jaroslav@1890: /*
jaroslav@1890: * DO NOT ALTER OR REMOVE COPYRIGHT NOTICES OR THIS FILE HEADER.
jaroslav@1890: *
jaroslav@1890: * This code is free software; you can redistribute it and/or modify it
jaroslav@1890: * under the terms of the GNU General Public License version 2 only, as
jaroslav@1890: * published by the Free Software Foundation. Oracle designates this
jaroslav@1890: * particular file as subject to the "Classpath" exception as provided
jaroslav@1890: * by Oracle in the LICENSE file that accompanied this code.
jaroslav@1890: *
jaroslav@1890: * This code is distributed in the hope that it will be useful, but WITHOUT
jaroslav@1890: * ANY WARRANTY; without even the implied warranty of MERCHANTABILITY or
jaroslav@1890: * FITNESS FOR A PARTICULAR PURPOSE. See the GNU General Public License
jaroslav@1890: * version 2 for more details (a copy is included in the LICENSE file that
jaroslav@1890: * accompanied this code).
jaroslav@1890: *
jaroslav@1890: * You should have received a copy of the GNU General Public License version
jaroslav@1890: * 2 along with this work; if not, write to the Free Software Foundation,
jaroslav@1890: * Inc., 51 Franklin St, Fifth Floor, Boston, MA 02110-1301 USA.
jaroslav@1890: *
jaroslav@1890: * Please contact Oracle, 500 Oracle Parkway, Redwood Shores, CA 94065 USA
jaroslav@1890: * or visit www.oracle.com if you need additional information or have any
jaroslav@1890: * questions.
jaroslav@1890: */
jaroslav@1890:
jaroslav@1890: /*
jaroslav@1890: * This file is available under and governed by the GNU General Public
jaroslav@1890: * License version 2 only, as published by the Free Software Foundation.
jaroslav@1890: * However, the following notice accompanied the original version of this
jaroslav@1890: * file:
jaroslav@1890: *
jaroslav@1890: * Written by Doug Lea, Bill Scherer, and Michael Scott with
jaroslav@1890: * assistance from members of JCP JSR-166 Expert Group and released to
jaroslav@1890: * the public domain, as explained at
jaroslav@1890: * http://creativecommons.org/publicdomain/zero/1.0/
jaroslav@1890: */
jaroslav@1890:
jaroslav@1890: package java.util.concurrent;
jaroslav@1890: import java.util.concurrent.locks.*;
jaroslav@1890: import java.util.concurrent.atomic.*;
jaroslav@1890: import java.util.*;
jaroslav@1890:
jaroslav@1890: /**
jaroslav@1890: * A {@linkplain BlockingQueue blocking queue} in which each insert
jaroslav@1890: * operation must wait for a corresponding remove operation by another
jaroslav@1890: * thread, and vice versa. A synchronous queue does not have any
jaroslav@1890: * internal capacity, not even a capacity of one. You cannot
jaroslav@1890: * peek at a synchronous queue because an element is only
jaroslav@1890: * present when you try to remove it; you cannot insert an element
jaroslav@1890: * (using any method) unless another thread is trying to remove it;
jaroslav@1890: * you cannot iterate as there is nothing to iterate. The
jaroslav@1890: * head of the queue is the element that the first queued
jaroslav@1890: * inserting thread is trying to add to the queue; if there is no such
jaroslav@1890: * queued thread then no element is available for removal and
jaroslav@1890: * poll() will return null. For purposes of other
jaroslav@1890: * Collection methods (for example contains), a
jaroslav@1890: * SynchronousQueue acts as an empty collection. This queue
jaroslav@1890: * does not permit null elements.
jaroslav@1890: *
jaroslav@1890: *
Synchronous queues are similar to rendezvous channels used in
jaroslav@1890: * CSP and Ada. They are well suited for handoff designs, in which an
jaroslav@1890: * object running in one thread must sync up with an object running
jaroslav@1890: * in another thread in order to hand it some information, event, or
jaroslav@1890: * task.
jaroslav@1890: *
jaroslav@1890: *
This class supports an optional fairness policy for ordering
jaroslav@1890: * waiting producer and consumer threads. By default, this ordering
jaroslav@1890: * is not guaranteed. However, a queue constructed with fairness set
jaroslav@1890: * to true grants threads access in FIFO order.
jaroslav@1890: *
jaroslav@1890: *
This class and its iterator implement all of the
jaroslav@1890: * optional methods of the {@link Collection} and {@link
jaroslav@1890: * Iterator} interfaces.
jaroslav@1890: *
jaroslav@1890: *
This class is a member of the
jaroslav@1890: *
jaroslav@1890: * Java Collections Framework.
jaroslav@1890: *
jaroslav@1890: * @since 1.5
jaroslav@1890: * @author Doug Lea and Bill Scherer and Michael Scott
jaroslav@1890: * @param the type of elements held in this collection
jaroslav@1890: */
jaroslav@1890: public class SynchronousQueue extends AbstractQueue
jaroslav@1890: implements BlockingQueue, java.io.Serializable {
jaroslav@1890: private static final long serialVersionUID = -3223113410248163686L;
jaroslav@1890:
jaroslav@1890: /*
jaroslav@1890: * This class implements extensions of the dual stack and dual
jaroslav@1890: * queue algorithms described in "Nonblocking Concurrent Objects
jaroslav@1890: * with Condition Synchronization", by W. N. Scherer III and
jaroslav@1890: * M. L. Scott. 18th Annual Conf. on Distributed Computing,
jaroslav@1890: * Oct. 2004 (see also
jaroslav@1890: * http://www.cs.rochester.edu/u/scott/synchronization/pseudocode/duals.html).
jaroslav@1890: * The (Lifo) stack is used for non-fair mode, and the (Fifo)
jaroslav@1890: * queue for fair mode. The performance of the two is generally
jaroslav@1890: * similar. Fifo usually supports higher throughput under
jaroslav@1890: * contention but Lifo maintains higher thread locality in common
jaroslav@1890: * applications.
jaroslav@1890: *
jaroslav@1890: * A dual queue (and similarly stack) is one that at any given
jaroslav@1890: * time either holds "data" -- items provided by put operations,
jaroslav@1890: * or "requests" -- slots representing take operations, or is
jaroslav@1890: * empty. A call to "fulfill" (i.e., a call requesting an item
jaroslav@1890: * from a queue holding data or vice versa) dequeues a
jaroslav@1890: * complementary node. The most interesting feature of these
jaroslav@1890: * queues is that any operation can figure out which mode the
jaroslav@1890: * queue is in, and act accordingly without needing locks.
jaroslav@1890: *
jaroslav@1890: * Both the queue and stack extend abstract class Transferer
jaroslav@1890: * defining the single method transfer that does a put or a
jaroslav@1890: * take. These are unified into a single method because in dual
jaroslav@1890: * data structures, the put and take operations are symmetrical,
jaroslav@1890: * so nearly all code can be combined. The resulting transfer
jaroslav@1890: * methods are on the long side, but are easier to follow than
jaroslav@1890: * they would be if broken up into nearly-duplicated parts.
jaroslav@1890: *
jaroslav@1890: * The queue and stack data structures share many conceptual
jaroslav@1890: * similarities but very few concrete details. For simplicity,
jaroslav@1890: * they are kept distinct so that they can later evolve
jaroslav@1890: * separately.
