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36 package java.util.concurrent;
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;
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.
59 * <p>This implementation employs an efficient "wait-free"
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.
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.
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.
84 * <p>This class and its iterator implement all of the <em>optional</em>
85 * methods of the {@link Queue} and {@link Iterator} interfaces.
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.
94 * <p>This class is a member of the
95 * <a href="{@docRoot}/../technotes/guides/collections/index.html">
96 * Java Collections Framework</a>.
100 * @param <E> the type of elements held in this collection
103 public class ConcurrentLinkedQueue<E> extends AbstractQueue<E>
104 implements Queue<E>, java.io.Serializable {
105 private static final long serialVersionUID = 196745693267521676L;
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.
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.
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
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.
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.
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.
154 * Since head and tail are updated concurrently and independently,
155 * it is possible for tail to lag behind head (why not)?
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.
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.
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
178 private static class Node<E> {
180 volatile Node<E> next;
183 * Constructs a new node. Uses relaxed write because item can
184 * only be seen after publication via casNext.
190 boolean casItem(E cmp, E val) {
198 void lazySetNext(Node<E> val) {
202 boolean casNext(Node<E> cmp, Node<E> val) {
212 * A node from which the first live (non-deleted) node (if any)
213 * can be reached in O(1) time.
215 * - all live nodes are reachable from head via succ()
217 * - (tmp = head).next != tmp || tmp != head
219 * - head.item may or may not be null.
220 * - it is permitted for tail to lag behind head, that is, for tail
221 * to not be reachable from head!
223 private transient volatile Node<E> head;
226 * A node from which the last node on list (that is, the unique
227 * node with node.next == null) can be reached in O(1) time.
229 * - the last node is always reachable from tail via succ()
232 * - tail.item may or may not be null.
233 * - it is permitted for tail to lag behind head, that is, for tail
234 * to not be reachable from head!
235 * - tail.next may or may not be self-pointing to tail.
237 private transient volatile Node<E> tail;
241 * Creates a {@code ConcurrentLinkedQueue} that is initially empty.
243 public ConcurrentLinkedQueue() {
244 head = tail = new Node<E>(null);
248 * Creates a {@code ConcurrentLinkedQueue}
249 * initially containing the elements of the given collection,
250 * added in traversal order of the collection's iterator.
252 * @param c the collection of elements to initially contain
253 * @throws NullPointerException if the specified collection or any
254 * of its elements are null
256 public ConcurrentLinkedQueue(Collection<? extends E> c) {
257 Node<E> h = null, t = null;
260 Node<E> newNode = new Node<E>(e);
264 t.lazySetNext(newNode);
269 h = t = new Node<E>(null);
274 // Have to override just to update the javadoc
277 * Inserts the specified element at the tail of this queue.
278 * As the queue is unbounded, this method will never throw
279 * {@link IllegalStateException} or return {@code false}.
281 * @return {@code true} (as specified by {@link Collection#add})
282 * @throws NullPointerException if the specified element is null
284 public boolean add(E e) {
289 * Try to CAS head to p. If successful, repoint old head to itself
290 * as sentinel for succ(), below.
292 final void updateHead(Node<E> h, Node<E> p) {
293 if (h != p && casHead(h, p))
298 * Returns the successor of p, or the head node if p.next has been
299 * linked to self, which will only be true if traversing with a
300 * stale pointer that is now off the list.
302 final Node<E> succ(Node<E> p) {
303 Node<E> next = p.next;
304 return (p == next) ? head : next;
308 * Inserts the specified element at the tail of this queue.
309 * As the queue is unbounded, this method will never return {@code false}.
311 * @return {@code true} (as specified by {@link Queue#offer})
312 * @throws NullPointerException if the specified element is null
314 public boolean offer(E e) {
316 final Node<E> newNode = new Node<E>(e);
318 for (Node<E> t = tail, p = t;;) {
322 if (p.casNext(null, newNode)) {
323 // Successful CAS is the linearization point
324 // for e to become an element of this queue,
325 // and for newNode to become "live".
326 if (p != t) // hop two nodes at a time
327 casTail(t, newNode); // Failure is OK.
330 // Lost CAS race to another thread; re-read next
333 // We have fallen off list. If tail is unchanged, it
334 // will also be off-list, in which case we need to
335 // jump to head, from which all live nodes are always
336 // reachable. Else the new tail is a better bet.
337 p = (t != (t = tail)) ? t : head;
339 // Check for tail updates after two hops.
340 p = (p != t && t != (t = tail)) ? t : q;
347 for (Node<E> h = head, p = h, q;;) {
350 if (item != null && p.casItem(item, null)) {
351 // Successful CAS is the linearization point
352 // for item to be removed from this queue.
