1 /*
   2  * DO NOT ALTER OR REMOVE COPYRIGHT NOTICES OR THIS FILE HEADER.
   3  *
   4  * This code is free software; you can redistribute it and/or modify it
   5  * under the terms of the GNU General Public License version 2 only, as
   6  * published by the Free Software Foundation.  Oracle designates this
   7  * particular file as subject to the "Classpath" exception as provided
   8  * by Oracle in the LICENSE file that accompanied this code.
   9  *
  10  * This code is distributed in the hope that it will be useful, but WITHOUT
  11  * ANY WARRANTY; without even the implied warranty of MERCHANTABILITY or
  12  * FITNESS FOR A PARTICULAR PURPOSE.  See the GNU General Public License
  13  * version 2 for more details (a copy is included in the LICENSE file that
  14  * accompanied this code).
  15  *
  16  * You should have received a copy of the GNU General Public License version
  17  * 2 along with this work; if not, write to the Free Software Foundation,
  18  * Inc., 51 Franklin St, Fifth Floor, Boston, MA 02110-1301 USA.
  19  *
  20  * Please contact Oracle, 500 Oracle Parkway, Redwood Shores, CA 94065 USA
  21  * or visit www.oracle.com if you need additional information or have any
  22  * questions.
  23  */
  24 
  25 /*
  26  * This file is available under and governed by the GNU General Public
  27  * License version 2 only, as published by the Free Software Foundation.
  28  * However, the following notice accompanied the original version of this
  29  * file:
  30  *
  31  * Written by Doug Lea and Martin Buchholz with assistance from members of
  32  * JCP JSR-166 Expert Group and released to the public domain, as explained
  33  * at http://creativecommons.org/licenses/publicdomain
  34  */
  35 
  36 package java.util.concurrent;
  37 
  38 import java.util.AbstractQueue;
  39 import java.util.ArrayList;
  40 import java.util.Collection;
  41 import java.util.Iterator;
  42 import java.util.NoSuchElementException;
  43 import java.util.Queue;
  44 
  45 /**
  46  * An unbounded thread-safe {@linkplain Queue queue} based on linked nodes.
  47  * This queue orders elements FIFO (first-in-first-out).
  48  * The <em>head</em> of the queue is that element that has been on the
  49  * queue the longest time.
  50  * The <em>tail</em> of the queue is that element that has been on the
  51  * queue the shortest time. New elements
  52  * are inserted at the tail of the queue, and the queue retrieval
  53  * operations obtain elements at the head of the queue.
  54  * A {@code ConcurrentLinkedQueue} is an appropriate choice when
  55  * many threads will share access to a common collection.
  56  * Like most other concurrent collection implementations, this class
  57  * does not permit the use of {@code null} elements.
  58  *
  59  * <p>This implementation employs an efficient &quot;wait-free&quot;
  60  * algorithm based on one described in <a
  61  * href="http://www.cs.rochester.edu/u/michael/PODC96.html"> Simple,
  62  * Fast, and Practical Non-Blocking and Blocking Concurrent Queue
  63  * Algorithms</a> by Maged M. Michael and Michael L. Scott.
  64  *
  65  * <p>Iterators are <i>weakly consistent</i>, returning elements
  66  * reflecting the state of the queue at some point at or since the
  67  * creation of the iterator.  They do <em>not</em> throw {@link
  68  * ConcurrentModificationException}, and may proceed concurrently with
  69  * other operations.  Elements contained in the queue since the creation
  70  * of the iterator will be returned exactly once.
  71  *
  72  * <p>Beware that, unlike in most collections, the {@code size} method
  73  * is <em>NOT</em> a constant-time operation. Because of the
  74  * asynchronous nature of these queues, determining the current number
  75  * of elements requires a traversal of the elements.
  76  *
  77  * <p>This class and its iterator implement all of the <em>optional</em>
  78  * methods of the {@link Queue} and {@link Iterator} interfaces.
  79  *
  80  * <p>Memory consistency effects: As with other concurrent
  81  * collections, actions in a thread prior to placing an object into a
  82  * {@code ConcurrentLinkedQueue}
  83  * <a href="package-summary.html#MemoryVisibility"><i>happen-before</i></a>
  84  * actions subsequent to the access or removal of that element from
  85  * the {@code ConcurrentLinkedQueue} in another thread.