jaroslav@1890: *
jaroslav@1890: * The algorithms here differ from the versions in the above paper
jaroslav@1890: * in extending them for use in synchronous queues, as well as
jaroslav@1890: * dealing with cancellation. The main differences include:
jaroslav@1890: *
jaroslav@1890: * 1. The original algorithms used bit-marked pointers, but
jaroslav@1890: * the ones here use mode bits in nodes, leading to a number
jaroslav@1890: * of further adaptations.
jaroslav@1890: * 2. SynchronousQueues must block threads waiting to become
jaroslav@1890: * fulfilled.
jaroslav@1890: * 3. Support for cancellation via timeout and interrupts,
jaroslav@1890: * including cleaning out cancelled nodes/threads
jaroslav@1890: * from lists to avoid garbage retention and memory depletion.
jaroslav@1890: *
jaroslav@1890: * Blocking is mainly accomplished using LockSupport park/unpark,
jaroslav@1890: * except that nodes that appear to be the next ones to become
jaroslav@1890: * fulfilled first spin a bit (on multiprocessors only). On very
jaroslav@1890: * busy synchronous queues, spinning can dramatically improve
jaroslav@1890: * throughput. And on less busy ones, the amount of spinning is
jaroslav@1890: * small enough not to be noticeable.
jaroslav@1890: *
jaroslav@1890: * Cleaning is done in different ways in queues vs stacks. For
jaroslav@1890: * queues, we can almost always remove a node immediately in O(1)
jaroslav@1890: * time (modulo retries for consistency checks) when it is
jaroslav@1890: * cancelled. But if it may be pinned as the current tail, it must
jaroslav@1890: * wait until some subsequent cancellation. For stacks, we need a
jaroslav@1890: * potentially O(n) traversal to be sure that we can remove the
jaroslav@1890: * node, but this can run concurrently with other threads
jaroslav@1890: * accessing the stack.
jaroslav@1890: *
jaroslav@1890: * While garbage collection takes care of most node reclamation
jaroslav@1890: * issues that otherwise complicate nonblocking algorithms, care
jaroslav@1890: * is taken to "forget" references to data, other nodes, and
jaroslav@1890: * threads that might be held on to long-term by blocked
jaroslav@1890: * threads. In cases where setting to null would otherwise
jaroslav@1890: * conflict with main algorithms, this is done by changing a
jaroslav@1890: * node's link to now point to the node itself. This doesn't arise
jaroslav@1890: * much for Stack nodes (because blocked threads do not hang on to
jaroslav@1890: * old head pointers), but references in Queue nodes must be
jaroslav@1890: * aggressively forgotten to avoid reachability of everything any
jaroslav@1890: * node has ever referred to since arrival.
jaroslav@1890: */
jaroslav@1890:
jaroslav@1890: /**
jaroslav@1890: * Shared internal API for dual stacks and queues.
jaroslav@1890: */
jaroslav@1890: abstract static class Transferer {
jaroslav@1890: /**
jaroslav@1890: * Performs a put or take.
jaroslav@1890: *
jaroslav@1890: * @param e if non-null, the item to be handed to a consumer;
jaroslav@1890: * if null, requests that transfer return an item
jaroslav@1890: * offered by producer.
jaroslav@1890: * @param timed if this operation should timeout
jaroslav@1890: * @param nanos the timeout, in nanoseconds
jaroslav@1890: * @return if non-null, the item provided or received; if null,
jaroslav@1890: * the operation failed due to timeout or interrupt --
jaroslav@1890: * the caller can distinguish which of these occurred
jaroslav@1890: * by checking Thread.interrupted.
jaroslav@1890: */
jaroslav@1890: abstract Object transfer(Object e, boolean timed, long nanos);
jaroslav@1890: }
jaroslav@1890:
jaroslav@1890: /** The number of CPUs, for spin control */
jaroslav@1890: static final int NCPUS = Runtime.getRuntime().availableProcessors();
jaroslav@1890:
jaroslav@1890: /**
jaroslav@1890: * The number of times to spin before blocking in timed waits.
jaroslav@1890: * The value is empirically derived -- it works well across a
jaroslav@1890: * variety of processors and OSes. Empirically, the best value
jaroslav@1890: * seems not to vary with number of CPUs (beyond 2) so is just
jaroslav@1890: * a constant.
jaroslav@1890: */
jaroslav@1890: static final int maxTimedSpins = (NCPUS < 2) ? 0 : 32;
jaroslav@1890:
jaroslav@1890: /**
jaroslav@1890: * The number of times to spin before blocking in untimed waits.
jaroslav@1890: * This is greater than timed value because untimed waits spin
jaroslav@1890: * faster since they don't need to check times on each spin.
jaroslav@1890: */
jaroslav@1890: static final int maxUntimedSpins = maxTimedSpins * 16;
jaroslav@1890:
jaroslav@1890: /**
jaroslav@1890: * The number of nanoseconds for which it is faster to spin
jaroslav@1890: * rather than to use timed park. A rough estimate suffices.
jaroslav@1890: */
jaroslav@1890: static final long spinForTimeoutThreshold = 1000L;
jaroslav@1890:
jaroslav@1890: /** Dual stack */
jaroslav@1890: static final class TransferStack extends Transferer {
jaroslav@1890: /*
jaroslav@1890: * This extends Scherer-Scott dual stack algorithm, differing,
jaroslav@1890: * among other ways, by using "covering" nodes rather than
jaroslav@1890: * bit-marked pointers: Fulfilling operations push on marker
jaroslav@1890: * nodes (with FULFILLING bit set in mode) to reserve a spot
jaroslav@1890: * to match a waiting node.
jaroslav@1890: */
jaroslav@1890:
jaroslav@1890: /* Modes for SNodes, ORed together in node fields */
jaroslav@1890: /** Node represents an unfulfilled consumer */
jaroslav@1890: static final int REQUEST = 0;
jaroslav@1890: /** Node represents an unfulfilled producer */
jaroslav@1890: static final int DATA = 1;
jaroslav@1890: /** Node is fulfilling another unfulfilled DATA or REQUEST */
jaroslav@1890: static final int FULFILLING = 2;
jaroslav@1890:
jaroslav@1890: /** Return true if m has fulfilling bit set */
jaroslav@1890: static boolean isFulfilling(int m) { return (m & FULFILLING) != 0; }
jaroslav@1890:
jaroslav@1890: /** Node class for TransferStacks. */
jaroslav@1890: static final class SNode {
jaroslav@1890: volatile SNode next; // next node in stack
jaroslav@1890: volatile SNode match; // the node matched to this
jaroslav@1890: volatile Thread waiter; // to control park/unpark
jaroslav@1890: Object item; // data; or null for REQUESTs
jaroslav@1890: int mode;
jaroslav@1890: // Note: item and mode fields don't need to be volatile
jaroslav@1890: // since they are always written before, and read after,
jaroslav@1890: // other volatile/atomic operations.
jaroslav@1890:
jaroslav@1890: SNode(Object item) {
jaroslav@1890: this.item = item;
jaroslav@1890: }
jaroslav@1890:
jaroslav@1890: boolean casNext(SNode cmp, SNode val) {
jaroslav@1890: return cmp == next &&
jaroslav@1890: UNSAFE.compareAndSwapObject(this, nextOffset, cmp, val);
jaroslav@1890: }
jaroslav@1890:
jaroslav@1890: /**
jaroslav@1890: * Tries to match node s to this node, if so, waking up thread.
jaroslav@1890: * Fulfillers call tryMatch to identify their waiters.
jaroslav@1890: * Waiters block until they have been matched.
