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.Arrays;
  40 import java.util.Collection;
  41 import java.util.Iterator;
  42 import java.util.NoSuchElementException;
  43 import java.util.Objects;
  44 import java.util.Queue;
  45 import java.util.Spliterator;
  46 import java.util.Spliterators;
  47 import java.util.function.Consumer;
  48 
  49 /**
  50  * An unbounded thread-safe {@linkplain Queue queue} based on linked nodes.
  51  * This queue orders elements FIFO (first-in-first-out).
  52  * The <em>head</em> of the queue is that element that has been on the
  53  * queue the longest time.
  54  * The <em>tail</em> of the queue is that element that has been on the
  55  * queue the shortest time. New elements
  56  * are inserted at the tail of the queue, and the queue retrieval
  57  * operations obtain elements at the head of the queue.
  58  * A {@code ConcurrentLinkedQueue} is an appropriate choice when
  59  * many threads will share access to a common collection.
  60  * Like most other concurrent collection implementations, this class
  61  * does not permit the use of {@code null} elements.
  62  *
  63  * <p>This implementation employs an efficient <em>non-blocking</em>
  64  * algorithm based on one described in
  65  * <a href="http://www.cs.rochester.edu/~scott/papers/1996_PODC_queues.pdf">
  66  * Simple, Fast, and Practical Non-Blocking and Blocking Concurrent Queue
  67  * Algorithms</a> by Maged M. Michael and Michael L. Scott.
  68  *
  69  * <p>Iterators are <i>weakly consistent</i>, returning elements
  70  * reflecting the state of the queue at some point at or since the
  71  * creation of the iterator.  They do <em>not</em> throw {@link
  72  * java.util.ConcurrentModificationException}, and may proceed concurrently
  73  * with other operations.  Elements contained in the queue since the creation
  74  * of the iterator will be returned exactly once.
  75  *
  76  * <p>Beware that, unlike in most collections, the {@code size} method
  77  * is <em>NOT</em> a constant-time operation. Because of the
  78  * asynchronous nature of these queues, determining the current number
  79  * of elements requires a traversal of the elements, and so may report
  80  * inaccurate results if this collection is modified during traversal.
  81  * Additionally, the bulk operations {@code addAll},
  82  * {@code removeAll}, {@code retainAll}, {@code containsAll},
  83  * {@code equals}, and {@code toArray} are <em>not</em> guaranteed
  84  * to be performed atomically. For example, an iterator operating
  85  * concurrently with an {@code addAll} operation might view only some
  86  * of the added elements.
  87  *
  88  * <p>This class and its iterator implement all of the <em>optional</em>
  89  * methods of the {@link Queue} and {@link Iterator} interfaces.
  90  *
  91  * <p>Memory consistency effects: As with other concurrent
  92  * collections, actions in a thread prior to placing an object into a
  93  * {@code ConcurrentLinkedQueue}
  94  * <a href="package-summary.html#MemoryVisibility"><i>happen-before</i></a>
  95  * actions subsequent to the access or removal of that element from
  96  * the {@code ConcurrentLinkedQueue} in another thread.
  97  *
  98  * <p>This class is a member of the
  99  * <a href="{@docRoot}/../technotes/guides/collections/index.html">
 100  * Java Collections Framework</a>.
 101  *
 102  * @since 1.5
 103  * @author Doug Lea
 104  * @param <E> the type of elements held in this queue
 105  */
 106 public class ConcurrentLinkedQueue<E> extends AbstractQueue<E>
 107         implements Queue<E>, java.io.Serializable {
 108     private static final long serialVersionUID = 196745693267521676L;
 109 
 110     /*
 111      * This is a modification of the Michael & Scott algorithm,
 112      * adapted for a garbage-collected environment, with support for
 113      * interior node deletion (to support remove(Object)).  For
 114      * explanation, read the paper.
 115      *
 116      * Note that like most non-blocking algorithms in this package,
 117      * this implementation relies on the fact that in garbage
 118      * collected systems, there is no possibility of ABA problems due
 119      * to recycled nodes, so there is no need to use "counted
 120      * pointers" or related techniques seen in versions used in
 121      * non-GC'ed settings.
