ConcurrentLinkedQueue.java

/*
 * DO NOT ALTER OR REMOVE COPYRIGHT NOTICES OR THIS FILE HEADER.
 *
 * This code is free software; you can redistribute it and/or modify it
 * under the terms of the GNU General Public License version 2 only, as
 * published by the Free Software Foundation.  Sun designates this
 * particular file as subject to the "Classpath" exception as provided
 * by Sun in the LICENSE file that accompanied this code.
 *
 * This code is distributed in the hope that it will be useful, but WITHOUT
 * ANY WARRANTY; without even the implied warranty of MERCHANTABILITY or
 * FITNESS FOR A PARTICULAR PURPOSE.  See the GNU General Public License
 * version 2 for more details (a copy is included in the LICENSE file that
 * accompanied this code).
 *
 * You should have received a copy of the GNU General Public License version
 * 2 along with this work; if not, write to the Free Software Foundation,
 * Inc., 51 Franklin St, Fifth Floor, Boston, MA 02110-1301 USA.
 *
 * Please contact Sun Microsystems, Inc., 4150 Network Circle, Santa Clara,
 * CA 95054 USA or visit www.sun.com if you need additional information or
 * have any questions.
 */

/*
 * This file is available under and governed by the GNU General Public
 * License version 2 only, as published by the Free Software Foundation.
 * However, the following notice accompanied the original version of this
 * file:
 *
 * Written by Doug Lea with assistance from members of JCP JSR-166
 * Expert Group and released to the public domain, as explained at
 * http://creativecommons.org/licenses/publicdomain
 */

package java.util.concurrent;

import java.util.AbstractQueue;
import java.util.ArrayList;
import java.util.Collection;
import java.util.Iterator;
import java.util.NoSuchElementException;
import java.util.Queue;

/**
 * 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.
 * This queue does not permit {@code null} elements.
 *
 * <p>This implementation employs an efficient &quot;wait-free&quot;
 * algorithm based on one described in <a
 * href="http://www.cs.rochester.edu/u/michael/PODC96.html"> Simple,
 * Fast, and Practical Non-Blocking and Blocking Concurrent Queue
 * Algorithms</a> by Maged M. Michael and Michael L. Scott.
 *
 * <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.
 *
 * <p>This class and its iterator implement all of the
 * <em>optional</em> methods of the {@link Collection} 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.
 *
 * <p>This class is a member of the
 * <a href="{@docRoot}/../technotes/guides/collections/index.html">
 * Java Collections Framework</a>.
 *
 * @since 1.5
 * @author Doug Lea
 * @param <E> the type of elements held in this collection
 *
 */
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). This is controlled by local "hops" variables
     * that only trigger helping-CASes after experiencing multiple
     * lags.
     *
     * 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 lazySet 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> {
        private volatile E item;
        private volatile Node<E> next;

        Node(E item) {
            // Piggyback on imminent casNext()
            lazySetItem(item);
        }

        E getItem() {
            return item;
        }

        boolean casItem(E cmp, E val) {
            return UNSAFE.compareAndSwapObject(this, itemOffset, cmp, val);
        }

        void setItem(E val) {
            item = val;
        }

        void lazySetItem(E val) {
            UNSAFE.putOrderedObject(this, itemOffset, val);
        }

        void lazySetNext(Node<E> val) {
            UNSAFE.putOrderedObject(this, nextOffset, val);
        }

        Node<E> getNext() {
            return next;
        }

        boolean casNext(Node<E> cmp, Node<E> val) {
            return UNSAFE.compareAndSwapObject(this, nextOffset, cmp, val);
        }

        // Unsafe mechanics

        private static final sun.misc.Unsafe UNSAFE =
            sun.misc.Unsafe.getUnsafe();
        private static final long nextOffset =
            objectFieldOffset(UNSAFE, "next", Node.class);
        private static final long itemOffset =
            objectFieldOffset(UNSAFE, "item", Node.class);
    }

    /**
     * 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!
     */
    private transient volatile Node<E> head = new Node<E>(null);

    /**
     * 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 = head;


    /**
     * Creates a {@code ConcurrentLinkedQueue} that is initially empty.
     */
    public ConcurrentLinkedQueue() {}

    /**
     * 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) {
        for (E e : c)
            add(e);
    }

    // Have to override just to update the javadoc

    /**
     * Inserts the specified element at the tail of this queue.
     *
     * @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);
    }

    /**
     * We don't bother to update head or tail pointers if fewer than
     * HOPS links from "true" location.  We assume that volatile
     * writes are significantly more expensive than volatile reads.
     */
    private static final int HOPS = 1;

    /**
     * Try 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) {
        if (h != p && casHead(h, p))
            h.lazySetNext(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.getNext();
        return (p == next) ? head : next;
    }

    /**
     * Inserts the specified element at the tail of this queue.
     *
     * @return {@code true} (as specified by {@link Queue#offer})
     * @throws NullPointerException if the specified element is null
     */
    public boolean offer(E e) {
        if (e == null) throw new NullPointerException();
        Node<E> n = new Node<E>(e);
        retry:
        for (;;) {
            Node<E> t = tail;
            Node<E> p = t;
            for (int hops = 0; ; hops++) {
                Node<E> next = succ(p);
                if (next != null) {
                    if (hops > HOPS && t != tail)
                        continue retry;
                    p = next;
                } else if (p.casNext(null, n)) {
                    if (hops >= HOPS)
                        casTail(t, n);  // Failure is OK.
                    return true;
                } else {
                    p = succ(p);
                }
            }
        }
    }

    public E poll() {
        Node<E> h = head;
        Node<E> p = h;
        for (int hops = 0; ; hops++) {
            E item = p.getItem();

            if (item != null && p.casItem(item, null)) {
                if (hops >= HOPS) {
                    Node<E> q = p.getNext();
                    updateHead(h, (q != null) ? q : p);
                }
                return item;
            }
            Node<E> next = succ(p);
            if (next == null) {
                updateHead(h, p);
                break;
            }
            p = next;
        }
        return null;
    }

    public E peek() {
        Node<E> h = head;
        Node<E> p = h;
        E item;
        for (;;) {
            item = p.getItem();
            if (item != null)
                break;
            Node<E> next = succ(p);
            if (next == null) {
                break;
            }
            p = next;
        }
        updateHead(h, p);
        return item;
    }

