2 * DO NOT ALTER OR REMOVE COPYRIGHT NOTICES OR THIS FILE HEADER.
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.
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).
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.
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
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
31 * Written by Doug Lea, Bill Scherer, and Michael Scott with
32 * assistance from members of JCP JSR-166 Expert Group and released to
33 * the public domain, as explained at
34 * http://creativecommons.org/publicdomain/zero/1.0/
37 package java.util.concurrent;
38 import java.util.concurrent.locks.*;
39 import java.util.concurrent.atomic.*;
43 * A {@linkplain BlockingQueue blocking queue} in which each insert
44 * operation must wait for a corresponding remove operation by another
45 * thread, and vice versa. A synchronous queue does not have any
46 * internal capacity, not even a capacity of one. You cannot
47 * <tt>peek</tt> at a synchronous queue because an element is only
48 * present when you try to remove it; you cannot insert an element
49 * (using any method) unless another thread is trying to remove it;
50 * you cannot iterate as there is nothing to iterate. The
51 * <em>head</em> of the queue is the element that the first queued
52 * inserting thread is trying to add to the queue; if there is no such
53 * queued thread then no element is available for removal and
54 * <tt>poll()</tt> will return <tt>null</tt>. For purposes of other
55 * <tt>Collection</tt> methods (for example <tt>contains</tt>), a
56 * <tt>SynchronousQueue</tt> acts as an empty collection. This queue
57 * does not permit <tt>null</tt> elements.
59 * <p>Synchronous queues are similar to rendezvous channels used in
60 * CSP and Ada. They are well suited for handoff designs, in which an
61 * object running in one thread must sync up with an object running
62 * in another thread in order to hand it some information, event, or
65 * <p> This class supports an optional fairness policy for ordering
66 * waiting producer and consumer threads. By default, this ordering
67 * is not guaranteed. However, a queue constructed with fairness set
68 * to <tt>true</tt> grants threads access in FIFO order.
70 * <p>This class and its iterator implement all of the
71 * <em>optional</em> methods of the {@link Collection} and {@link
72 * Iterator} interfaces.
74 * <p>This class is a member of the
75 * <a href="{@docRoot}/../technotes/guides/collections/index.html">
76 * Java Collections Framework</a>.
79 * @author Doug Lea and Bill Scherer and Michael Scott
80 * @param <E> the type of elements held in this collection
82 public class SynchronousQueue<E> extends AbstractQueue<E>
83 implements BlockingQueue<E>, java.io.Serializable {
84 private static final long serialVersionUID = -3223113410248163686L;
87 * This class implements extensions of the dual stack and dual
88 * queue algorithms described in "Nonblocking Concurrent Objects
89 * with Condition Synchronization", by W. N. Scherer III and
90 * M. L. Scott. 18th Annual Conf. on Distributed Computing,
92 * http://www.cs.rochester.edu/u/scott/synchronization/pseudocode/duals.html).
93 * The (Lifo) stack is used for non-fair mode, and the (Fifo)
94 * queue for fair mode. The performance of the two is generally
95 * similar. Fifo usually supports higher throughput under
96 * contention but Lifo maintains higher thread locality in common
99 * A dual queue (and similarly stack) is one that at any given
100 * time either holds "data" -- items provided by put operations,
101 * or "requests" -- slots representing take operations, or is
102 * empty. A call to "fulfill" (i.e., a call requesting an item
103 * from a queue holding data or vice versa) dequeues a
104 * complementary node. The most interesting feature of these
105 * queues is that any operation can figure out which mode the
106 * queue is in, and act accordingly without needing locks.
108 * Both the queue and stack extend abstract class Transferer
109 * defining the single method transfer that does a put or a
110 * take. These are unified into a single method because in dual
111 * data structures, the put and take operations are symmetrical,
112 * so nearly all code can be combined. The resulting transfer
113 * methods are on the long side, but are easier to follow than
114 * they would be if broken up into nearly-duplicated parts.
