xref: /libcore/luni/src/main/java/java/util/concurrent/ConcurrentLinkedQueue.java

/*
 * Written by Doug Lea and Martin Buchholz with assistance from members of
 * JCP JSR-166 Expert Group and released to the public domain, as explained
 * at http://creativecommons.org/publicdomain/zero/1.0/
 */

package java.util.concurrent;

import java.util.AbstractQueue;
import java.util.Arrays;
import java.util.Collection;
import java.util.Iterator;
import java.util.NoSuchElementException;
import java.util.Objects;
import java.util.Queue;
import java.util.Spliterator;
import java.util.Spliterators;
import java.util.function.Consumer;

// BEGIN android-note
// removed link to collections framework docs
// END android-note

/**
 * An unbounded thread-safe {@linkplain Queue queue} based on linked nodes.
 * This queue orders elements FIFO (first-in-first-out).
 * The <em>head</em> of the queue is that element that has been on the
 * queue the longest time.
 * The <em>tail</em> of the queue is that element that has been on the
 * queue the shortest time. New elements
 * are inserted at the tail of the queue, and the queue retrieval
 * operations obtain elements at the head of the queue.
 * A {@code ConcurrentLinkedQueue} is an appropriate choice when
 * many threads will share access to a common collection.
 * Like most other concurrent collection implementations, this class
 * does not permit the use of {@code null} elements.
 *
 * <p>This implementation employs an efficient <em>non-blocking</em>
 * algorithm based on one described in
 * <a href="http://www.cs.rochester.edu/~scott/papers/1996_PODC_queues.pdf">
 * Simple, Fast, and Practical Non-Blocking and Blocking Concurrent Queue
 * Algorithms</a> by Maged M. Michael and Michael L. Scott.
 *
 * <p>Iterators are <i>weakly consistent</i>, returning elements
 * reflecting the state of the queue at some point at or since the
 * creation of the iterator.  They do <em>not</em> throw {@link
 * java.util.ConcurrentModificationException}, and may proceed concurrently
 * with other operations.  Elements contained in the queue since the creation
 * of the iterator will be returned exactly once.
 *
 * <p>Beware that, unlike in most collections, the {@code size} method
 * is <em>NOT</em> a constant-time operation. Because of the
 * asynchronous nature of these queues, determining the current number
 * of elements requires a traversal of the elements, and so may report
 * inaccurate results if this collection is modified during traversal.
 * Additionally, the bulk operations {@code addAll},
 * {@code removeAll}, {@code retainAll}, {@code containsAll},
 * {@code equals}, and {@code toArray} are <em>not</em> guaranteed
 * to be performed atomically. For example, an iterator operating
 * concurrently with an {@code addAll} operation might view only some
 * of the added elements.
 *
 * <p>This class and its iterator implement all of the <em>optional</em>
 * methods of the {@link Queue} and {@link Iterator} interfaces.
 *
 * <p>Memory consistency effects: As with other concurrent
 * collections, actions in a thread prior to placing an object into a
 * {@code ConcurrentLinkedQueue}
 * <a href="package-summary.html#MemoryVisibility"><i>happen-before</i></a>
 * actions subsequent to the access or removal of that element from
 * the {@code ConcurrentLinkedQueue} in another thread.
 *
 * @since 1.5
 * @author Doug Lea
 * @param <E> the type of elements held in this queue
 */
public class ConcurrentLinkedQueue<E> extends AbstractQueue<E>
        implements Queue<E>, java.io.Serializable {
    private static final long serialVersionUID = 196745693267521676L;

