LinkedBlockingQueue解析

Queue接口定义：http://donald-draper.iteye.com/blog/2363491
AbstractQueue简介：http://donald-draper.iteye.com/blog/2363608
ConcurrentLinkedQueue解析：http://donald-draper.iteye.com/blog/2363874
BlockingQueue接口的定义：http://donald-draper.iteye.com/blog/2363942

package java.util.concurrent;

import java.util.concurrent.atomic.AtomicInteger;
import java.util.concurrent.locks.Condition;
import java.util.concurrent.locks.ReentrantLock;
import java.util.AbstractQueue;
import java.util.Collection;
import java.util.Iterator;
import java.util.NoSuchElementException;

/**
 * An optionally-bounded {@linkplain BlockingQueue blocking queue} based on
 * linked nodes.
 LinkedBlockingQueue是一个基于节点链接的可选是否有界的阻塞队列。
 * 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.
 * 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.
 * Linked queues typically have higher throughput than array-based queues but
 * less predictable performance in most concurrent applications.
 *
 队列元素的顺序是FIFO，head是待在队列中，最久的元素；tail则是最短的元素。新元素
 插入时放在队列尾，消费时，则从head获取。相对基于数组的队列，链接队列有一个高效的吞吐量，
 但是在大多数的并发应用中，性能是不可预测的。
 * <p> The optional capacity bound constructor argument serves as a
 * way to prevent excessive queue expansion. The capacity, if unspecified,
 * is equal to {@link Integer#MAX_VALUE}.  Linked nodes are
 * dynamically created upon each insertion unless this would bring the
 * queue above capacity.
 *
LinkedBlockingQueue有一个待容量参数的构造函数，以防止扩展。如果没有明确指定容量参数，
则容量最大值为nteger#MAX_VALUE，在容量可用的情况下，每次插入队列元素时，动态创建新链接节点。
 * <p>This class and its iterator implement all of the
 * [i]optional[/i] methods of the {@link Collection} and {@link
 * Iterator} interfaces.
 *
LinkedBlockingQueue实现了所有Collection和Iterator接口
 * <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 LinkedBlockingQueue<E> extends AbstractQueue<E>
        implements BlockingQueue<E>, java.io.Serializable {
    private static final long serialVersionUID = -6903933977591709194L;
     /*
     * A variant of the "two lock queue" algorithm.  The putLock gates
     * entry to put (and offer), and has an associated condition for
     * waiting puts.  Similarly for the takeLock.  The "count" field
     * that they both rely on is maintained as an atomic to avoid
     * needing to get both locks in most cases. Also, to minimize need
     * for puts to get takeLock and vice-versa, cascading notifies are
     * used. When a put notices that it has enabled at least one take,
     * it signals taker. That taker in turn signals others if more
     * items have been entered since the signal. And symmetrically for
     * takes signalling puts. Operations such as remove(Object) and
     * iterators acquire both locks.
     *
    LinkedBlockingQueue所使用的算法为two lock queue的变种。一把锁为putLock用于
    put&offer，同时关联一个等待put等待条件。另一把锁为takeLock。为了避免在大多数的
    可能需要两种锁的情况，用count属性来维持原子性。为了最小化这种需要，比如puts操作
    需要takeLock，反之亦然，我们使用了cascading notifies（级联通知）。当有一个线程take
    等待时，put通知，竟会在操作完时，唤醒take线程。take线程将会唤醒需要put的等待线程。
    同样批量takes将会唤醒puts，比如remove操作和iterators，将获取两种锁。
     * Visibility between writers and readers is provided as follows:
     *
     在读写线程之间的可见性如下：
     * Whenever an element is enqueued, the putLock is acquired and
     * count updated.  A subsequent reader guarantees visibility to the
     * enqueued Node by either acquiring the putLock (via fullyLock)
     * or by acquiring the takeLock, and then reading n = count.get();
     * this gives visibility to the first n items.
     *
     当一个元素进入队列时，将会获取putLock，并更新Count。通过fullyLock获取putLock或
     获取takeLock保证读线程可见进入队列的元素节点，然后读取当前count值；这种机制保证了
     元素的可见性。
     * To implement weakly consistent iterators, it appears we need to
     * keep all Nodes GC-reachable from a predecessor dequeued Node.
     * That would cause two problems:
     为了实现弱一致性的iterators，当前驱节点出队列时，我们需要保证所有的节点GC可达。
     这样会出现两种问题：
     * - 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.next.
     */
     1.允许rogue（游手好闲，无用）Iterator引起内存泄漏
     2.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。 
    /**
     * Linked list node class，节点
     */
    static class Node<E> {
        E item;

