This classic algorithm due to Floyd (1964) is known to be O(size).
437 */
438 private void heapify() {
439 final Object[] es = queue;
440 int n = size, i = (n >>> 1) - 1;
441 final Comparator<? super E> cmp;
442 if ((cmp = comparator) == null)
443 for (; i >= 0; i--)
444 siftDownComparable(i, (E) es[i], es, n);
445 else
446 for (; i >= 0; i--)
447 siftDownUsingComparator(i, (E) es[i], es, n, cmp);
448 }
449
450 /**
451 * Inserts the specified element into this priority queue.
452 *
453 * @param e the element to add
454 * @return {@code true} (as specified by {@link Collection#add})
455 * @throws ClassCastException if the specified element cannot be compared
456 * with elements currently in the priority queue according to the
457 * priority queue's ordering
458 * @throws NullPointerException if the specified element is null
459 */
460 public boolean add(E e) {
461 return offer(e);
462 }
463
464 /**
465 * Inserts the specified element into this priority queue.
466 * As the queue is unbounded, this method will never return {@code false}.
467 *
468 * @param e the element to add
469 * @return {@code true} (as specified by {@link Queue#offer})
470 * @throws ClassCastException if the specified element cannot be compared
471 * with elements currently in the priority queue according to the
472 * priority queue's ordering
473 * @throws NullPointerException if the specified element is null
474 */
475 public boolean offer(E e) {
476 if (e == null)
477 throw new NullPointerException();
478 final ReentrantLock lock = this.lock;
479 lock.lock();
480 int n, cap;
481 Object[] es;
482 while ((n = size) >= (cap = (es = queue).length))
483 tryGrow(es, cap);
484 try {
485 final Comparator<? super E> cmp;
486 if ((cmp = comparator) == null)
487 siftUpComparable(n, e, es);
488 else
489 siftUpUsingComparator(n, e, es, cmp);
490 size = n + 1;
491 notEmpty.signal();
492 } finally {
493 lock.unlock();
494 }
495 return true;
496 }
497
498 /**
499 * Inserts the specified element into this priority queue.
500 * As the queue is unbounded, this method will never block.
501 *
502 * @param e the element to add
503 * @throws ClassCastException if the specified element cannot be compared
504 * with elements currently in the priority queue according to the
505 * priority queue's ordering
506 * @throws NullPointerException if the specified element is null
507 */
508 public void put(E e) {
509 offer(e); // never need to block
510 }
511
512 /**
513 * Inserts the specified element into this priority queue.
514 * As the queue is unbounded, this method will never block or
515 * return {@code false}.
516 *
517 * @param e the element to add
518 * @param timeout This parameter is ignored as the method never blocks
519 * @param unit This parameter is ignored as the method never blocks
520 * @return {@code true} (as specified by
521 * {@link BlockingQueue#offer(Object,long,TimeUnit) BlockingQueue.offer})
522 * @throws ClassCastException if the specified element cannot be compared
523 * with elements currently in the priority queue according to the
524 * priority queue's ordering
525 * @throws NullPointerException if the specified element is null
526 */
527 public boolean offer(E e, long timeout, TimeUnit unit) {
528 return offer(e); // never need to block
529 }
530
531 public E poll() {
532 final ReentrantLock lock = this.lock;
533 lock.lock();
534 try {
535 return dequeue();
536 } finally {
537 lock.unlock();
538 }
539 }
540
541 public E take() throws InterruptedException {
542 final ReentrantLock lock = this.lock;
543 lock.lockInterruptibly();
544 E result;
545 try {
546 while ( (result = dequeue()) == null)
547 notEmpty.await();
548 } finally {
549 lock.unlock();
550 }
551 return result;
552 }
553
554 public E poll(long timeout, TimeUnit unit) throws InterruptedException {
555 long nanos = unit.toNanos(timeout);
556 final ReentrantLock lock = this.lock;
557 lock.lockInterruptibly();
558 E result;
559 try {
560 while ( (result = dequeue()) == null && nanos > 0)
561 nanos = notEmpty.awaitNanos(nanos);
562 } finally {
563 lock.unlock();
564 }
565 return result;
566 }
567
568 public E peek() {
569 final ReentrantLock lock = this.lock;
570 lock.lock();
571 try {
572 return (E) queue[0];
573 } finally {
574 lock.unlock();
575 }
576 }
577
578 /**
579 * Returns the comparator used to order the elements in this queue,
580 * or {@code null} if this queue uses the {@linkplain Comparable
581 * natural ordering} of its elements.