    /*
     * This is a modification of the Michael & Scott algorithm,
     * adapted for a garbage-collected environment, with support for
     * interior node deletion (to support remove(Object)).  For
     * explanation, read the paper.
     *
     * Note that like most non-blocking algorithms in this package,
     * this implementation relies on the fact that in garbage
     * collected systems, there is no possibility of ABA problems due
     * to recycled nodes, so there is no need to use "counted
     * pointers" or related techniques seen in versions used in
     * non-GC'ed settings.
     *
     * The fundamental invariants are:
     * - There is exactly one (last) Node with a null next reference,
     *   which is CASed when enqueueing.  This last Node can be
     *   reached in O(1) time from tail, but tail is merely an
     *   optimization - it can always be reached in O(N) time from
     *   head as well.
     * - The elements contained in the queue are the non-null items in
     *   Nodes that are reachable from head.  CASing the item
     *   reference of a Node to null atomically removes it from the
     *   queue.  Reachability of all elements from head must remain
     *   true even in the case of concurrent modifications that cause
     *   head to advance.  A dequeued Node may remain in use
     *   indefinitely due to creation of an Iterator or simply a
     *   poll() that has lost its time slice.
     *
     * The above might appear to imply that all Nodes are GC-reachable
     * from a predecessor dequeued Node.  That would cause two problems:
     * - allow a rogue Iterator to cause unbounded memory retention
     * - cause cross-generational linking of old Nodes to new Nodes if
     *   a Node was tenured while live, which generational GCs have a
     *   hard time dealing with, causing repeated major collections.
     * However, only non-deleted Nodes need to be reachable from
     * dequeued Nodes, and reachability does not necessarily have to
     * be of the kind understood by the GC.  We use the trick of
     * linking a Node that has just been dequeued to itself.  Such a
     * self-link implicitly means to advance to head.
     *
     * Both head and tail are permitted to lag.  In fact, failing to
     * update them every time one could is a significant optimization
     * (fewer CASes). As with LinkedTransferQueue (see the internal
     * documentation for that class), we use a slack threshold of two;
     * that is, we update head/tail when the current pointer appears
     * to be two or more steps away from the first/last node.
     *
     * Since head and tail are updated concurrently and independently,
     * it is possible for tail to lag behind head (why not)?
     *
     * CASing a Node's item reference to null atomically removes the
     * element from the queue.  Iterators skip over Nodes with null
     * items.  Prior implementations of this class had a race between
     * poll() and remove(Object) where the same element would appear
     * to be successfully removed by two concurrent operations.  The
     * method remove(Object) also lazily unlinks deleted Nodes, but
     * this is merely an optimization.
     *
     * When constructing a Node (before enqueuing it) we avoid paying
     * for a volatile write to item by using Unsafe.putObject instead
     * of a normal write.  This allows the cost of enqueue to be
     * "one-and-a-half" CASes.
     *
     * Both head and tail may or may not point to a Node with a
     * non-null item.  If the queue is empty, all items must of course
     * be null.  Upon creation, both head and tail refer to a dummy
     * Node with null item.  Both head and tail are only updated using
     * CAS, so they never regress, although again this is merely an
     * optimization.
     */

    private static class Node<E> {
        volatile E item;
        volatile Node<E> next;
    }

    /**
     * Returns a new node holding item.  Uses relaxed write because item
     * can only be seen after piggy-backing publication via casNext.
     */
    static <E> Node<E> newNode(E item) {
        Node<E> node = new Node<E>();
        U.putObject(node, ITEM, item);
        return node;
    }

    static <E> boolean casItem(Node<E> node, E cmp, E val) {
        return U.compareAndSwapObject(node, ITEM, cmp, val);
    }

    static <E> void lazySetNext(Node<E> node, Node<E> val) {
        U.putOrderedObject(node, NEXT, val);
    }

    static <E> boolean casNext(Node<E> node, Node<E> cmp, Node<E> val) {
        return U.compareAndSwapObject(node, NEXT, cmp, val);
    }

    /**
     * A node from which the first live (non-deleted) node (if any)
     * can be reached in O(1) time.
     * Invariants:
     * - all live nodes are reachable from head via succ()
     * - head != null
     * - (tmp = head).next != tmp || tmp != head
     * Non-invariants:
     * - head.item may or may not be null.
     * - it is permitted for tail to lag behind head, that is, for tail
     *   to not be reachable from head!
     */
    transient volatile Node<E> head;

    /**
     * A node from which the last node on list (that is, the unique
     * node with node.next == null) can be reached in O(1) time.
     * Invariants:
     * - the last node is always reachable from tail via succ()
     * - tail != null
     * Non-invariants:
     * - tail.item may or may not be null.
     * - it is permitted for tail to lag behind head, that is, for tail
     *   to not be reachable from head!
     * - tail.next may or may not be self-pointing to tail.
     */
    private transient volatile Node<E> tail;

