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    1   /*
    2    * Copyright 2012 The Netty Project
    3    *
    4    * The Netty Project licenses this file to you under the Apache License,
    5    * version 2.0 (the "License"); you may not use this file except in compliance
    6    * with the License. You may obtain a copy of the License at:
    7    *
    8    *   http://www.apache.org/licenses/LICENSE-2.0
    9    *
    10   * Unless required by applicable law or agreed to in writing, software
    11   * distributed under the License is distributed on an "AS IS" BASIS, WITHOUT
    12   * WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied. See the
    13   * License for the specific language governing permissions and limitations
    14   * under the License.
    15   */
    16
    17  /*
    18   * Written by Doug Lea with assistance from members of JCP JSR-166
    19   * Expert Group and released to the public domain, as explained at
    20   * http://creativecommons.org/licenses/publicdomain
    21   */
    22
    23  package org.jboss.netty.util.internal;
    24
    25  import java.util.AbstractQueue;
    26  import java.util.Collection;
    27  import java.util.ConcurrentModificationException;
    28  import java.util.Iterator;
    29  import java.util.NoSuchElementException;
    30  import java.util.concurrent.BlockingQueue;
    31  import java.util.concurrent.TimeUnit;
    32  import java.util.concurrent.atomic.AtomicIntegerFieldUpdater;
    33  import java.util.concurrent.atomic.AtomicReferenceFieldUpdater;
    34  import java.util.concurrent.locks.LockSupport;
    35
    36  /**
    37   * <strong>
    38   * This version does work even if sun.misc.Unsafe is not found in the classpath. So this is kept for compatibility reasons.
    39   * Please use {@link QueueFactory} to create a Queue as it will use the "optimal" implementation depending on the JVM
    40   * </strong>
    41   * <br>
    42   * <br>
    43   *
    44   * An unbounded {@link BlockingQueue} based on linked nodes.
    45   * This queue orders elements FIFO (first-in-first-out) with respect
    46   * to any given producer.  The <em>head</em> of the queue is that
    47   * element that has been on the queue the longest time for some
    48   * producer.  The <em>tail</em> of the queue is that element that has
    49   * been on the queue the shortest time for some producer.
    50   *
    51   * <p>Beware that, unlike in most collections, the {@code size}
    52   * method is <em>NOT</em> a constant-time operation. Because of the
    53   * asynchronous nature of these queues, determining the current number
    54   * of elements requires a traversal of the elements.
    55   *
    56   * <p>This class and its iterator implement all of the
    57   * <em>optional</em> methods of the {@link Collection} and {@link
    58   * Iterator} interfaces.
    59   *
    60   * <p>Memory consistency effects: As with other concurrent
    61   * collections, actions in a thread prior to placing an object into a
    62   * {@code LinkedTransferQueue}
    63   * <a href="package-summary.html#MemoryVisibility"><i>happen-before</i></a>
    64   * actions subsequent to the access or removal of that element from
    65   * the {@code LinkedTransferQueue} in another thread.
    66   *
    67   * <p>This class is a member of the
    68   * <a href="{@docRoot}/../technotes/guides/collections/index.html">
    69   * Java Collections Framework</a>.
    70   * @param <E> the type of elements held in this collection
    71   */
    72  @Deprecated
    73  public class LegacyLinkedTransferQueue<E> extends AbstractQueue<E>
    74      implements BlockingQueue<E>, java.io.Serializable {
    75      private static final long serialVersionUID = -3223113410248163686L;
    76
    77      /*
    78       * *** Overview of Dual Queues with Slack ***
    79       *
    80       * Dual Queues, introduced by Scherer and Scott
    81       * (http://www.cs.rice.edu/~wns1/papers/2004-DISC-DDS.pdf) are
    82       * (linked) queues in which nodes may represent either data or
    83       * requests.  When a thread tries to enqueue a data node, but
    84       * encounters a request node, it instead "matches" and removes it;
    85       * and vice versa for enqueuing requests. Blocking Dual Queues
    86       * arrange that threads enqueuing unmatched requests block until
    87       * other threads provide the match. Dual Synchronous Queues (see
    88       * Scherer, Lea, & Scott
    89       * http://www.cs.rochester.edu/u/scott/papers/2009_Scherer_CACM_SSQ.pdf)
    90       * additionally arrange that threads enqueuing unmatched data also
    91       * block.  Dual Transfer Queues support all of these modes, as
    92       * dictated by callers.
    93       *
    94       * A FIFO dual queue may be implemented using a variation of the
    95       * Michael & Scott (M&S) lock-free queue algorithm
    96       * (http://www.cs.rochester.edu/u/scott/papers/1996_PODC_queues.pdf).
    97       * It maintains two pointer fields, "head", pointing to a
    98       * (matched) node that in turn points to the first actual
    99       * (unmatched) queue node (or null if empty); and "tail" that
    100      * points to the last node on the queue (or again null if
    101      * empty). For example, here is a possible queue with four data
    102      * elements:
    103      *
    104      *  head                tail
    105      *    |                   |
    106      *    v                   v
    107      *    M -> U -> U -> U -> U
    108      *
    109      * The M&S queue algorithm is known to be prone to scalability and
    110      * overhead limitations when maintaining (via CAS) these head and
    111      * tail pointers. This has led to the development of
    112      * contention-reducing variants such as elimination arrays (see
    113      * Moir et al http://portal.acm.org/citation.cfm?id=1074013) and
    114      * optimistic back pointers (see Ladan-Mozes & Shavit
    115      * http://people.csail.mit.edu/edya/publications/OptimisticFIFOQueue-journal.pdf).
    116      * However, the nature of dual queues enables a simpler tactic for
    117      * improving M&S-style implementations when dual-ness is needed.
    118      *
    119      * In a dual queue, each node must atomically maintain its match
    120      * status. While there are other possible variants, we implement
    121      * this here as: for a data-mode node, matching entails CASing an
    122      * "item" field from a non-null data value to null upon match, and
    123      * vice-versa for request nodes, CASing from null to a data
    124      * value. (Note that the linearization properties of this style of
    125      * queue are easy to verify -- elements are made available by
    126      * linking, and unavailable by matching.) Compared to plain M&S
    127      * queues, this property of dual queues requires one additional
    128      * successful atomic operation per enq/deq pair. But it also
    129      * enables lower cost variants of queue maintenance mechanics. (A
    130      * variation of this idea applies even for non-dual queues that
    131      * support deletion of interior elements, such as
    132      * j.u.c.ConcurrentLinkedQueue.)