582 *
583 * @return the comparator used to order the elements in this queue,
584 * or {@code null} if this queue uses the natural
585 * ordering of its elements
586 */
587 public Comparator<? super E> comparator() {
588 return comparator;
589 }
590
591 public int size() {
592 final ReentrantLock lock = this.lock;
593 lock.lock();
594 try {
595 return size;
596 } finally {
597 lock.unlock();
598 }
599 }
600
601 /**
602 * Always returns {@code Integer.MAX_VALUE} because
603 * a {@code PriorityBlockingQueue} is not capacity constrained.
604 * @return {@code Integer.MAX_VALUE} always
605 */
606 public int remainingCapacity() {
607 return Integer.MAX_VALUE;
608 }
609
610 private int indexOf(Object o) {
611 if (o != null) {
612 final Object[] es = queue;
613 for (int i = 0, n = size; i < n; i++)
614 if (o.equals(es[i]))
615 return i;
616 }
617 return -1;
618 }
619
620 /**
621 * Removes the ith element from queue.
622 */
623 private void removeAt(int i) {
624 final Object[] es = queue;
625 final int n = size - 1;
626 if (n == i) // removed last element
627 es[i] = null;
628 else {
629 E moved = (E) es[n];
630 es[n] = null;
631 final Comparator<? super E> cmp;
632 if ((cmp = comparator) == null)
633 siftDownComparable(i, moved, es, n);
634 else
635 siftDownUsingComparator(i, moved, es, n, cmp);
636 if (es[i] == moved) {
637 if (cmp == null)
638 siftUpComparable(i, moved, es);
639 else
640 siftUpUsingComparator(i, moved, es, cmp);
641 }
642 }
643 size = n;
644 }
645
646 /**
647 * Removes a single instance of the specified element from this queue,
648 * if it is present. More formally, removes an element {@code e} such
649 * that {@code o.equals(e)}, if this queue contains one or more such
650 * elements. Returns {@code true} if and only if this queue contained
651 * the specified element (or equivalently, if this queue changed as a
652 * result of the call).
653 *
654 * @param o element to be removed from this queue, if present
655 * @return {@code true} if this queue changed as a result of the call
656 */
657 public boolean remove(Object o) {
658 final ReentrantLock lock = this.lock;
659 lock.lock();
660 try {
661 int i = indexOf(o);
662 if (i == -1)
663 return false;
664 removeAt(i);
665 return true;
666 } finally {
667 lock.unlock();
668 }
669 }
670
671 /**
672 * Identity-based version for use in Itr.remove.
673 *
674 * @param o element to be removed from this queue, if present
675 */
676 void removeEq(Object o) {
677 final ReentrantLock lock = this.lock;
678 lock.lock();
679 try {
680 final Object[] es = queue;
681 for (int i = 0, n = size; i < n; i++) {
682 if (o == es[i]) {
683 removeAt(i);
684 break;
685 }
686 }
687 } finally {
688 lock.unlock();
689 }
690 }
691
692 /**
693 * Returns {@code true} if this queue contains the specified element.
694 * More formally, returns {@code true} if and only if this queue contains
695 * at least one element {@code e} such that {@code o.equals(e)}.
696 *
697 * @param o object to be checked for containment in this queue
698 * @return {@code true} if this queue contains the specified element
699 */
700 public boolean contains(Object o) {
701 final ReentrantLock lock = this.lock;
702 lock.lock();
703 try {
704 return indexOf(o) != -1;
705 } finally {
706 lock.unlock();
707 }
708 }
709
710 public String toString() {
711 return Helpers.collectionToString(this);
712 }
713
714 /**
715 * @throws UnsupportedOperationException {@inheritDoc}
716 * @throws ClassCastException {@inheritDoc}
717 * @throws NullPointerException {@inheritDoc}
718 * @throws IllegalArgumentException {@inheritDoc}
719 */
720 public int drainTo(Collection<? super E> c) {
721 return drainTo(c, Integer.MAX_VALUE);
722 }
723
724 /**
725 * @throws UnsupportedOperationException {@inheritDoc}
726 * @throws ClassCastException {@inheritDoc}
727 * @throws NullPointerException {@inheritDoc}
728 * @throws IllegalArgumentException {@inheritDoc}
729 */
730 public int drainTo(Collection<? super E> c, int maxElements) {
731 Objects.requireNonNull(c);
732 if (c == this)
733 throw new IllegalArgumentException();
734 if (maxElements <= 0)
735 return 0;
736 final ReentrantLock lock = this.lock;
737 lock.lock();
738 try {
739 int n = Math.min(size, maxElements);
740 for (int i = 0; i < n; i++) {
741 c.add((E) queue[0]); // In this order, in case add() throws.
