ConcurrentLinkedQueue解析

Queue接口定义：http://donald-draper.iteye.com/blog/2363491
AbstractQueue简介：http://donald-draper.iteye.com/blog/2363608

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 [i]head[/i] of the queue is that element that has been on the
 * queue the longest time.
ConcurrentLinkedQueue一个基于链接节点线程安全的无界队列。队列的元素顺序为FIFO。
队列的头部，是在队列上时间最久的元素。
 * The [i]tail[/i] 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.
 *
 队列的尾元素是在队列中时间最短的元素。新元素添加到队列的尾部，队列获取元素，从
 队列头部获取。ConcurrentLinkedQueue适用于多个线程需要同时访问一个相同集合的场景。
与大多数并发集合一样，不允许插入null元素。
 * <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.
 *
 队列的实现是基于一个有效的wait-free算法，具体参见链接：简单快速实用的并发
 阻塞与非阻塞算法
 * <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 [i]not[/i] 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.
 *
Iterators是弱一致性，只反映了队列在某一点的状态，比如创建iterator的时间点。
Iterators不会抛出异常，可以处理其他的并发操作。在队列创建Iterators时，队列中的
元素，全都在Iterators中。
 * <p>Beware that, unlike in most collections, the {@code size} method
 * is [i]NOT[/i] 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 [i]not[/i] guaranteed
 * to be performed atomically. For example, an iterator operating
 * concurrently with an {@code addAll} operation might view only some
 * of the added elements.
 *
 不像大多数的集合，size操作时间复杂度不是一个常量，因为队列异步操作的天性，
 决定了遍历队列元素时，有可能其他线程修改队列，导致最终size的数量可能不准确。
 另外addAll，removeAll，retainAll，containsAll，equals，toArray都不能保证原子性。
 不如遍历操作和addAll操作同时发生，可能会看到一些相同的新增元素。
 * <p>This class and its iterator implement all of the [i]optional[/i]
 * methods of the {@link Queue} and {@link Iterator} interfaces.
 *
ConcurrentLinkedQueue实现了所有Queue和Iterator接口的所有方法。
 * <p>Memory consistency effects: As with other concurrent
 * collections, actions in a thread prior to placing an object into a
 * {@code ConcurrentLinkedQueue}
 * [url=package-summary.html#MemoryVisibility]<i>happen-before</i>[/url]
 * actions subsequent to the access or removal of that element from
 * the {@code ConcurrentLinkedQueue} in another thread.
 *
 内存一致性：与其他并发线程集合一样，线程添加一个元素到队列发生在其他线程访问
 或移除队列元素之前。
 * <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.
     *
     这是一个Michael & Scott 算法的修改，以适应内部节点的删除，引起的垃圾回收。
     * 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.
     *
     像其他非阻塞算法一样，本队列的实现依赖于实际的系统垃圾回收器，
     由于会回收节点，所以不可能发生ABA问题，所以不需要用，计数指针和non-GC
     设置的相关技术。

     * 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.
     基本原理是不变的：当以CAS方式入队列时，队尾的元素的next指针为null。
     我们用tail节点可，以常量1的速度到达尾部元素，tail仅仅是一个优化，也可从
     对头以时间 O(N) 到达队尾。
     * - 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.
     *
     队列中的元素都是非null的，同时可以从队列头到达。CAS操作一个元素为null，
     则直接从队列移除元素。队列头到队列中的其他元素，必须可达，以防并发修改，
     导致的队列头前移。一个已经出队列的元素可能仍在被用，比如创建Iterator，
     或poll操作失去的时间片。
     * 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.
     *
     以上情况的出现，预示着所有节点从前驱的出队列元素都GC可达。这样
     可能会引起来个问题：运行一个无赖Iterator引起内存垃圾；当节点存活在
     老年代，可能存在旧节点到新节点的交叉代连接，新生和老年的垃圾回收器
     很难处理，这样就会引起重复的FULL GC。然而，已删除的节点，可以从出队列
     节点达到，可达性不需要到达GC可以理解的那种。为了避免这种情况的发生，
     我们出队列的节点只会连接到它自己。自连接意味着促使队列头元素前进。

     * 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.
     *
     对头和队尾都允许滞后，最优化的更新队头和队尾可能会失败，不如fewer CASes。
     对于LinkedTransferQueue，我们用一个非严谨的临界条件2，我们从当前节点
     更新head/tail，至少需要2步以上。
     * Since head and tail are updated concurrently and independently,
     * it is possible for tail to lag behind head (why not)?
     *
     由于head and tail是并发独立更新的，队尾的更新可能在队头后面。
     * 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.
     *
     CAS操作一个节点引用为null，将自动从队列中删除。Iterators将会跳过
     null元素。当poll和remove操作并发时，优先移除元素。remove也为懒操作
     去除链接，这样一个优化。
     * 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.
     *
     在节点入队列前，构造节点时，要避免用可见性的Unsafe.putObject，
     而是用正常的write。进队列的消耗允许是CAS操作的1.5倍。
     * 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.
     */
     head and tail可能，也可能不指向一个非null元素。当队列为null时，
     所有的元素自然为null。当队列创建时，head and tail引用一个null的
     傀儡节点。head and tail仅仅用CAS操作更新，所以不会后退，这是一个优化。
     //队列节点元素
      private static class Node<E> {
        volatile E item;节点
        volatile Node<E> next;后继