 122      *
 123      * The fundamental invariants are:
 124      * - There is exactly one (last) Node with a null next reference,
 125      *   which is CASed when enqueueing.  This last Node can be
 126      *   reached in O(1) time from tail, but tail is merely an
 127      *   optimization - it can always be reached in O(N) time from
 128      *   head as well.
 129      * - The elements contained in the queue are the non-null items in
 130      *   Nodes that are reachable from head.  CASing the item
 131      *   reference of a Node to null atomically removes it from the
 132      *   queue.  Reachability of all elements from head must remain
 133      *   true even in the case of concurrent modifications that cause
 134      *   head to advance.  A dequeued Node may remain in use
 135      *   indefinitely due to creation of an Iterator or simply a
 136      *   poll() that has lost its time slice.
 137      *
 138      * The above might appear to imply that all Nodes are GC-reachable
 139      * from a predecessor dequeued Node.  That would cause two problems:
 140      * - allow a rogue Iterator to cause unbounded memory retention
 141      * - cause cross-generational linking of old Nodes to new Nodes if
 142      *   a Node was tenured while live, which generational GCs have a
 143      *   hard time dealing with, causing repeated major collections.
 144      * However, only non-deleted Nodes need to be reachable from
 145      * dequeued Nodes, and reachability does not necessarily have to
 146      * be of the kind understood by the GC.  We use the trick of
 147      * linking a Node that has just been dequeued to itself.  Such a
 148      * self-link implicitly means to advance to head.
 149      *
 150      * Both head and tail are permitted to lag.  In fact, failing to
 151      * update them every time one could is a significant optimization
 152      * (fewer CASes). As with LinkedTransferQueue (see the internal
 153      * documentation for that class), we use a slack threshold of two;
 154      * that is, we update head/tail when the current pointer appears
 155      * to be two or more steps away from the first/last node.
 156      *
 157      * Since head and tail are updated concurrently and independently,
 158      * it is possible for tail to lag behind head (why not)?
 159      *
 160      * CASing a Node's item reference to null atomically removes the
 161      * element from the queue.  Iterators skip over Nodes with null
 162      * items.  Prior implementations of this class had a race between
 163      * poll() and remove(Object) where the same element would appear
 164      * to be successfully removed by two concurrent operations.  The
 165      * method remove(Object) also lazily unlinks deleted Nodes, but
 166      * this is merely an optimization.
 167      *
 168      * When constructing a Node (before enqueuing it) we avoid paying
 169      * for a volatile write to item by using Unsafe.putObject instead
 170      * of a normal write.  This allows the cost of enqueue to be
 171      * "one-and-a-half" CASes.
 172      *
 173      * Both head and tail may or may not point to a Node with a
 174      * non-null item.  If the queue is empty, all items must of course
 175      * be null.  Upon creation, both head and tail refer to a dummy
 176      * Node with null item.  Both head and tail are only updated using
 177      * CAS, so they never regress, although again this is merely an
 178      * optimization.
 179      */
 180 
 181     private static class Node<E> {
 182         volatile E item;
 183         volatile Node<E> next;
 184     }
 185 
 186     /**
 187      * Returns a new node holding item.  Uses relaxed write because item
 188      * can only be seen after piggy-backing publication via casNext.
 189      */
 190     static <E> Node<E> newNode(E item) {
 191         Node<E> node = new Node<E>();
 192         U.putObject(node, ITEM, item);
 193         return node;
 194     }
 195 
 196     static <E> boolean casItem(Node<E> node, E cmp, E val) {
 197         return U.compareAndSwapObject(node, ITEM, cmp, val);
 198     }
 199 
 200     static <E> void lazySetNext(Node<E> node, Node<E> val) {
 201         U.putOrderedObject(node, NEXT, val);
 202     }
 203 
 204     static <E> boolean casNext(Node<E> node, Node<E> cmp, Node<E> val) {
 205         return U.compareAndSwapObject(node, NEXT, cmp, val);
 206     }
 207 
 208     /**
 209      * A node from which the first live (non-deleted) node (if any)
 210      * can be reached in O(1) time.
 211      * Invariants:
 212      * - all live nodes are reachable from head via succ()
 213      * - head != null
 214      * - (tmp = head).next != tmp || tmp != head
 215      * Non-invariants:
 216      * - head.item may or may not be null.
 217      * - it is permitted for tail to lag behind head, that is, for tail
 218      *   to not be reachable from head!