742 dequeue();
743 }
744 return n;
745 } finally {
746 lock.unlock();
747 }
748 }
749
750 /**
751 * Atomically removes all of the elements from this queue.
752 * The queue will be empty after this call returns.
753 */
754 public void clear() {
755 final ReentrantLock lock = this.lock;
756 lock.lock();
757 try {
758 final Object[] es = queue;
759 for (int i = 0, n = size; i < n; i++)
760 es[i] = null;
761 size = 0;
762 } finally {
763 lock.unlock();
764 }
765 }
766
767 /**
768 * Returns an array containing all of the elements in this queue.
769 * The returned array elements are in no particular order.
770 *
771 * <p>The returned array will be "safe" in that no references to it are
772 * maintained by this queue. (In other words, this method must allocate
773 * a new array). The caller is thus free to modify the returned array.
774 *
775 * <p>This method acts as bridge between array-based and collection-based
776 * APIs.
777 *
778 * @return an array containing all of the elements in this queue
779 */
780 public Object[] toArray() {
781 final ReentrantLock lock = this.lock;
782 lock.lock();
783 try {
784 return Arrays.copyOf(queue, size);
785 } finally {
786 lock.unlock();
787 }
788 }
789
790 /**
791 * Returns an array containing all of the elements in this queue; the
792 * runtime type of the returned array is that of the specified array.
793 * The returned array elements are in no particular order.
794 * If the queue fits in the specified array, it is returned therein.
795 * Otherwise, a new array is allocated with the runtime type of the
796 * specified array and the size of this queue.
797 *
798 * <p>If this queue fits in the specified array with room to spare
799 * (i.e., the array has more elements than this queue), the element in
800 * the array immediately following the end of the queue is set to
801 * {@code null}.
802 *
803 * <p>Like the {@link #toArray()} method, this method acts as bridge between
804 * array-based and collection-based APIs. Further, this method allows
805 * precise control over the runtime type of the output array, and may,
806 * under certain circumstances, be used to save allocation costs.
807 *
808 * <p>Suppose {@code x} is a queue known to contain only strings.
809 * The following code can be used to dump the queue into a newly
810 * allocated array of {@code String}:
811 *
812 * <pre> {@code String[] y = x.toArray(new String[0]);}</pre>
813 *
814 * Note that {@code toArray(new Object[0])} is identical in function to
815 * {@code toArray()}.
816 *
817 * @param a the array into which the elements of the queue are to
818 * be stored, if it is big enough; otherwise, a new array of the
819 * same runtime type is allocated for this purpose
820 * @return an array containing all of the elements in this queue
821 * @throws ArrayStoreException if the runtime type of the specified array
822 * is not a supertype of the runtime type of every element in
823 * this queue
824 * @throws NullPointerException if the specified array is null
825 */
826 public <T> T[] toArray(T[] a) {
827 final ReentrantLock lock = this.lock;
828 lock.lock();
829 try {
830 int n = size;
831 if (a.length < n)
832 // Make a new array of a's runtime type, but my contents:
833 return (T[]) Arrays.copyOf(queue, size, a.getClass());
834 System.arraycopy(queue, 0, a, 0, n);
835 if (a.length > n)
836 a[n] = null;
837 return a;
838 } finally {
839 lock.unlock();
840 }
841 }
842
843 /**
844 * Returns an iterator over the elements in this queue. The
845 * iterator does not return the elements in any particular order.
846 *
847 * <p>The returned iterator is
848 * <a href="package-summary.html#Weakly"><i>weakly consistent</i></a>.
