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/publicdomain/zero/1.0/
  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  * java.util.ConcurrentModificationException}, and may proceed concurrently
  69  * with 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, and so may report
  76  * inaccurate results if this collection is modified during traversal.
  77  * Additionally, the bulk operations {@code addAll},
  78  * {@code removeAll}, {@code retainAll}, {@code containsAll},
  79  * {@code equals}, and {@code toArray} are <em>not</em> guaranteed
  80  * to be performed atomically. For example, an iterator operating
  81  * concurrently with an {@code addAll} operation might view only some
  82  * of the added elements.
  83  *
  84  * <p>This class and its iterator implement all of the <em>optional</em>
  85  * methods of the {@link Queue} and {@link Iterator} interfaces.
  86  *
  87  * <p>Memory consistency effects: As with other concurrent
  88  * collections, actions in a thread prior to placing an object into a
  89  * {@code ConcurrentLinkedQueue}
  90  * <a href="package-summary.html#MemoryVisibility"><i>happen-before</i></a>
  91  * actions subsequent to the access or removal of that element from
  92  * the {@code ConcurrentLinkedQueue} in another thread.
  93  *
  94  * <p>This class is a member of the
  95  * <a href="{@docRoot}/../technotes/guides/collections/index.html">
  96  * Java Collections Framework</a>.
  97  *
  98  * @since 1.5
  99  * @author Doug Lea
 100  * @param <E> the type of elements held in this collection
 101  *
 102  */
 103 public class ConcurrentLinkedQueue<E> extends AbstractQueue<E>
 104         implements Queue<E>, java.io.Serializable {
 105     private static final long serialVersionUID = 196745693267521676L;
 106 
 107     /*
 108      * This is a modification of the Michael & Scott algorithm,
 109      * adapted for a garbage-collected environment, with support for
 110      * interior node deletion (to support remove(Object)).  For
 111      * explanation, read the paper.
 112      *
 113      * Note that like most non-blocking algorithms in this package,
 114      * this implementation relies on the fact that in garbage
 115      * collected systems, there is no possibility of ABA problems due
 116      * to recycled nodes, so there is no need to use "counted
 117      * pointers" or related techniques seen in versions used in
 118      * non-GC'ed settings.
 119      *
 120      * The fundamental invariants are:
 121      * - There is exactly one (last) Node with a null next reference,
 122      *   which is CASed when enqueueing.  This last Node can be
 123      *   reached in O(1) time from tail, but tail is merely an
 124      *   optimization - it can always be reached in O(N) time from
 125      *   head as well.
 126      * - The elements contained in the queue are the non-null items in
 127      *   Nodes that are reachable from head.  CASing the item
 128      *   reference of a Node to null atomically removes it from the
 129      *   queue.  Reachability of all elements from head must remain
 130      *   true even in the case of concurrent modifications that cause
 131      *   head to advance.  A dequeued Node may remain in use
 132      *   indefinitely due to creation of an Iterator or simply a
 133      *   poll() that has lost its time slice.
 134      *
 135      * The above might appear to imply that all Nodes are GC-reachable
 136      * from a predecessor dequeued Node.  That would cause two problems:
 137      * - allow a rogue Iterator to cause unbounded memory retention
 138      * - cause cross-generational linking of old Nodes to new Nodes if
 139      *   a Node was tenured while live, which generational GCs have a
 140      *   hard time dealing with, causing repeated major collections.
 141      * However, only non-deleted Nodes need to be reachable from
 142      * dequeued Nodes, and reachability does not necessarily have to
 143      * be of the kind understood by the GC.  We use the trick of
 144      * linking a Node that has just been dequeued to itself.  Such a
 145      * self-link implicitly means to advance to head.
 146      *
 147      * Both head and tail are permitted to lag.  In fact, failing to
 148      * update them every time one could is a significant optimization
 149      * (fewer CASes). As with LinkedTransferQueue (see the internal
 150      * documentation for that class), we use a slack threshold of two;
 151      * that is, we update head/tail when the current pointer appears
 152      * to be two or more steps away from the first/last node.