    /**
     * Creates a {@code ConcurrentLinkedQueue} that is initially empty.
     */
    public ConcurrentLinkedQueue() {
        head = tail = newNode(null);
    }

    /**
     * Creates a {@code ConcurrentLinkedQueue}
     * initially containing the elements of the given collection,
     * added in traversal order of the collection's iterator.
     *
     * @param c the collection of elements to initially contain
     * @throws NullPointerException if the specified collection or any
     *         of its elements are null
     */
    public ConcurrentLinkedQueue(Collection<? extends E> c) {
        Node<E> h = null, t = null;
        for (E e : c) {
            Node<E> newNode = newNode(Objects.requireNonNull(e));
            if (h == null)
                h = t = newNode;
            else {
                lazySetNext(t, newNode);
                t = newNode;
            }
        }
        if (h == null)
            h = t = newNode(null);
        head = h;
        tail = t;
    }

    // Have to override just to update the javadoc

    /**
     * Inserts the specified element at the tail of this queue.
     * As the queue is unbounded, this method will never throw
     * {@link IllegalStateException} or return {@code false}.
     *
     * @return {@code true} (as specified by {@link Collection#add})
     * @throws NullPointerException if the specified element is null
     */
    public boolean add(E e) {
        return offer(e);
    }

    /**
     * Tries to CAS head to p. If successful, repoint old head to itself
     * as sentinel for succ(), below.
     */
    final void updateHead(Node<E> h, Node<E> p) {
        // assert h != null && p != null && (h == p || h.item == null);
        if (h != p && casHead(h, p))
            lazySetNext(h, h);
    }

    /**
     * Returns the successor of p, or the head node if p.next has been
     * linked to self, which will only be true if traversing with a
     * stale pointer that is now off the list.
     */
    final Node<E> succ(Node<E> p) {
        Node<E> next = p.next;
        return (p == next) ? head : next;
    }

    /**
     * Inserts the specified element at the tail of this queue.
     * As the queue is unbounded, this method will never return {@code false}.
     *
     * @return {@code true} (as specified by {@link Queue#offer})
     * @throws NullPointerException if the specified element is null
     */
    public boolean offer(E e) {
        final Node<E> newNode = newNode(Objects.requireNonNull(e));

        for (Node<E> t = tail, p = t;;) {
            Node<E> q = p.next;
            if (q == null) {
                // p is last node
                if (casNext(p, null, newNode)) {
                    // Successful CAS is the linearization point
                    // for e to become an element of this queue,
                    // and for newNode to become "live".
                    if (p != t) // hop two nodes at a time
                        casTail(t, newNode);  // Failure is OK.
                    return true;
                }
                // Lost CAS race to another thread; re-read next
            }
            else if (p == q)
                // We have fallen off list.  If tail is unchanged, it
                // will also be off-list, in which case we need to
                // jump to head, from which all live nodes are always
                // reachable.  Else the new tail is a better bet.
                p = (t != (t = tail)) ? t : head;
            else
                // Check for tail updates after two hops.
                p = (p != t && t != (t = tail)) ? t : q;
        }
    }

    public E poll() {
        restartFromHead:
        for (;;) {
            for (Node<E> h = head, p = h, q;;) {
                E item = p.item;

                if (item != null && casItem(p, item, null)) {
                    // Successful CAS is the linearization point
                    // for item to be removed from this queue.
                    if (p != h) // hop two nodes at a time
                        updateHead(h, ((q = p.next) != null) ? q : p);
                    return item;
                }
                else if ((q = p.next) == null) {
                    updateHead(h, p);
                    return null;
                }
                else if (p == q)
                    continue restartFromHead;
                else
                    p = q;
            }
        }
    }

    public E peek() {
        restartFromHead:
        for (;;) {
            for (Node<E> h = head, p = h, q;;) {
                E item = p.item;
                if (item != null || (q = p.next) == null) {
                    updateHead(h, p);
                    return item;
                }
                else if (p == q)
                    continue restartFromHead;
                else
                    p = q;
            }
        }
    }

