Java多線程系列--“JUC集合”10之 ConcurrentLinkedQueue

 

概要

本章對Java.util.concurrent包中的ConcurrentHashMap類進行詳細的介紹。內容包括：
ConcurrentLinkedQueue介紹
ConcurrentLinkedQueue原理和數據結構
ConcurrentLinkedQueue函數列表
ConcurrentLinkedQueue源碼分析(JDK1.7.0_40版本)
ConcurrentLinkedQueue示例

轉載請注明出處：http://www.cnblogs.com/skywang12345/p/3498995.html

 

ConcurrentLinkedQueue介紹

ConcurrentLinkedQueue是線程安全的隊列，它適用於“高並發”的場景。
它是一個基於鏈接節點的無界線程安全隊列，按照 FIFO（先進先出）原則對元素進行排序。隊列元素中不可以放置null元素（內部實現的特殊節點除外）。

 

ConcurrentLinkedQueue原理和數據結構

ConcurrentLinkedQueue的數據結構，如下圖所示：

說明：
1. ConcurrentLinkedQueue繼承於AbstractQueue。
2. ConcurrentLinkedQueue內部是通過鏈表來實現的。它同時包含鏈表的頭節點head和尾節點tail。ConcurrentLinkedQueue按照 FIFO（先進先出）原則對元素進行排序。元素都是從尾部插入到鏈表，從頭部開始返回。
3. ConcurrentLinkedQueue的鏈表Node中的next的類型是volatile，而且鏈表數據item的類型也是volatile。關於volatile，我們知道它的語義包含：“即對一個volatile變量的讀，總是能看到（任意線程）對這個volatile變量最后的寫入”。ConcurrentLinkedQueue就是通過volatile來實現多線程對競爭資源的互斥訪問的。

 

ConcurrentLinkedQueue函數列表

// 創建一個最初為空的 ConcurrentLinkedQueue。
ConcurrentLinkedQueue()
// 創建一個最初包含給定 collection 元素的 ConcurrentLinkedQueue，按照此 collection 迭代器的遍歷順序來添加元素。
ConcurrentLinkedQueue(Collection<? extends E> c)

// 將指定元素插入此隊列的尾部。
boolean add(E e)
// 如果此隊列包含指定元素，則返回 true。
boolean contains(Object o)
// 如果此隊列不包含任何元素，則返回 true。
boolean isEmpty()
// 返回在此隊列元素上以恰當順序進行迭代的迭代器。
Iterator<E> iterator()
// 將指定元素插入此隊列的尾部。
boolean offer(E e)
// 獲取但不移除此隊列的頭；如果此隊列為空，則返回 null。
E peek()
// 獲取並移除此隊列的頭，如果此隊列為空，則返回 null。
E poll()
// 從隊列中移除指定元素的單個實例（如果存在）。
boolean remove(Object o)
// 返回此隊列中的元素數量。
int size()
// 返回以恰當順序包含此隊列所有元素的數組。
Object[] toArray()
// 返回以恰當順序包含此隊列所有元素的數組；返回數組的運行時類型是指定數組的運行時類型。
<T> T[] toArray(T[] a)

 

ConcurrentLinkedQueue源碼分析(JDK1.7.0_40版本)

ConcurrentLinkedQueue的完整源碼如下：

/*
 * ORACLE PROPRIETARY/CONFIDENTIAL. Use is subject to license terms.
 *
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/*
 *
 *
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 *
 * 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.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.
 * Like most other concurrent collection implementations, this class
 * does not permit the use of {@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>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.
 *
 * <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). 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;

        /**
         * Constructs a new node.  Uses relaxed write because item can
         * only be seen after publication via casNext.
         */
        Node(E item) {
            UNSAFE.putObject(this, itemOffset, item);
        }

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

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

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

        // Unsafe mechanics

        private static final sun.misc.Unsafe UNSAFE;
        private static final long itemOffset;
        private static final long nextOffset;

        static {
            try {
                UNSAFE = sun.misc.Unsafe.getUnsafe();
                Class k = Node.class;
                itemOffset = UNSAFE.objectFieldOffset
                    (k.getDeclaredField("item"));
                nextOffset = UNSAFE.objectFieldOffset
                    (k.getDeclaredField("next"));
            } catch (Exception e) {
                throw new Error(e);
            }
        }
    }