 153      *
 154      * Since head and tail are updated concurrently and independently,
 155      * it is possible for tail to lag behind head (why not)?
 156      *
 157      * CASing a Node's item reference to null atomically removes the
 158      * element from the queue.  Iterators skip over Nodes with null
 159      * items.  Prior implementations of this class had a race between
 160      * poll() and remove(Object) where the same element would appear
 161      * to be successfully removed by two concurrent operations.  The
 162      * method remove(Object) also lazily unlinks deleted Nodes, but
 163      * this is merely an optimization.
 164      *
 165      * When constructing a Node (before enqueuing it) we avoid paying
 166      * for a volatile write to item by using Unsafe.putObject instead
 167      * of a normal write.  This allows the cost of enqueue to be
 168      * "one-and-a-half" CASes.
 169      *
 170      * Both head and tail may or may not point to a Node with a
 171      * non-null item.  If the queue is empty, all items must of course
 172      * be null.  Upon creation, both head and tail refer to a dummy
 173      * Node with null item.  Both head and tail are only updated using
 174      * CAS, so they never regress, although again this is merely an
 175      * optimization.
 176      */
 177 
 178     private static class Node<E> {
 179         volatile E item;
 180         volatile Node<E> next;
 181 
 182         /**
 183          * Constructs a new node.  Uses relaxed write because item can
 184          * only be seen after publication via casNext.
 185          */
 186         Node(E item) {
 187             UNSAFE.putObject(this, itemOffset, item);
 188         }
 189 
 190         boolean casItem(E cmp, E val) {
 191             return UNSAFE.compareAndSwapObject(this, itemOffset, cmp, val);
 192         }
 193 
 194         void lazySetNext(Node<E> val) {
 195             UNSAFE.putOrderedObject(this, nextOffset, val);
 196         }
 197 
 198         boolean casNext(Node<E> cmp, Node<E> val) {
 199             return UNSAFE.compareAndSwapObject(this, nextOffset, cmp, val);
 200         }
 201 
 202         // Unsafe mechanics
 203 
 204         private static final sun.misc.Unsafe UNSAFE;
 205         private static final long itemOffset;
 206         private static final long nextOffset;
 207 
 208         static {
 209             try {
 210                 UNSAFE = sun.misc.Unsafe.getUnsafe();
 211                 Class k = Node.class;
 212                 itemOffset = UNSAFE.objectFieldOffset
 213                     (k.getDeclaredField("item"));
 214                 nextOffset = UNSAFE.objectFieldOffset
 215                     (k.getDeclaredField("next"));
 216             } catch (Exception e) {
 217                 throw new Error(e);
 218             }
 219         }
 220     }
 221 
 222     /**
 223      * A node from which the first live (non-deleted) node (if any)
 224      * can be reached in O(1) time.
 225      * Invariants:
 226      * - all live nodes are reachable from head via succ()
 227      * - head != null
 228      * - (tmp = head).next != tmp || tmp != head
 229      * Non-invariants:
 230      * - head.item may or may not be null.
 231      * - it is permitted for tail to lag behind head, that is, for tail
 232      *   to not be reachable from head!
 233      */
 234     private transient volatile Node<E> head;
 235 
 236     /**
 237      * A node from which the last node on list (that is, the unique
 238      * node with node.next == null) can be reached in O(1) time.
 239      * Invariants:
 240      * - the last node is always reachable from tail via succ()
 241      * - tail != null
 242      * Non-invariants:
 243      * - tail.item may or may not be null.
 244      * - it is permitted for tail to lag behind head, that is, for tail
 245      *   to not be reachable from head!
 246      * - tail.next may or may not be self-pointing to tail.
 247      */
 248     private transient volatile Node<E> tail;
 249 
 250 
 251     /**
 252      * Creates a {@code ConcurrentLinkedQueue} that is initially empty.