116 * The queue and stack data structures share many conceptual
117 * similarities but very few concrete details. For simplicity,
118 * they are kept distinct so that they can later evolve
121 * The algorithms here differ from the versions in the above paper
122 * in extending them for use in synchronous queues, as well as
123 * dealing with cancellation. The main differences include:
125 * 1. The original algorithms used bit-marked pointers, but
126 * the ones here use mode bits in nodes, leading to a number
127 * of further adaptations.
128 * 2. SynchronousQueues must block threads waiting to become
130 * 3. Support for cancellation via timeout and interrupts,
131 * including cleaning out cancelled nodes/threads
132 * from lists to avoid garbage retention and memory depletion.
134 * Blocking is mainly accomplished using LockSupport park/unpark,
135 * except that nodes that appear to be the next ones to become
136 * fulfilled first spin a bit (on multiprocessors only). On very
137 * busy synchronous queues, spinning can dramatically improve
138 * throughput. And on less busy ones, the amount of spinning is
139 * small enough not to be noticeable.
141 * Cleaning is done in different ways in queues vs stacks. For
142 * queues, we can almost always remove a node immediately in O(1)
143 * time (modulo retries for consistency checks) when it is
144 * cancelled. But if it may be pinned as the current tail, it must
145 * wait until some subsequent cancellation. For stacks, we need a
146 * potentially O(n) traversal to be sure that we can remove the
147 * node, but this can run concurrently with other threads
148 * accessing the stack.
150 * While garbage collection takes care of most node reclamation
151 * issues that otherwise complicate nonblocking algorithms, care
152 * is taken to "forget" references to data, other nodes, and
153 * threads that might be held on to long-term by blocked
154 * threads. In cases where setting to null would otherwise
155 * conflict with main algorithms, this is done by changing a
156 * node's link to now point to the node itself. This doesn't arise
157 * much for Stack nodes (because blocked threads do not hang on to
158 * old head pointers), but references in Queue nodes must be
159 * aggressively forgotten to avoid reachability of everything any
160 * node has ever referred to since arrival.
164 * Shared internal API for dual stacks and queues.
166 abstract static class Transferer {
168 * Performs a put or take.
170 * @param e if non-null, the item to be handed to a consumer;
171 * if null, requests that transfer return an item
172 * offered by producer.
173 * @param timed if this operation should timeout
174 * @param nanos the timeout, in nanoseconds
175 * @return if non-null, the item provided or received; if null,
176 * the operation failed due to timeout or interrupt --
177 * the caller can distinguish which of these occurred
178 * by checking Thread.interrupted.
180 abstract Object transfer(Object e, boolean timed, long nanos);
183 /** The number of CPUs, for spin control */
184 static final int NCPUS = Runtime.getRuntime().availableProcessors();
187 * The number of times to spin before blocking in timed waits.
188 * The value is empirically derived -- it works well across a
189 * variety of processors and OSes. Empirically, the best value
190 * seems not to vary with number of CPUs (beyond 2) so is just
193 static final int maxTimedSpins = (NCPUS < 2) ? 0 : 32;
196 * The number of times to spin before blocking in untimed waits.
197 * This is greater than timed value because untimed waits spin
198 * faster since they don't need to check times on each spin.
200 static final int maxUntimedSpins = maxTimedSpins * 16;
203 * The number of nanoseconds for which it is faster to spin
204 * rather than to use timed park. A rough estimate suffices.
206 static final long spinForTimeoutThreshold = 1000L;
209 static final class TransferStack extends Transferer {
211 * This extends Scherer-Scott dual stack algorithm, differing,
212 * among other ways, by using "covering" nodes rather than
213 * bit-marked pointers: Fulfilling operations push on marker
214 * nodes (with FULFILLING bit set in mode) to reserve a spot
215 * to match a waiting node.