    133      *
    134      * Once a node is matched, its match status can never again
    135      * change.  We may thus arrange that the linked list of them
    136      * contain a prefix of zero or more matched nodes, followed by a
    137      * suffix of zero or more unmatched nodes. (Note that we allow
    138      * both the prefix and suffix to be zero length, which in turn
    139      * means that we do not use a dummy header.)  If we were not
    140      * concerned with either time or space efficiency, we could
    141      * correctly perform enqueue and dequeue operations by traversing
    142      * from a pointer to the initial node; CASing the item of the
    143      * first unmatched node on match and CASing the next field of the
    144      * trailing node on appends. (Plus some special-casing when
    145      * initially empty).  While this would be a terrible idea in
    146      * itself, it does have the benefit of not requiring ANY atomic
    147      * updates on head/tail fields.
    148      *
    149      * We introduce here an approach that lies between the extremes of
    150      * never versus always updating queue (head and tail) pointers.
    151      * This offers a tradeoff between sometimes requiring extra
    152      * traversal steps to locate the first and/or last unmatched
    153      * nodes, versus the reduced overhead and contention of fewer
    154      * updates to queue pointers. For example, a possible snapshot of
    155      * a queue is:
    156      *
    157      *  head           tail
    158      *    |              |
    159      *    v              v
    160      *    M -> M -> U -> U -> U -> U
    161      *
    162      * The best value for this "slack" (the targeted maximum distance
    163      * between the value of "head" and the first unmatched node, and
    164      * similarly for "tail") is an empirical matter. We have found
    165      * that using very small constants in the range of 1-3 work best
    166      * over a range of platforms. Larger values introduce increasing
    167      * costs of cache misses and risks of long traversal chains, while
    168      * smaller values increase CAS contention and overhead.
    169      *
    170      * Dual queues with slack differ from plain M&S dual queues by
    171      * virtue of only sometimes updating head or tail pointers when
    172      * matching, appending, or even traversing nodes; in order to
    173      * maintain a targeted slack.  The idea of "sometimes" may be
    174      * operationalized in several ways. The simplest is to use a
    175      * per-operation counter incremented on each traversal step, and
    176      * to try (via CAS) to update the associated queue pointer
    177      * whenever the count exceeds a threshold. Another, that requires
    178      * more overhead, is to use random number generators to update
    179      * with a given probability per traversal step.
    180      *
    181      * In any strategy along these lines, because CASes updating
    182      * fields may fail, the actual slack may exceed targeted
    183      * slack. However, they may be retried at any time to maintain
    184      * targets.  Even when using very small slack values, this
    185      * approach works well for dual queues because it allows all
    186      * operations up to the point of matching or appending an item
    187      * (hence potentially allowing progress by another thread) to be
    188      * read-only, thus not introducing any further contention. As
    189      * described below, we implement this by performing slack
    190      * maintenance retries only after these points.
    191      *
    192      * As an accompaniment to such techniques, traversal overhead can
    193      * be further reduced without increasing contention of head
    194      * pointer updates: Threads may sometimes shortcut the "next" link
    195      * path from the current "head" node to be closer to the currently
    196      * known first unmatched node, and similarly for tail. Again, this
    197      * may be triggered with using thresholds or randomization.
    198      *
    199      * These ideas must be further extended to avoid unbounded amounts
    200      * of costly-to-reclaim garbage caused by the sequential "next"
    201      * links of nodes starting at old forgotten head nodes: As first
    202      * described in detail by Boehm
    203      * (http://portal.acm.org/citation.cfm?doid=503272.503282) if a GC
    204      * delays noticing that any arbitrarily old node has become
    205      * garbage, all newer dead nodes will also be unreclaimed.
    206      * (Similar issues arise in non-GC environments.)  To cope with
    207      * this in our implementation, upon CASing to advance the head
    208      * pointer, we set the "next" link of the previous head to point
    209      * only to itself; thus limiting the length of connected dead lists.
    210      * (We also take similar care to wipe out possibly garbage
    211      * retaining values held in other Node fields.)  However, doing so
    212      * adds some further complexity to traversal: If any "next"
    213      * pointer links to itself, it indicates that the current thread
    214      * has lagged behind a head-update, and so the traversal must
    215      * continue from the "head".  Traversals trying to find the
    216      * current tail starting from "tail" may also encounter
    217      * self-links, in which case they also continue at "head".
    218      *
    219      * It is tempting in slack-based scheme to not even use CAS for
    220      * updates (similarly to Ladan-Mozes & Shavit). However, this
    221      * cannot be done for head updates under the above link-forgetting
    222      * mechanics because an update may leave head at a detached node.
    223      * And while direct writes are possible for tail updates, they
    224      * increase the risk of long retraversals, and hence long garbage
    225      * chains, which can be much more costly than is worthwhile
    226      * considering that the cost difference of performing a CAS vs
    227      * write is smaller when they are not triggered on each operation
    228      * (especially considering that writes and CASes equally require
    229      * additional GC bookkeeping ("write barriers") that are sometimes
    230      * more costly than the writes themselves because of contention).
    231      *
    232      * *** Overview of implementation ***
    233      *
    234      * We use a threshold-based approach to updates, with a slack
    235      * threshold of two -- that is, we update head/tail when the
    236      * current pointer appears to be two or more steps away from the
    237      * first/last node. The slack value is hard-wired: a path greater
    238      * than one is naturally implemented by checking equality of
    239      * traversal pointers except when the list has only one element,
    240      * in which case we keep slack threshold at one. Avoiding tracking
    241      * explicit counts across method calls slightly simplifies an
    242      * already-messy implementation. Using randomization would
    243      * probably work better if there were a low-quality dirt-cheap
    244      * per-thread one available, but even ThreadLocalRandom is too
    245      * heavy for these purposes.
    246      *
    247      * With such a small slack threshold value, it is not worthwhile
    248      * to augment this with path short-circuiting (i.e., unsplicing
    249      * interior nodes) except in the case of cancellation/removal (see
    250      * below).
    251      *
    252      * We allow both the head and tail fields to be null before any
    253      * nodes are enqueued; initializing upon first append.  This
    254      * simplifies some other logic, as well as providing more
    255      * efficient explicit control paths instead of letting JVMs insert
    256      * implicit NullPointerExceptions when they are null.  While not
    257      * currently fully implemented, we also leave open the possibility
    258      * of re-nulling these fields when empty (which is complicated to
    259      * arrange, for little benefit.)
    260      *
    261      * All enqueue/dequeue operations are handled by the single method
    262      * "xfer" with parameters indicating whether to act as some form
    263      * of offer, put, poll, take, or transfer (each possibly with
    264      * timeout). The relative complexity of using one monolithic
    265      * method outweighs the code bulk and maintenance problems of
    266      * using separate methods for each case.