 253      */
 254     public ConcurrentLinkedQueue() {
 255         head = tail = new Node<E>(null);
 256     }
 257 
 258     /**
 259      * Creates a {@code ConcurrentLinkedQueue}
 260      * initially containing the elements of the given collection,
 261      * added in traversal order of the collection's iterator.
 262      *
 263      * @param c the collection of elements to initially contain
 264      * @throws NullPointerException if the specified collection or any
 265      *         of its elements are null
 266      */
 267     public ConcurrentLinkedQueue(Collection<? extends E> c) {
 268         Node<E> h = null, t = null;
 269         for (E e : c) {
 270             checkNotNull(e);
 271             Node<E> newNode = new Node<E>(e);
 272             if (h == null)
 273                 h = t = newNode;
 274             else {
 275                 t.lazySetNext(newNode);
 276                 t = newNode;
 277             }
 278         }
 279         if (h == null)
 280             h = t = new Node<E>(null);
 281         head = h;
 282         tail = t;
 283     }
 284 
 285     // Have to override just to update the javadoc
 286 
 287     /**
 288      * Inserts the specified element at the tail of this queue.
 289      * As the queue is unbounded, this method will never throw
 290      * {@link IllegalStateException} or return {@code false}.
 291      *
 292      * @return {@code true} (as specified by {@link Collection#add})
 293      * @throws NullPointerException if the specified element is null
 294      */
 295     public boolean add(E e) {
 296         return offer(e);
 297     }
 298 
 299     /**
 300      * Try to CAS head to p. If successful, repoint old head to itself
 301      * as sentinel for succ(), below.
 302      */
 303     final void updateHead(Node<E> h, Node<E> p) {
 304         if (h != p && casHead(h, p))
 305             h.lazySetNext(h);
 306     }
 307 
 308     /**
 309      * Returns the successor of p, or the head node if p.next has been
 310      * linked to self, which will only be true if traversing with a
 311      * stale pointer that is now off the list.
 312      */
 313     final Node<E> succ(Node<E> p) {
 314         Node<E> next = p.next;
 315         return (p == next) ? head : next;
 316     }
 317 
 318     /**
 319      * Inserts the specified element at the tail of this queue.
 320      * As the queue is unbounded, this method will never return {@code false}.
 321      *
 322      * @return {@code true} (as specified by {@link Queue#offer})
 323      * @throws NullPointerException if the specified element is null
 324      */
 325     public boolean offer(E e) {
 326         checkNotNull(e);
 327         final Node<E> newNode = new Node<E>(e);
 328 
 329         for (Node<E> t = tail, p = t;;) {
 330             Node<E> q = p.next;
 331             if (q == null) {
 332                 // p is last node
 333                 if (p.casNext(null, newNode)) {
 334                     // Successful CAS is the linearization point
 335                     // for e to become an element of this queue,
 336                     // and for newNode to become "live".
 337                     if (p != t) // hop two nodes at a time
 338                         casTail(t, newNode);  // Failure is OK.
 339                     return true;
 340                 }
 341                 // Lost CAS race to another thread; re-read next
 342             }
 343             else if (p == q)
 344                 // We have fallen off list.  If tail is unchanged, it
 345                 // will also be off-list, in which case we need to
 346                 // jump to head, from which all live nodes are always
 347                 // reachable.  Else the new tail is a better bet.
 348                 p = (t != (t = tail)) ? t : head;
 349             else
 350                 // Check for tail updates after two hops.
 351                 p = (p != t && t != (t = tail)) ? t : q;
 352         }
 353     }
 354 
 355     public E poll() {
 356         restartFromHead:
 357         for (;;) {
 358             for (Node<E> h = head, p = h, q;;) {
 359                 E item = p.item;
 360 
 361                 if (item != null && p.casItem(item, null)) {
 362                     // Successful CAS is the linearization point
 363                     // for item to be removed from this queue.
