https://openjdk.java.net/ 
/*
 * DO NOT ALTER OR REMOVE COPYRIGHT NOTICES OR THIS FILE HEADER.
 *
 * This code is free software; you can redistribute it and/or modify it
 * under the terms of the GNU General Public License version 2 only, as
 * published by the Free Software Foundation.  Oracle designates this
 * particular file as subject to the "Classpath" exception as provided
 * by Oracle in the LICENSE file that accompanied this code.
 *
 * This code is distributed in the hope that it will be useful, but WITHOUT
 * ANY WARRANTY; without even the implied warranty of MERCHANTABILITY or
 * FITNESS FOR A PARTICULAR PURPOSE.  See the GNU General Public License
 * version 2 for more details (a copy is included in the LICENSE file that
 * accompanied this code).
 *
 * You should have received a copy of the GNU General Public License version
 * 2 along with this work; if not, write to the Free Software Foundation,
 * Inc., 51 Franklin St, Fifth Floor, Boston, MA 02110-1301 USA.
 *
 * Please contact Oracle, 500 Oracle Parkway, Redwood Shores, CA 94065 USA
 * or visit www.oracle.com if you need additional information or have any
 * questions.
 */

/*
 * This file is available under and governed by the GNU General Public
 * License version 2 only, as published by the Free Software Foundation.
 * However, the following notice accompanied the original version of this
 * file:
 *
 * Written by Doug Lea, Bill Scherer, and Michael Scott with
 * assistance from members of JCP JSR-166 Expert Group and released to
 * the public domain, as explained at
 * http://creativecommons.org/publicdomain/zero/1.0/
 */

package java.util.concurrent;

import java.lang.invoke.MethodHandles;
import java.lang.invoke.VarHandle;
import java.util.AbstractQueue;
import java.util.Collection;
import java.util.Collections;
import java.util.Iterator;
import java.util.Objects;
import java.util.Spliterator;
import java.util.Spliterators;
import java.util.concurrent.locks.LockSupport;
import java.util.concurrent.locks.ReentrantLock;


A blocking queue in which each insert operation must wait for a corresponding remove operation by another thread, and vice versa. A synchronous queue does not have any internal capacity, not even a capacity of one. You cannot peek at a synchronous queue because an element is only present when you try to remove it; you cannot insert an element (using any method) unless another thread is trying to remove it; you cannot iterate as there is nothing to iterate. The head of the queue is the element that the first queued inserting thread is trying to add to the queue; if there is no such queued thread then no element is available for removal and poll() will return null. For purposes of other Collection methods (for example contains), a SynchronousQueue acts as an empty collection. This queue does not permit null elements. 
Synchronous queues are similar to rendezvous channels used in
CSP and Ada. They are well suited for handoff designs, in which an
object running in one thread must sync up with an object running
in another thread in order to hand it some information, event, or
task.

This class supports an optional fairness policy for ordering waiting producer and consumer threads. By default, this ordering is not guaranteed. However, a queue constructed with fairness set to true grants threads access in FIFO order. 
This class and its iterator implement all of the optional methods of the Collection and Iterator interfaces. 
This class is a member of the

Java Collections Framework.

Author:Doug Lea and Bill Scherer and Michael Scott
Type parameters:
  • <E> – the type of elements held in this queue
Since:1.5
/**
 * A {@linkplain BlockingQueue blocking queue} in which each insert
 * operation must wait for a corresponding remove operation by another
 * thread, and vice versa.  A synchronous queue does not have any
 * internal capacity, not even a capacity of one.  You cannot
 * {@code peek} at a synchronous queue because an element is only
 * present when you try to remove it; you cannot insert an element
 * (using any method) unless another thread is trying to remove it;
 * you cannot iterate as there is nothing to iterate.  The
 * <em>head</em> of the queue is the element that the first queued
 * inserting thread is trying to add to the queue; if there is no such
 * queued thread then no element is available for removal and
 * {@code poll()} will return {@code null}.  For purposes of other
 * {@code Collection} methods (for example {@code contains}), a
 * {@code SynchronousQueue} acts as an empty collection.  This queue
 * does not permit {@code null} elements.
 *
 * <p>Synchronous queues are similar to rendezvous channels used in
 * CSP and Ada. They are well suited for handoff designs, in which an
 * object running in one thread must sync up with an object running
 * in another thread in order to hand it some information, event, or
 * task.
 *
 * <p>This class supports an optional fairness policy for ordering
 * waiting producer and consumer threads.  By default, this ordering
 * is not guaranteed. However, a queue constructed with fairness set
 * to {@code true} grants threads access in FIFO order.
 *
 * <p>This class and its iterator implement all of the <em>optional</em>
 * methods of the {@link Collection} and {@link Iterator} interfaces.
 *
 * <p>This class is a member of the
 * <a href="{@docRoot}/java.base/java/util/package-summary.html#CollectionsFramework">
 * Java Collections Framework</a>.
 *
 * @since 1.5
 * @author Doug Lea and Bill Scherer and Michael Scott
 * @param <E> the type of elements held in this queue
 */
public class SynchronousQueue<E> extends AbstractQueue<E>
    implements BlockingQueue<E>, java.io.Serializable {
    private static final long serialVersionUID = -3223113410248163686L;