    /**
     * Returns the first live (non-deleted) node on list, or null if none.
     * This is yet another variant of poll/peek; here returning the
     * first node, not element.  We could make peek() a wrapper around
     * first(), but that would cost an extra volatile read of item,
     * and the need to add a retry loop to deal with the possibility
     * of losing a race to a concurrent poll().
     */
    Node<E> first() {
        restartFromHead:
        for (;;) {
            for (Node<E> h = head, p = h, q;;) {
                boolean hasItem = (p.item != null);
                if (hasItem || (q = p.next) == null) {
                    updateHead(h, p);
                    return hasItem ? p : null;
                }
                else if (p == q)
                    continue restartFromHead;
                else
                    p = q;
            }
        }
    }

    /**
     * Returns {@code true} if this queue contains no elements.
     *
     * @return {@code true} if this queue contains no elements
     */
    public boolean isEmpty() {
        return first() == null;
    }

    /**
     * Returns the number of elements in this queue.  If this queue
     * contains more than {@code Integer.MAX_VALUE} elements, returns
     * {@code Integer.MAX_VALUE}.
     *
     * <p>Beware that, unlike in most collections, this method is
     * <em>NOT</em> a constant-time operation. Because of the
     * asynchronous nature of these queues, determining the current
     * number of elements requires an O(n) traversal.
     * Additionally, if elements are added or removed during execution
     * of this method, the returned result may be inaccurate.  Thus,
     * this method is typically not very useful in concurrent
     * applications.
     *
     * @return the number of elements in this queue
     */
    public int size() {
        restartFromHead: for (;;) {
            int count = 0;
            for (Node<E> p = first(); p != null;) {
                if (p.item != null)
                    if (++count == Integer.MAX_VALUE)
                        break;  // @see Collection.size()
                if (p == (p = p.next))
                    continue restartFromHead;
            }
            return count;
        }
    }

    /**
     * Returns {@code true} if this queue contains the specified element.
     * More formally, returns {@code true} if and only if this queue contains
     * at least one element {@code e} such that {@code o.equals(e)}.
     *
     * @param o object to be checked for containment in this queue
     * @return {@code true} if this queue contains the specified element
     */
    public boolean contains(Object o) {
        if (o != null) {
            for (Node<E> p = first(); p != null; p = succ(p)) {
                E item = p.item;
                if (item != null && o.equals(item))
                    return true;
            }
        }
        return false;
    }

    /**
     * Removes a single instance of the specified element from this queue,
     * if it is present.  More formally, removes an element {@code e} such
     * that {@code o.equals(e)}, if this queue contains one or more such
     * elements.
     * Returns {@code true} if this queue contained the specified element
     * (or equivalently, if this queue changed as a result of the call).
     *
     * @param o element to be removed from this queue, if present
     * @return {@code true} if this queue changed as a result of the call
     */
    public boolean remove(Object o) {
        if (o != null) {
            Node<E> next, pred = null;
            for (Node<E> p = first(); p != null; pred = p, p = next) {
                boolean removed = false;
                E item = p.item;
                if (item != null) {
                    if (!o.equals(item)) {
                        next = succ(p);
                        continue;
                    }
                    removed = casItem(p, item, null);
                }

                next = succ(p);
                if (pred != null && next != null) // unlink
                    casNext(pred, p, next);
                if (removed)
                    return true;
            }
        }
        return false;
    }

    /**
     * Appends all of the elements in the specified collection to the end of
     * this queue, in the order that they are returned by the specified
     * collection's iterator.  Attempts to {@code addAll} of a queue to
     * itself result in {@code IllegalArgumentException}.
     *
     * @param c the elements to be inserted into this queue
     * @return {@code true} if this queue changed as a result of the call
     * @throws NullPointerException if the specified collection or any
     *         of its elements are null
     * @throws IllegalArgumentException if the collection is this queue
     */
    public boolean addAll(Collection<? extends E> c) {
        if (c == this)
            // As historically specified in AbstractQueue#addAll
            throw new IllegalArgumentException();

        // Copy c into a private chain of Nodes
        Node<E> beginningOfTheEnd = null, last = null;
        for (E e : c) {
            Node<E> newNode = newNode(Objects.requireNonNull(e));
            if (beginningOfTheEnd == null)
                beginningOfTheEnd = last = newNode;
            else {
                lazySetNext(last, newNode);
                last = newNode;
            }
        }
        if (beginningOfTheEnd == null)
            return false;

