Java多线程系列--“JUC集合”10之 ConcurrentLinkedQueue

概要

本章对Java.util.concurrent包中的ConcurrentHashMap类进行详细的介绍。内容包括:
ConcurrentLinkedQueue介绍
ConcurrentLinkedQueue原理和数据结构
ConcurrentLinkedQueue函数列表
ConcurrentLinkedQueue源码分析(JDK1.7.0_40版本)
ConcurrentLinkedQueue示例

转载请注明出处:http://www.cnblogs.com/skywang12345/p/3498995.html

 

ConcurrentLinkedQueue介绍

ConcurrentLinkedQueue是线程安全的队列,它适用于“高并发”的场景。
它是一个基于链接节点的无界线程安全队列,按照 FIFO(先进先出)原则对元素进行排序。队列元素中不可以放置null元素(内部实现的特殊节点除外)。

 

ConcurrentLinkedQueue原理和数据结构

ConcurrentLinkedQueue的数据结构,如下图所示:

说明
1. ConcurrentLinkedQueue继承于AbstractQueue。
2. ConcurrentLinkedQueue内部是通过链表来实现的。它同时包含链表的头节点head和尾节点tail。ConcurrentLinkedQueue按照 FIFO(先进先出)原则对元素进行排序。元素都是从尾部插入到链表,从头部开始返回。
3. ConcurrentLinkedQueue的链表Node中的next的类型是volatile,而且链表数据item的类型也是volatile。关于volatile,我们知道它的语义包含:“即对一个volatile变量的读,总是能看到(任意线程)对这个volatile变量最后的写入”。ConcurrentLinkedQueue就是通过volatile来实现多线程对竞争资源的互斥访问的。

 

ConcurrentLinkedQueue函数列表

// 创建一个最初为空的 ConcurrentLinkedQueue。
ConcurrentLinkedQueue()
// 创建一个最初包含给定 collection 元素的 ConcurrentLinkedQueue,按照此 collection 迭代器的遍历顺序来添加元素。
ConcurrentLinkedQueue(Collection<? extends E> c)

// 将指定元素插入此队列的尾部。
boolean add(E e)
// 如果此队列包含指定元素,则返回 true。
boolean contains(Object o)
// 如果此队列不包含任何元素,则返回 true。
boolean isEmpty()
// 返回在此队列元素上以恰当顺序进行迭代的迭代器。
Iterator<E> iterator()
// 将指定元素插入此队列的尾部。
boolean offer(E e)
// 获取但不移除此队列的头;如果此队列为空,则返回 null。
E peek()
// 获取并移除此队列的头,如果此队列为空,则返回 null。
E poll()
// 从队列中移除指定元素的单个实例(如果存在)。
boolean remove(Object o)
// 返回此队列中的元素数量。
int size()
// 返回以恰当顺序包含此队列所有元素的数组。
Object[] toArray()
// 返回以恰当顺序包含此队列所有元素的数组;返回数组的运行时类型是指定数组的运行时类型。
<T> T[] toArray(T[] a)

 

ConcurrentLinkedQueue源码分析(JDK1.7.0_40版本)

ConcurrentLinkedQueue的完整源码如下:

/*
 * ORACLE PROPRIETARY/CONFIDENTIAL. Use is subject to license terms.
 *
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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 "wait-free"
 * 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);
        }
    }
}

  

下面从ConcurrentLinkedQueue的创建,添加,删除这几个方面对它进行分析。

1 创建

下面以ConcurrentLinkedQueue()来进行说明。

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

说明:在构造函数中,新建了一个“内容为null的节点”,并设置表头head和表尾tail的值为新节点。

head和tail的定义如下:

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

head和tail都是volatile类型,他们具有volatile赋予的含义:“即对一个volatile变量的读,总是能看到(任意线程)对这个volatile变量最后的写入”。


Node的声明如下:

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);
        }
    }
}

说明
Node是个单向链表节点,next用于指向下一个Node,item用于存储数据。Node中操作节点数据的API,都是通过Unsafe机制的CAS函数实现的;例如casNext()是通过CAS函数“比较并设置节点的下一个节点”。

 

2. 添加

下面以add(E e)为例对ConcurrentLinkedQueue中的添加进行说明。

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

说明:add()实际上是调用的offer()来完成添加操作的。

offer()的源码如下:

public boolean offer(E e) {
    // 检查e是不是null,是的话抛出NullPointerException异常。
    checkNotNull(e);
    // 创建新的节点
    final Node<E> newNode = new Node<E>(e);

    // 将“新的节点”添加到链表的末尾。
    for (Node<E> t = tail, p = t;;) {
        Node<E> q = p.next;
        // 情况1:q为空
        if (q == null) {
            // CAS操作:如果“p的下一个节点为null”(即p为尾节点),则设置p的下一个节点为newNode。
            // 如果该CAS操作成功的话,则比较“p和t”(若p不等于t,则设置newNode为新的尾节点),然后返回true。
            // 如果该CAS操作失败,这意味着“其它线程对尾节点进行了修改”,则重新循环。
            if (p.casNext(null, newNode)) {
                if (p != t) // hop two nodes at a time
                    casTail(t, newNode);  // Failure is OK.
                return true;
            }
        }
        // 情况2:p和q相等
        else if (p == q)
            p = (t != (t = tail)) ? t : head;
        // 情况3:其它
        else
            p = (p != t && t != (t = tail)) ? t : q;
    }
}