jaroslav@1890: *
jaroslav@1890: * @param s the node to match
jaroslav@1890: * @return true if successfully matched to s
jaroslav@1890: */
jaroslav@1890: boolean tryMatch(SNode s) {
jaroslav@1890: if (match == null &&
jaroslav@1890: UNSAFE.compareAndSwapObject(this, matchOffset, null, s)) {
jaroslav@1890: Thread w = waiter;
jaroslav@1890: if (w != null) { // waiters need at most one unpark
jaroslav@1890: waiter = null;
jaroslav@1890: LockSupport.unpark(w);
jaroslav@1890: }
jaroslav@1890: return true;
jaroslav@1890: }
jaroslav@1890: return match == s;
jaroslav@1890: }
jaroslav@1890:
jaroslav@1890: /**
jaroslav@1890: * Tries to cancel a wait by matching node to itself.
jaroslav@1890: */
jaroslav@1890: void tryCancel() {
jaroslav@1890: UNSAFE.compareAndSwapObject(this, matchOffset, null, this);
jaroslav@1890: }
jaroslav@1890:
jaroslav@1890: boolean isCancelled() {
jaroslav@1890: return match == this;
jaroslav@1890: }
jaroslav@1890:
jaroslav@1890: // Unsafe mechanics
jaroslav@1890: private static final sun.misc.Unsafe UNSAFE;
jaroslav@1890: private static final long matchOffset;
jaroslav@1890: private static final long nextOffset;
jaroslav@1890:
jaroslav@1890: static {
jaroslav@1890: try {
jaroslav@1890: UNSAFE = sun.misc.Unsafe.getUnsafe();
jaroslav@1890: Class k = SNode.class;
jaroslav@1890: matchOffset = UNSAFE.objectFieldOffset
jaroslav@1890: (k.getDeclaredField("match"));
jaroslav@1890: nextOffset = UNSAFE.objectFieldOffset
jaroslav@1890: (k.getDeclaredField("next"));
jaroslav@1890: } catch (Exception e) {
jaroslav@1890: throw new Error(e);
jaroslav@1890: }
jaroslav@1890: }
jaroslav@1890: }
jaroslav@1890:
jaroslav@1890: /** The head (top) of the stack */
jaroslav@1890: volatile SNode head;
jaroslav@1890:
jaroslav@1890: boolean casHead(SNode h, SNode nh) {
jaroslav@1890: return h == head &&
jaroslav@1890: UNSAFE.compareAndSwapObject(this, headOffset, h, nh);
jaroslav@1890: }
jaroslav@1890:
jaroslav@1890: /**
jaroslav@1890: * Creates or resets fields of a node. Called only from transfer
jaroslav@1890: * where the node to push on stack is lazily created and
jaroslav@1890: * reused when possible to help reduce intervals between reads
jaroslav@1890: * and CASes of head and to avoid surges of garbage when CASes
jaroslav@1890: * to push nodes fail due to contention.
jaroslav@1890: */
jaroslav@1890: static SNode snode(SNode s, Object e, SNode next, int mode) {
jaroslav@1890: if (s == null) s = new SNode(e);
jaroslav@1890: s.mode = mode;
jaroslav@1890: s.next = next;
jaroslav@1890: return s;
jaroslav@1890: }
jaroslav@1890:
jaroslav@1890: /**
jaroslav@1890: * Puts or takes an item.
jaroslav@1890: */
jaroslav@1890: Object transfer(Object e, boolean timed, long nanos) {
jaroslav@1890: /*
jaroslav@1890: * Basic algorithm is to loop trying one of three actions:
jaroslav@1890: *
jaroslav@1890: * 1. If apparently empty or already containing nodes of same
jaroslav@1890: * mode, try to push node on stack and wait for a match,
jaroslav@1890: * returning it, or null if cancelled.
jaroslav@1890: *
jaroslav@1890: * 2. If apparently containing node of complementary mode,
jaroslav@1890: * try to push a fulfilling node on to stack, match
jaroslav@1890: * with corresponding waiting node, pop both from
jaroslav@1890: * stack, and return matched item. The matching or
jaroslav@1890: * unlinking might not actually be necessary because of
jaroslav@1890: * other threads performing action 3:
jaroslav@1890: *
jaroslav@1890: * 3. If top of stack already holds another fulfilling node,
jaroslav@1890: * help it out by doing its match and/or pop
jaroslav@1890: * operations, and then continue. The code for helping
jaroslav@1890: * is essentially the same as for fulfilling, except
jaroslav@1890: * that it doesn't return the item.
jaroslav@1890: */
jaroslav@1890:
jaroslav@1890: SNode s = null; // constructed/reused as needed
jaroslav@1890: int mode = (e == null) ? REQUEST : DATA;
jaroslav@1890:
jaroslav@1890: for (;;) {
jaroslav@1890: SNode h = head;
jaroslav@1890: if (h == null || h.mode == mode) { // empty or same-mode
jaroslav@1890: if (timed && nanos <= 0) { // can't wait
jaroslav@1890: if (h != null && h.isCancelled())
jaroslav@1890: casHead(h, h.next); // pop cancelled node
jaroslav@1890: else
jaroslav@1890: return null;
jaroslav@1890: } else if (casHead(h, s = snode(s, e, h, mode))) {
jaroslav@1890: SNode m = awaitFulfill(s, timed, nanos);
jaroslav@1890: if (m == s) { // wait was cancelled
jaroslav@1890: clean(s);
jaroslav@1890: return null;
jaroslav@1890: }
jaroslav@1890: if ((h = head) != null && h.next == s)
jaroslav@1890: casHead(h, s.next); // help s's fulfiller
jaroslav@1890: return (mode == REQUEST) ? m.item : s.item;
jaroslav@1890: }
jaroslav@1890: } else if (!isFulfilling(h.mode)) { // try to fulfill
jaroslav@1890: if (h.isCancelled()) // already cancelled
jaroslav@1890: casHead(h, h.next); // pop and retry
jaroslav@1890: else if (casHead(h, s=snode(s, e, h, FULFILLING|mode))) {
jaroslav@1890: for (;;) { // loop until matched or waiters disappear
jaroslav@1890: SNode m = s.next; // m is s's match
jaroslav@1890: if (m == null) { // all waiters are gone
jaroslav@1890: casHead(s, null); // pop fulfill node
jaroslav@1890: s = null; // use new node next time
jaroslav@1890: break; // restart main loop
jaroslav@1890: }
jaroslav@1890: SNode mn = m.next;
jaroslav@1890: if (m.tryMatch(s)) {
jaroslav@1890: casHead(s, mn); // pop both s and m
jaroslav@1890: return (mode == REQUEST) ? m.item : s.item;
jaroslav@1890: } else // lost match
jaroslav@1890: s.casNext(m, mn); // help unlink
jaroslav@1890: }
jaroslav@1890: }
jaroslav@1890: } else { // help a fulfiller
jaroslav@1890: SNode m = h.next; // m is h's match
jaroslav@1890: if (m == null) // waiter is gone
jaroslav@1890: casHead(h, null); // pop fulfilling node
jaroslav@1890: else {
jaroslav@1890: SNode mn = m.next;
jaroslav@1890: if (m.tryMatch(h)) // help match
jaroslav@1890: casHead(h, mn); // pop both h and m
jaroslav@1890: else // lost match
jaroslav@1890: h.casNext(m, mn); // help unlink
jaroslav@1890: }
jaroslav@1890: }
jaroslav@1890: }
jaroslav@1890: }
jaroslav@1890:
jaroslav@1890: /**
jaroslav@1890: * Spins/blocks until node s is matched by a fulfill operation.
