Java多线程系列--“JUC集合”09之 LinkedBlockingDeque

概要

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

转载请注明出处：http://www.cnblogs.com/skywang12345/p/3503480.html

LinkedBlockingDeque介绍

LinkedBlockingDeque是双向链表实现的双向并发阻塞队列。该阻塞队列同时支持FIFO和FILO两种操作方式，即可以从队列的头和尾同时操作(插入/删除)；并且，该阻塞队列是支持线程安全。

此外，LinkedBlockingDeque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。

LinkedBlockingDeque原理和数据结构

LinkedBlockingDeque的数据结构，如下图所示：

说明：
1. LinkedBlockingDeque继承于AbstractQueue，它本质上是一个支持FIFO和FILO的双向的队列。
2. LinkedBlockingDeque实现了BlockingDeque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. LinkedBlockingDeque是通过双向链表实现的。
3.1 first是双向链表的表头。
3.2 last是双向链表的表尾。
3.3 count是LinkedBlockingDeque的实际大小，即双向链表中当前节点个数。
3.4 capacity是LinkedBlockingDeque的容量，它是在创建LinkedBlockingDeque时指定的。
3.5 lock是控制对LinkedBlockingDeque的互斥锁，当多个线程竞争同时访问LinkedBlockingDeque时，某线程获取到了互斥锁lock，其它线程则需要阻塞等待，直到该线程释放lock，其它线程才有机会获取lock从而获取cpu执行权。
3.6 notEmpty和notFull分别是“非空条件”和“未满条件”。通过它们能够更加细腻进行并发控制。

     -- 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

LinkedBlockingDeque函数列表

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

// 在不违反容量限制的情况下，将指定的元素插入此双端队列的末尾。
boolean add(E e)
// 如果立即可行且不违反容量限制，则将指定的元素插入此双端队列的开头；如果当前没有空间可用，则抛出 IllegalStateException。
void addFirst(E e)
// 如果立即可行且不违反容量限制，则将指定的元素插入此双端队列的末尾；如果当前没有空间可用，则抛出 IllegalStateException。
void addLast(E e)
// 以原子方式 (atomically) 从此双端队列移除所有元素。
void clear()
// 如果此双端队列包含指定的元素，则返回 true。
boolean contains(Object o)
// 返回在此双端队列的元素上以逆向连续顺序进行迭代的迭代器。
Iterator<E> descendingIterator()
// 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)
// 获取但不移除此双端队列表示的队列的头部。
E element()
// 获取，但不移除此双端队列的第一个元素。
E getFirst()
// 获取，但不移除此双端队列的最后一个元素。
E getLast()
// 返回在此双端队列元素上以恰当顺序进行迭代的迭代器。
Iterator<E> iterator()
// 如果立即可行且不违反容量限制，则将指定的元素插入此双端队列表示的队列中（即此双端队列的尾部），并在成功时返回 true；如果当前没有空间可用，则返回 false。
boolean offer(E e)
// 将指定的元素插入此双端队列表示的队列中（即此双端队列的尾部），必要时将在指定的等待时间内一直等待可用空间。
boolean offer(E e, long timeout, TimeUnit unit)
// 如果立即可行且不违反容量限制，则将指定的元素插入此双端队列的开头，并在成功时返回 true；如果当前没有空间可用，则返回 false。
boolean offerFirst(E e)
// 将指定的元素插入此双端队列的开头，必要时将在指定的等待时间内等待可用空间。
boolean offerFirst(E e, long timeout, TimeUnit unit)
// 如果立即可行且不违反容量限制，则将指定的元素插入此双端队列的末尾，并在成功时返回 true；如果当前没有空间可用，则返回 false。
boolean offerLast(E e)
// 将指定的元素插入此双端队列的末尾，必要时将在指定的等待时间内等待可用空间。
boolean offerLast(E e, long timeout, TimeUnit unit)
// 获取但不移除此双端队列表示的队列的头部（即此双端队列的第一个元素）；如果此双端队列为空，则返回 null。
E peek()
// 获取，但不移除此双端队列的第一个元素；如果此双端队列为空，则返回 null。
E peekFirst()
// 获取，但不移除此双端队列的最后一个元素；如果此双端队列为空，则返回 null。
E peekLast()
// 获取并移除此双端队列表示的队列的头部（即此双端队列的第一个元素）；如果此双端队列为空，则返回 null。
E poll()
// 获取并移除此双端队列表示的队列的头部（即此双端队列的第一个元素），如有必要将在指定的等待时间内等待可用元素。
E poll(long timeout, TimeUnit unit)
// 获取并移除此双端队列的第一个元素；如果此双端队列为空，则返回 null。
E pollFirst()
// 获取并移除此双端队列的第一个元素，必要时将在指定的等待时间等待可用元素。
E pollFirst(long timeout, TimeUnit unit)
// 获取并移除此双端队列的最后一个元素；如果此双端队列为空，则返回 null。
E pollLast()
// 获取并移除此双端队列的最后一个元素，必要时将在指定的等待时间内等待可用元素。
E pollLast(long timeout, TimeUnit unit)
// 从此双端队列所表示的堆栈中弹出一个元素。
E pop()
// 将元素推入此双端队列表示的栈。
void push(E e)
// 将指定的元素插入此双端队列表示的队列中（即此双端队列的尾部），必要时将一直等待可用空间。
void put(E e)
// 将指定的元素插入此双端队列的开头，必要时将一直等待可用空间。
void putFirst(E e)
// 将指定的元素插入此双端队列的末尾，必要时将一直等待可用空间。
void putLast(E e)
// 返回理想情况下（没有内存和资源约束）此双端队列可不受阻塞地接受的额外元素数。
int remainingCapacity()
// 获取并移除此双端队列表示的队列的头部。
E remove()
// 从此双端队列移除第一次出现的指定元素。
boolean remove(Object o)
// 获取并移除此双端队列第一个元素。
E removeFirst()
// 从此双端队列移除第一次出现的指定元素。
boolean removeFirstOccurrence(Object o)
// 获取并移除此双端队列的最后一个元素。
E removeLast()
// 从此双端队列移除最后一次出现的指定元素。
boolean removeLastOccurrence(Object o)
// 返回此双端队列中的元素数。
int size()
// 获取并移除此双端队列表示的队列的头部（即此双端队列的第一个元素），必要时将一直等待可用元素。
E take()
// 获取并移除此双端队列的第一个元素，必要时将一直等待可用元素。
E takeFirst()
// 获取并移除此双端队列的最后一个元素，必要时将一直等待可用元素。
E takeLast()
// 返回以恰当顺序（从第一个元素到最后一个元素）包含此双端队列所有元素的数组。
Object[] toArray()
// 返回以恰当顺序包含此双端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)
// 返回此 collection 的字符串表示形式。
String toString()