218 /* Modes for SNodes, ORed together in node fields */
219 /** Node represents an unfulfilled consumer */
220 static final int REQUEST = 0;
221 /** Node represents an unfulfilled producer */
222 static final int DATA = 1;
223 /** Node is fulfilling another unfulfilled DATA or REQUEST */
224 static final int FULFILLING = 2;
226 /** Return true if m has fulfilling bit set */
227 static boolean isFulfilling(int m) { return (m & FULFILLING) != 0; }
229 /** Node class for TransferStacks. */
230 static final class SNode {
231 volatile SNode next; // next node in stack
232 volatile SNode match; // the node matched to this
233 volatile Thread waiter; // to control park/unpark
234 Object item; // data; or null for REQUESTs
236 // Note: item and mode fields don't need to be volatile
237 // since they are always written before, and read after,
238 // other volatile/atomic operations.
244 boolean casNext(SNode cmp, SNode val) {
245 return cmp == next &&
246 UNSAFE.compareAndSwapObject(this, nextOffset, cmp, val);
250 * Tries to match node s to this node, if so, waking up thread.
251 * Fulfillers call tryMatch to identify their waiters.
252 * Waiters block until they have been matched.
254 * @param s the node to match
255 * @return true if successfully matched to s
257 boolean tryMatch(SNode s) {
259 UNSAFE.compareAndSwapObject(this, matchOffset, null, s)) {
261 if (w != null) { // waiters need at most one unpark
263 LockSupport.unpark(w);
271 * Tries to cancel a wait by matching node to itself.
274 UNSAFE.compareAndSwapObject(this, matchOffset, null, this);
277 boolean isCancelled() {
278 return match == this;
282 private static final sun.misc.Unsafe UNSAFE;
283 private static final long matchOffset;
284 private static final long nextOffset;
288 UNSAFE = sun.misc.Unsafe.getUnsafe();
289 Class k = SNode.class;
290 matchOffset = UNSAFE.objectFieldOffset
291 (k.getDeclaredField("match"));
292 nextOffset = UNSAFE.objectFieldOffset
293 (k.getDeclaredField("next"));
294 } catch (Exception e) {
300 /** The head (top) of the stack */
303 boolean casHead(SNode h, SNode nh) {
305 UNSAFE.compareAndSwapObject(this, headOffset, h, nh);
309 * Creates or resets fields of a node. Called only from transfer
310 * where the node to push on stack is lazily created and
311 * reused when possible to help reduce intervals between reads
312 * and CASes of head and to avoid surges of garbage when CASes
313 * to push nodes fail due to contention.
315 static SNode snode(SNode s, Object e, SNode next, int mode) {
316 if (s == null) s = new SNode(e);
323 * Puts or takes an item.
325 Object transfer(Object e, boolean timed, long nanos) {
327 * Basic algorithm is to loop trying one of three actions:
329 * 1. If apparently empty or already containing nodes of same
330 * mode, try to push node on stack and wait for a match,
331 * returning it, or null if cancelled.
333 * 2. If apparently containing node of complementary mode,
334 * try to push a fulfilling node on to stack, match
335 * with corresponding waiting node, pop both from
336 * stack, and return matched item. The matching or
337 * unlinking might not actually be necessary because of
338 * other threads performing action 3:
340 * 3. If top of stack already holds another fulfilling node,
341 * help it out by doing its match and/or pop
342 * operations, and then continue. The code for helping
343 * is essentially the same as for fulfilling, except
344 * that it doesn't return the item.
347 SNode s = null; // constructed/reused as needed
348 int mode = (e == null) ? REQUEST : DATA;
352 if (h == null || h.mode == mode) { // empty or same-mode
353 if (timed && nanos <= 0) { // can't wait
354 if (h != null && h.isCancelled())
355 casHead(h, h.next); // pop cancelled node
358 } else if (casHead(h, s = snode(s, e, h, mode))) {
359 SNode m = awaitFulfill(s, timed, nanos);
360 if (m == s) { // wait was cancelled
364 if ((h = head) != null && h.next == s)
365 casHead(h, s.next); // help s's fulfiller
366 return (mode == REQUEST) ? m.item : s.item;
368 } else if (!isFulfilling(h.mode)) { // try to fulfill
369 if (h.isCancelled()) // already cancelled
370 casHead(h, h.next); // pop and retry
371 else if (casHead(h, s=snode(s, e, h, FULFILLING|mode))) {
372 for (;;) { // loop until matched or waiters disappear
373 SNode m = s.next; // m is s's match
374 if (m == null) { // all waiters are gone
375 casHead(s, null); // pop fulfill node
376 s = null; // use new node next time
377 break; // restart main loop
381 casHead(s, mn); // pop both s and m
382 return (mode == REQUEST) ? m.item : s.item;
384 s.casNext(m, mn); // help unlink
387 } else { // help a fulfiller
388 SNode m = h.next; // m is h's match
389 if (m == null) // waiter is gone
390 casHead(h, null); // pop fulfilling node
393 if (m.tryMatch(h)) // help match
394 casHead(h, mn); // pop both h and m
396 h.casNext(m, mn); // help unlink
403 * Spins/blocks until node s is matched by a fulfill operation.