jaroslav@1890: *
jaroslav@1890: * @param s the waiting node
jaroslav@1890: * @param timed true if timed wait
jaroslav@1890: * @param nanos timeout value
jaroslav@1890: * @return matched node, or s if cancelled
jaroslav@1890: */
jaroslav@1890: SNode awaitFulfill(SNode s, boolean timed, long nanos) {
jaroslav@1890: /*
jaroslav@1890: * When a node/thread is about to block, it sets its waiter
jaroslav@1890: * field and then rechecks state at least one more time
jaroslav@1890: * before actually parking, thus covering race vs
jaroslav@1890: * fulfiller noticing that waiter is non-null so should be
jaroslav@1890: * woken.
jaroslav@1890: *
jaroslav@1890: * When invoked by nodes that appear at the point of call
jaroslav@1890: * to be at the head of the stack, calls to park are
jaroslav@1890: * preceded by spins to avoid blocking when producers and
jaroslav@1890: * consumers are arriving very close in time. This can
jaroslav@1890: * happen enough to bother only on multiprocessors.
jaroslav@1890: *
jaroslav@1890: * The order of checks for returning out of main loop
jaroslav@1890: * reflects fact that interrupts have precedence over
jaroslav@1890: * normal returns, which have precedence over
jaroslav@1890: * timeouts. (So, on timeout, one last check for match is
jaroslav@1890: * done before giving up.) Except that calls from untimed
jaroslav@1890: * SynchronousQueue.{poll/offer} don't check interrupts
jaroslav@1890: * and don't wait at all, so are trapped in transfer
jaroslav@1890: * method rather than calling awaitFulfill.
jaroslav@1890: */
jaroslav@1890: long lastTime = timed ? System.nanoTime() : 0;
jaroslav@1890: Thread w = Thread.currentThread();
jaroslav@1890: SNode h = head;
jaroslav@1890: int spins = (shouldSpin(s) ?
jaroslav@1890: (timed ? maxTimedSpins : maxUntimedSpins) : 0);
jaroslav@1890: for (;;) {
jaroslav@1890: if (w.isInterrupted())
jaroslav@1890: s.tryCancel();
jaroslav@1890: SNode m = s.match;
jaroslav@1890: if (m != null)
jaroslav@1890: return m;
jaroslav@1890: if (timed) {
jaroslav@1890: long now = System.nanoTime();
jaroslav@1890: nanos -= now - lastTime;
jaroslav@1890: lastTime = now;
jaroslav@1890: if (nanos <= 0) {
jaroslav@1890: s.tryCancel();
jaroslav@1890: continue;
jaroslav@1890: }
jaroslav@1890: }
jaroslav@1890: if (spins > 0)
jaroslav@1890: spins = shouldSpin(s) ? (spins-1) : 0;
jaroslav@1890: else if (s.waiter == null)
jaroslav@1890: s.waiter = w; // establish waiter so can park next iter
jaroslav@1890: else if (!timed)
jaroslav@1890: LockSupport.park(this);
jaroslav@1890: else if (nanos > spinForTimeoutThreshold)
jaroslav@1890: LockSupport.parkNanos(this, nanos);
jaroslav@1890: }
jaroslav@1890: }
jaroslav@1890:
jaroslav@1890: /**
jaroslav@1890: * Returns true if node s is at head or there is an active
jaroslav@1890: * fulfiller.
jaroslav@1890: */
jaroslav@1890: boolean shouldSpin(SNode s) {
jaroslav@1890: SNode h = head;
jaroslav@1890: return (h == s || h == null || isFulfilling(h.mode));
jaroslav@1890: }
jaroslav@1890:
jaroslav@1890: /**
jaroslav@1890: * Unlinks s from the stack.
jaroslav@1890: */
jaroslav@1890: void clean(SNode s) {
jaroslav@1890: s.item = null; // forget item
jaroslav@1890: s.waiter = null; // forget thread
jaroslav@1890:
jaroslav@1890: /*
jaroslav@1890: * At worst we may need to traverse entire stack to unlink
jaroslav@1890: * s. If there are multiple concurrent calls to clean, we
jaroslav@1890: * might not see s if another thread has already removed
jaroslav@1890: * it. But we can stop when we see any node known to
jaroslav@1890: * follow s. We use s.next unless it too is cancelled, in
jaroslav@1890: * which case we try the node one past. We don't check any
jaroslav@1890: * further because we don't want to doubly traverse just to
jaroslav@1890: * find sentinel.
jaroslav@1890: */
jaroslav@1890:
jaroslav@1890: SNode past = s.next;
jaroslav@1890: if (past != null && past.isCancelled())
jaroslav@1890: past = past.next;
jaroslav@1890:
jaroslav@1890: // Absorb cancelled nodes at head
jaroslav@1890: SNode p;
jaroslav@1890: while ((p = head) != null && p != past && p.isCancelled())
jaroslav@1890: casHead(p, p.next);
jaroslav@1890:
jaroslav@1890: // Unsplice embedded nodes
jaroslav@1890: while (p != null && p != past) {
jaroslav@1890: SNode n = p.next;
jaroslav@1890: if (n != null && n.isCancelled())
jaroslav@1890: p.casNext(n, n.next);
jaroslav@1890: else
jaroslav@1890: p = n;
jaroslav@1890: }
jaroslav@1890: }
jaroslav@1890:
jaroslav@1890: // Unsafe mechanics
jaroslav@1890: private static final sun.misc.Unsafe UNSAFE;
jaroslav@1890: private static final long headOffset;
jaroslav@1890: static {
jaroslav@1890: try {
jaroslav@1890: UNSAFE = sun.misc.Unsafe.getUnsafe();
jaroslav@1890: Class k = TransferStack.class;
jaroslav@1890: headOffset = UNSAFE.objectFieldOffset
jaroslav@1890: (k.getDeclaredField("head"));
jaroslav@1890: } catch (Exception e) {
jaroslav@1890: throw new Error(e);
jaroslav@1890: }
jaroslav@1890: }
jaroslav@1890: }
jaroslav@1890:
jaroslav@1890: /** Dual Queue */
jaroslav@1890: static final class TransferQueue extends Transferer {
jaroslav@1890: /*
jaroslav@1890: * This extends Scherer-Scott dual queue algorithm, differing,
jaroslav@1890: * among other ways, by using modes within nodes rather than
jaroslav@1890: * marked pointers. The algorithm is a little simpler than
jaroslav@1890: * that for stacks because fulfillers do not need explicit
jaroslav@1890: * nodes, and matching is done by CAS'ing QNode.item field
jaroslav@1890: * from non-null to null (for put) or vice versa (for take).
jaroslav@1890: */
jaroslav@1890:
jaroslav@1890: /** Node class for TransferQueue. */
jaroslav@1890: static final class QNode {
jaroslav@1890: volatile QNode next; // next node in queue
jaroslav@1890: volatile Object item; // CAS'ed to or from null
jaroslav@1890: volatile Thread waiter; // to control park/unpark
jaroslav@1890: final boolean isData;
jaroslav@1890:
jaroslav@1890: QNode(Object item, boolean isData) {
jaroslav@1890: this.item = item;
jaroslav@1890: this.isData = isData;
jaroslav@1890: }
jaroslav@1890:
jaroslav@1890: boolean casNext(QNode cmp, QNode val) {
jaroslav@1890: return next == cmp &&
jaroslav@1890: UNSAFE.compareAndSwapObject(this, nextOffset, cmp, val);
jaroslav@1890: }
jaroslav@1890:
jaroslav@1890: boolean casItem(Object cmp, Object val) {
jaroslav@1890: return item == cmp &&
jaroslav@1890: UNSAFE.compareAndSwapObject(this, itemOffset, cmp, val);
jaroslav@1890: }
jaroslav@1890:
jaroslav@1890: /**
jaroslav@1890: * Tries to cancel by CAS'ing ref to this as item.