 219      */
 220     transient volatile Node<E> head;
 221 
 222     /**
 223      * A node from which the last node on list (that is, the unique
 224      * node with node.next == null) can be reached in O(1) time.
 225      * Invariants:
 226      * - the last node is always reachable from tail via succ()
 227      * - tail != null
 228      * Non-invariants:
 229      * - tail.item may or may not be null.
 230      * - it is permitted for tail to lag behind head, that is, for tail
 231      *   to not be reachable from head!
 232      * - tail.next may or may not be self-pointing to tail.
 233      */
 234     private transient volatile Node<E> tail;
 235 
 236     /**
 237      * Creates a {@code ConcurrentLinkedQueue} that is initially empty.
 238      */
 239     public ConcurrentLinkedQueue() {
 240         head = tail = newNode(null);
 241     }
 242 
 243     /**
 244      * Creates a {@code ConcurrentLinkedQueue}
 245      * initially containing the elements of the given collection,
 246      * added in traversal order of the collection's iterator.
 247      *
 248      * @param c the collection of elements to initially contain
 249      * @throws NullPointerException if the specified collection or any
 250      *         of its elements are null
 251      */
 252     public ConcurrentLinkedQueue(Collection<? extends E> c) {
 253         Node<E> h = null, t = null;
 254         for (E e : c) {
 255             Node<E> newNode = newNode(Objects.requireNonNull(e));
 256             if (h == null)
 257                 h = t = newNode;
 258             else {
 259                 lazySetNext(t, newNode);
 260                 t = newNode;
 261             }
 262         }
 263         if (h == null)
 264             h = t = newNode(null);
 265         head = h;
 266         tail = t;
 267     }
 268 
 269     // Have to override just to update the javadoc
 270 
 271     /**
 272      * Inserts the specified element at the tail of this queue.
 273      * As the queue is unbounded, this method will never throw
 274      * {@link IllegalStateException} or return {@code false}.
 275      *
 276      * @return {@code true} (as specified by {@link Collection#add})
 277      * @throws NullPointerException if the specified element is null
 278      */
 279     public boolean add(E e) {
 280         return offer(e);
 281     }
 282 
 283     /**
 284      * Tries to CAS head to p. If successful, repoint old head to itself
 285      * as sentinel for succ(), below.
 286      */
 287     final void updateHead(Node<E> h, Node<E> p) {
 288         // assert h != null && p != null && (h == p || h.item == null);
 289         if (h != p && casHead(h, p))
 290             lazySetNext(h, h);
 291     }
 292 
 293     /**
 294      * Returns the successor of p, or the head node if p.next has been
 295      * linked to self, which will only be true if traversing with a
 296      * stale pointer that is now off the list.
 297      */
 298     final Node<E> succ(Node<E> p) {
 299         Node<E> next = p.next;
 300         return (p == next) ? head : next;
 301     }
 302 
 303     /**
 304      * Inserts the specified element at the tail of this queue.
 305      * As the queue is unbounded, this method will never return {@code false}.
 306      *
 307      * @return {@code true} (as specified by {@link Queue#offer})
 308      * @throws NullPointerException if the specified element is null
 309      */
 310     public boolean offer(E e) {
 311         final Node<E> newNode = newNode(Objects.requireNonNull(e));
 312 
 313         for (Node<E> t = tail, p = t;;) {
 314             Node<E> q = p.next;
 315             if (q == null) {
 316                 // p is last node
 317                 if (casNext(p, null, newNode)) {
 318                     // Successful CAS is the linearization point
 319                     // for e to become an element of this queue,
 320                     // and for newNode to become "live".
 321                     if (p != t) // hop two nodes at a time
 322                         casTail(t, newNode);  // Failure is OK.
 323                     return true;
 324                 }
 325                 // Lost CAS race to another thread; re-read next
 326             }
 327             else if (p == q)
 328                 // We have fallen off list.  If tail is unchanged, it
 329                 // will also be off-list, in which case we need to
 330                 // jump to head, from which all live nodes are always
 331                 // reachable.  Else the new tail is a better bet.
 332                 p = (t != (t = tail)) ? t : head;
 333             else
 334                 // Check for tail updates after two hops.
