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	/*
	* 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 {@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.
	*
	* The above steps improve throughput when many threads produce
	* and/or consume data. But they don't help much with
	* single-source / single-sink usages in which one side or the
	* other is always transiently blocked, and so throughput is
	* mainly a function of thread scheduling. This is not usually
	* noticeably improved with bounded short spin-waits. Instead both
	* forms of transfer try Thread.yield if apparently the sole
	* waiter. This works well when there are more tasks that cores,
	* which is expected to be the main usage context of this mode. In
	* other cases, waiters may help with some bookkeeping, then
	* park/unpark.
	*/

	/**
	* Shared internal API for dual stacks and queues.
	*/
	abstract static class Transferer<E> {
	/**
	* 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 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 = 1023L;

	/** 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 */
	static final int REQUEST = 0;
	/** Node represents an unfulfilled producer */
	static final int DATA = 1;
	/** Node is fulfilling another unfulfilled DATA or REQUEST */
	static final int FULFILLING = 2;

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

	/** Node class for TransferStacks. */
	static final class SNode implements ForkJoinPool.ManagedBlocker {
	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.
	*
	* @param s the node to match
	* @return true if successfully matched to s
	*/
	boolean tryMatch(SNode s) {
	SNode m; Thread w;
	if ((m = match) == null) {
	if (SMATCH.compareAndSet(this, null, s)) {
	if ((w = waiter) != null)
	LockSupport.unpark(w);
	return true;
	}
	else
	m = match;
	}
	return m == s;
	}

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

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

	public final boolean isReleasable() {
	return match != null \|\| Thread.currentThread().isInterrupted();
	}

	public final boolean block() {
	while (!isReleasable()) LockSupport.park();
	return true;
	}

	void forgetWaiter() {
	SWAITER.setOpaque(this, null);
	}

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

	/** 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.
	*/
	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.
	*/
	@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))) {
	long deadline = timed ? System.nanoTime() + nanos : 0L;
	Thread w = Thread.currentThread();
	int stat = -1; // -1: may yield, +1: park, else 0
	SNode m; // await fulfill or cancel
	while ((m = s.match) == null) {
	if ((timed &&
	(nanos = deadline - System.nanoTime()) <= 0) \|\|
	w.isInterrupted()) {
	if (s.tryCancel()) {
	clean(s); // wait cancelled
	return null;
	}
	} else if ((m = s.match) != null) {
	break; // recheck
	} else if (stat <= 0) {
	if (stat < 0 && h == null && head == s) {
	stat = 0; // yield once if was empty
	Thread.yield();
	} else {
	stat = 1;
	s.waiter = w; // enable signal
	}
	} else if (!timed) {
	LockSupport.setCurrentBlocker(this);
	try {
	ForkJoinPool.managedBlock(s);
	} catch (InterruptedException cannotHappen) { }
	LockSupport.setCurrentBlocker(null);
	} else if (nanos > SPIN_FOR_TIMEOUT_THRESHOLD)
	LockSupport.parkNanos(this, nanos);
	}
	if (stat == 1)
	s.forgetWaiter();
	Object result = (mode == REQUEST) ? m.item : s.item;
	if (h != null && h.next == s)
	casHead(h, s.next); // help fulfiller
	return (E) result;
	}
	} 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
	}
	}
	}
	}

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

	/*
	* 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 */
	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. */
	static final class QNode implements ForkJoinPool.ManagedBlocker {
	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.
	*/
	boolean tryCancel(Object cmp) {
	return 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.
	*/
	boolean isOffList() {
	return next == this;
	}

	void forgetWaiter() {
	QWAITER.setOpaque(this, null);
	}

	boolean isFulfilled() {
	Object x;
	return isData == ((x = item) == null) \|\| x == this;
	}

	public final boolean isReleasable() {
	Object x;
	return isData == ((x = item) == null) \|\| x == this \|\|
	Thread.currentThread().isInterrupted();
	}

	public final boolean block() {
	while (!isReleasable()) LockSupport.park();
	return true;
	}

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

	/** Head of queue */
	transient volatile QNode head;
	/** 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.
	*/
	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.
	*/
	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.
	*/
	void advanceTail(QNode t, QNode nt) {
	if (tail == t)
	QTAIL.compareAndSet(this, t, nt);
	}

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

	/**
	* 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, h = head, m, tn; // m is node to fulfill
	if (t == null \|\| h == null)
	; // inconsistent
	else if (h == t \|\| t.isData == isData) { // empty or same-mode
	if (t != tail) // inconsistent
	;
	else if ((tn = t.next) != null) // lagging tail
	advanceTail(t, tn);
	else if (timed && nanos <= 0L) // can't wait
	return null;
	else if (t.casNext(null, (s != null) ? s :
	(s = new QNode(e, isData)))) {
	advanceTail(t, s);
	long deadline = timed ? System.nanoTime() + nanos : 0L;
	Thread w = Thread.currentThread();
	int stat = -1; // same idea as TransferStack
	Object item;
	while ((item = s.item) == e) {
	if ((timed &&
	(nanos = deadline - System.nanoTime()) <= 0) \|\|
	w.isInterrupted()) {
	if (s.tryCancel(e)) {
	clean(t, s);
	return null;
	}
	} else if ((item = s.item) != e) {
	break; // recheck
	} else if (stat <= 0) {
	if (t.next == s) {
	if (stat < 0 && t.isFulfilled()) {
	stat = 0; // yield once if first
	Thread.yield();
	}
	else {
	stat = 1;
	s.waiter = w;
	}
	}
	} else if (!timed) {
	LockSupport.setCurrentBlocker(this);
	try {
	ForkJoinPool.managedBlock(s);
	} catch (InterruptedException cannotHappen) { }
	LockSupport.setCurrentBlocker(null);
	}
	else if (nanos > SPIN_FOR_TIMEOUT_THRESHOLD)
	LockSupport.parkNanos(this, nanos);
	}
	if (stat == 1)
	s.forgetWaiter();
	if (!s.isOffList()) { // not already unlinked
	advanceHead(t, s); // unlink if head
	if (item != null) // and forget fields
	s.item = s;
	}
	return (item != null) ? (E)item : e;
	}

	} else if ((m = h.next) != null && t == tail && h == head) {
	Thread waiter;
	Object x = m.item;
	boolean fulfilled = ((isData == (x == null)) &&
	x != m && m.casItem(x, e));
	advanceHead(h, m); // (help) dequeue
	if (fulfilled) {
	if ((waiter = m.waiter) != null)
	LockSupport.unpark(waiter);
	return (x != null) ? (E)x : e;
	}
	}
	}
	}

	/**
	* Gets rid of cancelled node s with original predecessor pred.
	*/
	void clean(QNode pred, QNode s) {
	s.forgetWaiter();
	/*
	* 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.
	*/
	private transient volatile Transferer<E> transferer;

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

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

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

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

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

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

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

	/**
	* @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 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).
	* @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).
	* @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;
	}
	}

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