        // Atomically append the chain at the tail of this collection
        for (Node<E> t = tail, p = t;;) {
            Node<E> q = p.next;
            if (q == null) {
                // p is last node
                if (casNext(p, null, beginningOfTheEnd)) {
                    // Successful CAS is the linearization point
                    // for all elements to be added to this queue.
                    if (!casTail(t, last)) {
                        // Try a little harder to update tail,
                        // since we may be adding many elements.
                        t = tail;
                        if (last.next == null)
                            casTail(t, last);
                    }
                    return true;
                }
                // Lost CAS race to another thread; re-read next
            }
            else if (p == q)
                // We have fallen off list.  If tail is unchanged, it
                // will also be off-list, in which case we need to
                // jump to head, from which all live nodes are always
                // reachable.  Else the new tail is a better bet.
                p = (t != (t = tail)) ? t : head;
            else
                // Check for tail updates after two hops.
                p = (p != t && t != (t = tail)) ? t : q;
        }
    }

    public String toString() {
        String[] a = null;
        restartFromHead: for (;;) {
            int charLength = 0;
            int size = 0;
            for (Node<E> p = first(); p != null;) {
                E item = p.item;
                if (item != null) {
                    if (a == null)
                        a = new String[4];
                    else if (size == a.length)
                        a = Arrays.copyOf(a, 2 * size);
                    String s = item.toString();
                    a[size++] = s;
                    charLength += s.length();
                }
                if (p == (p = p.next))
                    continue restartFromHead;
            }

            if (size == 0)
                return "[]";

            return Helpers.toString(a, size, charLength);
        }
    }

    private Object[] toArrayInternal(Object[] a) {
        Object[] x = a;
        restartFromHead: for (;;) {
            int size = 0;
            for (Node<E> p = first(); p != null;) {
                E item = p.item;
                if (item != null) {
                    if (x == null)
                        x = new Object[4];
                    else if (size == x.length)
                        x = Arrays.copyOf(x, 2 * (size + 4));
                    x[size++] = item;
                }
                if (p == (p = p.next))
                    continue restartFromHead;
            }
            if (x == null)
                return new Object[0];
            else if (a != null && size <= a.length) {
                if (a != x)
                    System.arraycopy(x, 0, a, 0, size);
                if (size < a.length)
                    a[size] = null;
                return a;
            }
            return (size == x.length) ? x : Arrays.copyOf(x, size);
        }
    }

    /**
     * Returns an array containing all of the elements in this queue, in
     * proper sequence.
     *
     * <p>The returned array will be "safe" in that no references to it are
     * maintained by this queue.  (In other words, this method must allocate
     * a new array).  The caller is thus free to modify the returned array.
     *
     * <p>This method acts as bridge between array-based and collection-based
     * APIs.
     *
     * @return an array containing all of the elements in this queue
     */
    public Object[] toArray() {
        return toArrayInternal(null);
    }

    /**
     * Returns an array containing all of the elements in this queue, in
     * proper sequence; the runtime type of the returned array is that of
     * the specified array.  If the queue fits in the specified array, it
     * is returned therein.  Otherwise, a new array is allocated with the
     * runtime type of the specified array and the size of this queue.
     *
     * <p>If this queue fits in the specified array with room to spare
     * (i.e., the array has more elements than this queue), the element in
     * the array immediately following the end of the queue is set to
     * {@code null}.
     *
     * <p>Like the {@link #toArray()} method, this method acts as bridge between
     * array-based and collection-based APIs.  Further, this method allows
     * precise control over the runtime type of the output array, and may,
     * under certain circumstances, be used to save allocation costs.
     *
     * <p>Suppose {@code x} is a queue known to contain only strings.
     * The following code can be used to dump the queue into a newly
     * allocated array of {@code String}:
     *
     * <pre> {@code String[] y = x.toArray(new String[0]);}</pre>
     *
     * Note that {@code toArray(new Object[0])} is identical in function to
     * {@code toArray()}.
     *
     * @param a the array into which the elements of the queue are to
     *          be stored, if it is big enough; otherwise, a new array of the
     *          same runtime type is allocated for this purpose
     * @return an array containing all of the elements in this queue
     * @throws ArrayStoreException if the runtime type of the specified array
     *         is not a supertype of the runtime type of every element in
     *         this queue
     * @throws NullPointerException if the specified array is null
     */
    @SuppressWarnings("unchecked")
    public <T> T[] toArray(T[] a) {
        if (a == null) throw new NullPointerException();
        return (T[]) toArrayInternal(a);
    }