 335                 p = (p != t && t != (t = tail)) ? t : q;
 336         }
 337     }
 338 
 339     public E poll() {
 340         restartFromHead:
 341         for (;;) {
 342             for (Node<E> h = head, p = h, q;;) {
 343                 E item = p.item;
 344 
 345                 if (item != null && casItem(p, item, null)) {
 346                     // Successful CAS is the linearization point
 347                     // for item to be removed from this queue.
 348                     if (p != h) // hop two nodes at a time
 349                         updateHead(h, ((q = p.next) != null) ? q : p);
 350                     return item;
 351                 }
 352                 else if ((q = p.next) == null) {
 353                     updateHead(h, p);
 354                     return null;
 355                 }
 356                 else if (p == q)
 357                     continue restartFromHead;
 358                 else
 359                     p = q;
 360             }
 361         }
 362     }
 363 
 364     public E peek() {
 365         restartFromHead:
 366         for (;;) {
 367             for (Node<E> h = head, p = h, q;;) {
 368                 E item = p.item;
 369                 if (item != null || (q = p.next) == null) {
 370                     updateHead(h, p);
 371                     return item;
 372                 }
 373                 else if (p == q)
 374                     continue restartFromHead;
 375                 else
 376                     p = q;
 377             }
 378         }
 379     }
 380 
 381     /**
 382      * Returns the first live (non-deleted) node on list, or null if none.
 383      * This is yet another variant of poll/peek; here returning the
 384      * first node, not element.  We could make peek() a wrapper around
 385      * first(), but that would cost an extra volatile read of item,
 386      * and the need to add a retry loop to deal with the possibility
 387      * of losing a race to a concurrent poll().
 388      */
 389     Node<E> first() {
 390         restartFromHead:
 391         for (;;) {
 392             for (Node<E> h = head, p = h, q;;) {
 393                 boolean hasItem = (p.item != null);
 394                 if (hasItem || (q = p.next) == null) {
 395                     updateHead(h, p);
 396                     return hasItem ? p : null;
 397                 }
 398                 else if (p == q)
 399                     continue restartFromHead;
 400                 else
 401                     p = q;
 402             }
 403         }
 404     }
 405 
 406     /**
 407      * Returns {@code true} if this queue contains no elements.
 408      *
 409      * @return {@code true} if this queue contains no elements
 410      */
 411     public boolean isEmpty() {
 412         return first() == null;
 413     }
 414 
 415     /**
 416      * Returns the number of elements in this queue.  If this queue
 417      * contains more than {@code Integer.MAX_VALUE} elements, returns
 418      * {@code Integer.MAX_VALUE}.
 419      *
 420      * <p>Beware that, unlike in most collections, this method is
 421      * <em>NOT</em> a constant-time operation. Because of the
 422      * asynchronous nature of these queues, determining the current
 423      * number of elements requires an O(n) traversal.
 424      * Additionally, if elements are added or removed during execution
 425      * of this method, the returned result may be inaccurate.  Thus,
 426      * this method is typically not very useful in concurrent
 427      * applications.
 428      *
 429      * @return the number of elements in this queue
 430      */
 431     public int size() {
 432         restartFromHead: for (;;) {
 433             int count = 0;
 434             for (Node<E> p = first(); p != null;) {
 435                 if (p.item != null)
 436                     if (++count == Integer.MAX_VALUE)
 437                         break;  // @see Collection.size()
 438                 if (p == (p = p.next))
 439                     continue restartFromHead;
 440             }
 441             return count;
 442         }
 443     }
 444 
 445     /**
 446      * Returns {@code true} if this queue contains the specified element.
 447      * More formally, returns {@code true} if and only if this queue contains
 448      * at least one element {@code e} such that {@code o.equals(e)}.
 449      *
 450      * @param o object to be checked for containment in this queue
 451      * @return {@code true} if this queue contains the specified element
 452      */
 453     public boolean contains(Object o) {
 454         if (o != null) {
 455             for (Node<E> p = first(); p != null; p = succ(p)) {
 456                 E item = p.item;
 457                 if (item != null && o.equals(item))
 458                     return true;
 459             }
 460         }
 461         return false;
 462     }
 463 
 464     /**
 465      * Removes a single instance of the specified element from this queue,
 466      * if it is present.  More formally, removes an element {@code e} such
 467      * that {@code o.equals(e)}, if this queue contains one or more such
 468      * elements.