849 *
850 * @return an iterator over the elements in this queue
851 */
852 public Iterator<E> iterator() {
853 return new Itr(toArray());
854 }
855
856 /**
857 * Snapshot iterator that works off copy of underlying q array.
858 */
859 final class Itr implements Iterator<E> {
860 final Object[] array; // Array of all elements
861 int cursor; // index of next element to return
862 int lastRet = -1; // index of last element, or -1 if no such
863
864 Itr(Object[] array) {
865 this.array = array;
866 }
867
868 public boolean hasNext() {
869 return cursor < array.length;
870 }
871
872 public E next() {
873 if (cursor >= array.length)
874 throw new NoSuchElementException();
875 return (E)array[lastRet = cursor++];
876 }
877
878 public void remove() {
879 if (lastRet < 0)
880 throw new IllegalStateException();
881 removeEq(array[lastRet]);
882 lastRet = -1;
883 }
884
885 public void forEachRemaining(Consumer<? super E> action) {
886 Objects.requireNonNull(action);
887 final Object[] es = array;
888 int i;
889 if ((i = cursor) < es.length) {
890 lastRet = -1;
891 cursor = es.length;
892 for (; i < es.length; i++)
893 action.accept((E) es[i]);
894 lastRet = es.length - 1;
895 }
896 }
897 }
898
899 /**
900 * Saves this queue to a stream (that is, serializes it).
901 *
902 * For compatibility with previous version of this class, elements
903 * are first copied to a java.util.PriorityQueue, which is then
904 * serialized.
905 *
906 * @param s the stream
907 * @throws java.io.IOException if an I/O error occurs
908 */
909 private void writeObject(java.io.ObjectOutputStream s)
910 throws java.io.IOException {
911 lock.lock();
912 try {
913 // avoid zero capacity argument
914 q = new PriorityQueue<E>(Math.max(size, 1), comparator);
915 q.addAll(this);
916 s.defaultWriteObject();
917 } finally {
918 q = null;
919 lock.unlock();
920 }
921 }
922
923 /**
924 * Reconstitutes this queue from a stream (that is, deserializes it).
925 * @param s the stream
926 * @throws ClassNotFoundException if the class of a serialized object
927 * could not be found
928 * @throws java.io.IOException if an I/O error occurs
929 */
930 private void readObject(java.io.ObjectInputStream s)
931 throws java.io.IOException, ClassNotFoundException {
932 try {
933 s.defaultReadObject();
934 int sz = q.size();
935 SharedSecrets.getJavaObjectInputStreamAccess().checkArray(s, Object[].class, sz);
936 this.queue = new Object[Math.max(1, sz)];
937 comparator = q.comparator();
938 addAll(q);
939 } finally {
940 q = null;
941 }
942 }
943
944 /**
945 * Immutable snapshot spliterator that binds to elements "late".
946 */
947 final class PBQSpliterator implements Spliterator<E> {
948 Object[] array; // null until late-bound-initialized
949 int index;
950 int fence;
951
952 PBQSpliterator() {}
953
954 PBQSpliterator(Object[] array, int index, int fence) {
955 this.array = array;
956 this.index = index;
957 this.fence = fence;
958 }
959
960 private int getFence() {
961 if (array == null)
962 fence = (array = toArray()).length;
963 return fence;
964 }
965
966 public PBQSpliterator trySplit() {
967 int hi = getFence(), lo = index, mid = (lo + hi) >>> 1;
968 return (lo >= mid) ? null :
969 new PBQSpliterator(array, lo, index = mid);
970 }
971
972 public void forEachRemaining(Consumer<? super E> action) {
973 Objects.requireNonNull(action);
974 final int hi = getFence(), lo = index;
975 final Object[] es = array;
976 index = hi; // ensure exhaustion
977 for (int i = lo; i < hi; i++)
978 action.accept((E) es[i]);
979 }
980
981 public boolean tryAdvance(Consumer<? super E> action) {
982 Objects.requireNonNull(action);
983 if (getFence() > index && index >= 0) {
984 action.accept((E) array[index++]);
985 return true;
986 }
987 return false;
988 }
989
990 public long estimateSize() { return getFence() - index; }
991
992 public int characteristics() {
993 return (Spliterator.NONNULL |
994 Spliterator.SIZED |
995 Spliterator.SUBSIZED);
996 }
997 }
998
999 /**
1000 * Returns a {@link Spliterator} over the elements in this queue.