  86  *
  87  * <p>This class is a member of the
  88  * <a href="{@docRoot}/../technotes/guides/collections/index.html">
  89  * Java Collections Framework</a>.
  90  *
  91  * @since 1.5
  92  * @author Doug Lea
  93  * @param <E> the type of elements held in this collection
  94  *
  95  */
  96 public class ConcurrentLinkedQueue<E> extends AbstractQueue<E>
  97         implements Queue<E>, java.io.Serializable {
  98     private static final long serialVersionUID = 196745693267521676L;
  99 
 100     /*
 101      * This is a modification of the Michael & Scott algorithm,
 102      * adapted for a garbage-collected environment, with support for
 103      * interior node deletion (to support remove(Object)).  For
 104      * explanation, read the paper.
 105      *
 106      * Note that like most non-blocking algorithms in this package,
 107      * this implementation relies on the fact that in garbage
 108      * collected systems, there is no possibility of ABA problems due
 109      * to recycled nodes, so there is no need to use "counted
 110      * pointers" or related techniques seen in versions used in
 111      * non-GC'ed settings.
 112      *
 113      * The fundamental invariants are:
 114      * - There is exactly one (last) Node with a null next reference,
 115      *   which is CASed when enqueueing.  This last Node can be
 116      *   reached in O(1) time from tail, but tail is merely an
 117      *   optimization - it can always be reached in O(N) time from
 118      *   head as well.
 119      * - The elements contained in the queue are the non-null items in
 120      *   Nodes that are reachable from head.  CASing the item
 121      *   reference of a Node to null atomically removes it from the
 122      *   queue.  Reachability of all elements from head must remain
 123      *   true even in the case of concurrent modifications that cause
 124      *   head to advance.  A dequeued Node may remain in use
 125      *   indefinitely due to creation of an Iterator or simply a
 126      *   poll() that has lost its time slice.
 127      *
 128      * The above might appear to imply that all Nodes are GC-reachable
 129      * from a predecessor dequeued Node.  That would cause two problems:
 130      * - allow a rogue Iterator to cause unbounded memory retention
 131      * - cause cross-generational linking of old Nodes to new Nodes if
 132      *   a Node was tenured while live, which generational GCs have a
 133      *   hard time dealing with, causing repeated major collections.
 134      * However, only non-deleted Nodes need to be reachable from
 135      * dequeued Nodes, and reachability does not necessarily have to
 136      * be of the kind understood by the GC.  We use the trick of
 137      * linking a Node that has just been dequeued to itself.  Such a
 138      * self-link implicitly means to advance to head.
 139      *
 140      * Both head and tail are permitted to lag.  In fact, failing to
 141      * update them every time one could is a significant optimization
 142      * (fewer CASes). As with LinkedTransferQueue (see the internal
 143      * documentation for that class), we use a slack threshold of two;
 144      * that is, we update head/tail when the current pointer appears
 145      * to be two or more steps away from the first/last node.
 146      *
 147      * Since head and tail are updated concurrently and independently,
 148      * it is possible for tail to lag behind head (why not)?
 149      *
 150      * CASing a Node's item reference to null atomically removes the
 151      * element from the queue.  Iterators skip over Nodes with null
 152      * items.  Prior implementations of this class had a race between
 153      * poll() and remove(Object) where the same element would appear
 154      * to be successfully removed by two concurrent operations.  The
 155      * method remove(Object) also lazily unlinks deleted Nodes, but
 156      * this is merely an optimization.
 157      *
 158      * When constructing a Node (before enqueuing it) we avoid paying
 159      * for a volatile write to item by using Unsafe.putObject instead
 160      * of a normal write.  This allows the cost of enqueue to be
 161      * "one-and-a-half" CASes.
 162      *
 163      * Both head and tail may or may not point to a Node with a
 164      * non-null item.  If the queue is empty, all items must of course
 165      * be null.  Upon creation, both head and tail refer to a dummy
 166      * Node with null item.  Both head and tail are only updated using
 167      * CAS, so they never regress, although again this is merely an
 168      * optimization.
 169      */
 170 
 171     private static class Node<E> {
 172         volatile E item;
 173         volatile Node<E> next;
 174 
 175         /**
 176          * Constructs a new node.  Uses relaxed write because item can
 177          * only be seen after publication via casNext.