405 * @param s the waiting node
406 * @param timed true if timed wait
407 * @param nanos timeout value
408 * @return matched node, or s if cancelled
410 SNode awaitFulfill(SNode s, boolean timed, long nanos) {
412 * When a node/thread is about to block, it sets its waiter
413 * field and then rechecks state at least one more time
414 * before actually parking, thus covering race vs
415 * fulfiller noticing that waiter is non-null so should be
418 * When invoked by nodes that appear at the point of call
419 * to be at the head of the stack, calls to park are
420 * preceded by spins to avoid blocking when producers and
421 * consumers are arriving very close in time. This can
422 * happen enough to bother only on multiprocessors.
424 * The order of checks for returning out of main loop
425 * reflects fact that interrupts have precedence over
426 * normal returns, which have precedence over
427 * timeouts. (So, on timeout, one last check for match is
428 * done before giving up.) Except that calls from untimed
429 * SynchronousQueue.{poll/offer} don't check interrupts
430 * and don't wait at all, so are trapped in transfer
431 * method rather than calling awaitFulfill.
433 long lastTime = timed ? System.nanoTime() : 0;
434 Thread w = Thread.currentThread();
436 int spins = (shouldSpin(s) ?
437 (timed ? maxTimedSpins : maxUntimedSpins) : 0);
439 if (w.isInterrupted())
445 long now = System.nanoTime();
446 nanos -= now - lastTime;
454 spins = shouldSpin(s) ? (spins-1) : 0;
455 else if (s.waiter == null)
456 s.waiter = w; // establish waiter so can park next iter
458 LockSupport.park(this);
459 else if (nanos > spinForTimeoutThreshold)
460 LockSupport.parkNanos(this, nanos);
465 * Returns true if node s is at head or there is an active
468 boolean shouldSpin(SNode s) {
470 return (h == s || h == null || isFulfilling(h.mode));
474 * Unlinks s from the stack.
476 void clean(SNode s) {
477 s.item = null; // forget item
478 s.waiter = null; // forget thread
481 * At worst we may need to traverse entire stack to unlink
482 * s. If there are multiple concurrent calls to clean, we
483 * might not see s if another thread has already removed
484 * it. But we can stop when we see any node known to
485 * follow s. We use s.next unless it too is cancelled, in
486 * which case we try the node one past. We don't check any
487 * further because we don't want to doubly traverse just to
492 if (past != null && past.isCancelled())
495 // Absorb cancelled nodes at head
497 while ((p = head) != null && p != past && p.isCancelled())
500 // Unsplice embedded nodes
501 while (p != null && p != past) {
503 if (n != null && n.isCancelled())
504 p.casNext(n, n.next);
511 private static final sun.misc.Unsafe UNSAFE;
512 private static final long headOffset;
515 UNSAFE = sun.misc.Unsafe.getUnsafe();
516 Class k = TransferStack.class;
517 headOffset = UNSAFE.objectFieldOffset
518 (k.getDeclaredField("head"));
519 } catch (Exception e) {
526 static final class TransferQueue extends Transferer {
528 * This extends Scherer-Scott dual queue algorithm, differing,
529 * among other ways, by using modes within nodes rather than
530 * marked pointers. The algorithm is a little simpler than
531 * that for stacks because fulfillers do not need explicit
532 * nodes, and matching is done by CAS'ing QNode.item field
533 * from non-null to null (for put) or vice versa (for take).