    /**
     * Returns an iterator over the elements in this queue in proper sequence.
     * The elements will be returned in order from first (head) to last (tail).
     *
     * <p>The returned iterator is
     * <a href="package-summary.html#Weakly"><i>weakly consistent</i></a>.
     *
     * @return an iterator over the elements in this queue in proper sequence
     */
    public Iterator<E> iterator() {
        return new Itr();
    }

    private class Itr implements Iterator<E> {
        /**
         * Next node to return item for.
         */
        private Node<E> nextNode;

        /**
         * nextItem holds on to item fields because once we claim
         * that an element exists in hasNext(), we must return it in
         * the following next() call even if it was in the process of
         * being removed when hasNext() was called.
         */
        private E nextItem;

        /**
         * Node of the last returned item, to support remove.
         */
        private Node<E> lastRet;

        Itr() {
            restartFromHead: for (;;) {
                Node<E> h, p, q;
                for (p = h = head;; p = q) {
                    E item;
                    if ((item = p.item) != null) {
                        nextNode = p;
                        nextItem = item;
                        break;
                    }
                    else if ((q = p.next) == null)
                        break;
                    else if (p == q)
                        continue restartFromHead;
                }
                updateHead(h, p);
                return;
            }
        }

        public boolean hasNext() {
            return nextItem != null;
        }

        public E next() {
            final Node<E> pred = nextNode;
            if (pred == null) throw new NoSuchElementException();
            // assert nextItem != null;
            lastRet = pred;
            E item = null;

            for (Node<E> p = succ(pred), q;; p = q) {
                if (p == null || (item = p.item) != null) {
                    nextNode = p;
                    E x = nextItem;
                    nextItem = item;
                    return x;
                }
                // unlink deleted nodes
                if ((q = succ(p)) != null)
                    casNext(pred, p, q);
            }
        }

        public void remove() {
            Node<E> l = lastRet;
            if (l == null) throw new IllegalStateException();
            // rely on a future traversal to relink.
            l.item = null;
            lastRet = null;
        }
    }

    /**
     * Saves this queue to a stream (that is, serializes it).
     *
     * @param s the stream
     * @throws java.io.IOException if an I/O error occurs
     * @serialData All of the elements (each an {@code E}) in
     * the proper order, followed by a null
     */
    private void writeObject(java.io.ObjectOutputStream s)
        throws java.io.IOException {

        // Write out any hidden stuff
        s.defaultWriteObject();

        // Write out all elements in the proper order.
        for (Node<E> p = first(); p != null; p = succ(p)) {
            Object item = p.item;
            if (item != null)
                s.writeObject(item);
        }

        // Use trailing null as sentinel
        s.writeObject(null);
    }

    /**
     * Reconstitutes this queue from a stream (that is, deserializes it).
     * @param s the stream
     * @throws ClassNotFoundException if the class of a serialized object
     *         could not be found
     * @throws java.io.IOException if an I/O error occurs
     */
    private void readObject(java.io.ObjectInputStream s)
        throws java.io.IOException, ClassNotFoundException {
        s.defaultReadObject();

        // Read in elements until trailing null sentinel found
        Node<E> h = null, t = null;
        for (Object item; (item = s.readObject()) != null; ) {
            @SuppressWarnings("unchecked")
            Node<E> newNode = newNode((E) item);
            if (h == null)
                h = t = newNode;
            else {
                lazySetNext(t, newNode);
                t = newNode;
            }
        }
        if (h == null)
            h = t = newNode(null);
        head = h;
        tail = t;
    }