 178          */
 179         Node(E item) {
 180             UNSAFE.putObject(this, itemOffset, item);
 181         }
 182 
 183         boolean casItem(E cmp, E val) {
 184             return UNSAFE.compareAndSwapObject(this, itemOffset, cmp, val);
 185         }
 186 
 187         void lazySetNext(Node<E> val) {
 188             UNSAFE.putOrderedObject(this, nextOffset, val);
 189         }
 190 
 191         boolean casNext(Node<E> cmp, Node<E> val) {
 192             return UNSAFE.compareAndSwapObject(this, nextOffset, cmp, val);
 193         }
 194 
 195         // Unsafe mechanics
 196 
 197         private static final sun.misc.Unsafe UNSAFE =
 198             sun.misc.Unsafe.getUnsafe();
 199         private static final long nextOffset =
 200             objectFieldOffset(UNSAFE, "next", Node.class);
 201         private static final long itemOffset =
 202             objectFieldOffset(UNSAFE, "item", Node.class);
 203     }
 204 
 205     /**
 206      * A node from which the first live (non-deleted) node (if any)
 207      * can be reached in O(1) time.
 208      * Invariants:
 209      * - all live nodes are reachable from head via succ()
 210      * - head != null
 211      * - (tmp = head).next != tmp || tmp != head
 212      * Non-invariants:
 213      * - head.item may or may not be null.
 214      * - it is permitted for tail to lag behind head, that is, for tail
 215      *   to not be reachable from head!
 216      */
 217     private transient volatile Node<E> head;
 218 
 219     /**
 220      * A node from which the last node on list (that is, the unique
 221      * node with node.next == null) can be reached in O(1) time.
 222      * Invariants:
 223      * - the last node is always reachable from tail via succ()
 224      * - tail != null
 225      * Non-invariants:
 226      * - tail.item may or may not be null.
 227      * - it is permitted for tail to lag behind head, that is, for tail
 228      *   to not be reachable from head!
 229      * - tail.next may or may not be self-pointing to tail.
 230      */
 231     private transient volatile Node<E> tail;
 232 
 233 
 234     /**
 235      * Creates a {@code ConcurrentLinkedQueue} that is initially empty.
 236      */
 237     public ConcurrentLinkedQueue() {
 238         head = tail = new Node<E>(null);
 239     }
 240 
 241     /**
 242      * Creates a {@code ConcurrentLinkedQueue}
 243      * initially containing the elements of the given collection,
 244      * added in traversal order of the collection's iterator.
 245      *
 246      * @param c the collection of elements to initially contain
 247      * @throws NullPointerException if the specified collection or any
 248      *         of its elements are null
 249      */
 250     public ConcurrentLinkedQueue(Collection<? extends E> c) {
 251         Node<E> h = null, t = null;
 252         for (E e : c) {
 253             checkNotNull(e);
 254             Node<E> newNode = new Node<E>(e);
 255             if (h == null)
 256                 h = t = newNode;
 257             else {
 258                 t.lazySetNext(newNode);
 259                 t = newNode;
 260             }
 261         }
 262         if (h == null)
 263             h = t = new Node<E>(null);
 264         head = h;
 265         tail = t;
 266     }
 267 
 268     // Have to override just to update the javadoc
 269 
 270     /**
 271      * Inserts the specified element at the tail of this queue.


 272      *
 273      * @return {@code true} (as specified by {@link Collection#add})
 274      * @throws NullPointerException if the specified element is null
 275      */
 276     public boolean add(E e) {
 277         return offer(e);
 278     }
 279 
 280     /**
 281      * Try to CAS head to p. If successful, repoint old head to itself
 282      * as sentinel for succ(), below.
 283      */
 284     final void updateHead(Node<E> h, Node<E> p) {
 285         if (h != p && casHead(h, p))
 286             h.lazySetNext(h);
 287     }
 288 
 289     /**
 290      * Returns the successor of p, or the head node if p.next has been
 291      * linked to self, which will only be true if traversing with a
 292      * stale pointer that is now off the list.
 293      */
 294     final Node<E> succ(Node<E> p) {
 295         Node<E> next = p.next;
 296         return (p == next) ? head : next;
 297     }
 298 
 299     /**
 300      * Inserts the specified element at the tail of this queue.

 301      *
 302      * @return {@code true} (as specified by {@link Queue#offer})
 303      * @throws NullPointerException if the specified element is null
 304      */
 305     public boolean offer(E e) {
 306         checkNotNull(e);
 307         final Node<E> newNode = new Node<E>(e);
 308 
 309         for (Node<E> t = tail, p = t;;) {
 310             Node<E> q = p.next;
 311             if (q == null) {
 312                 // p is last node
 313                 if (p.casNext(null, newNode)) {
 314                     // Successful CAS is the linearization point
 315                     // for e to become an element of this queue,
 316                     // and for newNode to become "live".