        /**
         * One of:
         * - the real successor Node
         * - this Node, meaning the successor is head.next
         * - null, meaning there is no successor (this is the last node)
	 实际的后继节点链接，null意味为最后一个节点，无后继。
         */
        Node<E> next;

        Node(E x) { item = x; }
    }

    /** The capacity bound, or Integer.MAX_VALUE if none */
    队列容量，最大为Integer.MAX_VALUE
    private final int capacity;

    /** Current number of elements */
    当前元素的数量
    private final AtomicInteger count = new AtomicInteger(0);

    /**
     * Head of linked list.
     * Invariant: head.item == null
     头结点，不变的是头结点元素为null
     */
    private transient Node<E> head;

    /**
     * Tail of linked list.
     * Invariant: last.next == null
     尾结点，不变的是尾结点元素为null
     */
    private transient Node<E> last;

    /** Lock held by take, poll, etc */
    消费者锁takeLock可以被take，poll等操作持有
    private final ReentrantLock takeLock = new ReentrantLock();

    /** Wait queue for waiting takes */
    当队列为null，消费者等待的条件，即队列非空条件notEmpty
    private final Condition notEmpty = takeLock.newCondition();

    /** Lock held by put, offer, etc */
    生产者锁putLock，可以被put，offer等操作持有
    private final ReentrantLock putLock = new ReentrantLock();

    /** Wait queue for waiting puts */
    当队列满时，生产者等待队列条件，即队列非满条件notFull
    private final Condition notFull = putLock.newCondition();
     /**
     * Creates a {@code LinkedBlockingQueue} with a capacity of
     * {@link Integer#MAX_VALUE}.
     */
    public LinkedBlockingQueue() {
        this(Integer.MAX_VALUE);
    }

    /**
     * Creates a {@code LinkedBlockingQueue} with the given (fixed) capacity.
     *
     待容量参数的构造，初始化队列头节点与尾节点
     * @param capacity the capacity of this queue
     * @throws IllegalArgumentException if {@code capacity} is not greater
     *         than zero
     */
    public LinkedBlockingQueue(int capacity) {
        if (capacity <= 0) throw new IllegalArgumentException();
        this.capacity = capacity;
        last = head = new Node<E>(null);
    }
}

小节：
LinkedBlockingQueue是一个线程安全的阻塞并发队列，队列的顺序为FIFO，队列的中
节点包装者原始元素E，有一个后继链接，所以队列是单向的，队列的队头head和队尾节点last是傀儡节点，元素为null。队列有两把锁一个是takeLock，一个为putLock；消费者锁takeLock可以被take，poll等操作持有；生产者锁putLock，可以被put，offer等操作持有；同时有两个条件notEmpty和notFull，notEmpty是takeLock锁的条件，当队列为null，消费者等待的队列非空条件notEmpty；notFull为putLock的条件，为当队列满时，生产者等待队列条件，即队列非满条件notFull。队列中有一个AtomicInteger类型count，用于记录当前队列中元素的个数。

下面我们先来看put操作：

   