    267      *
    268      * Operation consists of up to three phases. The first is
    269      * implemented within method xfer, the second in tryAppend, and
    270      * the third in method awaitMatch.
    271      *
    272      * 1. Try to match an existing node
    273      *
    274      *    Starting at head, skip already-matched nodes until finding
    275      *    an unmatched node of opposite mode, if one exists, in which
    276      *    case matching it and returning, also if necessary updating
    277      *    head to one past the matched node (or the node itself if the
    278      *    list has no other unmatched nodes). If the CAS misses, then
    279      *    a loop retries advancing head by two steps until either
    280      *    success or the slack is at most two. By requiring that each
    281      *    attempt advances head by two (if applicable), we ensure that
    282      *    the slack does not grow without bound. Traversals also check
    283      *    if the initial head is now off-list, in which case they
    284      *    start at the new head.
    285      *
    286      *    If no candidates are found and the call was untimed
    287      *    poll/offer, (argument "how" is NOW) return.
    288      *
    289      * 2. Try to append a new node (method tryAppend)
    290      *
    291      *    Starting at current tail pointer, find the actual last node
    292      *    and try to append a new node (or if head was null, establish
    293      *    the first node). Nodes can be appended only if their
    294      *    predecessors are either already matched or are of the same
    295      *    mode. If we detect otherwise, then a new node with opposite
    296      *    mode must have been appended during traversal, so we must
    297      *    restart at phase 1. The traversal and update steps are
    298      *    otherwise similar to phase 1: Retrying upon CAS misses and
    299      *    checking for staleness.  In particular, if a self-link is
    300      *    encountered, then we can safely jump to a node on the list
    301      *    by continuing the traversal at current head.
    302      *
    303      *    On successful append, if the call was ASYNC, return.
    304      *
    305      * 3. Await match or cancellation (method awaitMatch)
    306      *
    307      *    Wait for another thread to match node; instead cancelling if
    308      *    the current thread was interrupted or the wait timed out. On
    309      *    multiprocessors, we use front-of-queue spinning: If a node
    310      *    appears to be the first unmatched node in the queue, it
    311      *    spins a bit before blocking. In either case, before blocking
    312      *    it tries to unsplice any nodes between the current "head"
    313      *    and the first unmatched node.
    314      *
    315      *    Front-of-queue spinning vastly improves performance of
    316      *    heavily contended queues. And so long as it is relatively
    317      *    brief and "quiet", spinning does not much impact performance
    318      *    of less-contended queues.  During spins threads check their
    319      *    interrupt status and generate a thread-local random number
    320      *    to decide to occasionally perform a Thread.yield. While
    321      *    yield has underdefined specs, we assume that might it help,
    322      *    and will not hurt in limiting impact of spinning on busy
    323      *    systems.  We also use smaller (1/2) spins for nodes that are
    324      *    not known to be front but whose predecessors have not
    325      *    blocked -- these "chained" spins avoid artifacts of
    326      *    front-of-queue rules which otherwise lead to alternating
    327      *    nodes spinning vs blocking. Further, front threads that
    328      *    represent phase changes (from data to request node or vice
    329      *    versa) compared to their predecessors receive additional
    330      *    chained spins, reflecting longer paths typically required to
    331      *    unblock threads during phase changes.
    332      * ** Unlinking removed interior nodes **
    333      *
    334      * In addition to minimizing garbage retention via self-linking
    335      * described above, we also unlink removed interior nodes. These
    336      * may arise due to timed out or interrupted waits, or calls to
    337      * remove(x) or Iterator.remove.  Normally, given a node that was
    338      * at one time known to be the predecessor of some node s that is
    339      * to be removed, we can unsplice s by CASing the next field of
    340      * its predecessor if it still points to s (otherwise s must
    341      * already have been removed or is now offlist). But there are two
    342      * situations in which we cannot guarantee to make node s
    343      * unreachable in this way: (1) If s is the trailing node of list
    344      * (i.e., with null next), then it is pinned as the target node
    345      * for appends, so can only be removed later after other nodes are
    346      * appended. (2) We cannot necessarily unlink s given a
    347      * predecessor node that is matched (including the case of being
    348      * cancelled): the predecessor may already be unspliced, in which
    349      * case some previous reachable node may still point to s.
    350      * (For further explanation see Herlihy & Shavit "The Art of
    351      * Multiprocessor Programming" chapter 9).  Although, in both
    352      * cases, we can rule out the need for further action if either s
    353      * or its predecessor are (or can be made to be) at, or fall off
    354      * from, the head of list.
    355      *
    356      * Without taking these into account, it would be possible for an
    357      * unbounded number of supposedly removed nodes to remain
    358      * reachable.  Situations leading to such buildup are uncommon but
    359      * can occur in practice; for example when a series of short timed
    360      * calls to poll repeatedly time out but never otherwise fall off
    361      * the list because of an untimed call to take at the front of the
    362      * queue.
    363      *
    364      * When these cases arise, rather than always retraversing the
    365      * entire list to find an actual predecessor to unlink (which
    366      * won't help for case (1) anyway), we record a conservative
    367      * estimate of possible unsplice failures (in "sweepVotes").
    368      * We trigger a full sweep when the estimate exceeds a threshold
    369      * ("SWEEP_THRESHOLD") indicating the maximum number of estimated
    370      * removal failures to tolerate before sweeping through, unlinking
    371      * cancelled nodes that were not unlinked upon initial removal.
    372      * We perform sweeps by the thread hitting threshold (rather than
    373      * background threads or by spreading work to other threads)
    374      * because in the main contexts in which removal occurs, the
    375      * caller is already timed-out, cancelled, or performing a
    376      * potentially O(n) operation (e.g. remove(x)), none of which are
    377      * time-critical enough to warrant the overhead that alternatives
    378      * would impose on other threads.
    379      *
    380      * Because the sweepVotes estimate is conservative, and because
    381      * nodes become unlinked "naturally" as they fall off the head of
    382      * the queue, and because we allow votes to accumulate even while
    383      * sweeps are in progress, there are typically significantly fewer
    384      * such nodes than estimated.  Choice of a threshold value
    385      * balances the likelihood of wasted effort and contention, versus
    386      * providing a worst-case bound on retention of interior nodes in
    387      * quiescent queues. The value defined below was chosen
    388      * empirically to balance these under various timeout scenarios.
    389      *
    390      * Note that we cannot self-link unlinked interior nodes during
    391      * sweeps. However, the associated garbage chains terminate when
    392      * some successor ultimately falls off the head of the list and is
    393      * self-linked.