 317                     if (p != t) // hop two nodes at a time
 318                         casTail(t, newNode);  // Failure is OK.
 319                     return true;
 320                 }
 321                 // Lost CAS race to another thread; re-read next
 322             }
 323             else if (p == q)
 324                 // We have fallen off list.  If tail is unchanged, it
 325                 // will also be off-list, in which case we need to
 326                 // jump to head, from which all live nodes are always
 327                 // reachable.  Else the new tail is a better bet.
 328                 p = (t != (t = tail)) ? t : head;
 329             else
 330                 // Check for tail updates after two hops.
 331                 p = (p != t && t != (t = tail)) ? t : q;
 332         }
 333     }
 334 
 335     public E poll() {
 336         restartFromHead:
 337         for (;;) {
 338             for (Node<E> h = head, p = h, q;;) {
 339                 E item = p.item;
 340 
 341                 if (item != null && p.casItem(item, null)) {
 342                     // Successful CAS is the linearization point
 343                     // for item to be removed from this queue.
 344                     if (p != h) // hop two nodes at a time
 345                         updateHead(h, ((q = p.next) != null) ? q : p);
 346                     return item;
 347                 }
 348                 else if ((q = p.next) == null) {
 349                     updateHead(h, p);
 350                     return null;
 351                 }
 352                 else if (p == q)
 353                     continue restartFromHead;
 354                 else
 355                     p = q;
 356             }
 357         }
 358     }
 359 
 360     public E peek() {
 361         restartFromHead:
 362         for (;;) {
 363             for (Node<E> h = head, p = h, q;;) {
 364                 E item = p.item;
 365                 if (item != null || (q = p.next) == null) {
 366                     updateHead(h, p);
 367                     return item;
 368                 }
 369                 else if (p == q)
 370                     continue restartFromHead;
 371                 else
 372                     p = q;
 373             }
 374         }
 375     }
 376 
 377     /**
 378      * Returns the first live (non-deleted) node on list, or null if none.
 379      * This is yet another variant of poll/peek; here returning the
 380      * first node, not element.  We could make peek() a wrapper around
 381      * first(), but that would cost an extra volatile read of item,
 382      * and the need to add a retry loop to deal with the possibility
 383      * of losing a race to a concurrent poll().
 384      */
 385     Node<E> first() {
 386         restartFromHead:
 387         for (;;) {
 388             for (Node<E> h = head, p = h, q;;) {
 389                 boolean hasItem = (p.item != null);
 390                 if (hasItem || (q = p.next) == null) {
 391                     updateHead(h, p);
 392                     return hasItem ? p : null;
 393                 }
 394                 else if (p == q)
 395                     continue restartFromHead;
 396                 else
 397                     p = q;
 398             }
 399         }
 400     }
 401 
 402     /**
 403      * Returns {@code true} if this queue contains no elements.
 404      *
 405      * @return {@code true} if this queue contains no elements
 406      */
 407     public boolean isEmpty() {
 408         return first() == null;
 409     }
 410 
 411     /**
 412      * Returns the number of elements in this queue.  If this queue
 413      * contains more than {@code Integer.MAX_VALUE} elements, returns
 414      * {@code Integer.MAX_VALUE}.
 415      *
 416      * <p>Beware that, unlike in most collections, this method is
 417      * <em>NOT</em> a constant-time operation. Because of the
 418      * asynchronous nature of these queues, determining the current
 419      * number of elements requires an O(n) traversal.
 420      * Additionally, if elements are added or removed during execution
 421      * of this method, the returned result may be inaccurate.  Thus,
 422      * this method is typically not very useful in concurrent
 423      * applications.
 424      *
 425      * @return the number of elements in this queue
 426      */
 427     public int size() {
 428         int count = 0;
 429         for (Node<E> p = first(); p != null; p = succ(p))
 430             if (p.item != null)
 431                 // Collection.size() spec says to max out
 432                 if (++count == Integer.MAX_VALUE)
 433                     break;
 434         return count;
 435     }
 436 
 437     /**
 438      * Returns {@code true} if this queue contains the specified element.