353 if (p != h) // hop two nodes at a time
354 updateHead(h, ((q = p.next) != null) ? q : p);
357 else if ((q = p.next) == null) {
362 continue restartFromHead;
372 for (Node<E> h = head, p = h, q;;) {
374 if (item != null || (q = p.next) == null) {
379 continue restartFromHead;
387 * Returns the first live (non-deleted) node on list, or null if none.
388 * This is yet another variant of poll/peek; here returning the
389 * first node, not element. We could make peek() a wrapper around
390 * first(), but that would cost an extra volatile read of item,
391 * and the need to add a retry loop to deal with the possibility
392 * of losing a race to a concurrent poll().
397 for (Node<E> h = head, p = h, q;;) {
398 boolean hasItem = (p.item != null);
399 if (hasItem || (q = p.next) == null) {
401 return hasItem ? p : null;
404 continue restartFromHead;
412 * Returns {@code true} if this queue contains no elements.
414 * @return {@code true} if this queue contains no elements
416 public boolean isEmpty() {
417 return first() == null;
421 * Returns the number of elements in this queue. If this queue
422 * contains more than {@code Integer.MAX_VALUE} elements, returns
423 * {@code Integer.MAX_VALUE}.
425 * <p>Beware that, unlike in most collections, this method is
426 * <em>NOT</em> a constant-time operation. Because of the
427 * asynchronous nature of these queues, determining the current
428 * number of elements requires an O(n) traversal.
429 * Additionally, if elements are added or removed during execution
430 * of this method, the returned result may be inaccurate. Thus,
431 * this method is typically not very useful in concurrent
434 * @return the number of elements in this queue
438 for (Node<E> p = first(); p != null; p = succ(p))
440 // Collection.size() spec says to max out
441 if (++count == Integer.MAX_VALUE)
447 * Returns {@code true} if this queue contains the specified element.
448 * More formally, returns {@code true} if and only if this queue contains
449 * at least one element {@code e} such that {@code o.equals(e)}.
451 * @param o object to be checked for containment in this queue
452 * @return {@code true} if this queue contains the specified element
454 public boolean contains(Object o) {
455 if (o == null) return false;
456 for (Node<E> p = first(); p != null; p = succ(p)) {
458 if (item != null && o.equals(item))
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
469 * Returns {@code true} if this queue contained the specified element
470 * (or equivalently, if this queue changed as a result of the call).
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
475 public boolean remove(Object o) {
476 if (o == null) return false;
478 for (Node<E> p = first(); p != null; p = succ(p)) {
482 p.casItem(item, null)) {
483 Node<E> next = succ(p);
484 if (pred != null && next != null)
485 pred.casNext(p, next);
494 * Appends all of the elements in the specified collection to the end of
495 * this queue, in the order that they are returned by the specified
496 * collection's iterator. Attempts to {@code addAll} of a queue to
497 * itself result in {@code IllegalArgumentException}.
499 * @param c the elements to be inserted into this queue
500 * @return {@code true} if this queue changed as a result of the call
501 * @throws NullPointerException if the specified collection or any
502 * of its elements are null
503 * @throws IllegalArgumentException if the collection is this queue
505 public boolean addAll(Collection<? extends E> c) {
507 // As historically specified in AbstractQueue#addAll
508 throw new IllegalArgumentException();
510 // Copy c into a private chain of Nodes
511 Node<E> beginningOfTheEnd = null, last = null;
514 Node<E> newNode = new Node<E>(e);
515 if (beginningOfTheEnd == null)
516 beginningOfTheEnd = last = newNode;
518 last.lazySetNext(newNode);
522 if (beginningOfTheEnd == null)
525 // Atomically append the chain at the tail of this collection
526 for (Node<E> t = tail, p = t;;) {
530 if (p.casNext(null, beginningOfTheEnd)) {
531 // Successful CAS is the linearization point
532 // for all elements to be added to this queue.
533 if (!casTail(t, last)) {
534 // Try a little harder to update tail,
535 // since we may be adding many elements.
537 if (last.next == null)
542 // Lost CAS race to another thread; re-read next
545 // We have fallen off list. If tail is unchanged, it
546 // will also be off-list, in which case we need to
547 // jump to head, from which all live nodes are always
548 // reachable. Else the new tail is a better bet.
549 p = (t != (t = tail)) ? t : head;
551 // Check for tail updates after two hops.
552 p = (p != t && t != (t = tail)) ? t : q;
557 * Returns an array containing all of the elements in this queue, in
560 * <p>The returned array will be "safe" in that no references to it are
561 * maintained by this queue. (In other words, this method must allocate
562 * a new array). The caller is thus free to modify the returned array.
564 * <p>This method acts as bridge between array-based and collection-based
567 * @return an array containing all of the elements in this queue
569 public Object[] toArray() {
570 // Use ArrayList to deal with resizing.