    /*
     * This class implements extensions of the dual stack and dual
     * queue algorithms described in "Nonblocking Concurrent Objects
     * with Condition Synchronization", by W. N. Scherer III and
     * M. L. Scott.  18th Annual Conf. on Distributed Computing,
     * Oct. 2004 (see also
     * http://www.cs.rochester.edu/u/scott/synchronization/pseudocode/duals.html).
     * The (Lifo) stack is used for non-fair mode, and the (Fifo)
     * queue for fair mode. The performance of the two is generally
     * similar. Fifo usually supports higher throughput under
     * contention but Lifo maintains higher thread locality in common
     * applications.
     *
     * A dual queue (and similarly stack) is one that at any given
     * time either holds "data" -- items provided by put operations,
     * or "requests" -- slots representing take operations, or is
     * empty. A call to "fulfill" (i.e., a call requesting an item
     * from a queue holding data or vice versa) dequeues a
     * complementary node.  The most interesting feature of these
     * queues is that any operation can figure out which mode the
     * queue is in, and act accordingly without needing locks.
     *
     * Both the queue and stack extend abstract class Transferer
     * defining the single method transfer that does a put or a
     * take. These are unified into a single method because in dual
     * data structures, the put and take operations are symmetrical,
     * so nearly all code can be combined. The resulting transfer
     * methods are on the long side, but are easier to follow than
     * they would be if broken up into nearly-duplicated parts.
     *
     * The queue and stack data structures share many conceptual
     * similarities but very few concrete details. For simplicity,
     * they are kept distinct so that they can later evolve
     * separately.
     *
     * The algorithms here differ from the versions in the above paper
     * in extending them for use in synchronous queues, as well as
     * dealing with cancellation. The main differences include:
     *
     *  1. The original algorithms used bit-marked pointers, but
     *     the ones here use mode bits in nodes, leading to a number
     *     of further adaptations.
     *  2. SynchronousQueues must block threads waiting to become
     *     fulfilled.
     *  3. Support for cancellation via timeout and interrupts,
     *     including cleaning out cancelled nodes/threads
     *     from lists to avoid garbage retention and memory depletion.
     *
     * Blocking is mainly accomplished using LockSupport park/unpark,
     * except that nodes that appear to be the next ones to become
     * fulfilled first spin a bit (on multiprocessors only). On very
     * busy synchronous queues, spinning can dramatically improve
     * throughput. And on less busy ones, the amount of spinning is
     * small enough not to be noticeable.
     *
     * Cleaning is done in different ways in queues vs stacks.  For
     * queues, we can almost always remove a node immediately in O(1)
     * time (modulo retries for consistency checks) when it is
     * cancelled. But if it may be pinned as the current tail, it must
     * wait until some subsequent cancellation. For stacks, we need a
     * potentially O(n) traversal to be sure that we can remove the
     * node, but this can run concurrently with other threads
     * accessing the stack.
     *
     * While garbage collection takes care of most node reclamation
     * issues that otherwise complicate nonblocking algorithms, care
     * is taken to "forget" references to data, other nodes, and
     * threads that might be held on to long-term by blocked
     * threads. In cases where setting to null would otherwise
     * conflict with main algorithms, this is done by changing a
     * node's link to now point to the node itself. This doesn't arise
     * much for Stack nodes (because blocked threads do not hang on to
     * old head pointers), but references in Queue nodes must be
     * aggressively forgotten to avoid reachability of everything any
     * node has ever referred to since arrival.
     */

    
Shared internal API for dual stacks and queues.

/**
     * Shared internal API for dual stacks and queues.
     */
    abstract static class Transferer<E> {
        
Performs a put or take.

Params:
  • e – if non-null, the item to be handed to a consumer;
         if null, requests that transfer return an item
         offered by producer.
  • timed – if this operation should timeout
  • nanos – the timeout, in nanoseconds
Returns:if non-null, the item provided or received; if null,
        the operation failed due to timeout or interrupt --
        the caller can distinguish which of these occurred
        by checking Thread.interrupted.
/**
         * Performs a put or take.
         *
         * @param e if non-null, the item to be handed to a consumer;
         *          if null, requests that transfer return an item
         *          offered by producer.
         * @param timed if this operation should timeout
         * @param nanos the timeout, in nanoseconds
         * @return if non-null, the item provided or received; if null,
         *         the operation failed due to timeout or interrupt --
         *         the caller can distinguish which of these occurred
         *         by checking Thread.interrupted.
         */
        abstract E transfer(E e, boolean timed, long nanos);
    }

    
The number of times to spin before blocking in timed waits.
The value is empirically derived -- it works well across a
variety of processors and OSes. Empirically, the best value
seems not to vary with number of CPUs (beyond 2) so is just
a constant.

/**
     * The number of times to spin before blocking in timed waits.
     * The value is empirically derived -- it works well across a
     * variety of processors and OSes. Empirically, the best value
     * seems not to vary with number of CPUs (beyond 2) so is just
     * a constant.
     */
    static final int MAX_TIMED_SPINS =
        (Runtime.getRuntime().availableProcessors() < 2) ? 0 : 32;

    
The number of times to spin before blocking in untimed waits.
This is greater than timed value because untimed waits spin
faster since they don't need to check times on each spin.

/**
     * The number of times to spin before blocking in untimed waits.
     * This is greater than timed value because untimed waits spin
     * faster since they don't need to check times on each spin.
     */
    static final int MAX_UNTIMED_SPINS = MAX_TIMED_SPINS * 16;

    
The number of nanoseconds for which it is faster to spin
rather than to use timed park. A rough estimate suffices.