    394      */
    395
    396     /** True if on multiprocessor */
    397     private static final boolean MP =
    398         Runtime.getRuntime().availableProcessors() > 1;
    399
    400     /**
    401      * The number of times to spin (with randomly interspersed calls
    402      * to Thread.yield) on multiprocessor before blocking when a node
    403      * is apparently the first waiter in the queue.  See above for
    404      * explanation. Must be a power of two. The value is empirically
    405      * derived -- it works pretty well across a variety of processors,
    406      * numbers of CPUs, and OSes.
    407      */
    408     private static final int FRONT_SPINS   = 1 << 7;
    409
    410     /**
    411      * The number of times to spin before blocking when a node is
    412      * preceded by another node that is apparently spinning.  Also
    413      * serves as an increment to FRONT_SPINS on phase changes, and as
    414      * base average frequency for yielding during spins. Must be a
    415      * power of two.
    416      */
    417     private static final int CHAINED_SPINS = FRONT_SPINS >>> 1;
    418
    419     /**
    420      * The maximum number of estimated removal failures (sweepVotes)
    421      * to tolerate before sweeping through the queue unlinking
    422      * cancelled nodes that were not unlinked upon initial
    423      * removal. See above for explanation. The value must be at least
    424      * two to avoid useless sweeps when removing trailing nodes.
    425      */
    426     static final int SWEEP_THRESHOLD = 32;
    427
    428     /**
    429      * Queue nodes. Uses Object, not E, for items to allow forgetting
    430      * them after use.  Relies heavily on Unsafe mechanics to minimize
    431      * unnecessary ordering constraints: Writes that are intrinsically
    432      * ordered wrt other accesses or CASes use simple relaxed forms.
    433      */
    434     static final class Node {
    435         final boolean isData;   // false if this is a request node
    436         volatile Object item;   // initially non-null if isData; CASed to match
    437         volatile Node next;
    438         volatile Thread waiter; // null until waiting
    439
    440         // CAS methods for fields
    441         boolean casNext(Node cmp, Node val) {
    442             if (AtomicFieldUpdaterUtil.isAvailable()) {
    443                 return nextUpdater.compareAndSet(this, cmp, val);
    444             } else {
    445                 synchronized (this) {
    446                     if (next == cmp) {
    447                         next = val;
    448                         return true;
    449                     } else {
    450                         return false;
    451                     }
    452                 }
    453             }
    454         }
    455
    456         boolean casItem(Object cmp, Object val) {
    457             // assert cmp == null || cmp.getClass() != Node.class;
    458             if (AtomicFieldUpdaterUtil.isAvailable()) {
    459                 return itemUpdater.compareAndSet(this, cmp, val);
    460             } else {
    461                 synchronized (this) {
    462                     if (item == cmp) {
    463                         item = val;
    464                         return true;
    465                     } else {
    466                         return false;
    467                     }
    468                 }
    469             }
    470         }
    471
    472         /**
    473          * Constructs a new node.  Uses relaxed write because item can
    474          * only be seen after publication via casNext.
    475          */
    476         Node(Object item, boolean isData) {
    477             this.item = item;
    478             this.isData = isData;
    479         }
    480
    481         /**
    482          * Links node to itself to avoid garbage retention.  Called
    483          * only after CASing head field, so uses relaxed write.
    484          */
    485         void forgetNext() {
    486             this.next = this;
    487         }
    488
    489         /**
    490          * Sets item to self and waiter to null, to avoid garbage
    491          * retention after matching or cancelling. Uses relaxed writes
    492          * bacause order is already constrained in the only calling
    493          * contexts: item is forgotten only after volatile/atomic
    494          * mechanics that extract items.  Similarly, clearing waiter
    495          * follows either CAS or return from park (if ever parked;
    496          * else we don't care).
    497          */
    498         void forgetContents() {
    499             this.item = this;
    500             this.waiter = null;
    501         }
    502
    503         /**
    504          * Returns true if this node has been matched, including the
    505          * case of artificial matches due to cancellation.
    506          */
    507         boolean isMatched() {
    508             Object x = item;
    509             return x == this || x == null == isData;
    510         }
    511
    512         /**
    513          * Returns true if this is an unmatched request node.
    514          */
    515         boolean isUnmatchedRequest() {
    516             return !isData && item == null;
    517         }
    518
    519         /**
    520          * Returns true if a node with the given mode cannot be
    521          * appended to this node because this node is unmatched and
    522          * has opposite data mode.
    523          */
    524         boolean cannotPrecede(boolean haveData) {
    525             boolean d = isData;
    526             Object x;
    527             return d != haveData && (x = item) != this && x != null == d;
    528         }
    529
    530         /**
    531          * Tries to artificially match a data node -- used by remove.
    532          */
    533         boolean tryMatchData() {
    534             // assert isData;
    535             Object x = item;
    536             if (x != null && x != this && casItem(x, null)) {
    537                 LockSupport.unpark(waiter);
    538                 return true;
    539             }
    540             return false;
    541         }
    542
    543         private static final AtomicReferenceFieldUpdater<Node, Node> nextUpdater =
    544             AtomicFieldUpdaterUtil.newRefUpdater(Node.class, Node.class, "next");
    545         private static final AtomicReferenceFieldUpdater<Node, Object> itemUpdater =
    546             AtomicFieldUpdaterUtil.newRefUpdater(Node.class, Object.class, "item");
    547
    548     }
    549
    550     /** head of the queue; null until first enqueue */
    551     transient volatile Node head;
    552
    553     /** tail of the queue; null until first append */
    554     transient volatile Node tail;
    555
    556     /** The number of apparent failures to unsplice removed nodes */
    557     transient volatile int sweepVotes;
    558
    559     // CAS methods for fields
    560     private boolean casTail(Node cmp, Node val) {
    561         if (AtomicFieldUpdaterUtil.isAvailable()) {
    562             return tailUpdater.compareAndSet(this, cmp, val);
    563         } else {
    564             synchronized (this) {
    565                 if (tail == cmp) {
    566                     tail = val;
    567                     return true;
    568                 } else {
    569                     return false;
    570                 }
    571             }
    572         }
    573     }
    574
    575     private boolean casHead(Node cmp, Node val) {
    576         if (AtomicFieldUpdaterUtil.isAvailable()) {
    577             return headUpdater.compareAndSet(this, cmp, val);
    578         } else {
    579             synchronized (this) {
    580                 if (head == cmp) {
    581                     head = val;
    582                     return true;
    583                 } else {
    584                     return false;
    585                 }
    586             }
    587         }
    588     }
    589
    590     private boolean casSweepVotes(int cmp, int val) {
    591         if (AtomicFieldUpdaterUtil.isAvailable()) {
    592             return sweepVotesUpdater.compareAndSet(this, cmp, val);
    593         } else {
    594             synchronized (this) {
    595                 if (sweepVotes == cmp) {
    596                     sweepVotes = val;
    597                     return true;
    598                 } else {
    599                     return false;
    600                 }
    601             }
    602         }
    603     }
    604
    605     /*
    606      * Possible values for "how" argument in xfer method.