 439      * More formally, returns {@code true} if and only if this queue contains
 440      * at least one element {@code e} such that {@code o.equals(e)}.
 441      *
 442      * @param o object to be checked for containment in this queue
 443      * @return {@code true} if this queue contains the specified element
 444      */
 445     public boolean contains(Object o) {
 446         if (o == null) return false;
 447         for (Node<E> p = first(); p != null; p = succ(p)) {
 448             E item = p.item;
 449             if (item != null && o.equals(item))
 450                 return true;
 451         }
 452         return false;
 453     }
 454 
 455     /**
 456      * Removes a single instance of the specified element from this queue,
 457      * if it is present.  More formally, removes an element {@code e} such
 458      * that {@code o.equals(e)}, if this queue contains one or more such
 459      * elements.
 460      * Returns {@code true} if this queue contained the specified element
 461      * (or equivalently, if this queue changed as a result of the call).
 462      *
 463      * @param o element to be removed from this queue, if present
 464      * @return {@code true} if this queue changed as a result of the call
 465      */
 466     public boolean remove(Object o) {
 467         if (o == null) return false;
 468         Node<E> pred = null;
 469         for (Node<E> p = first(); p != null; p = succ(p)) {
 470             E item = p.item;
 471             if (item != null &&
 472                 o.equals(item) &&
 473                 p.casItem(item, null)) {
 474                 Node<E> next = succ(p);
 475                 if (pred != null && next != null)
 476                     pred.casNext(p, next);
 477                 return true;
 478             }
 479             pred = p;
 480         }
 481         return false;
 482     }
 483 
 484     /**
 485      * Appends all of the elements in the specified collection to the end of
 486      * this queue, in the order that they are returned by the specified
 487      * collection's iterator.  Attempts to {@code addAll} of a queue to
 488      * itself result in {@code IllegalArgumentException}.
 489      *
 490      * @param c the elements to be inserted into this queue
 491      * @return {@code true} if this queue changed as a result of the call
 492      * @throws NullPointerException if the specified collection or any
 493      *         of its elements are null
 494      * @throws IllegalArgumentException if the collection is this queue
 495      */
 496     public boolean addAll(Collection<? extends E> c) {
 497         if (c == this)
 498             // As historically specified in AbstractQueue#addAll
 499             throw new IllegalArgumentException();
 500 
 501         // Copy c into a private chain of Nodes
 502         Node<E> beginningOfTheEnd = null, last = null;
 503         for (E e : c) {
 504             checkNotNull(e);
 505             Node<E> newNode = new Node<E>(e);
 506             if (beginningOfTheEnd == null)
 507                 beginningOfTheEnd = last = newNode;
 508             else {
 509                 last.lazySetNext(newNode);
 510                 last = newNode;
 511             }
 512         }
 513         if (beginningOfTheEnd == null)
 514             return false;
 515 
 516         // Atomically append the chain at the tail of this collection
 517         for (Node<E> t = tail, p = t;;) {
 518             Node<E> q = p.next;
 519             if (q == null) {
 520                 // p is last node
 521                 if (p.casNext(null, beginningOfTheEnd)) {
 522                     // Successful CAS is the linearization point
 523                     // for all elements to be added to this queue.
 524                     if (!casTail(t, last)) {
 525                         // Try a little harder to update tail,
 526                         // since we may be adding many elements.
 527                         t = tail;
 528                         if (last.next == null)
 529                             casTail(t, last);
 530                     }
 531                     return true;
 532                 }
 533                 // Lost CAS race to another thread; re-read next
 534             }
 535             else if (p == q)
 536                 // We have fallen off list.  If tail is unchanged, it
 537                 // will also be off-list, in which case we need to
 538                 // jump to head, from which all live nodes are always
 539                 // reachable.  Else the new tail is a better bet.
 540                 p = (t != (t = tail)) ? t : head;
 541             else
 542                 // Check for tail updates after two hops.
 543                 p = (p != t && t != (t = tail)) ? t : q;
 544         }
 545     }
 546 
 547     /**
 548      * Returns an array containing all of the elements in this queue, in
 549      * proper sequence.
 550      *
 551      * <p>The returned array will be "safe" in that no references to it are
 552      * maintained by this queue.  (In other words, this method must allocate
 553      * a new array).  The caller is thus free to modify the returned array.
 554      *
 555      * <p>This method acts as bridge between array-based and collection-based
 556      * APIs.