/**
     * The number of nanoseconds for which it is faster to spin
     * rather than to use timed park. A rough estimate suffices.
     */
    static final long SPIN_FOR_TIMEOUT_THRESHOLD = 1000L;

    
Dual stack 
/** Dual stack */
    static final class TransferStack<E> extends Transferer<E> {
        /*
         * This extends Scherer-Scott dual stack algorithm, differing,
         * among other ways, by using "covering" nodes rather than
         * bit-marked pointers: Fulfilling operations push on marker
         * nodes (with FULFILLING bit set in mode) to reserve a spot
         * to match a waiting node.
         */

        /* Modes for SNodes, ORed together in node fields */
        
Node represents an unfulfilled consumer 
/** Node represents an unfulfilled consumer */
        static final int REQUEST    = 0;
        
Node represents an unfulfilled producer 
/** Node represents an unfulfilled producer */
        static final int DATA       = 1;
        
Node is fulfilling another unfulfilled DATA or REQUEST 
/** Node is fulfilling another unfulfilled DATA or REQUEST */
        static final int FULFILLING = 2;

        
Returns true if m has fulfilling bit set. 
/** Returns true if m has fulfilling bit set. */
        static boolean isFulfilling(int m) { return (m & FULFILLING) != 0; }

        
Node class for TransferStacks. 
/** Node class for TransferStacks. */
        static final class SNode {
            volatile SNode next;        // next node in stack
            volatile SNode match;       // the node matched to this
            volatile Thread waiter;     // to control park/unpark
            Object item;                // data; or null for REQUESTs
            int mode;
            // Note: item and mode fields don't need to be volatile
            // since they are always written before, and read after,
            // other volatile/atomic operations.

            SNode(Object item) {
                this.item = item;
            }

            boolean casNext(SNode cmp, SNode val) {
                return cmp == next &&
                    SNEXT.compareAndSet(this, cmp, val);
            }

            
Tries to match node s to this node, if so, waking up thread.
Fulfillers call tryMatch to identify their waiters.
Waiters block until they have been matched.

Params:
  • s – the node to match
Returns:true if successfully matched to s
/**
             * Tries to match node s to this node, if so, waking up thread.
             * Fulfillers call tryMatch to identify their waiters.
             * Waiters block until they have been matched.
             *
             * @param s the node to match
             * @return true if successfully matched to s
             */
            boolean tryMatch(SNode s) {
                if (match == null &&
                    SMATCH.compareAndSet(this, null, s)) {
                    Thread w = waiter;
                    if (w != null) {    // waiters need at most one unpark
                        waiter = null;
                        LockSupport.unpark(w);
                    }
                    return true;
                }
                return match == s;
            }

            
Tries to cancel a wait by matching node to itself.

/**
             * Tries to cancel a wait by matching node to itself.
             */
            void tryCancel() {
                SMATCH.compareAndSet(this, null, this);
            }

            boolean isCancelled() {
                return match == this;
            }

            // VarHandle mechanics
            private static final VarHandle SMATCH;
            private static final VarHandle SNEXT;
            static {
                try {
                    MethodHandles.Lookup l = MethodHandles.lookup();
                    SMATCH = l.findVarHandle(SNode.class, "match", SNode.class);
                    SNEXT = l.findVarHandle(SNode.class, "next", SNode.class);
                } catch (ReflectiveOperationException e) {
                    throw new ExceptionInInitializerError(e);
                }
            }
        }

        
The head (top) of the stack 
/** The head (top) of the stack */
        volatile SNode head;

        boolean casHead(SNode h, SNode nh) {
            return h == head &&
                SHEAD.compareAndSet(this, h, nh);
        }

        
Creates or resets fields of a node. Called only from transfer
where the node to push on stack is lazily created and
reused when possible to help reduce intervals between reads
and CASes of head and to avoid surges of garbage when CASes
to push nodes fail due to contention.

/**
         * Creates or resets fields of a node. Called only from transfer
         * where the node to push on stack is lazily created and
         * reused when possible to help reduce intervals between reads
         * and CASes of head and to avoid surges of garbage when CASes
         * to push nodes fail due to contention.
         */
        static SNode snode(SNode s, Object e, SNode next, int mode) {
            if (s == null) s = new SNode(e);
            s.mode = mode;
            s.next = next;
            return s;
        }

        
Puts or takes an item.

/**
         * Puts or takes an item.
         */
        @SuppressWarnings("unchecked")
        E transfer(E e, boolean timed, long nanos) {
            /*
             * Basic algorithm is to loop trying one of three actions:
             *
             * 1. If apparently empty or already containing nodes of same
             *    mode, try to push node on stack and wait for a match,
             *    returning it, or null if cancelled.
             *
             * 2. If apparently containing node of complementary mode,
             *    try to push a fulfilling node on to stack, match
             *    with corresponding waiting node, pop both from
             *    stack, and return matched item. The matching or
             *    unlinking might not actually be necessary because of
             *    other threads performing action 3:
             *
             * 3. If top of stack already holds another fulfilling node,
             *    help it out by doing its match and/or pop
             *    operations, and then continue. The code for helping
             *    is essentially the same as for fulfilling, except
             *    that it doesn't return the item.
             */

            SNode s = null; // constructed/reused as needed
            int mode = (e == null) ? REQUEST : DATA;