    607      */
    608     private static final int NOW   = 0; // for untimed poll, tryTransfer
    609     private static final int ASYNC = 1; // for offer, put, add
    610     private static final int SYNC  = 2; // for transfer, take
    611     private static final int TIMED = 3; // for timed poll, tryTransfer
    612
    613     @SuppressWarnings("unchecked")
    614     static <E> E cast(Object item) {
    615         // assert item == null || item.getClass() != Node.class;
    616         // Explicit cast, see http://bugs.sun.com/bugdatabase/view_bug.do?bug_id=6302954
    617         return (E) item;
    618     }
    619
    620     /**
    621      * Implements all queuing methods. See above for explanation.
    622      *
    623      * @param e the item or null for take
    624      * @param haveData true if this is a put, else a take
    625      * @param how NOW, ASYNC, SYNC, or TIMED
    626      * @param nanos timeout in nanosecs, used only if mode is TIMED
    627      * @return an item if matched, else e
    628      * @throws NullPointerException if haveData mode but e is null
    629      */
    630     private E xfer(E e, boolean haveData, int how, long nanos) {
    631         if (haveData && e == null) {
    632             throw new NullPointerException();
    633         }
    634         Node s = null;                        // the node to append, if needed
    635
    636         retry: for (;;) {                     // restart on append race
    637
    638             for (Node h = head, p = h; p != null;) { // find & match first node
    639                 boolean isData = p.isData;
    640                 Object item = p.item;
    641                 if (item != p && item != null == isData) { // unmatched
    642                     if (isData == haveData) { // can't match
    643                         break;
    644                     }
    645                     if (p.casItem(item, e)) { // match
    646                         for (Node q = p; q != h;) {
    647                             Node n = q.next;  // update by 2 unless singleton
    648                             if (head == h && casHead(h, n == null? q : n)) {
    649                                 h.forgetNext();
    650                                 break;
    651                             }                 // advance and retry
    652                             if ((h = head)   == null ||
    653                                 (q = h.next) == null || !q.isMatched()) {
    654                                 break;        // unless slack < 2
    655                             }
    656                         }
    657                         LockSupport.unpark(p.waiter);
    658                         // Explicit cast, see http://bugs.sun.com/bugdatabase/view_bug.do?bug_id=6302954
    659                         return LegacyLinkedTransferQueue.cast(item);
    660                     }
    661                 }
    662                 Node n = p.next;
    663                 p = p != n ? n : (h = head); // Use head if p offlist
    664             }
    665
    666             if (how != NOW) {                 // No matches available
    667                 if (s == null) {
    668                     s = new Node(e, haveData);
    669                 }
    670                 Node pred = tryAppend(s, haveData);
    671                 if (pred == null) {
    672                     continue retry;           // lost race vs opposite mode
    673                 }
    674                 if (how != ASYNC) {
    675                     return awaitMatch(s, pred, e, how == TIMED, nanos);
    676                 }
    677             }
    678             return e; // not waiting
    679         }
    680     }
    681
    682     /**
    683      * Tries to append node s as tail.
    684      *
    685      * @param s the node to append
    686      * @param haveData true if appending in data mode
    687      * @return null on failure due to losing race with append in
    688      * different mode, else s's predecessor, or s itself if no
    689      * predecessor
    690      */
    691     private Node tryAppend(Node s, boolean haveData) {
    692         for (Node t = tail, p = t;;) {        // move p to last node and append
    693             Node n, u;                        // temps for reads of next & tail
    694             if (p == null && (p = head) == null) {
    695                 if (casHead(null, s)) {
    696                     return s;                 // initialize
    697                 }
    698             }
    699             else if (p.cannotPrecede(haveData)) {
    700                 return null;                  // lost race vs opposite mode
    701             } else if ((n = p.next) != null) { // not last; keep traversing
    702                 p = p != t && t != (u = tail) ? (t = u) : // stale tail
    703                     p != n ? n : null;      // restart if off list
    704             } else if (!p.casNext(null, s)) {
    705                 p = p.next;                   // re-read on CAS failure
    706             } else {
    707                 if (p != t) {                 // update if slack now >= 2
    708                     while ((tail != t || !casTail(t, s)) &&
    709                            (t = tail)   != null &&
    710                            (s = t.next) != null && // advance and retry
    711                            (s = s.next) != null && s != t) {
    712                         continue;
    713                     }
    714                 }
    715                 return p;
    716             }
    717         }
    718     }
    719
    720     /**
    721      * Spins/yields/blocks until node s is matched or caller gives up.
    722      *
    723      * @param s the waiting node
    724      * @param pred the predecessor of s, or s itself if it has no
    725      * predecessor, or null if unknown (the null case does not occur
    726      * in any current calls but may in possible future extensions)
    727      * @param e the comparison value for checking match
    728      * @param timed if true, wait only until timeout elapses
    729      * @param nanos timeout in nanosecs, used only if timed is true
    730      * @return matched item, or e if unmatched on interrupt or timeout
    731      */
    732     private E awaitMatch(Node s, Node pred, E e, boolean timed, long nanos) {
    733         long lastTime = timed ? System.nanoTime() : 0L;
    734         Thread w = Thread.currentThread();
    735         int spins = -1; // initialized after first item and cancel checks
    736         ThreadLocalRandom randomYields = null; // bound if needed
    737
    738         for (;;) {
    739             Object item = s.item;
    740             if (item != e) {                  // matched
    741                 // assert item != s;
    742                 s.forgetContents();           // avoid garbage
    743                 return LegacyLinkedTransferQueue.cast(item);
    744             }
    745             if ((w.isInterrupted() || timed && nanos <= 0) &&
    746                     s.casItem(e, s)) {        // cancel
    747                 unsplice(pred, s);
    748                 return e;
    749             }
    750
    751             if (spins < 0) {                  // establish spins at/near front
    752                 if ((spins = spinsFor(pred, s.isData)) > 0) {
    753                     randomYields = ThreadLocalRandom.current();
    754                 }
    755             }
    756             else if (spins > 0) {             // spin
    757                 --spins;
    758                 if (randomYields.nextInt(CHAINED_SPINS) == 0) {
    759                     Thread.yield();           // occasionally yield
    760                 }
    761             }
    762             else if (s.waiter == null) {
    763                 s.waiter = w;                 // request unpark then recheck
    764             }
    765             else if (timed) {
    766                 long now = System.nanoTime();
    767                 if ((nanos -= now - lastTime) > 0) {
    768                     LockSupport.parkNanos(nanos);
    769                 }
    770                 lastTime = now;
    771             }
    772             else {
    773                 LockSupport.park();
    774             }
    775         }
    776     }
    777
    778     /**
    779      * Returns spin/yield value for a node with given predecessor and
    780      * data mode. See above for explanation.