 364                     if (p != h) // hop two nodes at a time
 365                         updateHead(h, ((q = p.next) != null) ? q : p);
 366                     return item;
 367                 }
 368                 else if ((q = p.next) == null) {
 369                     updateHead(h, p);
 370                     return null;
 371                 }
 372                 else if (p == q)
 373                     continue restartFromHead;
 374                 else
 375                     p = q;
 376             }
 377         }
 378     }
 379 
 380     public E peek() {
 381         restartFromHead:
 382         for (;;) {
 383             for (Node<E> h = head, p = h, q;;) {
 384                 E item = p.item;
 385                 if (item != null || (q = p.next) == null) {
 386                     updateHead(h, p);
 387                     return item;
 388                 }
 389                 else if (p == q)
 390                     continue restartFromHead;
 391                 else
 392                     p = q;
 393             }
 394         }
 395     }
 396 
 397     /**
 398      * Returns the first live (non-deleted) node on list, or null if none.
 399      * This is yet another variant of poll/peek; here returning the
 400      * first node, not element.  We could make peek() a wrapper around
 401      * first(), but that would cost an extra volatile read of item,
 402      * and the need to add a retry loop to deal with the possibility
 403      * of losing a race to a concurrent poll().
 404      */
 405     Node<E> first() {
 406         restartFromHead:
 407         for (;;) {
 408             for (Node<E> h = head, p = h, q;;) {
 409                 boolean hasItem = (p.item != null);
 410                 if (hasItem || (q = p.next) == null) {
 411                     updateHead(h, p);
 412                     return hasItem ? p : null;
 413                 }
 414                 else if (p == q)
 415                     continue restartFromHead;
 416                 else
 417                     p = q;
 418             }
 419         }
 420     }
 421 
 422     /**
 423      * Returns {@code true} if this queue contains no elements.
 424      *
 425      * @return {@code true} if this queue contains no elements
 426      */
 427     public boolean isEmpty() {
 428         return first() == null;
 429     }
 430 
 431     /**
 432      * Returns the number of elements in this queue.  If this queue
 433      * contains more than {@code Integer.MAX_VALUE} elements, returns
 434      * {@code Integer.MAX_VALUE}.
 435      *
 436      * <p>Beware that, unlike in most collections, this method is
 437      * <em>NOT</em> a constant-time operation. Because of the
 438      * asynchronous nature of these queues, determining the current
 439      * number of elements requires an O(n) traversal.
 440      * Additionally, if elements are added or removed during execution
 441      * of this method, the returned result may be inaccurate.  Thus,
 442      * this method is typically not very useful in concurrent
 443      * applications.
 444      *
 445      * @return the number of elements in this queue
 446      */
 447     public int size() {
 448         int count = 0;
 449         for (Node<E> p = first(); p != null; p = succ(p))
 450             if (p.item != null)
 451                 // Collection.size() spec says to max out
 452                 if (++count == Integer.MAX_VALUE)
 453                     break;
 454         return count;
 455     }
 456 
 457     /**
 458      * Returns {@code true} if this queue contains the specified element.
 459      * More formally, returns {@code true} if and only if this queue contains
 460      * at least one element {@code e} such that {@code o.equals(e)}.
 461      *
 462      * @param o object to be checked for containment in this queue
 463      * @return {@code true} if this queue contains the specified element
 464      */
 465     public boolean contains(Object o) {
 466         if (o == null) return false;
 467         for (Node<E> p = first(); p != null; p = succ(p)) {
 468             E item = p.item;
 469             if (item != null && o.equals(item))
 470                 return true;
 471         }
 472         return false;
 473     }
 474 
 475     /**
 476      * Removes a single instance of the specified element from this queue,
 477      * if it is present.  More formally, removes an element {@code e} such
 478      * that {@code o.equals(e)}, if this queue contains one or more such
 479      * elements.
 480      * Returns {@code true} if this queue contained the specified element
 481      * (or equivalently, if this queue changed as a result of the call).