jaroslav@1890: */
jaroslav@1890: void tryCancel(Object cmp) {
jaroslav@1890: UNSAFE.compareAndSwapObject(this, itemOffset, cmp, this);
jaroslav@1890: }
jaroslav@1890:
jaroslav@1890: boolean isCancelled() {
jaroslav@1890: return item == this;
jaroslav@1890: }
jaroslav@1890:
jaroslav@1890: /**
jaroslav@1890: * Returns true if this node is known to be off the queue
jaroslav@1890: * because its next pointer has been forgotten due to
jaroslav@1890: * an advanceHead operation.
jaroslav@1890: */
jaroslav@1890: boolean isOffList() {
jaroslav@1890: return next == this;
jaroslav@1890: }
jaroslav@1890:
jaroslav@1890: // Unsafe mechanics
jaroslav@1890: private static final sun.misc.Unsafe UNSAFE;
jaroslav@1890: private static final long itemOffset;
jaroslav@1890: private static final long nextOffset;
jaroslav@1890:
jaroslav@1890: static {
jaroslav@1890: try {
jaroslav@1890: UNSAFE = sun.misc.Unsafe.getUnsafe();
jaroslav@1890: Class k = QNode.class;
jaroslav@1890: itemOffset = UNSAFE.objectFieldOffset
jaroslav@1890: (k.getDeclaredField("item"));
jaroslav@1890: nextOffset = UNSAFE.objectFieldOffset
jaroslav@1890: (k.getDeclaredField("next"));
jaroslav@1890: } catch (Exception e) {
jaroslav@1890: throw new Error(e);
jaroslav@1890: }
jaroslav@1890: }
jaroslav@1890: }
jaroslav@1890:
jaroslav@1890: /** Head of queue */
jaroslav@1890: transient volatile QNode head;
jaroslav@1890: /** Tail of queue */
jaroslav@1890: transient volatile QNode tail;
jaroslav@1890: /**
jaroslav@1890: * Reference to a cancelled node that might not yet have been
jaroslav@1890: * unlinked from queue because it was the last inserted node
jaroslav@1890: * when it cancelled.
jaroslav@1890: */
jaroslav@1890: transient volatile QNode cleanMe;
jaroslav@1890:
jaroslav@1890: TransferQueue() {
jaroslav@1890: QNode h = new QNode(null, false); // initialize to dummy node.
jaroslav@1890: head = h;
jaroslav@1890: tail = h;
jaroslav@1890: }
jaroslav@1890:
jaroslav@1890: /**
jaroslav@1890: * Tries to cas nh as new head; if successful, unlink
jaroslav@1890: * old head's next node to avoid garbage retention.
jaroslav@1890: */
jaroslav@1890: void advanceHead(QNode h, QNode nh) {
jaroslav@1890: if (h == head &&
jaroslav@1890: UNSAFE.compareAndSwapObject(this, headOffset, h, nh))
jaroslav@1890: h.next = h; // forget old next
jaroslav@1890: }
jaroslav@1890:
jaroslav@1890: /**
jaroslav@1890: * Tries to cas nt as new tail.
jaroslav@1890: */
jaroslav@1890: void advanceTail(QNode t, QNode nt) {
jaroslav@1890: if (tail == t)
jaroslav@1890: UNSAFE.compareAndSwapObject(this, tailOffset, t, nt);
jaroslav@1890: }
jaroslav@1890:
jaroslav@1890: /**
jaroslav@1890: * Tries to CAS cleanMe slot.
jaroslav@1890: */
jaroslav@1890: boolean casCleanMe(QNode cmp, QNode val) {
jaroslav@1890: return cleanMe == cmp &&
jaroslav@1890: UNSAFE.compareAndSwapObject(this, cleanMeOffset, cmp, val);
jaroslav@1890: }
jaroslav@1890:
jaroslav@1890: /**
jaroslav@1890: * Puts or takes an item.
jaroslav@1890: */
jaroslav@1890: Object transfer(Object e, boolean timed, long nanos) {
jaroslav@1890: /* Basic algorithm is to loop trying to take either of
jaroslav@1890: * two actions:
jaroslav@1890: *
jaroslav@1890: * 1. If queue apparently empty or holding same-mode nodes,
jaroslav@1890: * try to add node to queue of waiters, wait to be
jaroslav@1890: * fulfilled (or cancelled) and return matching item.
jaroslav@1890: *
jaroslav@1890: * 2. If queue apparently contains waiting items, and this
jaroslav@1890: * call is of complementary mode, try to fulfill by CAS'ing
jaroslav@1890: * item field of waiting node and dequeuing it, and then
jaroslav@1890: * returning matching item.
jaroslav@1890: *
jaroslav@1890: * In each case, along the way, check for and try to help
jaroslav@1890: * advance head and tail on behalf of other stalled/slow
jaroslav@1890: * threads.
jaroslav@1890: *
jaroslav@1890: * The loop starts off with a null check guarding against
jaroslav@1890: * seeing uninitialized head or tail values. This never
jaroslav@1890: * happens in current SynchronousQueue, but could if
jaroslav@1890: * callers held non-volatile/final ref to the
jaroslav@1890: * transferer. The check is here anyway because it places
jaroslav@1890: * null checks at top of loop, which is usually faster
jaroslav@1890: * than having them implicitly interspersed.
jaroslav@1890: */
jaroslav@1890:
jaroslav@1890: QNode s = null; // constructed/reused as needed
jaroslav@1890: boolean isData = (e != null);
jaroslav@1890:
jaroslav@1890: for (;;) {
jaroslav@1890: QNode t = tail;
jaroslav@1890: QNode h = head;
jaroslav@1890: if (t == null || h == null) // saw uninitialized value
jaroslav@1890: continue; // spin
jaroslav@1890:
jaroslav@1890: if (h == t || t.isData == isData) { // empty or same-mode
jaroslav@1890: QNode tn = t.next;
jaroslav@1890: if (t != tail) // inconsistent read
jaroslav@1890: continue;
jaroslav@1890: if (tn != null) { // lagging tail
jaroslav@1890: advanceTail(t, tn);
jaroslav@1890: continue;
jaroslav@1890: }
jaroslav@1890: if (timed && nanos <= 0) // can't wait
jaroslav@1890: return null;
jaroslav@1890: if (s == null)
jaroslav@1890: s = new QNode(e, isData);
jaroslav@1890: if (!t.casNext(null, s)) // failed to link in
jaroslav@1890: continue;
jaroslav@1890:
jaroslav@1890: advanceTail(t, s); // swing tail and wait
jaroslav@1890: Object x = awaitFulfill(s, e, timed, nanos);
jaroslav@1890: if (x == s) { // wait was cancelled
jaroslav@1890: clean(t, s);
jaroslav@1890: return null;
jaroslav@1890: }
jaroslav@1890:
jaroslav@1890: if (!s.isOffList()) { // not already unlinked
jaroslav@1890: advanceHead(t, s); // unlink if head
jaroslav@1890: if (x != null) // and forget fields
jaroslav@1890: s.item = s;
jaroslav@1890: s.waiter = null;
jaroslav@1890: }
jaroslav@1890: return (x != null) ? x : e;
jaroslav@1890:
jaroslav@1890: } else { // complementary-mode
jaroslav@1890: QNode m = h.next; // node to fulfill
jaroslav@1890: if (t != tail || m == null || h != head)
jaroslav@1890: continue; // inconsistent read
jaroslav@1890:
jaroslav@1890: Object x = m.item;
jaroslav@1890: if (isData == (x != null) || // m already fulfilled
jaroslav@1890: x == m || // m cancelled
jaroslav@1890: !m.casItem(x, e)) { // lost CAS
jaroslav@1890: advanceHead(h, m); // dequeue and retry
jaroslav@1890: continue;
jaroslav@1890: }
jaroslav@1890:
jaroslav@1890: advanceHead(h, m); // successfully fulfilled
jaroslav@1890: LockSupport.unpark(m.waiter);
jaroslav@1890: return (x != null) ? x : e;
jaroslav@1890: }
jaroslav@1890: }
jaroslav@1890: }
jaroslav@1890:
jaroslav@1890: /**
jaroslav@1890: * Spins/blocks until node s is fulfilled.