    781      */
    782     private static int spinsFor(Node pred, boolean haveData) {
    783         if (MP && pred != null) {
    784             if (pred.isData != haveData) {    // phase change
    785                 return FRONT_SPINS + CHAINED_SPINS;
    786             }
    787             if (pred.isMatched()) {           // probably at front
    788                 return FRONT_SPINS;
    789             }
    790             if (pred.waiter == null) {        // pred apparently spinning
    791                 return CHAINED_SPINS;
    792             }
    793         }
    794         return 0;
    795     }
    796
    797     /* -------------- Traversal methods -------------- */
    798
    799     /**
    800      * Returns the successor of p, or the head node if p.next has been
    801      * linked to self, which will only be true if traversing with a
    802      * stale pointer that is now off the list.
    803      */
    804     final Node succ(Node p) {
    805         Node next = p.next;
    806         return p == next ? head : next;
    807     }
    808
    809     /**
    810      * Returns the first unmatched node of the given mode, or null if
    811      * none.  Used by methods isEmpty, hasWaitingConsumer.
    812      */
    813     private Node firstOfMode(boolean isData) {
    814         for (Node p = head; p != null; p = succ(p)) {
    815             if (!p.isMatched()) {
    816                 return p.isData == isData ? p : null;
    817             }
    818         }
    819         return null;
    820     }
    821
    822     /**
    823      * Returns the item in the first unmatched node with isData; or
    824      * null if none.  Used by peek.
    825      */
    826     private E firstDataItem() {
    827         for (Node p = head; p != null; p = succ(p)) {
    828             Object item = p.item;
    829             if (p.isData) {
    830                 if (item != null && item != p) {
    831                     return LegacyLinkedTransferQueue.cast(item);
    832                 }
    833             }
    834             else if (item == null) {
    835                 return null;
    836             }
    837         }
    838         return null;
    839     }
    840
    841     /**
    842      * Traverses and counts unmatched nodes of the given mode.
    843      * Used by methods size and getWaitingConsumerCount.
    844      */
    845     private int countOfMode(boolean data) {
    846         int count = 0;
    847         for (Node p = head; p != null; ) {
    848             if (!p.isMatched()) {
    849                 if (p.isData != data) {
    850                     return 0;
    851                 }
    852                 if (++count == Integer.MAX_VALUE) { // saturated
    853                     break;
    854                 }
    855             }
    856             Node n = p.next;
    857             if (n != p) {
    858                 p = n;
    859             } else {
    860                 count = 0;
    861                 p = head;
    862             }
    863         }
    864         return count;
    865     }
    866
    867     final class Itr implements Iterator<E> {
    868         private Node nextNode;   // next node to return item for
    869         private E nextItem;      // the corresponding item
    870         private Node lastRet;    // last returned node, to support remove
    871         private Node lastPred;   // predecessor to unlink lastRet
    872
    873         /**
    874          * Moves to next node after prev, or first node if prev null.
    875          */
    876         private void advance(Node prev) {
    877             lastPred = lastRet;
    878             lastRet = prev;
    879             for (Node p = prev == null ? head : succ(prev);
    880                  p != null; p = succ(p)) {
    881                 Object item = p.item;
    882                 if (p.isData) {
    883                     if (item != null && item != p) {
    884                         nextItem = LegacyLinkedTransferQueue.cast(item);
    885                         nextNode = p;
    886                         return;
    887                     }
    888                 }
    889                 else if (item == null) {
    890                     break;
    891                 }
    892             }
    893             nextNode = null;
    894         }
    895
    896         Itr() {
    897             advance(null);
    898         }
    899
    900         public boolean hasNext() {
    901             return nextNode != null;
    902         }
    903
    904         public E next() {
    905             Node p = nextNode;
    906             if (p == null) {
    907                 throw new NoSuchElementException();
    908             }
    909             E e = nextItem;
    910             advance(p);
    911             return e;
    912         }
    913
    914         public void remove() {
    915             Node p = lastRet;
    916             if (p == null) {
    917                 throw new IllegalStateException();
    918             }
    919             if (p.tryMatchData()) {
    920                 unsplice(lastPred, p);
    921             }
    922         }
    923     }
    924
    925     /* -------------- Removal methods -------------- */
    926
    927     /**
    928      * Unsplices (now or later) the given deleted/cancelled node with
    929      * the given predecessor.
    930      *
    931      * @param pred a node that was at one time known to be the
    932      * predecessor of s, or null or s itself if s is/was at head
    933      * @param s the node to be unspliced
    934      */
    935     final void unsplice(Node pred, Node s) {
    936         s.forgetContents(); // forget unneeded fields
    937         /*
    938          * See above for rationale. Briefly: if pred still points to
    939          * s, try to unlink s.  If s cannot be unlinked, because it is
    940          * trailing node or pred might be unlinked, and neither pred
    941          * nor s are head or offlist, add to sweepVotes, and if enough
    942          * votes have accumulated, sweep.
    943          */
    944         if (pred != null && pred != s && pred.next == s) {
    945             Node n = s.next;
    946             if (n == null ||
    947                 n != s && pred.casNext(s, n) && pred.isMatched()) {
    948                 for (;;) {               // check if at, or could be, head
    949                     Node h = head;
    950                     if (h == pred || h == s || h == null) {
    951                         return;          // at head or list empty
    952                     }
    953                     if (!h.isMatched()) {
    954                         break;
    955                     }
    956                     Node hn = h.next;
    957                     if (hn == null) {
    958                         return;          // now empty
    959                     }
    960                     if (hn != h && casHead(h, hn)) {
    961                         h.forgetNext();  // advance head
    962                     }
    963                 }
    964                 if (pred.next != pred && s.next != s) { // recheck if offlist
    965                     for (;;) {           // sweep now if enough votes
    966                         int v = sweepVotes;
    967                         if (v < SWEEP_THRESHOLD) {
    968                             if (casSweepVotes(v, v + 1)) {
    969                                 break;
    970                             }
    971                         }
    972                         else if (casSweepVotes(v, 0)) {
    973                             sweep();
    974                             break;
    975                         }
    976                     }
    977                 }
    978             }
    979         }
    980     }
    981
    982     /**
    983      * Unlinks matched (typically cancelled) nodes encountered in a
    984      * traversal from head.
