Java多线程系列--“JUC集合”08之 LinkedBlockingQueue

概要

本章介绍JUC包中的LinkedBlockingQueue。内容包括：
LinkedBlockingQueue介绍
LinkedBlockingQueue原理和数据结构
LinkedBlockingQueue函数列表
LinkedBlockingQueue源码分析(JDK1.7.0_40版本)
LinkedBlockingQueue示例

转载请注明出处：http://www.cnblogs.com/skywang12345/p/3503458.html

LinkedBlockingQueue介绍

LinkedBlockingQueue是一个单向链表实现的阻塞队列。该队列按 FIFO（先进先出）排序元素，新元素插入到队列的尾部，并且队列获取操作会获得位于队列头部的元素。链接队列的吞吐量通常要高于基于数组的队列，但是在大多数并发应用程序中，其可预知的性能要低。

此外，LinkedBlockingQueue还是可选容量的(防止过度膨胀)，即可以指定队列的容量。如果不指定，默认容量大小等于Integer.MAX_VALUE。

LinkedBlockingQueue原理和数据结构

LinkedBlockingQueue的数据结构，如下图所示：

说明：
1. LinkedBlockingQueue继承于AbstractQueue，它本质上是一个FIFO(先进先出)的队列。
2. LinkedBlockingQueue实现了BlockingQueue接口，它支持多线程并发。当多线程竞争同一个资源时，某线程获取到该资源之后，其它线程需要阻塞等待。
3. LinkedBlockingQueue是通过单链表实现的。
(01) head是链表的表头。取出数据时，都是从表头head处插入。
(02) last是链表的表尾。新增数据时，都是从表尾last处插入。
(03) count是链表的实际大小，即当前链表中包含的节点个数。
(04) capacity是列表的容量，它是在创建链表时指定的。
(05) putLock是插入锁，takeLock是取出锁；notEmpty是“非空条件”，notFull是“未满条件”。通过它们对链表进行并发控制。
       LinkedBlockingQueue在实现“多线程对竞争资源的互斥访问”时，对于“插入”和“取出(删除)”操作分别使用了不同的锁。对于插入操作，通过“插入锁putLock”进行同步；对于取出操作，通过“取出锁takeLock”进行同步。
       此外，插入锁putLock和“非满条件notFull”相关联，取出锁takeLock和“非空条件notEmpty”相关联。通过notFull和notEmpty更细腻的控制锁。

     -- 若某线程(线程A)要取出数据时，队列正好为空，则该线程会执行notEmpty.await()进行等待；当其它某个线程(线程B)向队列中插入了数据之后，会调用notEmpty.signal()唤醒“notEmpty上的等待线程”。此时，线程A会被唤醒从而得以继续运行。 此外，线程A在执行取操作前，会获取takeLock，在取操作执行完毕再释放takeLock。
     -- 若某线程(线程H)要插入数据时，队列已满，则该线程会它执行notFull.await()进行等待；当其它某个线程(线程I)取出数据之后，会调用notFull.signal()唤醒“notFull上的等待线程”。此时，线程H就会被唤醒从而得以继续运行。 此外，线程H在执行插入操作前，会获取putLock，在插入操作执行完毕才释放putLock。

关于ReentrantLock 和 Condition等更多的内容，可以参考：
    (01) Java多线程系列--“JUC锁”02之 互斥锁ReentrantLock
    (02) Java多线程系列--“JUC锁”03之 公平锁(一)
    (03) Java多线程系列--“JUC锁”04之 公平锁(二)
    (04) Java多线程系列--“JUC锁”05之 非公平锁
    (05) Java多线程系列--“JUC锁”06之 Condition条件

LinkedBlockingQueue函数列表

// 创建一个容量为 Integer.MAX_VALUE 的 LinkedBlockingQueue。
LinkedBlockingQueue()
// 创建一个容量是 Integer.MAX_VALUE 的 LinkedBlockingQueue，最初包含给定 collection 的元素，元素按该 collection 迭代器的遍历顺序添加。
LinkedBlockingQueue(Collection<? extends E> c)
// 创建一个具有给定（固定）容量的 LinkedBlockingQueue。
LinkedBlockingQueue(int capacity)