            for (;;) {
                SNode h = head;
                if (h == null || h.mode == mode) {  // empty or same-mode
                    if (timed && nanos <= 0L) {     // can't wait
                        if (h != null && h.isCancelled())
                            casHead(h, h.next);     // pop cancelled node
                        else
                            return null;
                    } else if (casHead(h, s = snode(s, e, h, mode))) {
                        SNode m = awaitFulfill(s, timed, nanos);
                        if (m == s) {               // wait was cancelled
                            clean(s);
                            return null;
                        }
                        if ((h = head) != null && h.next == s)
                            casHead(h, s.next);     // help s's fulfiller
                        return (E) ((mode == REQUEST) ? m.item : s.item);
                    }
                } else if (!isFulfilling(h.mode)) { // try to fulfill
                    if (h.isCancelled())            // already cancelled
                        casHead(h, h.next);         // pop and retry
                    else if (casHead(h, s=snode(s, e, h, FULFILLING|mode))) {
                        for (;;) { // loop until matched or waiters disappear
                            SNode m = s.next;       // m is s's match
                            if (m == null) {        // all waiters are gone
                                casHead(s, null);   // pop fulfill node
                                s = null;           // use new node next time
                                break;              // restart main loop
                            }
                            SNode mn = m.next;
                            if (m.tryMatch(s)) {
                                casHead(s, mn);     // pop both s and m
                                return (E) ((mode == REQUEST) ? m.item : s.item);
                            } else                  // lost match
                                s.casNext(m, mn);   // help unlink
                        }
                    }
                } else {                            // help a fulfiller
                    SNode m = h.next;               // m is h's match
                    if (m == null)                  // waiter is gone
                        casHead(h, null);           // pop fulfilling node
                    else {
                        SNode mn = m.next;
                        if (m.tryMatch(h))          // help match
                            casHead(h, mn);         // pop both h and m
                        else                        // lost match
                            h.casNext(m, mn);       // help unlink
                    }
                }
            }
        }

        
Spins/blocks until node s is matched by a fulfill operation.

Params:
  • s – the waiting node
  • timed – true if timed wait
  • nanos – timeout value
Returns:matched node, or s if cancelled
/**
         * Spins/blocks until node s is matched by a fulfill operation.
         *
         * @param s the waiting node
         * @param timed true if timed wait
         * @param nanos timeout value
         * @return matched node, or s if cancelled
         */
        SNode awaitFulfill(SNode s, boolean timed, long nanos) {
            /*
             * When a node/thread is about to block, it sets its waiter
             * field and then rechecks state at least one more time
             * before actually parking, thus covering race vs
             * fulfiller noticing that waiter is non-null so should be
             * woken.
             *
             * When invoked by nodes that appear at the point of call
             * to be at the head of the stack, calls to park are
             * preceded by spins to avoid blocking when producers and
             * consumers are arriving very close in time.  This can
             * happen enough to bother only on multiprocessors.
             *
             * The order of checks for returning out of main loop
             * reflects fact that interrupts have precedence over
             * normal returns, which have precedence over
             * timeouts. (So, on timeout, one last check for match is
             * done before giving up.) Except that calls from untimed
             * SynchronousQueue.{poll/offer} don't check interrupts
             * and don't wait at all, so are trapped in transfer
             * method rather than calling awaitFulfill.
             */
            final long deadline = timed ? System.nanoTime() + nanos : 0L;
            Thread w = Thread.currentThread();
            int spins = shouldSpin(s)
                ? (timed ? MAX_TIMED_SPINS : MAX_UNTIMED_SPINS)
                : 0;
            for (;;) {
                if (w.isInterrupted())
                    s.tryCancel();
                SNode m = s.match;
                if (m != null)
                    return m;
                if (timed) {
                    nanos = deadline - System.nanoTime();
                    if (nanos <= 0L) {
                        s.tryCancel();
                        continue;
                    }
                }
                if (spins > 0) {
                    Thread.onSpinWait();
                    spins = shouldSpin(s) ? (spins - 1) : 0;
                }
                else if (s.waiter == null)
                    s.waiter = w; // establish waiter so can park next iter
                else if (!timed)
                    LockSupport.park(this);
                else if (nanos > SPIN_FOR_TIMEOUT_THRESHOLD)
                    LockSupport.parkNanos(this, nanos);
            }
        }

        
Returns true if node s is at head or there is an active
fulfiller.

/**
         * Returns true if node s is at head or there is an active
         * fulfiller.
         */
        boolean shouldSpin(SNode s) {
            SNode h = head;
            return (h == s || h == null || isFulfilling(h.mode));
        }

        
Unlinks s from the stack.

/**
         * Unlinks s from the stack.
         */
        void clean(SNode s) {
            s.item = null;   // forget item
            s.waiter = null; // forget thread

            /*
             * At worst we may need to traverse entire stack to unlink
             * s. If there are multiple concurrent calls to clean, we
             * might not see s if another thread has already removed
             * it. But we can stop when we see any node known to
             * follow s. We use s.next unless it too is cancelled, in
             * which case we try the node one past. We don't check any
             * further because we don't want to doubly traverse just to
             * find sentinel.
             */

            SNode past = s.next;
            if (past != null && past.isCancelled())
                past = past.next;

            // Absorb cancelled nodes at head
            SNode p;
            while ((p = head) != null && p != past && p.isCancelled())
                casHead(p, p.next);

            // Unsplice embedded nodes
            while (p != null && p != past) {
                SNode n = p.next;
                if (n != null && n.isCancelled())
                    p.casNext(n, n.next);
                else
                    p = n;
            }
        }

        // VarHandle mechanics
        private static final VarHandle SHEAD;
        static {
            try {
                MethodHandles.Lookup l = MethodHandles.lookup();
                SHEAD = l.findVarHandle(TransferStack.class, "head", SNode.class);
            } catch (ReflectiveOperationException e) {
                throw new ExceptionInInitializerError(e);
            }
        }
    }

    
Dual Queue 
/** Dual Queue */
    static final class TransferQueue<E> extends Transferer<E> {
        /*
         * This extends Scherer-Scott dual queue algorithm, differing,
         * among other ways, by using modes within nodes rather than
         * marked pointers. The algorithm is a little simpler than
         * that for stacks because fulfillers do not need explicit
         * nodes, and matching is done by CAS'ing QNode.item field
         * from non-null to null (for put) or vice versa (for take).
         */

        
Node class for TransferQueue. 
/** Node class for TransferQueue. */
        static final class QNode {
            volatile QNode next;          // next node in queue
            volatile Object item;         // CAS'ed to or from null
            volatile Thread waiter;       // to control park/unpark
            final boolean isData;

            QNode(Object item, boolean isData) {
                this.item = item;
                this.isData = isData;
            }

            boolean casNext(QNode cmp, QNode val) {
                return next == cmp &&
                    QNEXT.compareAndSet(this, cmp, val);
            }

            boolean casItem(Object cmp, Object val) {
                return item == cmp &&
                    QITEM.compareAndSet(this, cmp, val);
            }

            
Tries to cancel by CAS'ing ref to this as item.