    /**
     * 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() {
        Node<E> h = head;
        Node<E> p = h;
        Node<E> result;
        for (;;) {
            E item = p.getItem();
            if (item != null) {
                result = p;
                break;
            }
            Node<E> next = succ(p);
            if (next == null) {
                result = null;
                break;
            }
            p = next;
        }
        updateHead(h, p);
        return result;
    }

    /**
     * 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.
     *
     * @return the number of elements in this queue
     */
    public int size() {
        int count = 0;
        for (Node<E> p = first(); p != null; p = succ(p)) {
            if (p.getItem() != null) {
                // Collections.size() spec says to max out
                if (++count == Integer.MAX_VALUE)
                    break;
            }
        }
        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) return false;
        for (Node<E> p = first(); p != null; p = succ(p)) {
            E item = p.getItem();
            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) return false;
        Node<E> pred = null;
        for (Node<E> p = first(); p != null; p = succ(p)) {
            E item = p.getItem();
            if (item != null &&
                o.equals(item) &&
                p.casItem(item, null)) {
                Node<E> next = succ(p);
                if (pred != null && next != null)
                    pred.casNext(p, next);
                return true;
            }
            pred = p;
        }
        return false;
    }

    /**
     * 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() {
        // Use ArrayList to deal with resizing.
        ArrayList<E> al = new ArrayList<E>();
        for (Node<E> p = first(); p != null; p = succ(p)) {
            E item = p.getItem();
            if (item != null)
                al.add(item);
        }
        return al.toArray();
    }

    /**
     * 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>
     *     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) {
        // try to use sent-in array
        int k = 0;
        Node<E> p;
        for (p = first(); p != null && k < a.length; p = succ(p)) {
            E item = p.getItem();
            if (item != null)
                a[k++] = (T)item;
        }
        if (p == null) {
            if (k < a.length)
                a[k] = null;
            return a;
        }

        // If won't fit, use ArrayList version
        ArrayList<E> al = new ArrayList<E>();
        for (Node<E> q = first(); q != null; q = succ(q)) {
            E item = q.getItem();
            if (item != null)
                al.add(item);
        }
        return al.toArray(a);
    }

    /**
     * Returns an iterator over the elements in this queue in proper sequence.
     * The returned iterator is a "weakly consistent" iterator that
     * will never throw {@link java.util.ConcurrentModificationException
     * ConcurrentModificationException},
     * and guarantees to traverse elements as they existed upon
     * construction of the iterator, and may (but is not guaranteed to)
     * reflect any modifications subsequent to construction.
     *
     * @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() {
            advance();
        }

        /**
         * Moves to next valid node and returns item to return for
         * next(), or null if no such.
         */
        private E advance() {
            lastRet = nextNode;
            E x = nextItem;

            Node<E> pred, p;
            if (nextNode == null) {
                p = first();
                pred = null;
            } else {
                pred = nextNode;
                p = succ(nextNode);
            }

            for (;;) {
                if (p == null) {
                    nextNode = null;
                    nextItem = null;
                    return x;
                }
                E item = p.getItem();
                if (item != null) {
                    nextNode = p;
                    nextItem = item;
                    return x;
                } else {
                    // skip over nulls
                    Node<E> next = succ(p);
                    if (pred != null && next != null)
                        pred.casNext(p, next);
                    p = next;
                }
            }
        }

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

        public E next() {
            if (nextNode == null) throw new NoSuchElementException();
            return advance();
        }

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

    /**
     * Save the state to a stream (that is, serialize it).
     *
     * @serialData All of the elements (each an {@code E}) in
     * the proper order, followed by a null
     * @param s the stream
     */
    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.getItem();
            if (item != null)
                s.writeObject(item);
        }

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

    /**
     * Reconstitute the Queue instance from a stream (that is,
     * deserialize it).
     * @param s the stream
     */
    private void readObject(java.io.ObjectInputStream s)
        throws java.io.IOException, ClassNotFoundException {
        // Read in capacity, and any hidden stuff
        s.defaultReadObject();
        head = new Node<E>(null);
        tail = head;
        // Read in all elements and place in queue
        for (;;) {
            @SuppressWarnings("unchecked")
            E item = (E)s.readObject();
            if (item == null)
                break;
            else
                offer(item);
        }
    }

    // Unsafe mechanics

    private static final sun.misc.Unsafe UNSAFE = sun.misc.Unsafe.getUnsafe();
    private static final long headOffset =
        objectFieldOffset(UNSAFE, "head", ConcurrentLinkedQueue.class);
    private static final long tailOffset =
        objectFieldOffset(UNSAFE, "tail", ConcurrentLinkedQueue.class);

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

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

    private void lazySetHead(Node<E> val) {
        UNSAFE.putOrderedObject(this, headOffset, val);
    }

    static long objectFieldOffset(sun.misc.Unsafe UNSAFE,
                                  String field, Class<?> klazz) {
        try {
            return UNSAFE.objectFieldOffset(klazz.getDeclaredField(field));
        } catch (NoSuchFieldException e) {
            // Convert Exception to corresponding Error
            NoSuchFieldError error = new NoSuchFieldError(field);
            error.initCause(e);
            throw error;
        }
    }
}