    985      */
    986     private void sweep() {
    987         for (Node p = head, s, n; p != null && (s = p.next) != null; ) {
    988             if (!s.isMatched()) {
    989                 // Unmatched nodes are never self-linked
    990                 p = s;
    991             } else if ((n = s.next) == null) { // trailing node is pinned
    992                 break;
    993             } else if (s == n) { // stale
    994                 // No need to also check for p == s, since that implies s == n
    995                 p = head;
    996             } else {
    997                 p.casNext(s, n);
    998             }
    999         }
    1000     }
    1001
    1002     /**
    1003      * Main implementation of remove(Object)
    1004      */
    1005     private boolean findAndRemove(Object e) {
    1006         if (e != null) {
    1007             for (Node pred = null, p = head; p != null; ) {
    1008                 Object item = p.item;
    1009                 if (p.isData) {
    1010                     if (item != null && item != p && e.equals(item) &&
    1011                         p.tryMatchData()) {
    1012                         unsplice(pred, p);
    1013                         return true;
    1014                     }
    1015                 }
    1016                 else if (item == null) {
    1017                     break;
    1018                 }
    1019                 pred = p;
    1020                 if ((p = p.next) == pred) { // stale
    1021                     pred = null;
    1022                     p = head;
    1023                 }
    1024             }
    1025         }
    1026         return false;
    1027     }
    1028
    1029
    1030     /**
    1031      * Creates an initially empty {@code LinkedTransferQueue}.
    1032      */
    1033     public LegacyLinkedTransferQueue() {
    1034     }
    1035
    1036     /**
    1037      * Creates a {@code LinkedTransferQueue}
    1038      * initially containing the elements of the given collection,
    1039      * added in traversal order of the collection's iterator.
    1040      *
    1041      * @param c the collection of elements to initially contain
    1042      * @throws NullPointerException if the specified collection or any
    1043      *         of its elements are null
    1044      */
    1045     public LegacyLinkedTransferQueue(Collection<? extends E> c) {
    1046         this();
    1047         addAll(c);
    1048     }
    1049
    1050     /**
    1051      * Inserts the specified element at the tail of this queue.
    1052      * As the queue is unbounded, this method will never block.
    1053      *
    1054      * @throws NullPointerException if the specified element is null
    1055      */
    1056     public void put(E e) {
    1057         xfer(e, true, ASYNC, 0);
    1058     }
    1059
    1060     /**
    1061      * Inserts the specified element at the tail of this queue.
    1062      * As the queue is unbounded, this method will never block or
    1063      * return {@code false}.
    1064      *
    1065      * @return {@code true} (as specified by
    1066      *  {@link BlockingQueue#offer(Object,long,TimeUnit) BlockingQueue.offer})
    1067      * @throws NullPointerException if the specified element is null
    1068      */
    1069     public boolean offer(E e, long timeout, TimeUnit unit) {
    1070         xfer(e, true, ASYNC, 0);
    1071         return true;
    1072     }
    1073
    1074     /**
    1075      * Inserts the specified element at the tail of this queue.
    1076      * As the queue is unbounded, this method will never return {@code false}.
    1077      *
    1078      * @return {@code true} (as specified by
    1079      *         {@link BlockingQueue#offer(Object) BlockingQueue.offer})
    1080      * @throws NullPointerException if the specified element is null
    1081      */
    1082     public boolean offer(E e) {
    1083         xfer(e, true, ASYNC, 0);
    1084         return true;
    1085     }
    1086
    1087     /**
    1088      * Inserts the specified element at the tail of this queue.
    1089      * As the queue is unbounded, this method will never throw
    1090      * {@link IllegalStateException} or return {@code false}.
    1091      *
    1092      * @return {@code true} (as specified by {@link Collection#add})
    1093      * @throws NullPointerException if the specified element is null
    1094      */
    1095     @Override
    1096     public boolean add(E e) {
    1097         xfer(e, true, ASYNC, 0);
    1098         return true;
    1099     }
    1100
    1101     /**
    1102      * Transfers the element to a waiting consumer immediately, if possible.
    1103      *
    1104      * <p>More precisely, transfers the specified element immediately
    1105      * if there exists a consumer already waiting to receive it (in
    1106      * {@link #take} or timed {@link #poll(long,TimeUnit) poll}),
    1107      * otherwise returning {@code false} without enqueuing the element.
    1108      *
    1109      * @throws NullPointerException if the specified element is null
    1110      */
    1111     public boolean tryTransfer(E e) {
    1112         return xfer(e, true, NOW, 0) == null;
    1113     }
    1114
    1115     /**
    1116      * Transfers the element to a consumer, waiting if necessary to do so.
    1117      *
    1118      * <p>More precisely, transfers the specified element immediately
    1119      * if there exists a consumer already waiting to receive it (in
    1120      * {@link #take} or timed {@link #poll(long,TimeUnit) poll}),
    1121      * else inserts the specified element at the tail of this queue
    1122      * and waits until the element is received by a consumer.
    1123      *
    1124      * @throws NullPointerException if the specified element is null
    1125      */
    1126     public void transfer(E e) throws InterruptedException {
    1127         if (xfer(e, true, SYNC, 0) != null) {
    1128             Thread.interrupted(); // failure possible only due to interrupt
    1129             throw new InterruptedException();
    1130         }
    1131     }
    1132
    1133     /**
    1134      * Transfers the element to a consumer if it is possible to do so
    1135      * before the timeout elapses.
    1136      *
    1137      * <p>More precisely, transfers the specified element immediately
    1138      * if there exists a consumer already waiting to receive it (in
    1139      * {@link #take} or timed {@link #poll(long,TimeUnit) poll}),
    1140      * else inserts the specified element at the tail of this queue
    1141      * and waits until the element is received by a consumer,
    1142      * returning {@code false} if the specified wait time elapses
    1143      * before the element can be transferred.