 469      * Returns {@code true} if this queue contained the specified element
 470      * (or equivalently, if this queue changed as a result of the call).
 471      *
 472      * @param o element to be removed from this queue, if present
 473      * @return {@code true} if this queue changed as a result of the call
 474      */
 475     public boolean remove(Object o) {
 476         if (o != null) {
 477             Node<E> next, pred = null;
 478             for (Node<E> p = first(); p != null; pred = p, p = next) {
 479                 boolean removed = false;
 480                 E item = p.item;
 481                 if (item != null) {
 482                     if (!o.equals(item)) {
 483                         next = succ(p);
 484                         continue;
 485                     }
 486                     removed = casItem(p, item, null);
 487                 }
 488 
 489                 next = succ(p);
 490                 if (pred != null && next != null) // unlink
 491                     casNext(pred, p, next);
 492                 if (removed)
 493                     return true;
 494             }
 495         }
 496         return false;
 497     }
 498 
 499     /**
 500      * Appends all of the elements in the specified collection to the end of
 501      * this queue, in the order that they are returned by the specified
 502      * collection's iterator.  Attempts to {@code addAll} of a queue to
 503      * itself result in {@code IllegalArgumentException}.
 504      *
 505      * @param c the elements to be inserted into this queue
 506      * @return {@code true} if this queue changed as a result of the call
 507      * @throws NullPointerException if the specified collection or any
 508      *         of its elements are null
 509      * @throws IllegalArgumentException if the collection is this queue
 510      */
 511     public boolean addAll(Collection<? extends E> c) {
 512         if (c == this)
 513             // As historically specified in AbstractQueue#addAll
 514             throw new IllegalArgumentException();
 515 
 516         // Copy c into a private chain of Nodes
 517         Node<E> beginningOfTheEnd = null, last = null;
 518         for (E e : c) {
 519             Node<E> newNode = newNode(Objects.requireNonNull(e));
 520             if (beginningOfTheEnd == null)
 521                 beginningOfTheEnd = last = newNode;
 522             else {
 523                 lazySetNext(last, newNode);
 524                 last = newNode;
 525             }
 526         }
 527         if (beginningOfTheEnd == null)
 528             return false;
 529 
 530         // Atomically append the chain at the tail of this collection
 531         for (Node<E> t = tail, p = t;;) {
 532             Node<E> q = p.next;
 533             if (q == null) {
 534                 // p is last node
 535                 if (casNext(p, null, beginningOfTheEnd)) {
 536                     // Successful CAS is the linearization point
 537                     // for all elements to be added to this queue.
 538                     if (!casTail(t, last)) {
 539                         // Try a little harder to update tail,
 540                         // since we may be adding many elements.
 541                         t = tail;
 542                         if (last.next == null)
 543                             casTail(t, last);
 544                     }
 545                     return true;
 546                 }
 547                 // Lost CAS race to another thread; re-read next
 548             }
 549             else if (p == q)
 550                 // We have fallen off list.  If tail is unchanged, it
 551                 // will also be off-list, in which case we need to
 552                 // jump to head, from which all live nodes are always
 553                 // reachable.  Else the new tail is a better bet.
 554                 p = (t != (t = tail)) ? t : head;
 555             else
 556                 // Check for tail updates after two hops.
 557                 p = (p != t && t != (t = tail)) ? t : q;
 558         }
 559     }
 560 
 561     public String toString() {
 562         String[] a = null;
 563         restartFromHead: for (;;) {
 564             int charLength = 0;
 565             int size = 0;
 566             for (Node<E> p = first(); p != null;) {
 567                 E item = p.item;
 568                 if (item != null) {
 569                     if (a == null)
 570                         a = new String[4];
 571                     else if (size == a.length)
 572                         a = Arrays.copyOf(a, 2 * size);
 573                     String s = item.toString();
 574                     a[size++] = s;
 575                     charLength += s.length();
 576                 }
 577                 if (p == (p = p.next))
 578                     continue restartFromHead;
 579             }
 580 
 581             if (size == 0)
 582                 return "[]";
 583 
 584             return Helpers.toString(a, size, charLength);
 585         }
 586     }
 587 
 588     private Object[] toArrayInternal(Object[] a) {
 589         Object[] x = a;
 590         restartFromHead: for (;;) {
 591             int size = 0;
 592             for (Node<E> p = first(); p != null;) {
 593                 E item = p.item;
 594                 if (item != null) {
 595                     if (x == null)
 596                         x = new Object[4];
 597                     else if (size == x.length)
 598                         x = Arrays.copyOf(x, 2 * (size + 4));
 599                     x[size++] = item;
 600                 }
 601                 if (p == (p = p.next))
 602                     continue restartFromHead;
 603             }
 604             if (x == null)
 605                 return new Object[0];
 606             else if (a != null && size <= a.length) {
 607                 if (a != x)
 608                     System.arraycopy(x, 0, a, 0, size);
 609                 if (size < a.length)
 610                     a[size] = null;
 611                 return a;
 612             }
 613             return (size == x.length) ? x : Arrays.copyOf(x, size);
 614         }
 615     }
 616 
 617     /**
 618      * Returns an array containing all of the elements in this queue, in
 619      * proper sequence.