536 /** Node class for TransferQueue. */
537 static final class QNode {
538 volatile QNode next; // next node in queue
539 volatile Object item; // CAS'ed to or from null
540 volatile Thread waiter; // to control park/unpark
541 final boolean isData;
543 QNode(Object item, boolean isData) {
545 this.isData = isData;
548 boolean casNext(QNode cmp, QNode val) {
549 return next == cmp &&
550 UNSAFE.compareAndSwapObject(this, nextOffset, cmp, val);
553 boolean casItem(Object cmp, Object val) {
554 return item == cmp &&
555 UNSAFE.compareAndSwapObject(this, itemOffset, cmp, val);
559 * Tries to cancel by CAS'ing ref to this as item.
561 void tryCancel(Object cmp) {
562 UNSAFE.compareAndSwapObject(this, itemOffset, cmp, this);
565 boolean isCancelled() {
570 * Returns true if this node is known to be off the queue
571 * because its next pointer has been forgotten due to
572 * an advanceHead operation.
574 boolean isOffList() {
579 private static final sun.misc.Unsafe UNSAFE;
580 private static final long itemOffset;
581 private static final long nextOffset;
585 UNSAFE = sun.misc.Unsafe.getUnsafe();
586 Class k = QNode.class;
587 itemOffset = UNSAFE.objectFieldOffset
588 (k.getDeclaredField("item"));
589 nextOffset = UNSAFE.objectFieldOffset
590 (k.getDeclaredField("next"));
591 } catch (Exception e) {
598 transient volatile QNode head;
600 transient volatile QNode tail;
602 * Reference to a cancelled node that might not yet have been
603 * unlinked from queue because it was the last inserted node
606 transient volatile QNode cleanMe;
609 QNode h = new QNode(null, false); // initialize to dummy node.
615 * Tries to cas nh as new head; if successful, unlink
616 * old head's next node to avoid garbage retention.
618 void advanceHead(QNode h, QNode nh) {
620 UNSAFE.compareAndSwapObject(this, headOffset, h, nh))
621 h.next = h; // forget old next
625 * Tries to cas nt as new tail.
627 void advanceTail(QNode t, QNode nt) {
629 UNSAFE.compareAndSwapObject(this, tailOffset, t, nt);
633 * Tries to CAS cleanMe slot.
635 boolean casCleanMe(QNode cmp, QNode val) {
636 return cleanMe == cmp &&
637 UNSAFE.compareAndSwapObject(this, cleanMeOffset, cmp, val);
641 * Puts or takes an item.
643 Object transfer(Object e, boolean timed, long nanos) {
644 /* Basic algorithm is to loop trying to take either of
647 * 1. If queue apparently empty or holding same-mode nodes,
648 * try to add node to queue of waiters, wait to be
649 * fulfilled (or cancelled) and return matching item.
651 * 2. If queue apparently contains waiting items, and this
652 * call is of complementary mode, try to fulfill by CAS'ing
653 * item field of waiting node and dequeuing it, and then
654 * returning matching item.
656 * In each case, along the way, check for and try to help
657 * advance head and tail on behalf of other stalled/slow
660 * The loop starts off with a null check guarding against
661 * seeing uninitialized head or tail values. This never
662 * happens in current SynchronousQueue, but could if
663 * callers held non-volatile/final ref to the
664 * transferer. The check is here anyway because it places
665 * null checks at top of loop, which is usually faster
666 * than having them implicitly interspersed.