 482      *
 483      * @param o element to be removed from this queue, if present
 484      * @return {@code true} if this queue changed as a result of the call
 485      */
 486     public boolean remove(Object o) {
 487         if (o == null) return false;
 488         Node<E> pred = null;
 489         for (Node<E> p = first(); p != null; p = succ(p)) {
 490             E item = p.item;
 491             if (item != null &&
 492                 o.equals(item) &&
 493                 p.casItem(item, null)) {
 494                 Node<E> next = succ(p);
 495                 if (pred != null && next != null)
 496                     pred.casNext(p, next);
 497                 return true;
 498             }
 499             pred = p;
 500         }
 501         return false;
 502     }
 503 
 504     /**
 505      * Appends all of the elements in the specified collection to the end of
 506      * this queue, in the order that they are returned by the specified
 507      * collection's iterator.  Attempts to {@code addAll} of a queue to
 508      * itself result in {@code IllegalArgumentException}.
 509      *
 510      * @param c the elements to be inserted into this queue
 511      * @return {@code true} if this queue changed as a result of the call
 512      * @throws NullPointerException if the specified collection or any
 513      *         of its elements are null
 514      * @throws IllegalArgumentException if the collection is this queue
 515      */
 516     public boolean addAll(Collection<? extends E> c) {
 517         if (c == this)
 518             // As historically specified in AbstractQueue#addAll
 519             throw new IllegalArgumentException();
 520 
 521         // Copy c into a private chain of Nodes
 522         Node<E> beginningOfTheEnd = null, last = null;
 523         for (E e : c) {
 524             checkNotNull(e);
 525             Node<E> newNode = new Node<E>(e);
 526             if (beginningOfTheEnd == null)
 527                 beginningOfTheEnd = last = newNode;
 528             else {
 529                 last.lazySetNext(newNode);
 530                 last = newNode;
 531             }
 532         }
 533         if (beginningOfTheEnd == null)
 534             return false;
 535 
 536         // Atomically append the chain at the tail of this collection
 537         for (Node<E> t = tail, p = t;;) {
 538             Node<E> q = p.next;
 539             if (q == null) {
 540                 // p is last node
 541                 if (p.casNext(null, beginningOfTheEnd)) {
 542                     // Successful CAS is the linearization point
 543                     // for all elements to be added to this queue.
 544                     if (!casTail(t, last)) {
 545                         // Try a little harder to update tail,
 546                         // since we may be adding many elements.
 547                         t = tail;
 548                         if (last.next == null)
 549                             casTail(t, last);
 550                     }
 551                     return true;
 552                 }
 553                 // Lost CAS race to another thread; re-read next
 554             }
 555             else if (p == q)
 556                 // We have fallen off list.  If tail is unchanged, it
 557                 // will also be off-list, in which case we need to
 558                 // jump to head, from which all live nodes are always
 559                 // reachable.  Else the new tail is a better bet.
 560                 p = (t != (t = tail)) ? t : head;
 561             else
 562                 // Check for tail updates after two hops.
 563                 p = (p != t && t != (t = tail)) ? t : q;
 564         }
 565     }
 566 
 567     /**
 568      * Returns an array containing all of the elements in this queue, in
 569      * proper sequence.
 570      *
 571      * <p>The returned array will be "safe" in that no references to it are
 572      * maintained by this queue.  (In other words, this method must allocate
 573      * a new array).  The caller is thus free to modify the returned array.
 574      *
 575      * <p>This method acts as bridge between array-based and collection-based
 576      * APIs.
 577      *
 578      * @return an array containing all of the elements in this queue
 579      */
 580     public Object[] toArray() {
 581         // Use ArrayList to deal with resizing.
 582         ArrayList<E> al = new ArrayList<E>();
 583         for (Node<E> p = first(); p != null; p = succ(p)) {
 584             E item = p.item;
 585             if (item != null)
 586                 al.add(item);
 587         }
 588         return al.toArray();
 589     }
 590 
 591     /**
 592      * Returns an array containing all of the elements in this queue, in
 593      * proper sequence; the runtime type of the returned array is that of
 594      * the specified array.  If the queue fits in the specified array, it
 595      * is returned therein.  Otherwise, a new array is allocated with the
 596      * runtime type of the specified array and the size of this queue.