    /** A customized variant of Spliterators.IteratorSpliterator */
    static final class CLQSpliterator<E> implements Spliterator<E> {
        static final int MAX_BATCH = 1 << 25;  // max batch array size;
        final ConcurrentLinkedQueue<E> queue;
        Node<E> current;    // current node; null until initialized
        int batch;          // batch size for splits
        boolean exhausted;  // true when no more nodes
        CLQSpliterator(ConcurrentLinkedQueue<E> queue) {
            this.queue = queue;
        }

        public Spliterator<E> trySplit() {
            Node<E> p;
            final ConcurrentLinkedQueue<E> q = this.queue;
            int b = batch;
            int n = (b <= 0) ? 1 : (b >= MAX_BATCH) ? MAX_BATCH : b + 1;
            if (!exhausted &&
                ((p = current) != null || (p = q.first()) != null) &&
                p.next != null) {
                Object[] a = new Object[n];
                int i = 0;
                do {
                    if ((a[i] = p.item) != null)
                        ++i;
                    if (p == (p = p.next))
                        p = q.first();
                } while (p != null && i < n);
                if ((current = p) == null)
                    exhausted = true;
                if (i > 0) {
                    batch = i;
                    return Spliterators.spliterator
                        (a, 0, i, (Spliterator.ORDERED |
                                   Spliterator.NONNULL |
                                   Spliterator.CONCURRENT));
                }
            }
            return null;
        }

        public void forEachRemaining(Consumer<? super E> action) {
            Node<E> p;
            if (action == null) throw new NullPointerException();
            final ConcurrentLinkedQueue<E> q = this.queue;
            if (!exhausted &&
                ((p = current) != null || (p = q.first()) != null)) {
                exhausted = true;
                do {
                    E e = p.item;
                    if (p == (p = p.next))
                        p = q.first();
                    if (e != null)
                        action.accept(e);
                } while (p != null);
            }
        }

        public boolean tryAdvance(Consumer<? super E> action) {
            Node<E> p;
            if (action == null) throw new NullPointerException();
            final ConcurrentLinkedQueue<E> q = this.queue;
            if (!exhausted &&
                ((p = current) != null || (p = q.first()) != null)) {
                E e;
                do {
                    e = p.item;
                    if (p == (p = p.next))
                        p = q.first();
                } while (e == null && p != null);
                if ((current = p) == null)
                    exhausted = true;
                if (e != null) {
                    action.accept(e);
                    return true;
                }
            }
            return false;
        }

        public long estimateSize() { return Long.MAX_VALUE; }

        public int characteristics() {
            return Spliterator.ORDERED | Spliterator.NONNULL |
                Spliterator.CONCURRENT;
        }
    }

    /**
     * Returns a {@link Spliterator} over the elements in this queue.
     *
     * <p>The returned spliterator is
     * <a href="package-summary.html#Weakly"><i>weakly consistent</i></a>.
     *
     * <p>The {@code Spliterator} reports {@link Spliterator#CONCURRENT},
     * {@link Spliterator#ORDERED}, and {@link Spliterator#NONNULL}.
     *
     * @implNote
     * The {@code Spliterator} implements {@code trySplit} to permit limited
     * parallelism.
     *
     * @return a {@code Spliterator} over the elements in this queue
     * @since 1.8
     */
    @Override
    public Spliterator<E> spliterator() {
        return new CLQSpliterator<E>(this);
    }

    private boolean casTail(Node<E> cmp, Node<E> val) {
        return U.compareAndSwapObject(this, TAIL, cmp, val);
    }

    private boolean casHead(Node<E> cmp, Node<E> val) {
        return U.compareAndSwapObject(this, HEAD, cmp, val);
    }

    // Unsafe mechanics

    private static final sun.misc.Unsafe U = sun.misc.Unsafe.getUnsafe();
    private static final long HEAD;
    private static final long TAIL;
    private static final long ITEM;
    private static final long NEXT;
    static {
        try {
            HEAD = U.objectFieldOffset
                (ConcurrentLinkedQueue.class.getDeclaredField("head"));
            TAIL = U.objectFieldOffset
                (ConcurrentLinkedQueue.class.getDeclaredField("tail"));
            ITEM = U.objectFieldOffset
                (Node.class.getDeclaredField("item"));
            NEXT = U.objectFieldOffset
                (Node.class.getDeclaredField("next"));
        } catch (ReflectiveOperationException e) {
            throw new Error(e);
        }
    }
}