 557      *
 558      * @return an array containing all of the elements in this queue
 559      */
 560     public Object[] toArray() {
 561         // Use ArrayList to deal with resizing.
 562         ArrayList<E> al = new ArrayList<E>();
 563         for (Node<E> p = first(); p != null; p = succ(p)) {
 564             E item = p.item;
 565             if (item != null)
 566                 al.add(item);
 567         }
 568         return al.toArray();
 569     }
 570 
 571     /**
 572      * Returns an array containing all of the elements in this queue, in
 573      * proper sequence; the runtime type of the returned array is that of
 574      * the specified array.  If the queue fits in the specified array, it
 575      * is returned therein.  Otherwise, a new array is allocated with the
 576      * runtime type of the specified array and the size of this queue.
 577      *
 578      * <p>If this queue fits in the specified array with room to spare
 579      * (i.e., the array has more elements than this queue), the element in
 580      * the array immediately following the end of the queue is set to
 581      * {@code null}.
 582      *
 583      * <p>Like the {@link #toArray()} method, this method acts as bridge between
 584      * array-based and collection-based APIs.  Further, this method allows
 585      * precise control over the runtime type of the output array, and may,
 586      * under certain circumstances, be used to save allocation costs.
 587      *
 588      * <p>Suppose {@code x} is a queue known to contain only strings.
 589      * The following code can be used to dump the queue into a newly
 590      * allocated array of {@code String}:
 591      *
 592      * <pre>
 593      *     String[] y = x.toArray(new String[0]);</pre>
 594      *
 595      * Note that {@code toArray(new Object[0])} is identical in function to
 596      * {@code toArray()}.
 597      *
 598      * @param a the array into which the elements of the queue are to
 599      *          be stored, if it is big enough; otherwise, a new array of the
 600      *          same runtime type is allocated for this purpose
 601      * @return an array containing all of the elements in this queue
 602      * @throws ArrayStoreException if the runtime type of the specified array
 603      *         is not a supertype of the runtime type of every element in
 604      *         this queue
 605      * @throws NullPointerException if the specified array is null
 606      */
 607     @SuppressWarnings("unchecked")
 608     public <T> T[] toArray(T[] a) {
 609         // try to use sent-in array
 610         int k = 0;
 611         Node<E> p;
 612         for (p = first(); p != null && k < a.length; p = succ(p)) {
 613             E item = p.item;
 614             if (item != null)
 615                 a[k++] = (T)item;
 616         }
 617         if (p == null) {
 618             if (k < a.length)
 619                 a[k] = null;
 620             return a;
 621         }
 622 
 623         // If won't fit, use ArrayList version
 624         ArrayList<E> al = new ArrayList<E>();
 625         for (Node<E> q = first(); q != null; q = succ(q)) {
 626             E item = q.item;
 627             if (item != null)
 628                 al.add(item);
 629         }
 630         return al.toArray(a);
 631     }
 632 
 633     /**
 634      * Returns an iterator over the elements in this queue in proper sequence.
 635      * The elements will be returned in order from first (head) to last (tail).
 636      *
 637      * <p>The returned {@code Iterator} is a "weakly consistent" iterator that
 638      * will never throw {@link java.util.ConcurrentModificationException
 639      * ConcurrentModificationException},
 640      * and guarantees to traverse elements as they existed upon
 641      * construction of the iterator, and may (but is not guaranteed to)
 642      * reflect any modifications subsequent to construction.
 643      *
 644      * @return an iterator over the elements in this queue in proper sequence
 645      */
 646     public Iterator<E> iterator() {
 647         return new Itr();
 648     }
 649 
 650     private class Itr implements Iterator<E> {
 651         /**
 652          * Next node to return item for.
 653          */
 654         private Node<E> nextNode;
 655 
 656         /**
 657          * nextItem holds on to item fields because once we claim
 658          * that an element exists in hasNext(), we must return it in
 659          * the following next() call even if it was in the process of
 660          * being removed when hasNext() was called.
 661          */
 662         private E nextItem;
 663 
 664         /**
 665          * Node of the last returned item, to support remove.
 666          */
 667         private Node<E> lastRet;
 668 
 669         Itr() {
 670             advance();
 671         }
 672 
 673         /**
 674          * Moves to next valid node and returns item to return for
 675          * next(), or null if no such.