/**
             * Tries to cancel by CAS'ing ref to this as item.
             */
            void tryCancel(Object cmp) {
                QITEM.compareAndSet(this, cmp, this);
            }

            boolean isCancelled() {
                return item == this;
            }

            
Returns true if this node is known to be off the queue
because its next pointer has been forgotten due to
an advanceHead operation.

/**
             * Returns true if this node is known to be off the queue
             * because its next pointer has been forgotten due to
             * an advanceHead operation.
             */
            boolean isOffList() {
                return next == this;
            }

            // VarHandle mechanics
            private static final VarHandle QITEM;
            private static final VarHandle QNEXT;
            static {
                try {
                    MethodHandles.Lookup l = MethodHandles.lookup();
                    QITEM = l.findVarHandle(QNode.class, "item", Object.class);
                    QNEXT = l.findVarHandle(QNode.class, "next", QNode.class);
                } catch (ReflectiveOperationException e) {
                    throw new ExceptionInInitializerError(e);
                }
            }
        }

        
Head of queue 
/** Head of queue */
        transient volatile QNode head;
        
Tail of queue 
/** Tail of queue */
        transient volatile QNode tail;
        
Reference to a cancelled node that might not yet have been
unlinked from queue because it was the last inserted node
when it was cancelled.

/**
         * Reference to a cancelled node that might not yet have been
         * unlinked from queue because it was the last inserted node
         * when it was cancelled.
         */
        transient volatile QNode cleanMe;

        TransferQueue() {
            QNode h = new QNode(null, false); // initialize to dummy node.
            head = h;
            tail = h;
        }

        
Tries to cas nh as new head; if successful, unlink
old head's next node to avoid garbage retention.

/**
         * Tries to cas nh as new head; if successful, unlink
         * old head's next node to avoid garbage retention.
         */
        void advanceHead(QNode h, QNode nh) {
            if (h == head &&
                QHEAD.compareAndSet(this, h, nh))
                h.next = h; // forget old next
        }

        
Tries to cas nt as new tail.

/**
         * Tries to cas nt as new tail.
         */
        void advanceTail(QNode t, QNode nt) {
            if (tail == t)
                QTAIL.compareAndSet(this, t, nt);
        }

        
Tries to CAS cleanMe slot.

/**
         * Tries to CAS cleanMe slot.
         */
        boolean casCleanMe(QNode cmp, QNode val) {
            return cleanMe == cmp &&
                QCLEANME.compareAndSet(this, cmp, val);
        }

        
Puts or takes an item.

/**
         * Puts or takes an item.
         */
        @SuppressWarnings("unchecked")
        E transfer(E e, boolean timed, long nanos) {
            /* Basic algorithm is to loop trying to take either of
             * two actions:
             *
             * 1. If queue apparently empty or holding same-mode nodes,
             *    try to add node to queue of waiters, wait to be
             *    fulfilled (or cancelled) and return matching item.
             *
             * 2. If queue apparently contains waiting items, and this
             *    call is of complementary mode, try to fulfill by CAS'ing
             *    item field of waiting node and dequeuing it, and then
             *    returning matching item.
             *
             * In each case, along the way, check for and try to help
             * advance head and tail on behalf of other stalled/slow
             * threads.
             *
             * The loop starts off with a null check guarding against
             * seeing uninitialized head or tail values. This never
             * happens in current SynchronousQueue, but could if
             * callers held non-volatile/final ref to the
             * transferer. The check is here anyway because it places
             * null checks at top of loop, which is usually faster
             * than having them implicitly interspersed.
             */

            QNode s = null; // constructed/reused as needed
            boolean isData = (e != null);

            for (;;) {
                QNode t = tail;
                QNode h = head;
                if (t == null || h == null)         // saw uninitialized value
                    continue;                       // spin

                if (h == t || t.isData == isData) { // empty or same-mode
                    QNode tn = t.next;
                    if (t != tail)                  // inconsistent read
                        continue;
                    if (tn != null) {               // lagging tail
                        advanceTail(t, tn);
                        continue;
                    }
                    if (timed && nanos <= 0L)       // can't wait
                        return null;
                    if (s == null)
                        s = new QNode(e, isData);
                    if (!t.casNext(null, s))        // failed to link in
                        continue;

                    advanceTail(t, s);              // swing tail and wait
                    Object x = awaitFulfill(s, e, timed, nanos);
                    if (x == s) {                   // wait was cancelled
                        clean(t, s);
                        return null;
                    }

                    if (!s.isOffList()) {           // not already unlinked
                        advanceHead(t, s);          // unlink if head
                        if (x != null)              // and forget fields
                            s.item = s;
                        s.waiter = null;
                    }
                    return (x != null) ? (E)x : e;

                } else {                            // complementary-mode
                    QNode m = h.next;               // node to fulfill
                    if (t != tail || m == null || h != head)
                        continue;                   // inconsistent read

                    Object x = m.item;
                    if (isData == (x != null) ||    // m already fulfilled
                        x == m ||                   // m cancelled
                        !m.casItem(x, e)) {         // lost CAS
                        advanceHead(h, m);          // dequeue and retry
                        continue;
                    }

                    advanceHead(h, m);              // successfully fulfilled
                    LockSupport.unpark(m.waiter);
                    return (x != null) ? (E)x : e;
                }
            }
        }

        
Spins/blocks until node s is fulfilled.