说明:offer(E e)的作用就是将元素e添加到链表的末尾。offer()比较的地方是理解for循环,下面区分3种情况对for进行分析。

情况1 -- q为空。这意味着q是尾节点的下一个节点。此时,通过p.casNext(null, newNode)将“p的下一个节点设为newNode”,若设置成功的话,则比较“p和t”(若p不等于t,则设置newNode为新的尾节点),然后返回true。否则的话(意味着“其它线程对尾节点进行了修改”),什么也不做,继续进行for循环。
p.casNext(null, newNode),是调用CAS对p进行操作。若“p的下一个节点等于null”,则设置“p的下一个节点等于newNode”;设置成功的话,返回true,失败的话返回false。

情况2 -- p和q相等。这种情况什么时候会发生呢?通过“情况3”,我们知道,经过“情况3”的处理后,p的值可能等于q。
此时,若尾节点没有发生变化的话,那么,应该是头节点发生了变化,则设置p为头节点,然后重新遍历链表;否则(尾节点变化的话),则设置p为尾节点。

情况3 -- 其它。
我们将p = (p != t && t != (t = tail)) ? t : q;转换成如下代码。

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,则设置p为q。否则的话,判断“尾节点是否发生变化”,没有变化的话,则设置p为q;否则,设置p为尾节点。


checkNotNull()的源码如下:

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

 

3. 删除

下面以poll()为例对ConcurrentLinkedQueue中的删除进行说明。

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

            // 情况1
            // 表头的数据不为null,并且“设置表头的数据为null”这个操作成功的话;
            // 则比较“p和h”(若p!=h,即表头发生了变化,则更新表头,即设置表头为p),然后返回原表头的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;
            }
            // 情况2
            // 表头的下一个节点为null,即链表只有一个“内容为null的表头节点”。则更新表头为p,并返回null。
            else if ((q = p.next) == null) {
                updateHead(h, p);
                return null;
            }
            // 情况3
            // 这可能到由于“情况4”的发生导致p=q,在该情况下跳转到restartFromHead标记重新操作。
            else if (p == q)
                continue restartFromHead;
            // 情况4
            // 设置p为q
            else
                p = q;
        }
    }
}

说明:poll()的作用就是删除链表的表头节点,并返回被删节点对应的值。poll()的实现原理和offer()比较类似,下面根将or循环划分为4种情况进行分析。

情况1:“表头节点的数据”不为null,并且“设置表头节点的数据为null”这个操作成功。
p.casItem(item, null) -- 调用CAS函数,比较“节点p的数据值”与item是否相等,是的话,设置节点p的数据值为null。
在情况1发生时,先比较“p和h”,若p!=h,即表头发生了变化,则调用updateHead()更新表头;然后返回删除节点的item值。
updateHead()的源码如下:

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

说明:updateHead()的最终目的是更新表头为p,并设置h的下一个节点为h本身。
casHead(h,p)是通过CAS函数设置表头,若表头等于h的话,则设置表头为p。
lazySetNext()的源码如下:

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

putOrderedObject()函数,我们在前面一章“TODO”中介绍过。h.lazySetNext(h)的作用是通过CAS函数设置h的下一个节点为h自身,该设置可能会延迟执行。

情况2:如果表头的下一个节点为null,即链表只有一个“内容为null的表头节点”。
则调用updateHead(h, p),将表头更新p;然后返回null。

情况3:p=q
在“情况4”的发生后,会导致p=q;此时,“情况3”就会发生。当“情况3”发生后,它会跳转到restartFromHead标记重新操作。

情况4:其它情况。
设置p=q。

 

ConcurrentLinkedQueue示例

 1 import java.util.*;
 2 import java.util.concurrent.*;
 3 
 4 /*
 5  *   ConcurrentLinkedQueue是“线程安全”的队列,而LinkedList是非线程安全的。
 6  *
 7  *   下面是“多个线程同时操作并且遍历queue”的示例
 8  *   (01) 当queue是ConcurrentLinkedQueue对象时,程序能正常运行。
 9  *   (02) 当queue是LinkedList对象时,程序会产生ConcurrentModificationException异常。
10  *
11  * @author skywang
12  */
13 public class ConcurrentLinkedQueueDemo1 {
14 
15     // TODO: queue是LinkedList对象时,程序会出错。
16     //private static Queue<String> queue = new LinkedList<String>();
17     private static Queue<String> queue = new ConcurrentLinkedQueue<String>();
18     public static void main(String[] args) {
19     
20         // 同时启动两个线程对queue进行操作!
21         new MyThread("ta").start();
22         new MyThread("tb").start();
23     }
24 
25     private static void printAll() {
26         String value;
27         Iterator iter = queue.iterator();
28         while(iter.hasNext()) {
29             value = (String)iter.next();
30             System.out.print(value+", ");
31         }
32         System.out.println();
33     }
34 
35     private static class MyThread extends Thread {
36         MyThread(String name) {
37             super(name);
38         }
39         @Override
40         public void run() {
41                 int i = 0;
42             while (i++ < 6) {
43                 // “线程名” + "-" + "序号"
44                 String val = Thread.currentThread().getName()+i;
45                 queue.add(val);
46                 // 通过“Iterator”遍历queue。
47                 printAll();
48             }
49         }
50     }
51 }

(某一次)运行结果

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, 

结果说明:如果将源码中的queue改成LinkedList对象时,程序会产生ConcurrentModificationException异常。

posted @ 2020-05-03 23:21  跃小云  阅读(689)  评论(0编辑  收藏  举报