    /**
     * 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;

    /**
     * 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 = new Node<E>(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) {
            checkNotNull(e);
            Node<E> newNode = new Node<E>(e);
            if (h == null)
                h = t = newNode;
            else {
                t.lazySetNext(newNode);
                t = newNode;
            }
        }
        if (h == null)
            h = t = new Node<E>(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);
    }

    /**
     * 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.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) {
        checkNotNull(e);
        final Node<E> newNode = new Node<E>(e);

        for (Node<E> t = tail, p = t;;) {
            Node<E> q = p.next;
            if (q == null) {
                // p is last node
                if (p.casNext(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 && p.casItem(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() {
        int count = 0;
        for (Node<E> p = first(); p != null; p = succ(p))
            if (p.item != null)
                // Collection.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.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) return false;
        Node<E> pred = null;
        for (Node<E> p = first(); p != null; p = succ(p)) {
            E item = p.item;
            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;
    }

    /**
     * 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) {
            checkNotNull(e);
            Node<E> newNode = new Node<E>(e);
            if (beginningOfTheEnd == null)
                beginningOfTheEnd = last = newNode;
            else {
                last.lazySetNext(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 (p.casNext(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;
        }
    }

    /**
     * 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.item;
            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.item;
            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.item;
            if (item != null)
                al.add(item);
        }
        return al.toArray(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 "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.item;
                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.item = null;
            lastRet = null;
        }
    }

    /**
     * Saves the state to a stream (that is, serializes 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.item;
            if (item != null)
                s.writeObject(item);
        }

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

    /**
     * Reconstitutes the instance from a stream (that is, deserializes it).
     * @param s the stream
     */
    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;
        Object item;
        while ((item = s.readObject()) != null) {
            @SuppressWarnings("unchecked")
            Node<E> newNode = new Node<E>((E) item);
            if (h == null)
                h = t = newNode;
            else {
                t.lazySetNext(newNode);
                t = newNode;
            }
        }
        if (h == null)
            h = t = new Node<E>(null);
        head = h;
        tail = t;
    }

    /**
     * Throws NullPointerException if argument is null.
     *
     * @param v the element
     */
    private static void checkNotNull(Object v) {
        if (v == null)
            throw new NullPointerException();
    }

    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);
    }

    // Unsafe mechanics

    private static final sun.misc.Unsafe UNSAFE;
    private static final long headOffset;
    private static final long tailOffset;
    static {
        try {
            UNSAFE = sun.misc.Unsafe.getUnsafe();
            Class k = ConcurrentLinkedQueue.class;
            headOffset = UNSAFE.objectFieldOffset
                (k.getDeclaredField("head"));
            tailOffset = UNSAFE.objectFieldOffset
                (k.getDeclaredField("tail"));
        } catch (Exception e) {
            throw new Error(e);
        }
    }
}

 

下面從ConcurrentLinkedQueue的創建，添加，刪除這幾個方面對它進行分析。

1 創建

下面以ConcurrentLinkedQueue()來進行說明。

public ConcurrentLinkedQueue() {
    head = tail = new Node<E>(null);
}

說明：在構造函數中，新建了一個“內容為null的節點”，並設置表頭head和表尾tail的值為新節點。

head和tail的定義如下：

private transient volatile Node<E> head;
private transient volatile Node<E> tail;

head和tail都是volatile類型，他們具有volatile賦予的含義：“即對一個volatile變量的讀，總是能看到（任意線程）對這個volatile變量最后的寫入”。

Node的聲明如下：

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

    Node(E item) {
        UNSAFE.putObject(this, itemOffset, item);
    }

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

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

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

    // Unsafe mechanics
    private static final sun.misc.Unsafe UNSAFE;
    private static final long itemOffset;
    private static final long nextOffset;

    static {
        try {
            UNSAFE = sun.misc.Unsafe.getUnsafe();
            Class k = Node.class;
            itemOffset = UNSAFE.objectFieldOffset
                (k.getDeclaredField("item"));
            nextOffset = UNSAFE.objectFieldOffset
                (k.getDeclaredField("next"));
        } catch (Exception e) {
            throw new Error(e);
        }
    }
}