 620      *
 621      * <p>The returned array will be "safe" in that no references to it are
 622      * maintained by this queue.  (In other words, this method must allocate
 623      * a new array).  The caller is thus free to modify the returned array.
 624      *
 625      * <p>This method acts as bridge between array-based and collection-based
 626      * APIs.
 627      *
 628      * @return an array containing all of the elements in this queue
 629      */
 630     public Object[] toArray() {
 631         return toArrayInternal(null);
 632     }
 633 
 634     /**
 635      * Returns an array containing all of the elements in this queue, in
 636      * proper sequence; the runtime type of the returned array is that of
 637      * the specified array.  If the queue fits in the specified array, it
 638      * is returned therein.  Otherwise, a new array is allocated with the
 639      * runtime type of the specified array and the size of this queue.
 640      *
 641      * <p>If this queue fits in the specified array with room to spare
 642      * (i.e., the array has more elements than this queue), the element in
 643      * the array immediately following the end of the queue is set to
 644      * {@code null}.
 645      *
 646      * <p>Like the {@link #toArray()} method, this method acts as bridge between
 647      * array-based and collection-based APIs.  Further, this method allows
 648      * precise control over the runtime type of the output array, and may,
 649      * under certain circumstances, be used to save allocation costs.
 650      *
 651      * <p>Suppose {@code x} is a queue known to contain only strings.
 652      * The following code can be used to dump the queue into a newly
 653      * allocated array of {@code String}:
 654      *
 655      * <pre> {@code String[] y = x.toArray(new String[0]);}</pre>
 656      *
 657      * Note that {@code toArray(new Object[0])} is identical in function to
 658      * {@code toArray()}.
 659      *
 660      * @param a the array into which the elements of the queue are to
 661      *          be stored, if it is big enough; otherwise, a new array of the
 662      *          same runtime type is allocated for this purpose
 663      * @return an array containing all of the elements in this queue
 664      * @throws ArrayStoreException if the runtime type of the specified array
 665      *         is not a supertype of the runtime type of every element in
 666      *         this queue
 667      * @throws NullPointerException if the specified array is null
 668      */
 669     @SuppressWarnings("unchecked")
 670     public <T> T[] toArray(T[] a) {
 671         if (a == null) throw new NullPointerException();
 672         return (T[]) toArrayInternal(a);
 673     }
 674 
 675     /**
 676      * Returns an iterator over the elements in this queue in proper sequence.
 677      * The elements will be returned in order from first (head) to last (tail).
 678      *
 679      * <p>The returned iterator is
 680      * <a href="package-summary.html#Weakly"><i>weakly consistent</i></a>.
 681      *
 682      * @return an iterator over the elements in this queue in proper sequence
 683      */
 684     public Iterator<E> iterator() {
 685         return new Itr();
 686     }
 687 
 688     private class Itr implements Iterator<E> {
 689         /**
 690          * Next node to return item for.
 691          */
 692         private Node<E> nextNode;
 693 
 694         /**
 695          * nextItem holds on to item fields because once we claim
 696          * that an element exists in hasNext(), we must return it in
 697          * the following next() call even if it was in the process of
 698          * being removed when hasNext() was called.
 699          */
 700         private E nextItem;
 701 
 702         /**
 703          * Node of the last returned item, to support remove.