 597      *
 598      * <p>If this queue fits in the specified array with room to spare
 599      * (i.e., the array has more elements than this queue), the element in
 600      * the array immediately following the end of the queue is set to
 601      * {@code null}.
 602      *
 603      * <p>Like the {@link #toArray()} method, this method acts as bridge between
 604      * array-based and collection-based APIs.  Further, this method allows
 605      * precise control over the runtime type of the output array, and may,
 606      * under certain circumstances, be used to save allocation costs.
 607      *
 608      * <p>Suppose {@code x} is a queue known to contain only strings.
 609      * The following code can be used to dump the queue into a newly
 610      * allocated array of {@code String}:
 611      *
 612      * <pre>
 613      *     String[] y = x.toArray(new String[0]);</pre>
 614      *
 615      * Note that {@code toArray(new Object[0])} is identical in function to
 616      * {@code toArray()}.
 617      *
 618      * @param a the array into which the elements of the queue are to
 619      *          be stored, if it is big enough; otherwise, a new array of the
 620      *          same runtime type is allocated for this purpose
 621      * @return an array containing all of the elements in this queue
 622      * @throws ArrayStoreException if the runtime type of the specified array
 623      *         is not a supertype of the runtime type of every element in
 624      *         this queue
 625      * @throws NullPointerException if the specified array is null
 626      */
 627     @SuppressWarnings("unchecked")
 628     public <T> T[] toArray(T[] a) {
 629         // try to use sent-in array
 630         int k = 0;
 631         Node<E> p;
 632         for (p = first(); p != null && k < a.length; p = succ(p)) {
 633             E item = p.item;
 634             if (item != null)
 635                 a[k++] = (T)item;
 636         }
 637         if (p == null) {
 638             if (k < a.length)
 639                 a[k] = null;
 640             return a;
 641         }
 642 
 643         // If won't fit, use ArrayList version
 644         ArrayList<E> al = new ArrayList<E>();
 645         for (Node<E> q = first(); q != null; q = succ(q)) {
 646             E item = q.item;
 647             if (item != null)
 648                 al.add(item);
 649         }
 650         return al.toArray(a);
 651     }
 652 
 653     /**
 654      * Returns an iterator over the elements in this queue in proper sequence.
 655      * The elements will be returned in order from first (head) to last (tail).
 656      *
 657      * <p>The returned iterator is a "weakly consistent" iterator that
 658      * will never throw {@link java.util.ConcurrentModificationException
 659      * ConcurrentModificationException}, and guarantees to traverse
 660      * elements as they existed upon construction of the iterator, and
 661      * may (but is not guaranteed to) reflect any modifications
 662      * subsequent to construction.
 663      *
 664      * @return an iterator over the elements in this queue in proper sequence
 665      */
 666     public Iterator<E> iterator() {
 667         return new Itr();
 668     }
 669 
 670     private class Itr implements Iterator<E> {
 671         /**
 672          * Next node to return item for.
 673          */
 674         private Node<E> nextNode;
 675 
 676         /**
 677          * nextItem holds on to item fields because once we claim
 678          * that an element exists in hasNext(), we must return it in
 679          * the following next() call even if it was in the process of
 680          * being removed when hasNext() was called.
 681          */
 682         private E nextItem;
 683 
 684         /**
 685          * Node of the last returned item, to support remove.
 686          */
 687         private Node<E> lastRet;
 688 
 689         Itr() {
 690             advance();
 691         }
 692 
 693         /**
 694          * Moves to next valid node and returns item to return for
 695          * next(), or null if no such.