/**
     * Inserts the specified element at the tail of this queue, waiting if
     * necessary for space to become available.
     *
     插入元素到队尾，如果需要，等待队列空间可利用，即非满
     * @throws InterruptedException {@inheritDoc}
     * @throws NullPointerException {@inheritDoc}
     */
    public void put(E e) throws InterruptedException {
        //元素为null，抛出异常
        if (e == null) throw new NullPointerException();
        // Note: convention in all put/take/etc is to preset local var
        // holding count negative to indicate failure unless set.
        int c = -1;
	//包装元素为节点，获取当前元素数量，同时以可中断方式获取putLock锁
        Node<E> node = new Node(e);
        final ReentrantLock putLock = this.putLock;
        final AtomicInteger count = this.count;
        putLock.lockInterruptibly();
        try {
            /*
             * Note that count is used in wait guard even though it is
             * not protected by lock. This works because count can
             * only decrease at this point (all other puts are shut
             * out by lock), and we (or some other waiting put) are
             * signalled if it ever changes from capacity. Similarly
             * for all other uses of count in other wait guards.
	     count计数器是条件等待的依据，不被lock锁保护。当其他puts线程
	     释放锁时，消费者可以消费元素，减少count，当其他线程等待put时，
	     如果容量空间可利用，则唤醒put等待线程。对于用count的等待线程，同样。
             */
            while (count.get() == capacity) {
	        //如果队列已满，则等待队列非满条件notFull
                notFull.await();
            }
	    //将节点添加到队列
            enqueue(node);
	    //容量自增1
            c = count.getAndIncrement();
            if (c + 1 < capacity)
	        //如果队列未满，则唤醒一个等待notFull条件的put线程
                notFull.signal();
        } finally {
            putLock.unlock();
        }
        if (c == 0)
	    //队列放入元素成功，则唤醒一个等待队列为notEmpty的take线程；
            signalNotEmpty();
    }

put操作首先以可中断方式获取锁，如果成功，则判断队列是否已满，
如果队列已满，则等待队列非满条件notFull，否则，添加元素节点到队列，
再次判断判断队列是否已满，如果没满，则唤醒一个等待notFull条件的put线程；
释放putLock。如果添加元素成功，则唤醒一个等待队列为notEmpty的take线程。
上面有两点需要关注：
1.

//将节点添加到队列
 enqueue(node);

  /**
     * Links node at end of queue.
     *
     * @param node the node
     */
    private void enqueue(Node<E> node) {
        // assert putLock.isHeldByCurrentThread();
        // assert last.next == null;
        last = last.next = node;
    }

2.

if (c == 0)
//队列放入元素成功，则唤醒一个等待队列为notEmpty的take线程；
signalNotEmpty();

 

/**
     * Signals a waiting take. Called only from put/offer (which do not
     * otherwise ordinarily lock takeLock.)
     */
    //唤醒一个等待take的线程，put/offer会调用此方法
    private void signalNotEmpty() {
        final ReentrantLock takeLock = this.takeLock;
        takeLock.lock();
        try {
            notEmpty.signal();
        } finally {
            takeLock.unlock();
        }
    }

从signalNotEmpty方法来看，先获取takeLock锁，再唤醒等待take的线程；为什么先获取
takeLock，而不是直接唤醒呢？这是为了在通知队列非空信息时，避免其他take线程的进入，
进行不必要的等待。
再来看Offer操作：

/**
     * Inserts the specified element at the tail of this queue if it is
     * possible to do so immediately without exceeding the queue's capacity,
     * returning {@code true} upon success and {@code false} if this queue
     * is full.
     * When using a capacity-restricted queue, this method is generally
     * preferable to method {@link BlockingQueue#add add}, which can fail to
     * insert an element only by throwing an exception.
     *
     * @throws NullPointerException if the specified element is null
     */
    public boolean offer(E e) {
        if (e == null) throw new NullPointerException();
        final AtomicInteger count = this.count;
	//如果队列已满则返回false
        if (count.get() == capacity)
            return false;
        int c = -1;
        Node<E> node = new Node(e);
        final ReentrantLock putLock = this.putLock;
        putLock.lock();
        try {
            if (count.get() < capacity) {
                enqueue(node);
                c = count.getAndIncrement();
                if (c + 1 < capacity)
                    notFull.signal();
            }
        } finally {
            putLock.unlock();
        }
        if (c == 0)
            signalNotEmpty();
        return c >= 0;
    }

从上面来看put操作和offer操作的区别时，put是先获取putLock锁，再判断队列是否已满，已满则等待notFull条件；而offer是先判断队列是否已满，如果已满，则返回false，未满则获取putLock，后续操作相同。从分析来看，在我们向队列中添加元素时，如果使用offer，当队列已满的情况下，我们需要重新将元素放入队列，而put不需要我们再次这样操作，当队列满时，等待队列nullFull条件。
具体选哪一种，根据具体的场景去选择。

 