jaroslav@1890: *
jaroslav@1890: * @param s the waiting node
jaroslav@1890: * @param e the comparison value for checking match
jaroslav@1890: * @param timed true if timed wait
jaroslav@1890: * @param nanos timeout value
jaroslav@1890: * @return matched item, or s if cancelled
jaroslav@1890: */
jaroslav@1890: Object awaitFulfill(QNode s, Object e, boolean timed, long nanos) {
jaroslav@1890: /* Same idea as TransferStack.awaitFulfill */
jaroslav@1890: long lastTime = timed ? System.nanoTime() : 0;
jaroslav@1890: Thread w = Thread.currentThread();
jaroslav@1890: int spins = ((head.next == s) ?
jaroslav@1890: (timed ? maxTimedSpins : maxUntimedSpins) : 0);
jaroslav@1890: for (;;) {
jaroslav@1890: if (w.isInterrupted())
jaroslav@1890: s.tryCancel(e);
jaroslav@1890: Object x = s.item;
jaroslav@1890: if (x != e)
jaroslav@1890: return x;
jaroslav@1890: if (timed) {
jaroslav@1890: long now = System.nanoTime();
jaroslav@1890: nanos -= now - lastTime;
jaroslav@1890: lastTime = now;
jaroslav@1890: if (nanos <= 0) {
jaroslav@1890: s.tryCancel(e);
jaroslav@1890: continue;
jaroslav@1890: }
jaroslav@1890: }
jaroslav@1890: if (spins > 0)
jaroslav@1890: --spins;
jaroslav@1890: else if (s.waiter == null)
jaroslav@1890: s.waiter = w;
jaroslav@1890: else if (!timed)
jaroslav@1890: LockSupport.park(this);
jaroslav@1890: else if (nanos > spinForTimeoutThreshold)
jaroslav@1890: LockSupport.parkNanos(this, nanos);
jaroslav@1890: }
jaroslav@1890: }
jaroslav@1890:
jaroslav@1890: /**
jaroslav@1890: * Gets rid of cancelled node s with original predecessor pred.
jaroslav@1890: */
jaroslav@1890: void clean(QNode pred, QNode s) {
jaroslav@1890: s.waiter = null; // forget thread
jaroslav@1890: /*
jaroslav@1890: * At any given time, exactly one node on list cannot be
jaroslav@1890: * deleted -- the last inserted node. To accommodate this,
jaroslav@1890: * if we cannot delete s, we save its predecessor as
jaroslav@1890: * "cleanMe", deleting the previously saved version
jaroslav@1890: * first. At least one of node s or the node previously
jaroslav@1890: * saved can always be deleted, so this always terminates.
jaroslav@1890: */
jaroslav@1890: while (pred.next == s) { // Return early if already unlinked
jaroslav@1890: QNode h = head;
jaroslav@1890: QNode hn = h.next; // Absorb cancelled first node as head
jaroslav@1890: if (hn != null && hn.isCancelled()) {
jaroslav@1890: advanceHead(h, hn);
jaroslav@1890: continue;
jaroslav@1890: }
jaroslav@1890: QNode t = tail; // Ensure consistent read for tail
jaroslav@1890: if (t == h)
jaroslav@1890: return;
jaroslav@1890: QNode tn = t.next;
jaroslav@1890: if (t != tail)
jaroslav@1890: continue;
jaroslav@1890: if (tn != null) {
jaroslav@1890: advanceTail(t, tn);
jaroslav@1890: continue;
jaroslav@1890: }
jaroslav@1890: if (s != t) { // If not tail, try to unsplice
jaroslav@1890: QNode sn = s.next;
jaroslav@1890: if (sn == s || pred.casNext(s, sn))
jaroslav@1890: return;
jaroslav@1890: }
jaroslav@1890: QNode dp = cleanMe;
jaroslav@1890: if (dp != null) { // Try unlinking previous cancelled node
jaroslav@1890: QNode d = dp.next;
jaroslav@1890: QNode dn;
jaroslav@1890: if (d == null || // d is gone or
jaroslav@1890: d == dp || // d is off list or
jaroslav@1890: !d.isCancelled() || // d not cancelled or
jaroslav@1890: (d != t && // d not tail and
jaroslav@1890: (dn = d.next) != null && // has successor
jaroslav@1890: dn != d && // that is on list
jaroslav@1890: dp.casNext(d, dn))) // d unspliced
jaroslav@1890: casCleanMe(dp, null);
jaroslav@1890: if (dp == pred)
jaroslav@1890: return; // s is already saved node
jaroslav@1890: } else if (casCleanMe(null, pred))
jaroslav@1890: return; // Postpone cleaning s
jaroslav@1890: }
jaroslav@1890: }
jaroslav@1890:
jaroslav@1890: private static final sun.misc.Unsafe UNSAFE;
jaroslav@1890: private static final long headOffset;
jaroslav@1890: private static final long tailOffset;
jaroslav@1890: private static final long cleanMeOffset;
jaroslav@1890: static {
jaroslav@1890: try {
jaroslav@1890: UNSAFE = sun.misc.Unsafe.getUnsafe();
jaroslav@1890: Class k = TransferQueue.class;
jaroslav@1890: headOffset = UNSAFE.objectFieldOffset
jaroslav@1890: (k.getDeclaredField("head"));
jaroslav@1890: tailOffset = UNSAFE.objectFieldOffset
jaroslav@1890: (k.getDeclaredField("tail"));
jaroslav@1890: cleanMeOffset = UNSAFE.objectFieldOffset
jaroslav@1890: (k.getDeclaredField("cleanMe"));
jaroslav@1890: } catch (Exception e) {
jaroslav@1890: throw new Error(e);
jaroslav@1890: }
jaroslav@1890: }
jaroslav@1890: }
jaroslav@1890:
jaroslav@1890: /**
jaroslav@1890: * The transferer. Set only in constructor, but cannot be declared
jaroslav@1890: * as final without further complicating serialization. Since
jaroslav@1890: * this is accessed only at most once per public method, there
jaroslav@1890: * isn't a noticeable performance penalty for using volatile
jaroslav@1890: * instead of final here.
jaroslav@1890: */
jaroslav@1890: private transient volatile Transferer transferer;
jaroslav@1890:
jaroslav@1890: /**
jaroslav@1890: * Creates a SynchronousQueue with nonfair access policy.
jaroslav@1890: */
jaroslav@1890: public SynchronousQueue() {
jaroslav@1890: this(false);
jaroslav@1890: }
jaroslav@1890:
jaroslav@1890: /**
jaroslav@1890: * Creates a SynchronousQueue with the specified fairness policy.
jaroslav@1890: *
jaroslav@1890: * @param fair if true, waiting threads contend in FIFO order for
jaroslav@1890: * access; otherwise the order is unspecified.