Params:
  • s – the waiting node
  • e – the comparison value for checking match
  • timed – true if timed wait
  • nanos – timeout value
Returns:matched item, or s if cancelled
/**
         * Spins/blocks until node s is fulfilled.
         *
         * @param s the waiting node
         * @param e the comparison value for checking match
         * @param timed true if timed wait
         * @param nanos timeout value
         * @return matched item, or s if cancelled
         */
        Object awaitFulfill(QNode s, E e, boolean timed, long nanos) {
            /* Same idea as TransferStack.awaitFulfill */
            final long deadline = timed ? System.nanoTime() + nanos : 0L;
            Thread w = Thread.currentThread();
            int spins = (head.next == s)
                ? (timed ? MAX_TIMED_SPINS : MAX_UNTIMED_SPINS)
                : 0;
            for (;;) {
                if (w.isInterrupted())
                    s.tryCancel(e);
                Object x = s.item;
                if (x != e)
                    return x;
                if (timed) {
                    nanos = deadline - System.nanoTime();
                    if (nanos <= 0L) {
                        s.tryCancel(e);
                        continue;
                    }
                }
                if (spins > 0) {
                    --spins;
                    Thread.onSpinWait();
                }
                else if (s.waiter == null)
                    s.waiter = w;
                else if (!timed)
                    LockSupport.park(this);
                else if (nanos > SPIN_FOR_TIMEOUT_THRESHOLD)
                    LockSupport.parkNanos(this, nanos);
            }
        }

        
Gets rid of cancelled node s with original predecessor pred.

/**
         * Gets rid of cancelled node s with original predecessor pred.
         */
        void clean(QNode pred, QNode s) {
            s.waiter = null; // forget thread
            /*
             * At any given time, exactly one node on list cannot be
             * deleted -- the last inserted node. To accommodate this,
             * if we cannot delete s, we save its predecessor as
             * "cleanMe", deleting the previously saved version
             * first. At least one of node s or the node previously
             * saved can always be deleted, so this always terminates.
             */
            while (pred.next == s) { // Return early if already unlinked
                QNode h = head;
                QNode hn = h.next;   // Absorb cancelled first node as head
                if (hn != null && hn.isCancelled()) {
                    advanceHead(h, hn);
                    continue;
                }
                QNode t = tail;      // Ensure consistent read for tail
                if (t == h)
                    return;
                QNode tn = t.next;
                if (t != tail)
                    continue;
                if (tn != null) {
                    advanceTail(t, tn);
                    continue;
                }
                if (s != t) {        // If not tail, try to unsplice
                    QNode sn = s.next;
                    if (sn == s || pred.casNext(s, sn))
                        return;
                }
                QNode dp = cleanMe;
                if (dp != null) {    // Try unlinking previous cancelled node
                    QNode d = dp.next;
                    QNode dn;
                    if (d == null ||               // d is gone or
                        d == dp ||                 // d is off list or
                        !d.isCancelled() ||        // d not cancelled or
                        (d != t &&                 // d not tail and
                         (dn = d.next) != null &&  //   has successor
                         dn != d &&                //   that is on list
                         dp.casNext(d, dn)))       // d unspliced
                        casCleanMe(dp, null);
                    if (dp == pred)
                        return;      // s is already saved node
                } else if (casCleanMe(null, pred))
                    return;          // Postpone cleaning s
            }
        }

        // VarHandle mechanics
        private static final VarHandle QHEAD;
        private static final VarHandle QTAIL;
        private static final VarHandle QCLEANME;
        static {
            try {
                MethodHandles.Lookup l = MethodHandles.lookup();
                QHEAD = l.findVarHandle(TransferQueue.class, "head",
                                        QNode.class);
                QTAIL = l.findVarHandle(TransferQueue.class, "tail",
                                        QNode.class);
                QCLEANME = l.findVarHandle(TransferQueue.class, "cleanMe",
                                           QNode.class);
            } catch (ReflectiveOperationException e) {
                throw new ExceptionInInitializerError(e);
            }
        }
    }

    
The transferer. Set only in constructor, but cannot be declared
as final without further complicating serialization.  Since
this is accessed only at most once per public method, there
isn't a noticeable performance penalty for using volatile
instead of final here.

/**
     * The transferer. Set only in constructor, but cannot be declared
     * as final without further complicating serialization.  Since
     * this is accessed only at most once per public method, there
     * isn't a noticeable performance penalty for using volatile
     * instead of final here.
     */
    private transient volatile Transferer<E> transferer;

    
Creates a SynchronousQueue with nonfair access policy. 
/**
     * Creates a {@code SynchronousQueue} with nonfair access policy.
     */
    public SynchronousQueue() {
        this(false);
    }

    
Creates a SynchronousQueue with the specified fairness policy. 
Params:
  • fair – if true, waiting threads contend in FIFO order for
       access; otherwise the order is unspecified.
/**
     * Creates a {@code SynchronousQueue} with the specified fairness policy.
     *
     * @param fair if true, waiting threads contend in FIFO order for
     *        access; otherwise the order is unspecified.
     */
    public SynchronousQueue(boolean fair) {
        transferer = fair ? new TransferQueue<E>() : new TransferStack<E>();
    }

    
Adds the specified element to this queue, waiting if necessary for
another thread to receive it.