669 QNode s = null; // constructed/reused as needed
670 boolean isData = (e != null);
675 if (t == null || h == null) // saw uninitialized value
678 if (h == t || t.isData == isData) { // empty or same-mode
680 if (t != tail) // inconsistent read
682 if (tn != null) { // lagging tail
686 if (timed && nanos <= 0) // can't wait
689 s = new QNode(e, isData);
690 if (!t.casNext(null, s)) // failed to link in
693 advanceTail(t, s); // swing tail and wait
694 Object x = awaitFulfill(s, e, timed, nanos);
695 if (x == s) { // wait was cancelled
700 if (!s.isOffList()) { // not already unlinked
701 advanceHead(t, s); // unlink if head
702 if (x != null) // and forget fields
706 return (x != null) ? x : e;
708 } else { // complementary-mode
709 QNode m = h.next; // node to fulfill
710 if (t != tail || m == null || h != head)
711 continue; // inconsistent read
714 if (isData == (x != null) || // m already fulfilled
715 x == m || // m cancelled
716 !m.casItem(x, e)) { // lost CAS
717 advanceHead(h, m); // dequeue and retry
721 advanceHead(h, m); // successfully fulfilled
722 LockSupport.unpark(m.waiter);
723 return (x != null) ? x : e;
729 * Spins/blocks until node s is fulfilled.
731 * @param s the waiting node
732 * @param e the comparison value for checking match
733 * @param timed true if timed wait
734 * @param nanos timeout value
735 * @return matched item, or s if cancelled
737 Object awaitFulfill(QNode s, Object e, boolean timed, long nanos) {
738 /* Same idea as TransferStack.awaitFulfill */
739 long lastTime = timed ? System.nanoTime() : 0;
740 Thread w = Thread.currentThread();
741 int spins = ((head.next == s) ?
742 (timed ? maxTimedSpins : maxUntimedSpins) : 0);
744 if (w.isInterrupted())
750 long now = System.nanoTime();
751 nanos -= now - lastTime;
760 else if (s.waiter == null)
763 LockSupport.park(this);
764 else if (nanos > spinForTimeoutThreshold)
765 LockSupport.parkNanos(this, nanos);
770 * Gets rid of cancelled node s with original predecessor pred.
772 void clean(QNode pred, QNode s) {
773 s.waiter = null; // forget thread
775 * At any given time, exactly one node on list cannot be
776 * deleted -- the last inserted node. To accommodate this,
777 * if we cannot delete s, we save its predecessor as
778 * "cleanMe", deleting the previously saved version
779 * first. At least one of node s or the node previously
780 * saved can always be deleted, so this always terminates.
782 while (pred.next == s) { // Return early if already unlinked
784 QNode hn = h.next; // Absorb cancelled first node as head
785 if (hn != null && hn.isCancelled()) {
789 QNode t = tail; // Ensure consistent read for tail
799 if (s != t) { // If not tail, try to unsplice
801 if (sn == s || pred.casNext(s, sn))
805 if (dp != null) { // Try unlinking previous cancelled node
808 if (d == null || // d is gone or
809 d == dp || // d is off list or
810 !d.isCancelled() || // d not cancelled or
811 (d != t && // d not tail and
812 (dn = d.next) != null && // has successor
813 dn != d && // that is on list
814 dp.casNext(d, dn))) // d unspliced
815 casCleanMe(dp, null);
817 return; // s is already saved node
818 } else if (casCleanMe(null, pred))
819 return; // Postpone cleaning s
823 private static final sun.misc.Unsafe UNSAFE;
824 private static final long headOffset;
825 private static final long tailOffset;
826 private static final long cleanMeOffset;
829 UNSAFE = sun.misc.Unsafe.getUnsafe();
830 Class k = TransferQueue.class;
831 headOffset = UNSAFE.objectFieldOffset
832 (k.getDeclaredField("head"));
833 tailOffset = UNSAFE.objectFieldOffset
834 (k.getDeclaredField("tail"));
835 cleanMeOffset = UNSAFE.objectFieldOffset
836 (k.getDeclaredField("cleanMe"));
837 } catch (Exception e) {
844 * The transferer. Set only in constructor, but cannot be declared
845 * as final without further complicating serialization. Since
846 * this is accessed only at most once per public method, there
847 * isn't a noticeable performance penalty for using volatile
848 * instead of final here.
850 private transient volatile Transferer transferer;
853 * Creates a <tt>SynchronousQueue</tt> with nonfair access policy.
855 public SynchronousQueue() {
860 * Creates a <tt>SynchronousQueue</tt> with the specified fairness policy.