 696          */
 697         private E advance() {
 698             lastRet = nextNode;
 699             E x = nextItem;
 700 
 701             Node<E> pred, p;
 702             if (nextNode == null) {
 703                 p = first();
 704                 pred = null;
 705             } else {
 706                 pred = nextNode;
 707                 p = succ(nextNode);
 708             }
 709 
 710             for (;;) {
 711                 if (p == null) {
 712                     nextNode = null;
 713                     nextItem = null;
 714                     return x;
 715                 }
 716                 E item = p.item;
 717                 if (item != null) {
 718                     nextNode = p;
 719                     nextItem = item;
 720                     return x;
 721                 } else {
 722                     // skip over nulls
 723                     Node<E> next = succ(p);
 724                     if (pred != null && next != null)
 725                         pred.casNext(p, next);
 726                     p = next;
 727                 }
 728             }
 729         }
 730 
 731         public boolean hasNext() {
 732             return nextNode != null;
 733         }
 734 
 735         public E next() {
 736             if (nextNode == null) throw new NoSuchElementException();
 737             return advance();
 738         }
 739 
 740         public void remove() {
 741             Node<E> l = lastRet;
 742             if (l == null) throw new IllegalStateException();
 743             // rely on a future traversal to relink.
 744             l.item = null;
 745             lastRet = null;
 746         }
 747     }
 748 
 749     /**
 750      * Saves the state to a stream (that is, serializes it).
 751      *
 752      * @serialData All of the elements (each an {@code E}) in
 753      * the proper order, followed by a null
 754      * @param s the stream
 755      */
 756     private void writeObject(java.io.ObjectOutputStream s)
 757         throws java.io.IOException {
 758 
 759         // Write out any hidden stuff
 760         s.defaultWriteObject();
 761 
 762         // Write out all elements in the proper order.
 763         for (Node<E> p = first(); p != null; p = succ(p)) {
 764             Object item = p.item;
 765             if (item != null)
 766                 s.writeObject(item);
 767         }
 768 
 769         // Use trailing null as sentinel
 770         s.writeObject(null);
 771     }
 772 
 773     /**
 774      * Reconstitutes the instance from a stream (that is, deserializes it).
 775      * @param s the stream
 776      */
 777     private void readObject(java.io.ObjectInputStream s)
 778         throws java.io.IOException, ClassNotFoundException {
 779         s.defaultReadObject();
 780 
 781         // Read in elements until trailing null sentinel found
 782         Node<E> h = null, t = null;
 783         Object item;
 784         while ((item = s.readObject()) != null) {
 785             @SuppressWarnings("unchecked")
 786             Node<E> newNode = new Node<E>((E) item);
 787             if (h == null)
 788                 h = t = newNode;
 789             else {
 790                 t.lazySetNext(newNode);
 791                 t = newNode;
 792             }
 793         }
 794         if (h == null)
 795             h = t = new Node<E>(null);
 796         head = h;
 797         tail = t;
 798     }
 799 
 800     /**
 801      * Throws NullPointerException if argument is null.
 802      *
 803      * @param v the element
 804      */
 805     private static void checkNotNull(Object v) {
 806         if (v == null)
 807             throw new NullPointerException();
 808     }
 809 
 810     private boolean casTail(Node<E> cmp, Node<E> val) {
 811         return UNSAFE.compareAndSwapObject(this, tailOffset, cmp, val);
 812     }
 813 
 814     private boolean casHead(Node<E> cmp, Node<E> val) {
 815         return UNSAFE.compareAndSwapObject(this, headOffset, cmp, val);
 816     }
 817 
 818     // Unsafe mechanics
 819 
 820     private static final sun.misc.Unsafe UNSAFE;
 821     private static final long headOffset;
 822     private static final long tailOffset;
 823     static {
 824         try {
 825             UNSAFE = sun.misc.Unsafe.getUnsafe();
 826             Class k = ConcurrentLinkedQueue.class;
 827             headOffset = UNSAFE.objectFieldOffset
 828                 (k.getDeclaredField("head"));
 829             tailOffset = UNSAFE.objectFieldOffset
 830                 (k.getDeclaredField("tail"));
 831         } catch (Exception e) {
 832             throw new Error(e);
 833         }
 834     }
 835 }
--- EOF ---