 704          */
 705         private Node<E> lastRet;
 706 
 707         Itr() {
 708             restartFromHead: for (;;) {
 709                 Node<E> h, p, q;
 710                 for (p = h = head;; p = q) {
 711                     E item;
 712                     if ((item = p.item) != null) {
 713                         nextNode = p;
 714                         nextItem = item;
 715                         break;
 716                     }
 717                     else if ((q = p.next) == null)
 718                         break;
 719                     else if (p == q)
 720                         continue restartFromHead;
 721                 }
 722                 updateHead(h, p);
 723                 return;
 724             }
 725         }
 726 
 727         public boolean hasNext() {
 728             return nextItem != null;
 729         }
 730 
 731         public E next() {
 732             final Node<E> pred = nextNode;
 733             if (pred == null) throw new NoSuchElementException();
 734             // assert nextItem != null;
 735             lastRet = pred;
 736             E item = null;
 737 
 738             for (Node<E> p = succ(pred), q;; p = q) {
 739                 if (p == null || (item = p.item) != null) {
 740                     nextNode = p;
 741                     E x = nextItem;
 742                     nextItem = item;
 743                     return x;
 744                 }
 745                 // unlink deleted nodes
 746                 if ((q = succ(p)) != null)
 747                     casNext(pred, p, q);
 748             }
 749         }
 750 
 751         public void remove() {
 752             Node<E> l = lastRet;
 753             if (l == null) throw new IllegalStateException();
 754             // rely on a future traversal to relink.
 755             l.item = null;
 756             lastRet = null;
 757         }
 758     }
 759 
 760     /**
 761      * Saves this queue to a stream (that is, serializes it).
 762      *
 763      * @param s the stream
 764      * @throws java.io.IOException if an I/O error occurs
 765      * @serialData All of the elements (each an {@code E}) in
 766      * the proper order, followed by a null
 767      */
 768     private void writeObject(java.io.ObjectOutputStream s)
 769         throws java.io.IOException {
 770 
 771         // Write out any hidden stuff
 772         s.defaultWriteObject();
 773 
 774         // Write out all elements in the proper order.
 775         for (Node<E> p = first(); p != null; p = succ(p)) {
 776             Object item = p.item;
 777             if (item != null)
 778                 s.writeObject(item);
 779         }
 780 
 781         // Use trailing null as sentinel
 782         s.writeObject(null);
 783     }
 784 
 785     /**
 786      * Reconstitutes this queue from a stream (that is, deserializes it).
 787      * @param s the stream
 788      * @throws ClassNotFoundException if the class of a serialized object
 789      *         could not be found
 790      * @throws java.io.IOException if an I/O error occurs
 791      */
 792     private void readObject(java.io.ObjectInputStream s)
 793         throws java.io.IOException, ClassNotFoundException {
 794         s.defaultReadObject();
 795 
 796         // Read in elements until trailing null sentinel found
 797         Node<E> h = null, t = null;
 798         for (Object item; (item = s.readObject()) != null; ) {
 799             @SuppressWarnings("unchecked")
 800             Node<E> newNode = newNode((E) item);
 801             if (h == null)
 802                 h = t = newNode;
 803             else {
 804                 lazySetNext(t, newNode);
 805                 t = newNode;
 806             }
 807         }
 808         if (h == null)
 809             h = t = newNode(null);
 810         head = h;
 811         tail = t;
 812     }
 813 
 814     /** A customized variant of Spliterators.IteratorSpliterator */
 815     static final class CLQSpliterator<E> implements Spliterator<E> {
 816         static final int MAX_BATCH = 1 << 25;  // max batch array size;
 817         final ConcurrentLinkedQueue<E> queue;
 818         Node<E> current;    // current node; null until initialized
 819         int batch;          // batch size for splits
 820         boolean exhausted;  // true when no more nodes
 821         CLQSpliterator(ConcurrentLinkedQueue<E> queue) {
 822             this.queue = queue;
 823         }
 824 
 825         public Spliterator<E> trySplit() {
 826             Node<E> p;
 827             final ConcurrentLinkedQueue<E> q = this.queue;
 828             int b = batch;
 829             int n = (b <= 0) ? 1 : (b >= MAX_BATCH) ? MAX_BATCH : b + 1;
 830             if (!exhausted &&