862 * @param fair if true, waiting threads contend in FIFO order for
863 * access; otherwise the order is unspecified.
865 public SynchronousQueue(boolean fair) {
866 transferer = fair ? new TransferQueue() : new TransferStack();
870 * Adds the specified element to this queue, waiting if necessary for
871 * another thread to receive it.
873 * @throws InterruptedException {@inheritDoc}
874 * @throws NullPointerException {@inheritDoc}
876 public void put(E o) throws InterruptedException {
877 if (o == null) throw new NullPointerException();
878 if (transferer.transfer(o, false, 0) == null) {
879 Thread.interrupted();
880 throw new InterruptedException();
885 * Inserts the specified element into this queue, waiting if necessary
886 * up to the specified wait time for another thread to receive it.
888 * @return <tt>true</tt> if successful, or <tt>false</tt> if the
889 * specified waiting time elapses before a consumer appears.
890 * @throws InterruptedException {@inheritDoc}
891 * @throws NullPointerException {@inheritDoc}
893 public boolean offer(E o, long timeout, TimeUnit unit)
894 throws InterruptedException {
895 if (o == null) throw new NullPointerException();
896 if (transferer.transfer(o, true, unit.toNanos(timeout)) != null)
898 if (!Thread.interrupted())
900 throw new InterruptedException();
904 * Inserts the specified element into this queue, if another thread is
905 * waiting to receive it.
907 * @param e the element to add
908 * @return <tt>true</tt> if the element was added to this queue, else
910 * @throws NullPointerException if the specified element is null
912 public boolean offer(E e) {
913 if (e == null) throw new NullPointerException();
914 return transferer.transfer(e, true, 0) != null;
918 * Retrieves and removes the head of this queue, waiting if necessary
919 * for another thread to insert it.
921 * @return the head of this queue
922 * @throws InterruptedException {@inheritDoc}
924 public E take() throws InterruptedException {
925 Object e = transferer.transfer(null, false, 0);
928 Thread.interrupted();
929 throw new InterruptedException();
933 * Retrieves and removes the head of this queue, waiting
934 * if necessary up to the specified wait time, for another thread
937 * @return the head of this queue, or <tt>null</tt> if the
938 * specified waiting time elapses before an element is present.
939 * @throws InterruptedException {@inheritDoc}
941 public E poll(long timeout, TimeUnit unit) throws InterruptedException {
942 Object e = transferer.transfer(null, true, unit.toNanos(timeout));
943 if (e != null || !Thread.interrupted())
945 throw new InterruptedException();
949 * Retrieves and removes the head of this queue, if another thread
950 * is currently making an element available.
952 * @return the head of this queue, or <tt>null</tt> if no
953 * element is available.
956 return (E)transferer.transfer(null, true, 0);
960 * Always returns <tt>true</tt>.
961 * A <tt>SynchronousQueue</tt> has no internal capacity.
963 * @return <tt>true</tt>
965 public boolean isEmpty() {
970 * Always returns zero.
971 * A <tt>SynchronousQueue</tt> has no internal capacity.
980 * Always returns zero.
981 * A <tt>SynchronousQueue</tt> has no internal capacity.
985 public int remainingCapacity() {
991 * A <tt>SynchronousQueue</tt> has no internal capacity.
993 public void clear() {
997 * Always returns <tt>false</tt>.
998 * A <tt>SynchronousQueue</tt> has no internal capacity.
1000 * @param o the element
1001 * @return <tt>false</tt>
1003 public boolean contains(Object o) {
1008 * Always returns <tt>false</tt>.
1009 * A <tt>SynchronousQueue</tt> has no internal capacity.
1011 * @param o the element to remove
1012 * @return <tt>false</tt>
1014 public boolean remove(Object o) {
1019 * Returns <tt>false</tt> unless the given collection is empty.
1020 * A <tt>SynchronousQueue</tt> has no internal capacity.
1022 * @param c the collection
1023 * @return <tt>false</tt> unless given collection is empty
1025 public boolean containsAll(Collection<?> c) {
1030 * Always returns <tt>false</tt>.
1031 * A <tt>SynchronousQueue</tt> has no internal capacity.