571 ArrayList<E> al = new ArrayList<E>();
572 for (Node<E> p = first(); p != null; p = succ(p)) {
581 * Returns an array containing all of the elements in this queue, in
582 * proper sequence; the runtime type of the returned array is that of
583 * the specified array. If the queue fits in the specified array, it
584 * is returned therein. Otherwise, a new array is allocated with the
585 * runtime type of the specified array and the size of this queue.
587 * <p>If this queue fits in the specified array with room to spare
588 * (i.e., the array has more elements than this queue), the element in
589 * the array immediately following the end of the queue is set to
592 * <p>Like the {@link #toArray()} method, this method acts as bridge between
593 * array-based and collection-based APIs. Further, this method allows
594 * precise control over the runtime type of the output array, and may,
595 * under certain circumstances, be used to save allocation costs.
597 * <p>Suppose {@code x} is a queue known to contain only strings.
598 * The following code can be used to dump the queue into a newly
599 * allocated array of {@code String}:
602 * String[] y = x.toArray(new String[0]);</pre>
604 * Note that {@code toArray(new Object[0])} is identical in function to
607 * @param a the array into which the elements of the queue are to
608 * be stored, if it is big enough; otherwise, a new array of the
609 * same runtime type is allocated for this purpose
610 * @return an array containing all of the elements in this queue
611 * @throws ArrayStoreException if the runtime type of the specified array
612 * is not a supertype of the runtime type of every element in
614 * @throws NullPointerException if the specified array is null
616 @SuppressWarnings("unchecked")
617 public <T> T[] toArray(T[] a) {
618 // try to use sent-in array
621 for (p = first(); p != null && k < a.length; p = succ(p)) {
632 // If won't fit, use ArrayList version
633 ArrayList<E> al = new ArrayList<E>();
634 for (Node<E> q = first(); q != null; q = succ(q)) {
639 return al.toArray(a);
643 * Returns an iterator over the elements in this queue in proper sequence.
644 * The elements will be returned in order from first (head) to last (tail).
646 * <p>The returned iterator is a "weakly consistent" iterator that
647 * will never throw {@link java.util.ConcurrentModificationException
648 * ConcurrentModificationException}, and guarantees to traverse
649 * elements as they existed upon construction of the iterator, and
650 * may (but is not guaranteed to) reflect any modifications
651 * subsequent to construction.
653 * @return an iterator over the elements in this queue in proper sequence
655 public Iterator<E> iterator() {
659 private class Itr implements Iterator<E> {
661 * Next node to return item for.
663 private Node<E> nextNode;
666 * nextItem holds on to item fields because once we claim
667 * that an element exists in hasNext(), we must return it in
668 * the following next() call even if it was in the process of
669 * being removed when hasNext() was called.
674 * Node of the last returned item, to support remove.
676 private Node<E> lastRet;
683 * Moves to next valid node and returns item to return for
684 * next(), or null if no such.
686 private E advance() {
691 if (nextNode == null) {
712 Node<E> next = succ(p);
713 if (pred != null && next != null)
714 pred.casNext(p, next);
720 public boolean hasNext() {
721 return nextNode != null;
725 if (nextNode == null) throw new NoSuchElementException();
729 public void remove() {
731 if (l == null) throw new IllegalStateException();
732 // rely on a future traversal to relink.
739 * Saves the state to a stream (that is, serializes it).
741 * @serialData All of the elements (each an {@code E}) in
742 * the proper order, followed by a null
743 * @param s the stream
745 private void writeObject(java.io.ObjectOutputStream s)
746 throws java.io.IOException {
748 // Write out any hidden stuff
749 s.defaultWriteObject();
751 // Write out all elements in the proper order.
752 for (Node<E> p = first(); p != null; p = succ(p)) {
753 Object item = p.item;
758 // Use trailing null as sentinel
763 * Reconstitutes the instance from a stream (that is, deserializes it).
764 * @param s the stream
766 private void readObject(java.io.ObjectInputStream s)
767 throws java.io.IOException, ClassNotFoundException {
768 s.defaultReadObject();
770 // Read in elements until trailing null sentinel found
771 Node<E> h = null, t = null;
773 while ((item = s.readObject()) != null) {
774 @SuppressWarnings("unchecked")
775 Node<E> newNode = new Node<E>((E) item);
779 t.lazySetNext(newNode);
784 h = t = new Node<E>(null);
790 * Throws NullPointerException if argument is null.
792 * @param v the element
794 private static void checkNotNull(Object v) {
796 throw new NullPointerException();
799 private boolean casTail(Node<E> cmp, Node<E> val) {
807 private boolean casHead(Node<E> cmp, Node<E> val) {