 831                 ((p = current) != null || (p = q.first()) != null) &&
 832                 p.next != null) {
 833                 Object[] a = new Object[n];
 834                 int i = 0;
 835                 do {
 836                     if ((a[i] = p.item) != null)
 837                         ++i;
 838                     if (p == (p = p.next))
 839                         p = q.first();
 840                 } while (p != null && i < n);
 841                 if ((current = p) == null)
 842                     exhausted = true;
 843                 if (i > 0) {
 844                     batch = i;
 845                     return Spliterators.spliterator
 846                         (a, 0, i, (Spliterator.ORDERED |
 847                                    Spliterator.NONNULL |
 848                                    Spliterator.CONCURRENT));
 849                 }
 850             }
 851             return null;
 852         }
 853 
 854         public void forEachRemaining(Consumer<? super E> action) {
 855             Node<E> p;
 856             if (action == null) throw new NullPointerException();
 857             final ConcurrentLinkedQueue<E> q = this.queue;
 858             if (!exhausted &&
 859                 ((p = current) != null || (p = q.first()) != null)) {
 860                 exhausted = true;
 861                 do {
 862                     E e = p.item;
 863                     if (p == (p = p.next))
 864                         p = q.first();
 865                     if (e != null)
 866                         action.accept(e);
 867                 } while (p != null);
 868             }
 869         }
 870 
 871         public boolean tryAdvance(Consumer<? super E> action) {
 872             Node<E> p;
 873             if (action == null) throw new NullPointerException();
 874             final ConcurrentLinkedQueue<E> q = this.queue;
 875             if (!exhausted &&
 876                 ((p = current) != null || (p = q.first()) != null)) {
 877                 E e;
 878                 do {
 879                     e = p.item;
 880                     if (p == (p = p.next))
 881                         p = q.first();
 882                 } while (e == null && p != null);
 883                 if ((current = p) == null)
 884                     exhausted = true;
 885                 if (e != null) {
 886                     action.accept(e);
 887                     return true;
 888                 }
 889             }
 890             return false;
 891         }
 892 
 893         public long estimateSize() { return Long.MAX_VALUE; }
 894 
 895         public int characteristics() {
 896             return Spliterator.ORDERED | Spliterator.NONNULL |
 897                 Spliterator.CONCURRENT;
 898         }
 899     }
 900 
 901     /**
 902      * Returns a {@link Spliterator} over the elements in this queue.
 903      *
 904      * <p>The returned spliterator is
 905      * <a href="package-summary.html#Weakly"><i>weakly consistent</i></a>.
 906      *
 907      * <p>The {@code Spliterator} reports {@link Spliterator#CONCURRENT},
 908      * {@link Spliterator#ORDERED}, and {@link Spliterator#NONNULL}.
 909      *
 910      * @implNote
 911      * The {@code Spliterator} implements {@code trySplit} to permit limited
 912      * parallelism.
 913      *
 914      * @return a {@code Spliterator} over the elements in this queue
 915      * @since 1.8
 916      */
 917     @Override
 918     public Spliterator<E> spliterator() {
 919         return new CLQSpliterator<E>(this);
 920     }
 921 
 922     private boolean casTail(Node<E> cmp, Node<E> val) {
 923         return U.compareAndSwapObject(this, TAIL, cmp, val);
 924     }
 925 
 926     private boolean casHead(Node<E> cmp, Node<E> val) {
 927         return U.compareAndSwapObject(this, HEAD, cmp, val);
 928     }
 929 
 930     // Unsafe mechanics
 931 
 932     private static final sun.misc.Unsafe U = sun.misc.Unsafe.getUnsafe();
 933     private static final long HEAD;
 934     private static final long TAIL;
 935     private static final long ITEM;
 936     private static final long NEXT;
 937     static {
 938         try {
 939             HEAD = U.objectFieldOffset
 940                 (ConcurrentLinkedQueue.class.getDeclaredField("head"));
 941             TAIL = U.objectFieldOffset
 942                 (ConcurrentLinkedQueue.class.getDeclaredField("tail"));
 943             ITEM = U.objectFieldOffset
 944                 (Node.class.getDeclaredField("item"));
 945             NEXT = U.objectFieldOffset
 946                 (Node.class.getDeclaredField("next"));
 947         } catch (ReflectiveOperationException e) {
 948             throw new Error(e);
 949         }
 950     }
 951 }