Throws:
/**
     * Adds the specified element to this queue, waiting if necessary for
     * another thread to receive it.
     *
     * @throws InterruptedException {@inheritDoc}
     * @throws NullPointerException {@inheritDoc}
     */
    public void put(E e) throws InterruptedException {
        if (e == null) throw new NullPointerException();
        if (transferer.transfer(e, false, 0) == null) {
            Thread.interrupted();
            throw new InterruptedException();
        }
    }

    
Inserts the specified element into this queue, waiting if necessary
up to the specified wait time for another thread to receive it.

Throws:
Returns:true if successful, or false if the specified waiting time elapses before a consumer appears
/**
     * Inserts the specified element into this queue, waiting if necessary
     * up to the specified wait time for another thread to receive it.
     *
     * @return {@code true} if successful, or {@code false} if the
     *         specified waiting time elapses before a consumer appears
     * @throws InterruptedException {@inheritDoc}
     * @throws NullPointerException {@inheritDoc}
     */
    public boolean offer(E e, long timeout, TimeUnit unit)
        throws InterruptedException {
        if (e == null) throw new NullPointerException();
        if (transferer.transfer(e, true, unit.toNanos(timeout)) != null)
            return true;
        if (!Thread.interrupted())
            return false;
        throw new InterruptedException();
    }

    
Inserts the specified element into this queue, if another thread is
waiting to receive it.

Params:
  • e – the element to add
Throws:
Returns:true if the element was added to this queue, else false
/**
     * Inserts the specified element into this queue, if another thread is
     * waiting to receive it.
     *
     * @param e the element to add
     * @return {@code true} if the element was added to this queue, else
     *         {@code false}
     * @throws NullPointerException if the specified element is null
     */
    public boolean offer(E e) {
        if (e == null) throw new NullPointerException();
        return transferer.transfer(e, true, 0) != null;
    }

    
Retrieves and removes the head of this queue, waiting if necessary
for another thread to insert it.

Throws:
Returns:the head of this queue
/**
     * Retrieves and removes the head of this queue, waiting if necessary
     * for another thread to insert it.
     *
     * @return the head of this queue
     * @throws InterruptedException {@inheritDoc}
     */
    public E take() throws InterruptedException {
        E e = transferer.transfer(null, false, 0);
        if (e != null)
            return e;
        Thread.interrupted();
        throw new InterruptedException();
    }

    
Retrieves and removes the head of this queue, waiting
if necessary up to the specified wait time, for another thread
to insert it.

Throws:
Returns:the head of this queue, or null if the specified waiting time elapses before an element is present
/**
     * Retrieves and removes the head of this queue, waiting
     * if necessary up to the specified wait time, for another thread
     * to insert it.
     *
     * @return the head of this queue, or {@code null} if the
     *         specified waiting time elapses before an element is present
     * @throws InterruptedException {@inheritDoc}
     */
    public E poll(long timeout, TimeUnit unit) throws InterruptedException {
        E e = transferer.transfer(null, true, unit.toNanos(timeout));
        if (e != null || !Thread.interrupted())
            return e;
        throw new InterruptedException();
    }

    
Retrieves and removes the head of this queue, if another thread
is currently making an element available.

Returns:the head of this queue, or null if no element is available
/**
     * Retrieves and removes the head of this queue, if another thread
     * is currently making an element available.
     *
     * @return the head of this queue, or {@code null} if no
     *         element is available
     */
    public E poll() {
        return transferer.transfer(null, true, 0);
    }

    
Always returns true. A SynchronousQueue has no internal capacity. 
Returns:true
/**
     * Always returns {@code true}.
     * A {@code SynchronousQueue} has no internal capacity.
     *
     * @return {@code true}
     */
    public boolean isEmpty() {
        return true;
    }

    
Always returns zero. A SynchronousQueue has no internal capacity. 
Returns:zero
/**
     * Always returns zero.
     * A {@code SynchronousQueue} has no internal capacity.
     *
     * @return zero
     */
    public int size() {
        return 0;
    }

    
Always returns zero. A SynchronousQueue has no internal capacity. 
Returns:zero
/**
     * Always returns zero.
     * A {@code SynchronousQueue} has no internal capacity.
     *
     * @return zero
     */
    public int remainingCapacity() {
        return 0;
    }

    
Does nothing. A SynchronousQueue has no internal capacity. 
/**
     * Does nothing.
     * A {@code SynchronousQueue} has no internal capacity.
     */
    public void clear() {
    }

    
Always returns false. A SynchronousQueue has no internal capacity. 
Params:
  • o – the element
Returns:false
/**
     * Always returns {@code false}.
     * A {@code SynchronousQueue} has no internal capacity.
     *
     * @param o the element
     * @return {@code false}
     */
    public boolean contains(Object o) {
        return false;
    }

    
Always returns false. A SynchronousQueue has no internal capacity. 
Params:
  • o – the element to remove
Returns:false
/**
     * Always returns {@code false}.
     * A {@code SynchronousQueue} has no internal capacity.
     *
     * @param o the element to remove
     * @return {@code false}
     */
    public boolean remove(Object o) {
        return false;
    }

    
Returns false unless the given collection is empty. A SynchronousQueue has no internal capacity. 
Params:
  • c – the collection
Returns:false unless given collection is empty
/**
     * Returns {@code false} unless the given collection is empty.
     * A {@code SynchronousQueue} has no internal capacity.
     *
     * @param c the collection
     * @return {@code false} unless given collection is empty
     */
    public boolean containsAll(Collection<?> c) {
        return c.isEmpty();
    }

    
Always returns false. A SynchronousQueue has no internal capacity. 
Params:
  • c – the collection
Returns:false
/**
     * Always returns {@code false}.
     * A {@code SynchronousQueue} has no internal capacity.
     *
     * @param c the collection
     * @return {@code false}
     */
    public boolean removeAll(Collection<?> c) {
        return false;
    }