1033 * @param c the collection
1034 * @return <tt>false</tt>
1036 public boolean removeAll(Collection<?> c) {
1041 * Always returns <tt>false</tt>.
1042 * A <tt>SynchronousQueue</tt> has no internal capacity.
1044 * @param c the collection
1045 * @return <tt>false</tt>
1047 public boolean retainAll(Collection<?> c) {
1052 * Always returns <tt>null</tt>.
1053 * A <tt>SynchronousQueue</tt> does not return elements
1054 * unless actively waited on.
1056 * @return <tt>null</tt>
1063 * Returns an empty iterator in which <tt>hasNext</tt> always returns
1066 * @return an empty iterator
1068 public Iterator<E> iterator() {
1069 return Collections.emptyIterator();
1073 * Returns a zero-length array.
1074 * @return a zero-length array
1076 public Object[] toArray() {
1077 return new Object[0];
1081 * Sets the zeroeth element of the specified array to <tt>null</tt>
1082 * (if the array has non-zero length) and returns it.
1084 * @param a the array
1085 * @return the specified array
1086 * @throws NullPointerException if the specified array is null
1088 public <T> T[] toArray(T[] a) {
1095 * @throws UnsupportedOperationException {@inheritDoc}
1096 * @throws ClassCastException {@inheritDoc}
1097 * @throws NullPointerException {@inheritDoc}
1098 * @throws IllegalArgumentException {@inheritDoc}
1100 public int drainTo(Collection<? super E> c) {
1102 throw new NullPointerException();
1104 throw new IllegalArgumentException();
1107 while ( (e = poll()) != null) {
1115 * @throws UnsupportedOperationException {@inheritDoc}
1116 * @throws ClassCastException {@inheritDoc}
1117 * @throws NullPointerException {@inheritDoc}
1118 * @throws IllegalArgumentException {@inheritDoc}
1120 public int drainTo(Collection<? super E> c, int maxElements) {
1122 throw new NullPointerException();
1124 throw new IllegalArgumentException();
1127 while (n < maxElements && (e = poll()) != null) {
1135 * To cope with serialization strategy in the 1.5 version of
1136 * SynchronousQueue, we declare some unused classes and fields
1137 * that exist solely to enable serializability across versions.
1138 * These fields are never used, so are initialized only if this
1139 * object is ever serialized or deserialized.
1142 static class WaitQueue implements java.io.Serializable { }
1143 static class LifoWaitQueue extends WaitQueue {
1144 private static final long serialVersionUID = -3633113410248163686L;
1146 static class FifoWaitQueue extends WaitQueue {
1147 private static final long serialVersionUID = -3623113410248163686L;
1149 private ReentrantLock qlock;
1150 private WaitQueue waitingProducers;
1151 private WaitQueue waitingConsumers;
1154 * Save the state to a stream (that is, serialize it).
1156 * @param s the stream
1158 private void writeObject(java.io.ObjectOutputStream s)
1159 throws java.io.IOException {
1160 boolean fair = transferer instanceof TransferQueue;
1162 qlock = new ReentrantLock(true);
1163 waitingProducers = new FifoWaitQueue();
1164 waitingConsumers = new FifoWaitQueue();
1167 qlock = new ReentrantLock();
1168 waitingProducers = new LifoWaitQueue();
1169 waitingConsumers = new LifoWaitQueue();
1171 s.defaultWriteObject();
1174 private void readObject(final java.io.ObjectInputStream s)
1175 throws java.io.IOException, ClassNotFoundException {
1176 s.defaultReadObject();
1177 if (waitingProducers instanceof FifoWaitQueue)
1178 transferer = new TransferQueue();
1180 transferer = new TransferStack();
1184 static long objectFieldOffset(sun.misc.Unsafe UNSAFE,
1185 String field, Class<?> klazz) {
1187 return UNSAFE.objectFieldOffset(klazz.getDeclaredField(field));
1188 } catch (NoSuchFieldException e) {
1189 // Convert Exception to corresponding Error
1190 NoSuchFieldError error = new NoSuchFieldError(field);