 676          */
 677         private E advance() {
 678             lastRet = nextNode;
 679             E x = nextItem;
 680 
 681             Node<E> pred, p;
 682             if (nextNode == null) {
 683                 p = first();
 684                 pred = null;
 685             } else {
 686                 pred = nextNode;
 687                 p = succ(nextNode);
 688             }
 689 
 690             for (;;) {
 691                 if (p == null) {
 692                     nextNode = null;
 693                     nextItem = null;
 694                     return x;
 695                 }
 696                 E item = p.item;
 697                 if (item != null) {
 698                     nextNode = p;
 699                     nextItem = item;
 700                     return x;
 701                 } else {
 702                     // skip over nulls
 703                     Node<E> next = succ(p);
 704                     if (pred != null && next != null)
 705                         pred.casNext(p, next);
 706                     p = next;
 707                 }
 708             }
 709         }
 710 
 711         public boolean hasNext() {
 712             return nextNode != null;
 713         }
 714 
 715         public E next() {
 716             if (nextNode == null) throw new NoSuchElementException();
 717             return advance();
 718         }
 719 
 720         public void remove() {
 721             Node<E> l = lastRet;
 722             if (l == null) throw new IllegalStateException();
 723             // rely on a future traversal to relink.
 724             l.item = null;
 725             lastRet = null;
 726         }
 727     }
 728 
 729     /**
 730      * Saves the state to a stream (that is, serializes it).
 731      *
 732      * @serialData All of the elements (each an {@code E}) in
 733      * the proper order, followed by a null
 734      * @param s the stream
 735      */
 736     private void writeObject(java.io.ObjectOutputStream s)
 737         throws java.io.IOException {
 738 
 739         // Write out any hidden stuff
 740         s.defaultWriteObject();
 741 
 742         // Write out all elements in the proper order.
 743         for (Node<E> p = first(); p != null; p = succ(p)) {
 744             Object item = p.item;
 745             if (item != null)
 746                 s.writeObject(item);
 747         }
 748 
 749         // Use trailing null as sentinel
 750         s.writeObject(null);
 751     }
 752 
 753     /**
 754      * Reconstitutes the instance from a stream (that is, deserializes it).
 755      * @param s the stream
 756      */
 757     private void readObject(java.io.ObjectInputStream s)
 758         throws java.io.IOException, ClassNotFoundException {
 759         s.defaultReadObject();
 760 
 761         // Read in elements until trailing null sentinel found
 762         Node<E> h = null, t = null;
 763         Object item;
 764         while ((item = s.readObject()) != null) {
 765             @SuppressWarnings("unchecked")
 766             Node<E> newNode = new Node<E>((E) item);
 767             if (h == null)
 768                 h = t = newNode;
 769             else {
 770                 t.lazySetNext(newNode);
 771                 t = newNode;
 772             }
 773         }
 774         if (h == null)
 775             h = t = new Node<E>(null);
 776         head = h;
 777         tail = t;
 778     }
 779 
 780     /**
 781      * Throws NullPointerException if argument is null.
 782      *
 783      * @param v the element
 784      */
 785     private static void checkNotNull(Object v) {
 786         if (v == null)
 787             throw new NullPointerException();
 788     }
 789 
 790     // Unsafe mechanics
 791 
 792     private static final sun.misc.Unsafe UNSAFE = sun.misc.Unsafe.getUnsafe();
 793     private static final long headOffset =
 794         objectFieldOffset(UNSAFE, "head", ConcurrentLinkedQueue.class);
 795     private static final long tailOffset =
 796         objectFieldOffset(UNSAFE, "tail", ConcurrentLinkedQueue.class);
 797 
 798     private boolean casTail(Node<E> cmp, Node<E> val) {
 799         return UNSAFE.compareAndSwapObject(this, tailOffset, cmp, val);
 800     }
 801 
 802     private boolean casHead(Node<E> cmp, Node<E> val) {
 803         return UNSAFE.compareAndSwapObject(this, headOffset, cmp, val);
 804     }
 805 
 806     static long objectFieldOffset(sun.misc.Unsafe UNSAFE,
 807                                   String field, Class<?> klazz) {
 808         try {
 809             return UNSAFE.objectFieldOffset(klazz.getDeclaredField(field));
 810         } catch (NoSuchFieldException e) {
 811             // Convert Exception to corresponding Error
 812             NoSuchFieldError error = new NoSuchFieldError(field);
 813             error.initCause(e);
 814             throw error;
 815         }
 816     }
 817 }
--- EOF ---