/**
     * Inserts the specified element at the tail of this queue, waiting if
     * necessary up to the specified wait time for space to become available.
     *
     * @return {@code true} if successful, or {@code false} if
     *         the specified waiting time elapses before space is available.
     * @throws InterruptedException {@inheritDoc}
     * @throws NullPointerException {@inheritDoc}
     */
    public boolean offer(E e, long timeout, TimeUnit unit)
        throws InterruptedException {

        if (e == null) throw new NullPointerException();
        long nanos = unit.toNanos(timeout);
        int c = -1;
        final ReentrantLock putLock = this.putLock;
        final AtomicInteger count = this.count;
        putLock.lockInterruptibly();
        try {
            while (count.get() == capacity) {
                if (nanos <= 0)
                    return false;
		 //超时等待
                nanos = notFull.awaitNanos(nanos);
            }
            enqueue(new Node<E>(e));
            c = count.getAndIncrement();
            if (c + 1 < capacity)
                notFull.signal();
        } finally {
            putLock.unlock();
        }
        if (c == 0)
            signalNotEmpty();
        return true;
    }

从上面来看，offer(E e, long timeout, TimeUnit unit)与put(E e,)更像，区别在于
当超时offer获取putLock锁成功后，如果队列已满，则超时等待notFull条件。
再来看take操作：

 public E take() throws InterruptedException {
        E x;
        int c = -1;
	//获取当前队列容量计数器,并以可中断方式获取takeLock,
        final AtomicInteger count = this.count;
        final ReentrantLock takeLock = this.takeLock;
        takeLock.lockInterruptibly();
        try {
            while (count.get() == 0) {
	        //如果队列为空，等待notEmpty
                notEmpty.await();
            }
	    //从队列头获取元素
            x = dequeue();
	    //计数器减1
            c = count.getAndDecrement();
            if (c > 1)
	        //如果队列中还有元素，则唤醒一个等待非空条件的take线程
                notEmpty.signal();
        } finally {
            takeLock.unlock();
        }
        if (c == capacity)
	    //如果在take前，队列容量已满，则成功take后，唤醒等待notFull条件的put线程。
            signalNotFull();
        return x;
    }

take操作，首先获取当前队列容量计数器,并以可中断方式获取takeLock,获取锁成功，则
判断队列是否为空，如果为空，则等待notEmpty条件，否则从队头取出元素，容量计数器减1，
如果队列中还有元素，则唤醒一个等待非空条件的take线程；如果在take前，队列容量已满，
则成功take后，唤醒等待notFull条件的put线程。
这里有两点要关注：
1.

//从队列头获取元素
x = dequeue();

/**
     * Removes a node from head of queue.
     *
     * @return the node
     */
    private E dequeue() {
        // assert takeLock.isHeldByCurrentThread();
        // assert head.item == null;
        Node<E> h = head;
        Node<E> first = h.next;
        h.next = h; // help GC
        head = first;
        E x = first.item;
        first.item = null;
        return x;
    }

2.

 if (c == capacity)
    //如果在take前，队列容量已满，则成功take后，唤醒等待notFull条件的put线程。
   signalNotFull();

 

 /**
     * Signals a waiting put. Called only from take/poll.
     */
    private void signalNotFull() {
        final ReentrantLock putLock = this.putLock;
	//先获取putLock
        putLock.lock();
        try {
            notFull.signal();
        } finally {
            putLock.unlock();
        }
    }

从signalNotFull可以看出，是先获取putLock，再唤醒等待put的线程，以防止
再唤醒的过程之前，有其他put线程进入，进行不必要的等待。
再看超时等待poll
 

public E poll(long timeout, TimeUnit unit) throws InterruptedException {
        E x = null;
        int c = -1;
        long nanos = unit.toNanos(timeout);
        final AtomicInteger count = this.count;
        final ReentrantLock takeLock = this.takeLock;
        takeLock.lockInterruptibly();
        try {
            while (count.get() == 0) {
                if (nanos <= 0)
                    return null;
                nanos = notEmpty.awaitNanos(nanos);
            }
            x = dequeue();
            c = count.getAndDecrement();
            if (c > 1)
                notEmpty.signal();
        } finally {
            takeLock.unlock();
        }
        if (c == capacity)
            signalNotFull();
        return x;
    }