// 从队列彻底移除所有元素。
void clear()
// 移除此队列中所有可用的元素，并将它们添加到给定 collection 中。
int drainTo(Collection<? super E> c)
// 最多从此队列中移除给定数量的可用元素，并将这些元素添加到给定 collection 中。
int drainTo(Collection<? super E> c, int maxElements)
// 返回在队列中的元素上按适当顺序进行迭代的迭代器。
Iterator<E> iterator()
// 将指定元素插入到此队列的尾部（如果立即可行且不会超出此队列的容量），在成功时返回 true，如果此队列已满，则返回 false。
boolean offer(E e)
// 将指定元素插入到此队列的尾部，如有必要，则等待指定的时间以使空间变得可用。
boolean offer(E e, long timeout, TimeUnit unit)
// 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()
// 获取并移除此队列的头部，在指定的等待时间前等待可用的元素（如果有必要）。
E poll(long timeout, TimeUnit unit)
// 将指定元素插入到此队列的尾部，如有必要，则等待空间变得可用。
void put(E e)
// 返回理想情况下（没有内存和资源约束）此队列可接受并且不会被阻塞的附加元素数量。
int remainingCapacity()
// 从此队列移除指定元素的单个实例（如果存在）。
boolean remove(Object o)
// 返回队列中的元素个数。
int size()
// 获取并移除此队列的头部，在元素变得可用之前一直等待（如果有必要）。
E take()
// 返回按适当顺序包含此队列中所有元素的数组。
Object[] toArray()
// 返回按适当顺序包含此队列中所有元素的数组；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)
// 返回此 collection 的字符串表示形式。
String toString()

LinkedBlockingQueue源码分析(JDK1.7.0_40版本)

LinkedBlockingQueue.java的完整源码如下：

 /*
  * ORACLE PROPRIETARY/CONFIDENTIAL. Use is subject to license terms.
  *
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 /*
  *
  *
  *
  *
  *
  * Written by Doug Lea with assistance from members of JCP JSR-166
  * Expert Group and released to the public domain, as explained at
  * http://creativecommons.org/publicdomain/zero/1.0/
  */

 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.
  * 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.
  * Linked queues typically have higher throughput than array-based queues but
  * less predictable performance in most concurrent applications.
  *
  * <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.
  *
  * <p>This class and its iterator implement all of the
  * <em>optional</em> methods of the {@link Collection} and {@link
  * Iterator} interfaces.
  *
  * <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.
      *
      * Visibility between writers and readers is provided as follows:
      *
      * 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.
      *
      * 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:
      * - 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.
      */

     /**
      * Linked list node class
      */
     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)
          */
         Node<E> next;

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

     /** The capacity bound, or Integer.MAX_VALUE if none */
     private final int capacity;

     /** Current number of elements */
     private final AtomicInteger count = new AtomicInteger(0);

     /**
      * Head of linked list.
      * Invariant: head.item == null
      */
     private transient Node<E> head;

     /**
      * Tail of linked list.
      * Invariant: last.next == null
      */
     private transient Node<E> last;

     /** Lock held by take, poll, etc */
     private final ReentrantLock takeLock = new ReentrantLock();

     /** Wait queue for waiting takes */
     private final Condition notEmpty = takeLock.newCondition();

     /** Lock held by put, offer, etc */
     private final ReentrantLock putLock = new ReentrantLock();

     /** Wait queue for waiting puts */
     private final Condition notFull = putLock.newCondition();

     /**
      * Signals a waiting take. Called only from put/offer (which do not
      * otherwise ordinarily lock takeLock.)
      */
     private void signalNotEmpty() {
         final ReentrantLock takeLock = this.takeLock;
         takeLock.lock();
         try {
             notEmpty.signal();
         } finally {
             takeLock.unlock();
         }
     }

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

     /**
      * 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;
     }

     /**
      * 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;
     }

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

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

 //     /**
 //      * Tells whether both locks are held by current thread.
 //      */
 //     boolean isFullyLocked() {
 //         return (putLock.isHeldByCurrentThread() &&
 //                 takeLock.isHeldByCurrentThread());
 //     }

     /**
      * 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.
      *
      * @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);
     }