1001 * The spliterator does not traverse elements in any particular order
1002 * (the {@link Spliterator#ORDERED ORDERED} characteristic is not reported).
1003 *
1004 * <p>The returned spliterator is
1005 * <a href="package-summary.html#Weakly"><i>weakly consistent</i></a>.
1006 *
1007 * <p>The {@code Spliterator} reports {@link Spliterator#SIZED} and
1008 * {@link Spliterator#NONNULL}.
1009 *
1010 * @implNote
1011 * The {@code Spliterator} additionally reports {@link Spliterator#SUBSIZED}.
1012 *
1013 * @return a {@code Spliterator} over the elements in this queue
1014 * @since 1.8
1015 */
1016 public Spliterator<E> spliterator() {
1017 return new PBQSpliterator();
1018 }
1019
1020 /**
1021 * @throws NullPointerException {@inheritDoc}
1022 */
1023 public boolean removeIf(Predicate<? super E> filter) {
1024 Objects.requireNonNull(filter);
1025 return bulkRemove(filter);
1026 }
1027
1028 /**
1029 * @throws NullPointerException {@inheritDoc}
1030 */
1031 public boolean removeAll(Collection<?> c) {
1032 Objects.requireNonNull(c);
1033 return bulkRemove(e -> c.contains(e));
1034 }
1035
1036 /**
1037 * @throws NullPointerException {@inheritDoc}
1038 */
1039 public boolean retainAll(Collection<?> c) {
1040 Objects.requireNonNull(c);
1041 return bulkRemove(e -> !c.contains(e));
1042 }
1043
1044 // A tiny bit set implementation
1045
1046 private static long[] nBits(int n) {
1047 return new long[((n - 1) >> 6) + 1];
1048 }
1049 private static void setBit(long[] bits, int i) {
1050 bits[i >> 6] |= 1L << i;
1051 }
1052 private static boolean isClear(long[] bits, int i) {
1053 return (bits[i >> 6] & (1L << i)) == 0;
1054 }
1055
1056 /** Implementation of bulk remove methods. */
1057 private boolean bulkRemove(Predicate<? super E> filter) {
1058 final ReentrantLock lock = this.lock;
1059 lock.lock();
1060 try {
1061 final Object[] es = queue;
1062 final int end = size;
1063 int i;
1064 // Optimize for initial run of survivors
1065 for (i = 0; i < end && !filter.test((E) es[i]); i++)
1066 ;
1067 if (i >= end)
1068 return false;
1069 // Tolerate predicates that reentrantly access the
1070 // collection for read, so traverse once to find elements
1071 // to delete, a second pass to physically expunge.
1072 final int beg = i;
1073 final long[] deathRow = nBits(end - beg);
1074 deathRow[0] = 1L; // set bit 0
1075 for (i = beg + 1; i < end; i++)
1076 if (filter.test((E) es[i]))
1077 setBit(deathRow, i - beg);
1078 int w = beg;
1079 for (i = beg; i < end; i++)
1080 if (isClear(deathRow, i - beg))
1081 es[w++] = es[i];
1082 for (i = size = w; i < end; i++)
1083 es[i] = null;
1084 heapify();
1085 return true;
1086 } finally {
1087 lock.unlock();
1088 }
1089 }
1090
1091 /**
1092 * @throws NullPointerException {@inheritDoc}
1093 */
1094 public void forEach(Consumer<? super E> action) {
1095 Objects.requireNonNull(action);
1096 final ReentrantLock lock = this.lock;
1097 lock.lock();
1098 try {
1099 final Object[] es = queue;
1100 for (int i = 0, n = size; i < n; i++)
1101 action.accept((E) es[i]);
1102 } finally {
1103 lock.unlock();
1104 }
1105 }
1106
1107 // VarHandle mechanics
1108 private static final VarHandle ALLOCATIONSPINLOCK;
1109 static {
1110 try {
1111 MethodHandles.Lookup l = MethodHandles.lookup();
1112 ALLOCATIONSPINLOCK = l.findVarHandle(PriorityBlockingQueue.class,
1113 "allocationSpinLock",
1114 int.class);
1115 } catch (ReflectiveOperationException e) {
1116 throw new ExceptionInInitializerError(e);
1117 }
1118 }
1119 }