从超时poll方法来看，与take方法的区别在于，当队列为null，进行超时等待。
再看poll方法：
  

public E poll() {
        final AtomicInteger count = this.count;
        if (count.get() == 0)
	    //先检查队列是否为空，为空，则返回null
            return null;
        E x = null;
        int c = -1;
        final ReentrantLock takeLock = this.takeLock;
        takeLock.lock();
        try {
            if (count.get() > 0) {
                x = dequeue();
                c = count.getAndDecrement();
                if (c > 1)
                    notEmpty.signal();
            }
        } finally {
            takeLock.unlock();
        }
        if (c == capacity)
            signalNotFull();
        return x;
    }

从poll方法来看，与take方法的最大区别为先检查先检查队列是否为空，为空，则返回null；
不为空，剩下的操作与take相同。

再看peek检查元素；
 

public E peek() {
        //首先检查队列是否为空，为空，则返回null
        if (count.get() == 0)
            return null;
        final ReentrantLock takeLock = this.takeLock;
        takeLock.lock();
        try {
	    //否则获取takeLock锁，取出队头元素
            Node<E> first = head.next;
            if (first == null)
                return null;
            else
                return first.item;
        } finally {
            takeLock.unlock();
        }
    }

peek操作首先检查队列是否为空，为空，则返回null，
否则获取takeLock锁，取出队头元素，并返回。

再看移除元素remove操作：

/**
     * 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) {
        //元素为null，返回false
        if (o == null) return false;
	//获取takeLock和putLock锁
        fullyLock();
        try {
            for (Node<E> trail = head, p = trail.next;
                 p != null;
                 trail = p, p = p.next) {
		 //遍历队列，比较元素是否相等，相等则移除
                if (o.equals(p.item)) {
                    unlink(p, trail);
                    return true;
                }
            }
            return false;
        } finally {
	    //释放takeLock和putLock锁
            fullyUnlock();
        }
    }

以上remove方法有3点要看：
1.

//获取takeLock和putLock锁
fullyLock();
   /**
     * Lock to prevent both puts and takes.
     */
    void fullyLock() {
        putLock.lock();
        takeLock.lock();
    }

2.

 //遍历队列，比较元素是否相等，相等则移除
 if (o.equals(p.item)) {
     unlink(p, trail);
     return true;
 }

  

/**
     * Unlinks interior Node p with predecessor trail.
     */
    void unlink(Node<E> p, Node<E> trail) {
        // assert isFullyLocked();
        // p.next is not changed, to allow iterators that are
        // traversing p to maintain their weak-consistency guarantee.
        p.item = null;
        trail.next = p.next;
        if (last == p)
            last = trail;
        if (count.getAndDecrement() == capacity)
	    //在移除元素的过程中，移除成功，则唤醒一个等待put的线程
            notFull.signal();
    }

3.

 //释放takeLock和putLock锁
fullyUnlock();

    /**
     * Unlock to allow both puts and takes.
     */
    void fullyUnlock() {
        takeLock.unlock();
        putLock.unlock();
    }

从remove方法来看，需要获取takeLock和putLock锁，遍历队列，比较元素是否相等，
相等则移除，则唤醒一个等待put的线程，最后释放takeLock和putLock锁。为什么要获取
两把锁呢，主要防止在移除的过程中，有线程消费元素，或生产元素，带来的不缺定性结果。

再来看包含contain操作：

public boolean contains(Object o) {
        if (o == null) return false;
        fullyLock();
        try {
            for (Node<E> p = head.next; p != null; p = p.next)
                if (o.equals(p.item))
                    return true;
            return false;
        } finally {
            fullyUnlock();
        }
    }

contain操作与remove思路一样。
clear操作：

 public void clear() {
        fullyLock();
        try {
            for (Node<E> p, h = head; (p = h.next) != null; h = p) {
                h.next = h;
                p.item = null;
            }
            head = last;
            // assert head.item == null && head.next == null;
            if (count.getAndSet(0) == capacity)
                notFull.signal();
        } finally {
            fullyUnlock();
        }
    }

再来看将元素移到另一个集合的操作

/**
     * @throws UnsupportedOperationException {@inheritDoc}
     * @throws ClassCastException            {@inheritDoc}
     * @throws NullPointerException          {@inheritDoc}
     * @throws IllegalArgumentException      {@inheritDoc}
     */
    public int drainTo(Collection<? super E> c) {
        //委托给drainTo(c, Integer.MAX_VALUE)
        return drainTo(c, Integer.MAX_VALUE);
    }