     /**
      * Creates a {@code LinkedBlockingQueue} with a capacity of
      * {@link Integer#MAX_VALUE}, 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 LinkedBlockingQueue(Collection<? extends E> c) {
         this(Integer.MAX_VALUE);
         final ReentrantLock putLock = this.putLock;
         putLock.lock(); // Never contended, but necessary for visibility
         try {
             int n = 0;
             for (E e : c) {
                 if (e == null)
                     throw new NullPointerException();
                 if (n == capacity)
                     throw new IllegalStateException("Queue full");
                 enqueue(new Node<E>(e));
                 ++n;
             }
             count.set(n);
         } finally {
             putLock.unlock();
         }
     }

     // this doc comment is overridden to remove the reference to collections
     // greater in size than Integer.MAX_VALUE
     /**
      * Returns the number of elements in this queue.
      *
      * @return the number of elements in this queue
      */
     public int size() {
         return count.get();
     }

     // this doc comment is a modified copy of the inherited doc comment,
     // without the reference to unlimited queues.
     /**
      * Returns the number of additional elements that this queue can ideally
      * (in the absence of memory or resource constraints) accept without
      * blocking. This is always equal to the initial capacity of this queue
      * less the current {@code size} of this queue.
      *
      * <p>Note that you <em>cannot</em> always tell if an attempt to insert
      * an element will succeed by inspecting {@code remainingCapacity}
      * because it may be the case that another thread is about to
      * insert or remove an element.
      */
     public int remainingCapacity() {
         return capacity - count.get();
     }

     /**
      * Inserts the specified element at the tail of this queue, waiting if
      * necessary for space to become available.
      *
      * @throws InterruptedException {@inheritDoc}
      * @throws NullPointerException {@inheritDoc}
      */
     public void put(E e) throws InterruptedException {
         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;
         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.
              */
             while (count.get() == capacity) {
                 notFull.await();
             }
             enqueue(node);
             c = count.getAndIncrement();
             if (c + 1 < capacity)
                 notFull.signal();
         } finally {
             putLock.unlock();
         }
         if (c == 0)
             signalNotEmpty();
     }

     /**
      * 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;
                 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;
     }

     /**
      * 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;
         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;
     }

     public E take() throws InterruptedException {
         E x;
         int c = -1;
         final AtomicInteger count = this.count;
         final ReentrantLock takeLock = this.takeLock;
         takeLock.lockInterruptibly();
         try {
             while (count.get() == 0) {
                 notEmpty.await();
             }
             x = dequeue();
             c = count.getAndDecrement();
             if (c > 1)
                 notEmpty.signal();
         } finally {
             takeLock.unlock();
         }
         if (c == capacity)
             signalNotFull();
         return x;
     }

     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;
     }

     public E poll() {
         final AtomicInteger count = this.count;
         if (count.get() == 0)
             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;
     }

     public E peek() {
         if (count.get() == 0)
             return null;
         final ReentrantLock takeLock = this.takeLock;
         takeLock.lock();
         try {
             Node<E> first = head.next;
             if (first == null)
                 return null;
             else
                 return first.item;
         } finally {
             takeLock.unlock();
         }
     }

     /**
      * 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)
             notFull.signal();
     }

     /**
      * 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;
         fullyLock();
         try {
             for (Node<E> trail = head, p = trail.next;
                  p != null;
                  trail = p, p = p.next) {
                 if (o.equals(p.item)) {
                     unlink(p, trail);
                     return true;
                 }
             }
             return false;
         } finally {
             fullyUnlock();
         }
     }

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

     /**
      * 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() {
         fullyLock();
         try {
             int size = count.get();
             Object[] a = new Object[size];
             int k = 0;
             for (Node<E> p = head.next; p != null; p = p.next)
                 a[k++] = p.item;
             return a;
         } finally {
             fullyUnlock();
         }
     }

     /**
      * 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) {
         fullyLock();
         try {
             int size = count.get();
             if (a.length < size)
                 a = (T[])java.lang.reflect.Array.newInstance
                     (a.getClass().getComponentType(), size);

             int k = 0;
             for (Node<E> p = head.next; p != null; p = p.next)
                 a[k++] = (T)p.item;
             if (a.length > k)
                 a[k] = null;
             return a;
         } finally {
             fullyUnlock();
         }
     }

     public String toString() {
         fullyLock();
         try {
             Node<E> p = head.next;
             if (p == null)
                 return "[]";