    1144      *
    1145      * @throws NullPointerException if the specified element is null
    1146      */
    1147     public boolean tryTransfer(E e, long timeout, TimeUnit unit)
    1148         throws InterruptedException {
    1149         if (xfer(e, true, TIMED, unit.toNanos(timeout)) == null) {
    1150             return true;
    1151         }
    1152         if (!Thread.interrupted()) {
    1153             return false;
    1154         }
    1155         throw new InterruptedException();
    1156     }
    1157
    1158     public E take() throws InterruptedException {
    1159         E e = xfer(null, false, SYNC, 0);
    1160         if (e != null) {
    1161             return e;
    1162         }
    1163         Thread.interrupted();
    1164         throw new InterruptedException();
    1165     }
    1166
    1167     public E poll(long timeout, TimeUnit unit) throws InterruptedException {
    1168         E e = xfer(null, false, TIMED, unit.toNanos(timeout));
    1169         if (e != null || !Thread.interrupted()) {
    1170             return e;
    1171         }
    1172         throw new InterruptedException();
    1173     }
    1174
    1175     public E poll() {
    1176         return xfer(null, false, NOW, 0);
    1177     }
    1178
    1179     /**
    1180      * @throws NullPointerException     {@inheritDoc}
    1181      * @throws IllegalArgumentException {@inheritDoc}
    1182      */
    1183     public int drainTo(Collection<? super E> c) {
    1184         if (c == null) {
    1185             throw new NullPointerException();
    1186         }
    1187         if (c == this) {
    1188             throw new IllegalArgumentException();
    1189         }
    1190         int n = 0;
    1191         E e;
    1192         while ( (e = poll()) != null) {
    1193             c.add(e);
    1194             ++n;
    1195         }
    1196         return n;
    1197     }
    1198
    1199     /**
    1200      * @throws NullPointerException     {@inheritDoc}
    1201      * @throws IllegalArgumentException {@inheritDoc}
    1202      */
    1203     public int drainTo(Collection<? super E> c, int maxElements) {
    1204         if (c == null) {
    1205             throw new NullPointerException();
    1206         }
    1207         if (c == this) {
    1208             throw new IllegalArgumentException();
    1209         }
    1210         int n = 0;
    1211         E e;
    1212         while (n < maxElements && (e = poll()) != null) {
    1213             c.add(e);
    1214             ++n;
    1215         }
    1216         return n;
    1217     }
    1218
    1219     /**
    1220      * Returns an iterator over the elements in this queue in proper
    1221      * sequence, from head to tail.
    1222      *
    1223      * <p>The returned iterator is a "weakly consistent" iterator that
    1224      * will never throw
    1225      * {@link ConcurrentModificationException ConcurrentModificationException},
    1226      * and guarantees to traverse elements as they existed upon
    1227      * construction of the iterator, and may (but is not guaranteed
    1228      * to) reflect any modifications subsequent to construction.
    1229      *
    1230      * @return an iterator over the elements in this queue in proper sequence
    1231      */
    1232     @Override
    1233     public Iterator<E> iterator() {
    1234         return new Itr();
    1235     }
    1236
    1237     public E peek() {
    1238         return firstDataItem();
    1239     }
    1240
    1241     /**
    1242      * Returns {@code true} if this queue contains no elements.
    1243      *
    1244      * @return {@code true} if this queue contains no elements
    1245      */
    1246     @Override
    1247     public boolean isEmpty() {
    1248         for (Node p = head; p != null; p = succ(p)) {
    1249             if (!p.isMatched()) {
    1250                 return !p.isData;
    1251             }
    1252         }
    1253         return true;
    1254     }
    1255
    1256     public boolean hasWaitingConsumer() {
    1257         return firstOfMode(false) != null;
    1258     }
    1259
    1260     /**
    1261      * Returns the number of elements in this queue.  If this queue
    1262      * contains more than {@code Integer.MAX_VALUE} elements, returns
    1263      * {@code Integer.MAX_VALUE}.
    1264      *
    1265      * <p>Beware that, unlike in most collections, this method is
    1266      * <em>NOT</em> a constant-time operation. Because of the
    1267      * asynchronous nature of these queues, determining the current
    1268      * number of elements requires an O(n) traversal.
    1269      *
    1270      * @return the number of elements in this queue
    1271      */
    1272     @Override
    1273     public int size() {
    1274         return countOfMode(true);
    1275     }
    1276
    1277     public int getWaitingConsumerCount() {
    1278         return countOfMode(false);
    1279     }
    1280
    1281     /**
    1282      * Removes a single instance of the specified element from this queue,
    1283      * if it is present.  More formally, removes an element {@code e} such
    1284      * that {@code o.equals(e)}, if this queue contains one or more such
    1285      * elements.
    1286      * Returns {@code true} if this queue contained the specified element
    1287      * (or equivalently, if this queue changed as a result of the call).
    1288      *
    1289      * @param o element to be removed from this queue, if present
    1290      * @return {@code true} if this queue changed as a result of the call
    1291      */
    1292     @Override
    1293     public boolean remove(Object o) {
    1294         return findAndRemove(o);
    1295     }
    1296
    1297     /**
    1298      * Always returns {@code Integer.MAX_VALUE} because a
    1299      * {@code LinkedTransferQueue} is not capacity constrained.
    1300      *
    1301      * @return {@code Integer.MAX_VALUE} (as specified by
    1302      *         {@link BlockingQueue#remainingCapacity()})
    1303      */
    1304     public int remainingCapacity() {
    1305         return Integer.MAX_VALUE;
    1306     }
    1307
    1308     /**
    1309      * Saves the state to a stream (that is, serializes it).
    1310      *
    1311      * @serialData All of the elements (each an {@code E}) in
    1312      * the proper order, followed by a null
    1313      * @param s the stream
    1314      */
    1315     private void writeObject(java.io.ObjectOutputStream s)
    1316         throws java.io.IOException {
    1317         s.defaultWriteObject();
    1318         for (E e : this) {
    1319             s.writeObject(e);
    1320         }
    1321         // Use trailing null as sentinel
    1322         s.writeObject(null);
    1323     }
    1324
    1325     /**
    1326      * Reconstitutes the Queue instance from a stream (that is,
    1327      * deserializes it).
    1328      *
    1329      * @param s the stream
    1330      */
    1331     private void readObject(java.io.ObjectInputStream s)
    1332         throws java.io.IOException, ClassNotFoundException {
    1333         s.defaultReadObject();
    1334         for (;;) {
    1335             E item = (E) s.readObject();
    1336             if (item == null) {
    1337                 break;
    1338             } else {
    1339                 offer(item);
    1340             }
    1341         }
    1342     }
    1343
    1344     @SuppressWarnings("rawtypes")
    1345     private static final AtomicReferenceFieldUpdater<LegacyLinkedTransferQueue, Node> headUpdater =
    1346         AtomicFieldUpdaterUtil.newRefUpdater(LegacyLinkedTransferQueue.class, Node.class, "head");
    1347     @SuppressWarnings("rawtypes")
    1348     private static final AtomicReferenceFieldUpdater<LegacyLinkedTransferQueue, Node> tailUpdater =
    1349         AtomicFieldUpdaterUtil.newRefUpdater(LegacyLinkedTransferQueue.class, Node.class, "tail");
    1350     @SuppressWarnings("rawtypes")
    1351     private static final AtomicIntegerFieldUpdater<LegacyLinkedTransferQueue> sweepVotesUpdater =
    1352         AtomicFieldUpdaterUtil.newIntUpdater(LegacyLinkedTransferQueue.class, "sweepVotes");
    1353 }
    1354