    /**
     * @throws UnsupportedOperationException {@inheritDoc}
     * @throws ClassCastException            {@inheritDoc}
     * @throws NullPointerException          {@inheritDoc}
     * @throws IllegalArgumentException      {@inheritDoc}
     */
    public int drainTo(Collection<? super E> c, int maxElements) {
        if (c == null)
            throw new NullPointerException();
        if (c == this)
            throw new IllegalArgumentException();
        boolean signalNotFull = false;
        final ReentrantLock takeLock = this.takeLock;
	//获取takeLock锁
        takeLock.lock();
        try {
            int n = Math.min(maxElements, count.get());
            // count.get provides visibility to first n Nodes
            Node<E> h = head;
            int i = 0;
            try {
	        //从队头take，n个元素，并添加集合中
                while (i < n) {
                    Node<E> p = h.next;
                    c.add(p.item);
                    p.item = null;
                    h.next = h;
                    h = p;
                    ++i;
                }
                return n;
            } finally {
                // Restore invariants even if c.add() threw
                if (i > 0) {
                    // assert h.item == null;
                    head = h;
                    signalNotFull = (count.getAndAdd(-i) == capacity);
                }
            }
        } finally {
            takeLock.unlock();
            if (signalNotFull)
	        //如果成功移除元素，则唤醒等待put的线程
                signalNotFull();
        }
    }

//获取当前队列容量

   public int size() {
        return count.get();
    }

//获取队列当前的剩余空间

  public int remainingCapacity() {
        return capacity - count.get();
    }

迭代器：

public Iterator<E> iterator() {
      return new Itr();
    }

    private class Itr implements Iterator<E> {
        /*
         * Basic weakly-consistent iterator.  At all times hold the next
         * item to hand out so that if hasNext() reports true, we will
         * still have it to return even if lost race with a take etc.
         */
        private Node<E> current;
        private Node<E> lastRet;
        private E currentElement;

        Itr() {
            fullyLock();
            try {
                current = head.next;
                if (current != null)
                    currentElement = current.item;
            } finally {
                fullyUnlock();
            }
        }

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

        /**
         * Returns the next live successor of p, or null if no such.
         *
         * Unlike other traversal methods, iterators need to handle both:
         * - dequeued nodes (p.next == p)
         * - (possibly multiple) interior removed nodes (p.item == null)
         */
        private Node<E> nextNode(Node<E> p) {
            for (;;) {
                Node<E> s = p.next;
                if (s == p)
                    return head.next;
                if (s == null || s.item != null)
                    return s;
                p = s;
            }
        }

        public E next() {
            fullyLock();
            try {
                if (current == null)
                    throw new NoSuchElementException();
                E x = currentElement;
                lastRet = current;
                current = nextNode(current);
                currentElement = (current == null) ? null : current.item;
                return x;
            } finally {
                fullyUnlock();
            }
        }

        public void remove() {
            if (lastRet == null)
                throw new IllegalStateException();
            fullyLock();
            try {
                Node<E> node = lastRet;
                lastRet = null;
                for (Node<E> trail = head, p = trail.next;
                     p != null;
                     trail = p, p = p.next) {
                    if (p == node) {
                        unlink(p, trail);
                        break;
                    }
                }
            } finally {
                fullyUnlock();
            }
        }
    }

从上来看，迭代器在构造时，需要两把锁put和take；获取next，也需要两把锁，
移除，则直接从队列中，移除。
序列化：

 private void writeObject(java.io.ObjectOutputStream s)
        throws java.io.IOException {

        fullyLock();
        try {
            // Write out any hidden stuff, plus capacity
            s.defaultWriteObject();

            // Write out all elements in the proper order.
            for (Node<E> p = head.next; p != null; p = p.next)
                s.writeObject(p.item);

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

反序列化：
  

 /**
     * Reconstitute this queue instance from a stream (that is,
     * deserialize it).
     *
     * @param s the stream
     */
    private void readObject(java.io.ObjectInputStream s)
        throws java.io.IOException, ClassNotFoundException {
        // Read in capacity, and any hidden stuff
        s.defaultReadObject();

        count.set(0);
        last = head = new Node<E>(null);