說明：
Node是個單向鏈表節點，next用於指向下一個Node，item用於存儲數據。Node中操作節點數據的API，都是通過Unsafe機制的CAS函數實現的；例如casNext()是通過CAS函數“比較並設置節點的下一個節點”。

 

2. 添加

下面以add(E e)為例對ConcurrentLinkedQueue中的添加進行說明。

public boolean add(E e) {
    return offer(e);
}

說明：add()實際上是調用的offer()來完成添加操作的。

offer()的源碼如下：

public boolean offer(E e) {
    // 檢查e是不是null，是的話拋出NullPointerException異常。
    checkNotNull(e);
    // 創建新的節點
    final Node<E> newNode = new Node<E>(e);

    // 將“新的節點”添加到鏈表的末尾。
    for (Node<E> t = tail, p = t;;) {
        Node<E> q = p.next;
        // 情況1：q為空
        if (q == null) {
            // CAS操作：如果“p的下一個節點為null”(即p為尾節點)，則設置p的下一個節點為newNode。
            // 如果該CAS操作成功的話，則比較“p和t”(若p不等於t，則設置newNode為新的尾節點)，然后返回true。
            // 如果該CAS操作失敗，這意味着“其它線程對尾節點進行了修改”，則重新循環。
            if (p.casNext(null, newNode)) {
                if (p != t) // hop two nodes at a time
                    casTail(t, newNode);  // Failure is OK.
                return true;
            }
        }
        // 情況2：p和q相等
        else if (p == q)
            p = (t != (t = tail)) ? t : head;
        // 情況3：其它
        else
            p = (p != t && t != (t = tail)) ? t : q;
    }
}

說明：offer(E e)的作用就是將元素e添加到鏈表的末尾。offer()比較的地方是理解for循環，下面區分3種情況對for進行分析。

情況1 -- q為空。這意味着q是尾節點的下一個節點。此時，通過p.casNext(null, newNode)將“p的下一個節點設為newNode”，若設置成功的話，則比較“p和t”(若p不等於t，則設置newNode為新的尾節點)，然后返回true。否則的話(意味着“其它線程對尾節點進行了修改”)，什么也不做，繼續進行for循環。
p.casNext(null, newNode)，是調用CAS對p進行操作。若“p的下一個節點等於null”，則設置“p的下一個節點等於newNode”；設置成功的話，返回true，失敗的話返回false。

情況2 -- p和q相等。這種情況什么時候會發生呢？通過“情況3”，我們知道，經過“情況3”的處理后，p的值可能等於q。
此時，若尾節點沒有發生變化的話，那么，應該是頭節點發生了變化，則設置p為頭節點，然后重新遍歷鏈表；否則(尾節點變化的話)，則設置p為尾節點。

情況3 -- 其它。
我們將p = (p != t && t != (t = tail)) ? t : q;轉換成如下代碼。

if (p==t) {
    p = q;
} else {
    Node<E> tmp=t;
    t = tail;
    if (tmp==t) {
        p=q;
    } else {
        p=t;
    }
}

如果p和t相等，則設置p為q。否則的話，判斷“尾節點是否發生變化”，沒有變化的話，則設置p為q；否則，設置p為尾節點。

checkNotNull()的源碼如下：

private static void checkNotNull(Object v) {
    if (v == null)
        throw new NullPointerException();
}

 

3. 刪除

下面以poll()為例對ConcurrentLinkedQueue中的刪除進行說明。

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

            // 情況1
            // 表頭的數據不為null，並且“設置表頭的數據為null”這個操作成功的話;
            // 則比較“p和h”(若p!=h，即表頭發生了變化，則更新表頭，即設置表頭為p)，然后返回原表頭的item值。
            if (item != null && p.casItem(item, null)) {
                if (p != h) // hop two nodes at a time
                    updateHead(h, ((q = p.next) != null) ? q : p);
                return item;
            }
            // 情況2
            // 表頭的下一個節點為null，即鏈表只有一個“內容為null的表頭節點”。則更新表頭為p，並返回null。
            else if ((q = p.next) == null) {
                updateHead(h, p);
                return null;
            }
            // 情況3
            // 這可能到由於“情況4”的發生導致p=q，在該情況下跳轉到restartFromHead標記重新操作。
            else if (p == q)
                continue restartFromHead;
            // 情況4
            // 設置p為q
            else
                p = q;
        }
    }
}