    
Always returns false. A SynchronousQueue has no internal capacity. 
Params:
  • c – the collection
Returns:false
/**
     * Always returns {@code false}.
     * A {@code SynchronousQueue} has no internal capacity.
     *
     * @param c the collection
     * @return {@code false}
     */
    public boolean retainAll(Collection<?> c) {
        return false;
    }

    
Always returns null. A SynchronousQueue does not return elements unless actively waited on. 
Returns:null
/**
     * Always returns {@code null}.
     * A {@code SynchronousQueue} does not return elements
     * unless actively waited on.
     *
     * @return {@code null}
     */
    public E peek() {
        return null;
    }

    
Returns an empty iterator in which hasNext always returns false. 
Returns:an empty iterator
/**
     * Returns an empty iterator in which {@code hasNext} always returns
     * {@code false}.
     *
     * @return an empty iterator
     */
    public Iterator<E> iterator() {
        return Collections.emptyIterator();
    }

    
Returns an empty spliterator in which calls to trySplit always return null. 
Returns:an empty spliterator
Since:1.8
/**
     * Returns an empty spliterator in which calls to
     * {@link Spliterator#trySplit() trySplit} always return {@code null}.
     *
     * @return an empty spliterator
     * @since 1.8
     */
    public Spliterator<E> spliterator() {
        return Spliterators.emptySpliterator();
    }

    
Returns a zero-length array.

Returns:a zero-length array
/**
     * Returns a zero-length array.
     * @return a zero-length array
     */
    public Object[] toArray() {
        return new Object[0];
    }

    
Sets the zeroth element of the specified array to null (if the array has non-zero length) and returns it. 
Params:
  • a – the array
Throws:
Returns:the specified array
/**
     * Sets the zeroth element of the specified array to {@code null}
     * (if the array has non-zero length) and returns it.
     *
     * @param a the array
     * @return the specified array
     * @throws NullPointerException if the specified array is null
     */
    public <T> T[] toArray(T[] a) {
        if (a.length > 0)
            a[0] = null;
        return a;
    }

    
Always returns "[]". 
Returns:"[]"
/**
     * Always returns {@code "[]"}.
     * @return {@code "[]"}
     */
    public String toString() {
        return "[]";
    }

    
Throws:
/**
     * @throws UnsupportedOperationException {@inheritDoc}
     * @throws ClassCastException            {@inheritDoc}
     * @throws NullPointerException          {@inheritDoc}
     * @throws IllegalArgumentException      {@inheritDoc}
     */
    public int drainTo(Collection<? super E> c) {
        Objects.requireNonNull(c);
        if (c == this)
            throw new IllegalArgumentException();
        int n = 0;
        for (E e; (e = poll()) != null; n++)
            c.add(e);
        return n;
    }

    
Throws:
/**
     * @throws UnsupportedOperationException {@inheritDoc}
     * @throws ClassCastException            {@inheritDoc}
     * @throws NullPointerException          {@inheritDoc}
     * @throws IllegalArgumentException      {@inheritDoc}
     */
    public int drainTo(Collection<? super E> c, int maxElements) {
        Objects.requireNonNull(c);
        if (c == this)
            throw new IllegalArgumentException();
        int n = 0;
        for (E e; n < maxElements && (e = poll()) != null; n++)
            c.add(e);
        return n;
    }

    /*
     * To cope with serialization strategy in the 1.5 version of
     * SynchronousQueue, we declare some unused classes and fields
     * that exist solely to enable serializability across versions.
     * These fields are never used, so are initialized only if this
     * object is ever serialized or deserialized.
     */

    @SuppressWarnings("serial")
    static class WaitQueue implements java.io.Serializable { }
    static class LifoWaitQueue extends WaitQueue {
        private static final long serialVersionUID = -3633113410248163686L;
    }
    static class FifoWaitQueue extends WaitQueue {
        private static final long serialVersionUID = -3623113410248163686L;
    }
    private ReentrantLock qlock;
    private WaitQueue waitingProducers;
    private WaitQueue waitingConsumers;

    
Saves this queue to a stream (that is, serializes it).

Params:
  • s – the stream
Throws:
/**
     * Saves this queue to a stream (that is, serializes it).
     * @param s the stream
     * @throws java.io.IOException if an I/O error occurs
     */
    private void writeObject(java.io.ObjectOutputStream s)
        throws java.io.IOException {
        boolean fair = transferer instanceof TransferQueue;
        if (fair) {
            qlock = new ReentrantLock(true);
            waitingProducers = new FifoWaitQueue();
            waitingConsumers = new FifoWaitQueue();
        }
        else {
            qlock = new ReentrantLock();
            waitingProducers = new LifoWaitQueue();
            waitingConsumers = new LifoWaitQueue();
        }
        s.defaultWriteObject();
    }

    
Reconstitutes this queue from a stream (that is, deserializes it).

Params:
  • s – the stream
Throws:
/**
     * Reconstitutes this queue from a stream (that is, deserializes it).
     * @param s the stream
     * @throws ClassNotFoundException if the class of a serialized object
     *         could not be found
     * @throws java.io.IOException if an I/O error occurs
     */
    private void readObject(java.io.ObjectInputStream s)
        throws java.io.IOException, ClassNotFoundException {
        s.defaultReadObject();
        if (waitingProducers instanceof FifoWaitQueue)
            transferer = new TransferQueue<E>();
        else
            transferer = new TransferStack<E>();
    }

    static {
        // Reduce the risk of rare disastrous classloading in first call to
        // LockSupport.park: https://bugs.openjdk.java.net/browse/JDK-8074773
        Class<?> ensureLoaded = LockSupport.class;
    }
}