jaroslav@1890: */
jaroslav@1890: public SynchronousQueue(boolean fair) {
jaroslav@1890: transferer = fair ? new TransferQueue() : new TransferStack();
jaroslav@1890: }
jaroslav@1890:
jaroslav@1890: /**
jaroslav@1890: * Adds the specified element to this queue, waiting if necessary for
jaroslav@1890: * another thread to receive it.
jaroslav@1890: *
jaroslav@1890: * @throws InterruptedException {@inheritDoc}
jaroslav@1890: * @throws NullPointerException {@inheritDoc}
jaroslav@1890: */
jaroslav@1890: public void put(E o) throws InterruptedException {
jaroslav@1890: if (o == null) throw new NullPointerException();
jaroslav@1890: if (transferer.transfer(o, false, 0) == null) {
jaroslav@1890: Thread.interrupted();
jaroslav@1890: throw new InterruptedException();
jaroslav@1890: }
jaroslav@1890: }
jaroslav@1890:
jaroslav@1890: /**
jaroslav@1890: * Inserts the specified element into this queue, waiting if necessary
jaroslav@1890: * up to the specified wait time for another thread to receive it.
jaroslav@1890: *
jaroslav@1890: * @return true if successful, or false if the
jaroslav@1890: * specified waiting time elapses before a consumer appears.
jaroslav@1890: * @throws InterruptedException {@inheritDoc}
jaroslav@1890: * @throws NullPointerException {@inheritDoc}
jaroslav@1890: */
jaroslav@1890: public boolean offer(E o, long timeout, TimeUnit unit)
jaroslav@1890: throws InterruptedException {
jaroslav@1890: if (o == null) throw new NullPointerException();
jaroslav@1890: if (transferer.transfer(o, true, unit.toNanos(timeout)) != null)
jaroslav@1890: return true;
jaroslav@1890: if (!Thread.interrupted())
jaroslav@1890: return false;
jaroslav@1890: throw new InterruptedException();
jaroslav@1890: }
jaroslav@1890:
jaroslav@1890: /**
jaroslav@1890: * Inserts the specified element into this queue, if another thread is
jaroslav@1890: * waiting to receive it.
jaroslav@1890: *
jaroslav@1890: * @param e the element to add
jaroslav@1890: * @return true if the element was added to this queue, else
jaroslav@1890: * false
jaroslav@1890: * @throws NullPointerException if the specified element is null
jaroslav@1890: */
jaroslav@1890: public boolean offer(E e) {
jaroslav@1890: if (e == null) throw new NullPointerException();
jaroslav@1890: return transferer.transfer(e, true, 0) != null;
jaroslav@1890: }
jaroslav@1890:
jaroslav@1890: /**
jaroslav@1890: * Retrieves and removes the head of this queue, waiting if necessary
jaroslav@1890: * for another thread to insert it.
jaroslav@1890: *
jaroslav@1890: * @return the head of this queue
jaroslav@1890: * @throws InterruptedException {@inheritDoc}
jaroslav@1890: */
jaroslav@1890: public E take() throws InterruptedException {
jaroslav@1890: Object e = transferer.transfer(null, false, 0);
jaroslav@1890: if (e != null)
jaroslav@1890: return (E)e;
jaroslav@1890: Thread.interrupted();
jaroslav@1890: throw new InterruptedException();
jaroslav@1890: }
jaroslav@1890:
jaroslav@1890: /**
jaroslav@1890: * Retrieves and removes the head of this queue, waiting
jaroslav@1890: * if necessary up to the specified wait time, for another thread
jaroslav@1890: * to insert it.
jaroslav@1890: *
jaroslav@1890: * @return the head of this queue, or null if the
jaroslav@1890: * specified waiting time elapses before an element is present.
jaroslav@1890: * @throws InterruptedException {@inheritDoc}
jaroslav@1890: */
jaroslav@1890: public E poll(long timeout, TimeUnit unit) throws InterruptedException {
jaroslav@1890: Object e = transferer.transfer(null, true, unit.toNanos(timeout));
jaroslav@1890: if (e != null || !Thread.interrupted())
jaroslav@1890: return (E)e;
jaroslav@1890: throw new InterruptedException();
jaroslav@1890: }
jaroslav@1890:
jaroslav@1890: /**
jaroslav@1890: * Retrieves and removes the head of this queue, if another thread
jaroslav@1890: * is currently making an element available.
jaroslav@1890: *
jaroslav@1890: * @return the head of this queue, or null if no
jaroslav@1890: * element is available.
jaroslav@1890: */
jaroslav@1890: public E poll() {
jaroslav@1890: return (E)transferer.transfer(null, true, 0);
jaroslav@1890: }
jaroslav@1890:
jaroslav@1890: /**
jaroslav@1890: * Always returns true.
jaroslav@1890: * A SynchronousQueue has no internal capacity.
jaroslav@1890: *
jaroslav@1890: * @return true
jaroslav@1890: */
jaroslav@1890: public boolean isEmpty() {
jaroslav@1890: return true;
jaroslav@1890: }
jaroslav@1890:
jaroslav@1890: /**
jaroslav@1890: * Always returns zero.
jaroslav@1890: * A SynchronousQueue has no internal capacity.
jaroslav@1890: *
jaroslav@1890: * @return zero.
jaroslav@1890: */
jaroslav@1890: public int size() {
jaroslav@1890: return 0;
jaroslav@1890: }
jaroslav@1890:
jaroslav@1890: /**
jaroslav@1890: * Always returns zero.
jaroslav@1890: * A SynchronousQueue has no internal capacity.
jaroslav@1890: *
jaroslav@1890: * @return zero.
jaroslav@1890: */
jaroslav@1890: public int remainingCapacity() {
jaroslav@1890: return 0;
jaroslav@1890: }
jaroslav@1890:
jaroslav@1890: /**
jaroslav@1890: * Does nothing.
jaroslav@1890: * A SynchronousQueue has no internal capacity.
jaroslav@1890: */
jaroslav@1890: public void clear() {
jaroslav@1890: }
jaroslav@1890:
jaroslav@1890: /**
jaroslav@1890: * Always returns false.
jaroslav@1890: * A SynchronousQueue has no internal capacity.
jaroslav@1890: *
jaroslav@1890: * @param o the element
jaroslav@1890: * @return false
jaroslav@1890: */
jaroslav@1890: public boolean contains(Object o) {
jaroslav@1890: return false;
jaroslav@1890: }
jaroslav@1890:
jaroslav@1890: /**
jaroslav@1890: * Always returns false.
jaroslav@1890: * A SynchronousQueue has no internal capacity.
jaroslav@1890: *
jaroslav@1890: * @param o the element to remove
jaroslav@1890: * @return false
jaroslav@1890: */
jaroslav@1890: public boolean remove(Object o) {
jaroslav@1890: return false;
jaroslav@1890: }
jaroslav@1890:
jaroslav@1890: /**
jaroslav@1890: * Returns false unless the given collection is empty.
jaroslav@1890: * A SynchronousQueue has no internal capacity.
jaroslav@1890: *
jaroslav@1890: * @param c the collection
jaroslav@1890: * @return false unless given collection is empty
jaroslav@1890: */
jaroslav@1890: public boolean containsAll(Collection> c) {
jaroslav@1890: return c.isEmpty();
jaroslav@1890: }
jaroslav@1890:
jaroslav@1890: /**
jaroslav@1890: * Always returns false.
jaroslav@1890: * A SynchronousQueue has no internal capacity.