說明：poll()的作用就是刪除鏈表的表頭節點，並返回被刪節點對應的值。poll()的實現原理和offer()比較類似，下面根將or循環划分為4種情況進行分析。

情況1：“表頭節點的數據”不為null，並且“設置表頭節點的數據為null”這個操作成功。
p.casItem(item, null) -- 調用CAS函數，比較“節點p的數據值”與item是否相等，是的話，設置節點p的數據值為null。
在情況1發生時，先比較“p和h”，若p!=h，即表頭發生了變化，則調用updateHead()更新表頭；然后返回刪除節點的item值。
updateHead()的源碼如下：

final void updateHead(Node<E> h, Node<E> p) {
    if (h != p && casHead(h, p))
        h.lazySetNext(h);
}

說明：updateHead()的最終目的是更新表頭為p，並設置h的下一個節點為h本身。
casHead(h,p)是通過CAS函數設置表頭，若表頭等於h的話，則設置表頭為p。
lazySetNext()的源碼如下：

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

putOrderedObject()函數，我們在前面一章“TODO”中介紹過。h.lazySetNext(h)的作用是通過CAS函數設置h的下一個節點為h自身，該設置可能會延遲執行。

情況2：如果表頭的下一個節點為null，即鏈表只有一個“內容為null的表頭節點”。
則調用updateHead(h, p)，將表頭更新p；然后返回null。

情況3：p=q
在“情況4”的發生后，會導致p=q；此時，“情況3”就會發生。當“情況3”發生后，它會跳轉到restartFromHead標記重新操作。

情況4：其它情況。
設置p=q。

 

ConcurrentLinkedQueue示例

import java.util.*;
import java.util.concurrent.*;

/*
 *   ConcurrentLinkedQueue是“線程安全”的隊列，而LinkedList是非線程安全的。
 *
 *   下面是“多個線程同時操作並且遍歷queue”的示例
 *   (01) 當queue是ConcurrentLinkedQueue對象時，程序能正常運行。
 *   (02) 當queue是LinkedList對象時，程序會產生ConcurrentModificationException異常。
 *
 * @author skywang
 */
public class ConcurrentLinkedQueueDemo1 {

    // TODO: queue是LinkedList對象時，程序會出錯。
    //private static Queue<String> queue = new LinkedList<String>();
    private static Queue<String> queue = new ConcurrentLinkedQueue<String>();
    public static void main(String[] args) {
    
        // 同時啟動兩個線程對queue進行操作！
        new MyThread("ta").start();
        new MyThread("tb").start();
    }

    private static void printAll() {
        String value;
        Iterator iter = queue.iterator();
        while(iter.hasNext()) {
            value = (String)iter.next();
            System.out.print(value+", ");
        }
        System.out.println();
    }

    private static class MyThread extends Thread {
        MyThread(String name) {
            super(name);
        }
        @Override
        public void run() {
                int i = 0;
            while (i++ < 6) {
                // “線程名” + "-" + "序號"
                String val = Thread.currentThread().getName()+i;
                queue.add(val);
                // 通過“Iterator”遍歷queue。
                printAll();
            }
        }
    }
}

(某一次)運行結果：

ta1, ta1, tb1, tb1,

ta1, ta1, tb1, tb1, ta2, ta2, tb2, 
tb2, 
ta1, ta1, tb1, tb1, ta2, ta2, tb2, tb2, ta3, tb3, 
ta3, ta1, tb3, tb1, ta4, 
ta2, ta1, tb2, tb1, ta3, ta2, tb3, tb2, ta4, ta3, tb4, 
tb3, ta1, ta4, tb1, tb4, ta2, ta5, 
tb2, ta1, ta3, tb1, tb3, ta2, ta4, tb2, tb4, ta3, ta5, tb3, tb5, 
ta4, ta1, tb4, tb1, ta5, ta2, tb5, tb2, ta6, 
ta3, ta1, tb3, tb1, ta4, ta2, tb4, tb2, ta5, ta3, tb5, tb3, ta6, ta4, tb6, 
tb4, ta5, tb5, ta6, tb6, 

結果說明：如果將源碼中的queue改成LinkedList對象時，程序會產生ConcurrentModificationException異常。

 

---

更多內容

1. Java多線程系列--“JUC集合”01之 框架 

2. Java多線程系列目錄(共xx篇)