             StringBuilder sb = new StringBuilder();
             sb.append('[');
             for (;;) {
                 E e = p.item;
                 sb.append(e == this ? "(this Collection)" : e);
                 p = p.next;
                 if (p == null)
                     return sb.append(']').toString();
                 sb.append(',').append(' ');
             }
         } finally {
             fullyUnlock();
         }
     }

     /**
      * Atomically removes all of the elements from this queue.
      * The queue will be empty after this call returns.
      */
     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();
         }
     }

     /**
      * @throws UnsupportedOperationException {@inheritDoc}
      * @throws ClassCastException            {@inheritDoc}
      * @throws NullPointerException          {@inheritDoc}
      * @throws IllegalArgumentException      {@inheritDoc}
      */
     public int drainTo(Collection<? super E> c) {
         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;
         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 {
                 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)
                 signalNotFull();
         }
     }

     /**
      * 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> {
         /*
          * 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();
             }
         }
     }

     /**
      * Save the state to a stream (that is, serialize it).
      *
      * @serialData The capacity is emitted (int), followed by all of
      * its elements (each an {@code Object}) in the proper order,
      * followed by a null
      * @param s the stream
      */
     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();
         }
     }

     /**
      * 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);
         }
     }
 }

下面从LinkedBlockingQueue的创建，添加，删除，遍历这几个方面对它进行分析。

1. 创建

下面以LinkedBlockingQueue(int capacity)来进行说明。

public LinkedBlockingQueue(int capacity) {
    if (capacity <= 0) throw new IllegalArgumentException();
    this.capacity = capacity;
    last = head = new Node<E>(null);
}

说明：
(01) capacity是“链式阻塞队列”的容量。
(02) head和last是“链式阻塞队列”的首节点和尾节点。它们在LinkedBlockingQueue中的声明如下：

// 容量
private final int capacity;
// 当前数量
private final AtomicInteger count = new AtomicInteger(0);
private transient Node<E> head; // 链表的表头
private transient Node<E> last; // 链表的表尾
// 用于控制“删除元素”的互斥锁takeLock 和 锁对应的“非空条件”notEmpty
private final ReentrantLock takeLock = new ReentrantLock();
private final Condition notEmpty = takeLock.newCondition();
// 用于控制“添加元素”的互斥锁putLock 和 锁对应的“非满条件”notFull
private final ReentrantLock putLock = new ReentrantLock();
private final Condition notFull = putLock.newCondition();

链表的节点定义如下：

static class Node<E> {
    E item;         // 数据
    Node<E> next;   // 下一个节点的指针

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

2. 添加

下面以offer(E e)为例，对LinkedBlockingQueue的添加方法进行说明。

public boolean offer(E e) {
    if (e == null) throw new NullPointerException();
    // 如果“队列已满”，则返回false，表示插入失败。
    final AtomicInteger count = this.count;
    if (count.get() == capacity)
        return false;
    int c = -1;
    // 新建“节点e”
    Node<E> node = new Node(e);
    final ReentrantLock putLock = this.putLock;
    // 获取“插入锁putLock”
    putLock.lock();
    try {
        // 再次对“队列是不是满”的进行判断。
        // 若“队列未满”，则插入节点。
        if (count.get() < capacity) {
            // 插入节点
            enqueue(node);
            // 将“当前节点数量”+1，并返回“原始的数量”
            c = count.getAndIncrement();
            // 如果在插入元素之后，队列仍然未满，则唤醒notFull上的等待线程。
            if (c + 1 < capacity)
                notFull.signal();
        }
    } finally {
        // 释放“插入锁putLock”
        putLock.unlock();
    }
    // 如果在插入节点前，队列为空；则插入节点后，唤醒notEmpty上的等待线程
    if (c == 0)
        signalNotEmpty();
    return c >= 0;
}