jaroslav@1890: *
jaroslav@1890: * @param c the collection
jaroslav@1890: * @return false
jaroslav@1890: */
jaroslav@1890: public boolean removeAll(Collection> c) {
jaroslav@1890: return false;
jaroslav@1890: }
jaroslav@1890:
jaroslav@1890: /**
jaroslav@1890: * Always returns false.
jaroslav@1890: * A SynchronousQueue has no internal capacity.
jaroslav@1890: *
jaroslav@1890: * @param c the collection
jaroslav@1890: * @return false
jaroslav@1890: */
jaroslav@1890: public boolean retainAll(Collection> c) {
jaroslav@1890: return false;
jaroslav@1890: }
jaroslav@1890:
jaroslav@1890: /**
jaroslav@1890: * Always returns null.
jaroslav@1890: * A SynchronousQueue does not return elements
jaroslav@1890: * unless actively waited on.
jaroslav@1890: *
jaroslav@1890: * @return null
jaroslav@1890: */
jaroslav@1890: public E peek() {
jaroslav@1890: return null;
jaroslav@1890: }
jaroslav@1890:
jaroslav@1890: /**
jaroslav@1890: * Returns an empty iterator in which hasNext always returns
jaroslav@1890: * false.
jaroslav@1890: *
jaroslav@1890: * @return an empty iterator
jaroslav@1890: */
jaroslav@1890: public Iterator iterator() {
jaroslav@1890: return Collections.emptyIterator();
jaroslav@1890: }
jaroslav@1890:
jaroslav@1890: /**
jaroslav@1890: * Returns a zero-length array.
jaroslav@1890: * @return a zero-length array
jaroslav@1890: */
jaroslav@1890: public Object[] toArray() {
jaroslav@1890: return new Object[0];
jaroslav@1890: }
jaroslav@1890:
jaroslav@1890: /**
jaroslav@1890: * Sets the zeroeth element of the specified array to null
jaroslav@1890: * (if the array has non-zero length) and returns it.
jaroslav@1890: *
jaroslav@1890: * @param a the array
jaroslav@1890: * @return the specified array
jaroslav@1890: * @throws NullPointerException if the specified array is null
jaroslav@1890: */
jaroslav@1890: public T[] toArray(T[] a) {
jaroslav@1890: if (a.length > 0)
jaroslav@1890: a[0] = null;
jaroslav@1890: return a;
jaroslav@1890: }
jaroslav@1890:
jaroslav@1890: /**
jaroslav@1890: * @throws UnsupportedOperationException {@inheritDoc}
jaroslav@1890: * @throws ClassCastException {@inheritDoc}
jaroslav@1890: * @throws NullPointerException {@inheritDoc}
jaroslav@1890: * @throws IllegalArgumentException {@inheritDoc}
jaroslav@1890: */
jaroslav@1890: public int drainTo(Collection super E> c) {
jaroslav@1890: if (c == null)
jaroslav@1890: throw new NullPointerException();
jaroslav@1890: if (c == this)
jaroslav@1890: throw new IllegalArgumentException();
jaroslav@1890: int n = 0;
jaroslav@1890: E e;
jaroslav@1890: while ( (e = poll()) != null) {
jaroslav@1890: c.add(e);
jaroslav@1890: ++n;
jaroslav@1890: }
jaroslav@1890: return n;
jaroslav@1890: }
jaroslav@1890:
jaroslav@1890: /**
jaroslav@1890: * @throws UnsupportedOperationException {@inheritDoc}
jaroslav@1890: * @throws ClassCastException {@inheritDoc}
jaroslav@1890: * @throws NullPointerException {@inheritDoc}
jaroslav@1890: * @throws IllegalArgumentException {@inheritDoc}
jaroslav@1890: */
jaroslav@1890: public int drainTo(Collection super E> c, int maxElements) {
jaroslav@1890: if (c == null)
jaroslav@1890: throw new NullPointerException();
jaroslav@1890: if (c == this)
jaroslav@1890: throw new IllegalArgumentException();
jaroslav@1890: int n = 0;
jaroslav@1890: E e;
jaroslav@1890: while (n < maxElements && (e = poll()) != null) {
jaroslav@1890: c.add(e);
jaroslav@1890: ++n;
jaroslav@1890: }
jaroslav@1890: return n;
jaroslav@1890: }
jaroslav@1890:
jaroslav@1890: /*
jaroslav@1890: * To cope with serialization strategy in the 1.5 version of
jaroslav@1890: * SynchronousQueue, we declare some unused classes and fields
jaroslav@1890: * that exist solely to enable serializability across versions.
jaroslav@1890: * These fields are never used, so are initialized only if this
jaroslav@1890: * object is ever serialized or deserialized.
jaroslav@1890: */
jaroslav@1890:
jaroslav@1890: static class WaitQueue implements java.io.Serializable { }
jaroslav@1890: static class LifoWaitQueue extends WaitQueue {
jaroslav@1890: private static final long serialVersionUID = -3633113410248163686L;
jaroslav@1890: }
jaroslav@1890: static class FifoWaitQueue extends WaitQueue {
jaroslav@1890: private static final long serialVersionUID = -3623113410248163686L;
jaroslav@1890: }
jaroslav@1890: private ReentrantLock qlock;
jaroslav@1890: private WaitQueue waitingProducers;
jaroslav@1890: private WaitQueue waitingConsumers;
jaroslav@1890:
jaroslav@1890: /**
jaroslav@1890: * Save the state to a stream (that is, serialize it).
jaroslav@1890: *
jaroslav@1890: * @param s the stream
jaroslav@1890: */
jaroslav@1890: private void writeObject(java.io.ObjectOutputStream s)
jaroslav@1890: throws java.io.IOException {
jaroslav@1890: boolean fair = transferer instanceof TransferQueue;
jaroslav@1890: if (fair) {
jaroslav@1890: qlock = new ReentrantLock(true);
jaroslav@1890: waitingProducers = new FifoWaitQueue();
jaroslav@1890: waitingConsumers = new FifoWaitQueue();
jaroslav@1890: }
jaroslav@1890: else {
jaroslav@1890: qlock = new ReentrantLock();
jaroslav@1890: waitingProducers = new LifoWaitQueue();
jaroslav@1890: waitingConsumers = new LifoWaitQueue();
jaroslav@1890: }
jaroslav@1890: s.defaultWriteObject();
jaroslav@1890: }
jaroslav@1890:
jaroslav@1890: private void readObject(final java.io.ObjectInputStream s)
jaroslav@1890: throws java.io.IOException, ClassNotFoundException {
jaroslav@1890: s.defaultReadObject();
jaroslav@1890: if (waitingProducers instanceof FifoWaitQueue)
jaroslav@1890: transferer = new TransferQueue();
jaroslav@1890: else
jaroslav@1890: transferer = new TransferStack();
jaroslav@1890: }
jaroslav@1890:
jaroslav@1890: // Unsafe mechanics
jaroslav@1890: static long objectFieldOffset(sun.misc.Unsafe UNSAFE,
jaroslav@1890: String field, Class> klazz) {
jaroslav@1890: try {
jaroslav@1890: return UNSAFE.objectFieldOffset(klazz.getDeclaredField(field));
jaroslav@1890: } catch (NoSuchFieldException e) {
jaroslav@1890: // Convert Exception to corresponding Error
jaroslav@1890: NoSuchFieldError error = new NoSuchFieldError(field);
jaroslav@1890: error.initCause(e);
jaroslav@1890: throw error;
jaroslav@1890: }
jaroslav@1890: }
jaroslav@1890:
jaroslav@1890: }