        // Read in all elements and place in queue
        for (;;) {
            @SuppressWarnings("unchecked")
            E item = (E)s.readObject();
            if (item == null)
                break;
            add(item);
        }
    }

总结：
  LinkedBlockingQueue是一个线程安全的阻塞并发队列，队列的顺序为FIFO，队列的中节点包装者原始元素E，有一个后继链接，所以队列是单向的，队列的队头head和队尾节点last是傀儡节点，元素为null。队列有两把锁一个是takeLock，一个为putLock；消费者锁takeLock可以被take，poll等操作持有；生产者锁putLock，可以被put，offer等操作持有；同时有两个条件notEmpty和notFull，notEmpty是takeLock锁的条件，当队列为null，消费者等待的队列非空条件notEmpty；notFull为putLock的条件，为当队列满时，生产者等待队列条件，即队列非满条件notFull。队列中有一个AtomicInteger类型count，用于记录当前队列中元素的个数。
    put操作首先以可中断方式获取锁，如果成功，则判断队列是否已满，如果队列已满，则等待队列非满条件notFull，否则，添加元素节点到队列，再次判断判断队列是否已满，如果没满，则唤醒一个等待notFull条件的put线程；释放putLock。如果添加元素成功，获取takeLock锁，成功，则唤醒一个等待队列为notEmpty的take线程。put操作和offer操作的区别时，put是先获取putLock锁，再判断队列是否已满，已满则等待notFull条件；而offer是先判断队列是否已满，如果已满，则返回false，未满则获取putLock，后续操作相同。从分析来看，在我们向队列中添加元素时，如果使用offer，当队列已满的情况下，我们需要重新将元素放入队列，而put不需要我们再次这样操作，当队列满时，等待队列nullFull条件。具体选哪一种，根据具体的场景去选择。offer(E e, long timeout, TimeUnit unit)与put(E e,)更像，区别在于当超时offer获取putLock锁成功后，如果队列已满，则超时等待notFull条件。
   take操作，首先获取当前队列容量计数器,并以可中断方式获取takeLock,获取锁成功，则
判断队列是否为空，如果为空，则等待notEmpty条件，否则从队头取出元素，容量计数器减1，如果队列中还有元素，则唤醒一个等待非空条件的take线程；如果在take前，队列容量已满，则成功take后，唤醒等待notFull条件的put线程。超时poll方法，与take方法的区别在于，当队列为null，进行超时等待。poll方法，与taket方法的最大区别为先检查先检查队列是否为空，为空，则返回null；不为空，剩下的操作与take相同。具体选哪一种，根据具体的场景去选择。
    peek操作首先检查队列是否为空，为空，则返回null，否则获取takeLock锁，取出队头元素，并返回。
    remove方法需要获取takeLock和putLock锁，遍历队列，比较元素是否相等，相等则移除，则唤醒一个等待put的线程，最后释放takeLock和putLock锁。为什么要获取两把锁呢，主要防止在移除的过程中，有线程消费元素，或生产元素，带来的不缺定性结果。contain操作与remove思路一样。
   drainTo操作首先获取takeLock锁，从队头take，n个元素，并添加集合中，
如果成功移除元素，则唤醒等待put的线程。
   在上面所有的操作中，我们看所有的唤醒都是signal而不是signalAll，那么为什么不总是使用signalAll替换signal呢？ 假设有N个线程在条件队列中等待，调用signalAll会唤醒所有线程，然后这N个线程竞争同一个锁，最多只有一个线程能够得到锁，于是其它线程又回到挂起状态。这意味每一次唤醒操作可能带来大量的上下文切换（如果N比较大的话），同时有大量的竞争锁的请求。这对于频繁的唤醒操作而言性能上可能是一种灾难。如果说总是只有一个线程被唤醒后能够拿到锁，那么为什么不使用signal呢？所以某些情况下使用signal的性能是要高于signalAll的。如果满足下面的条件，可以使用单一的signal取代signalAll操作： 相同的等待者，也就是说等待条件变量的线程操作相同，每一个从wait条件发生时，执行相同的逻辑，同时一个条件变量的通知至多只能唤醒一个线程。