说明：offer()的作用很简单，就是将元素E添加到队列的末尾。

enqueue()的源码如下：

private void enqueue(Node<E> node) {
    // assert putLock.isHeldByCurrentThread();
    // assert last.next == null;
    last = last.next = node;
}

enqueue()的作用是将node添加到队列末尾，并设置node为新的尾节点！

signalNotEmpty()的源码如下：

private void signalNotEmpty() {
    final ReentrantLock takeLock = this.takeLock;
    takeLock.lock();
    try {
        notEmpty.signal();
    } finally {
        takeLock.unlock();
    }
}

signalNotEmpty()的作用是唤醒notEmpty上的等待线程。

3. 取出

下面以take()为例，对LinkedBlockingQueue的取出方法进行说明。

public E take() throws InterruptedException {
    E x;
    int c = -1;
    final AtomicInteger count = this.count;
    final ReentrantLock takeLock = this.takeLock;
    // 获取“取出锁”，若当前线程是中断状态，则抛出InterruptedException异常
    takeLock.lockInterruptibly();
    try {
        // 若“队列为空”，则一直等待。
        while (count.get() == 0) {
            notEmpty.await();
        }
        // 取出元素
        x = dequeue();
        // 取出元素之后，将“节点数量”-1；并返回“原始的节点数量”。
        c = count.getAndDecrement();
        if (c > 1)
            notEmpty.signal();
    } finally {
        // 释放“取出锁”
        takeLock.unlock();
    }
    // 如果在“取出元素之前”，队列是满的；则在取出元素之后，唤醒notFull上的等待线程。
    if (c == capacity)
        signalNotFull();
    return x;
}

说明：take()的作用是取出并返回队列的头。若队列为空，则一直等待。

dequeue()的源码如下：

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;
}

dequeue()的作用就是删除队列的头节点，并将表头指向“原头节点的下一个节点”。

signalNotFull()的源码如下：

private void signalNotFull() {
    final ReentrantLock putLock = this.putLock;
    putLock.lock();
    try {
        notFull.signal();
    } finally {
        putLock.unlock();
    }
}

signalNotFull()的作用就是唤醒notFull上的等待线程。

4. 遍历

下面对LinkedBlockingQueue的遍历方法进行说明。

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

iterator()实际上是返回一个Iter对象。

Itr类的定义如下：

private class Itr implements Iterator<E> {
    // 当前节点
    private Node<E> current;
    // 上一次返回的节点
    private Node<E> lastRet;
    // 当前节点对应的值
    private E currentElement;

    Itr() {
        // 同时获取“插入锁putLock” 和 “取出锁takeLock”
        fullyLock();
        try {
            // 设置“当前元素”为“队列表头的下一节点”，即为队列的第一个有效节点
            current = head.next;
            if (current != null)
                currentElement = current.item;
        } finally {
            // 释放“插入锁putLock” 和 “取出锁takeLock”
            fullyUnlock();
        }
    }

    // 返回“下一个节点是否为null”
    public boolean hasNext() {
        return current != 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;
        }
    }

    // 返回下一个节点
    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();
        }
    }

    // 删除下一个节点
    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();
        }
    }
}

LinkedBlockingQueue示例

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

 /*
  *   LinkedBlockingQueue是“线程安全”的队列，而LinkedList是非线程安全的。
  *
  *   下面是“多个线程同时操作并且遍历queue”的示例
  *   (01) 当queue是LinkedBlockingQueue对象时，程序能正常运行。
  *   (02) 当queue是LinkedList对象时，程序会产生ConcurrentModificationException异常。
  *
  * @author skywang
  */
 public class LinkedBlockingQueueDemo1 {

     // TODO: queue是LinkedList对象时，程序会出错。
     //private static Queue<String> queue = new LinkedList<String>();
     private static Queue<String> queue = new LinkedBlockingQueue<String>();
     public static void main(String[] args) {

         // 同时启动两个线程对queue进行操作！
         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) {
                 // “线程名” + "-" + "序号"
                 String val = Thread.currentThread().getName()+i;
                 queue.add(val);
                 // 通过“Iterator”遍历queue。
                 printAll();
             }
         }
     }
 }

(某一次)运行结果：

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

结果说明：
示例程序中，启动两个线程(线程ta和线程tb)分别对LinkedBlockingQueue进行操作。以线程ta而言，它会先获取“线程名”+“序号”，然后将该字符串添加到LinkedBlockingQueue中；接着，遍历并输出LinkedBlockingQueue中的全部元素。 线程tb的操作和线程ta一样，只不过线程tb的名字和线程ta的名字不同。
当queue是LinkedBlockingQueue对象时，程序能正常运行。如果将queue改为LinkedList时，程序会产生ConcurrentModificationException异常。

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