        /**
         * Constructs a new node.  Uses relaxed write because item can
         * only be seen after publication via casNext.
         */
	//构造节点
        Node(E item) {
            UNSAFE.putObject(this, itemOffset, item);
        }
        //比较旧元素，相等则更新，CAS
        boolean casItem(E cmp, E val) {
            return UNSAFE.compareAndSwapObject(this, itemOffset, cmp, val);
        }
        //懒设置节点后继
        void lazySetNext(Node<E> val) {
            UNSAFE.putOrderedObject(this, nextOffset, val);
        }
        //CAS节点的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;
        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.
     队头元素，队列中第一个存活元素，可以，以常量1的速度到达
     * Invariants:
     不变的是：所以的节点都可以通过succ，到达头部
     * - all live nodes are reachable from head via succ()
     * - head != null
     * - (tmp = head).next != tmp || tmp != head
     * Non-invariants:
     可变的是，头结点元素可能为null，也可能不为null
     * - 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!
     */
    允许tail的更新晚于head，意味着队尾到队头不可达。
    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.
     队列中最后一个元素，后继为null，以常量1的速度到达
     * Invariants:
     不变的是：所以的节点都可以通过succ，从队尾到
     * - the last node is always reachable from tail via succ()
     * - tail != null
     * Non-invariants:
     可变的是：允许tail的更新晚于head，意味着队尾到队头不可达。
     * - 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.
     */
     tail可能也可能不会指向自己。
    private transient volatile Node<E> tail;
     /**
     * Creates a {@code ConcurrentLinkedQueue} that is initially empty.
     */
    构造ConcurrentLinkedQueue
    public ConcurrentLinkedQueue() {
        head = tail = new Node<E>(null);
    }
}

来看添加队列元素：
 

/**
     * 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) {
        委托给Offer
        return offer(e);
    }

/**
     * 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) {
        //检查是否为null
        checkNotNull(e);
	//根据元素创建节点
        final Node<E> newNode = new Node<E>(e);

        for (Node<E> t = tail, p = t;;) {
            Node<E> q = p.next;
            if (q == null) {
	        //如果为最后一个元素，且后继为null，则以CAS设置新节点为tail的后继
                // 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
		        //将新节点设为尾元素
                        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.
		//当tail指向自己，则不在队列中，则需要调到队头，以使其他节点可达
                p = (t != (t = tail)) ? t : head;
            else
                // Check for tail updates after two hops.在两步之后，检查队尾是否更新
                p = (p != t && t != (t = tail)) ? t : q;
        }
    }

检查是否为null

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

poll操作：

  public E poll() {
        restartFromHead:
        for (;;) {
            for (Node<E> h = head, p = h, q;;) {
	       //检查队头节点，如果不为null，则设置为null
                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，更新头部为其后继
                        updateHead(h, ((q = p.next) != null) ? q : p);
                    return item;
                }
                else if ((q = p.next) == null) {
		    //更新头部为p，队列为空
                    updateHead(h, p);
                    return null;
                }
                else if (p == q)
                    continue restartFromHead;
                else
                    p = q;
            }
        }
    }

更新头节点：
 

 /**
     * 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) {
       //懒设置队头节点为其后继，并将旧的节点指向自己，以便垃圾回收
        if (h != p && casHead(h, p))
            h.lazySetNext(h);
    }

peek操作：
  

 public E peek() {
        restartFromHead:
        for (;;) {
            for (Node<E> h = head, p = h, q;;) {
                E item = p.item;
                if (item != null || (q = p.next) == null) {
		    //返回队头元素，更新头结点
                    updateHead(h, p);
                    return item;
                }
                else if (p == q)
                    continue restartFromHead;
                else
                    p = q;
            }
        }
    }

//返回队列第一个节点

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;
            }
        }
    }

获取节点的后继节点
 

 final Node<E> succ(Node<E> p) {
        Node<E> next = p.next;
        return (p == next) ? head : next;
    }

移除元素

public boolean remove(Object o) {
        if (o == null) return false;
        Node<E> pred = null;
	//遍历队列，找到节点数据与o相等的节点，更新节点后继为null，更新前驱的后继
        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;
    }

//查询队列是否包含某元素

 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;
    }

//获取队列size

 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;
    }

//根据C构造ConcurrentLinkedQueue

 public ConcurrentLinkedQueue(Collection<? extends E> c) {
        Node<E> h = null, t = null;
	//遍历集合，将元素组装成节点链
        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);
	//初始化队列与队尾
        head = h;
        tail = t;
    }

添加集合元素到队列

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;
	//将集合元素组装成节点链
        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
	//将节点链，挂到队列尾
        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;
        }
    }

序列化：

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.
	//序列化所有元素
        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;
	//反序列化所有元素到队列
        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;
    }

总结：
ConcurrentLinkedQueue一个基于链接节点线程安全的单向无界队列。队列的元素顺序为FIFO。队列的头部，是在队列上时间最久的元素。队列的尾元素是在队列中时间最短的元素。新元素添加到队列的尾部，队列获取元素，从队列头部获取ConcurrentLinkedQueue适用于多个线程需要同时访问一个相同集合的场景。与大多数并发集合一样，不允许插入null元素。Iterators是弱一致性，只反映了队列在某一点的状态，比如创建iterator的时间点。
Iterators不会抛出异常，可以处理其他的并发操作。在队列创建Iterators时，队列中的元素，全都在Iterators中。不像大多数的集合，size操作时间复杂度不是一个常量，因为队列异步操作的天性，决定了遍历队列元素时，有可能其他线程修改队列，导致最终size的数量可能不准确。另外addAll，removeAll，retainAll，containsAll，equals，toArray都不能保证原子性。比如遍历操作和addAll操作同时发生，可能会看到一些相同的新增元素。