LinkedBlockingDeque源码分析(JDK1.7.0_40版本)

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

 /**
  * An optionally-bounded {@linkplain BlockingDeque blocking deque} based on
  * linked nodes.
  *
  * <p> The optional capacity bound constructor argument serves as a
  * way to prevent excessive 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
  * deque above capacity.
  *
  * <p>Most operations run in constant time (ignoring time spent
  * blocking).  Exceptions include {@link #remove(Object) remove},
  * {@link #removeFirstOccurrence removeFirstOccurrence}, {@link
  * #removeLastOccurrence removeLastOccurrence}, {@link #contains
  * contains}, {@link #iterator iterator.remove()}, and the bulk
  * operations, all of which run in linear time.
  *
  * <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 1.6
  * @author  Doug Lea
  * @param <E> the type of elements held in this collection
  */
 public class LinkedBlockingDeque<E>
     extends AbstractQueue<E>
     implements BlockingDeque<E>,  java.io.Serializable {

     /*
      * Implemented as a simple doubly-linked list protected by a
      * single lock and using conditions to manage blocking.
      *
      * 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 jump to "first" (for next links)
      * or "last" (for prev links).
      */

     /*
      * We have "diamond" multiple interface/abstract class inheritance
      * here, and that introduces ambiguities. Often we want the
      * BlockingDeque javadoc combined with the AbstractQueue
      * implementation, so a lot of method specs are duplicated here.
      */

     private static final long serialVersionUID = -387911632671998426L;

     /** Doubly-linked list node class */
     static final class Node<E> {
         /**
          * The item, or null if this node has been removed.
          */
         E item;

         /**
          * One of:
          * - the real predecessor Node
          * - this Node, meaning the predecessor is tail
          * - null, meaning there is no predecessor
          */
         Node<E> prev;

         /**
          * One of:
          * - the real successor Node
          * - this Node, meaning the successor is head
          * - null, meaning there is no successor
          */
         Node<E> next;

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

     /**
      * Pointer to first node.
      * Invariant: (first == null && last == null) ||
      *            (first.prev == null && first.item != null)
      */
     transient Node<E> first;

     /**
      * Pointer to last node.
      * Invariant: (first == null && last == null) ||
      *            (last.next == null && last.item != null)
      */
     transient Node<E> last;

     /** Number of items in the deque */
     private transient int count;

     /** Maximum number of items in the deque */
     private final int capacity;

     /** Main lock guarding all access */
     final ReentrantLock lock = new ReentrantLock();

     /** Condition for waiting takes */
     private final Condition notEmpty = lock.newCondition();

     /** Condition for waiting puts */
     private final Condition notFull = lock.newCondition();

     /**
      * Creates a {@code LinkedBlockingDeque} with a capacity of
      * {@link Integer#MAX_VALUE}.
      */
     public LinkedBlockingDeque() {
         this(Integer.MAX_VALUE);
     }

     /**
      * Creates a {@code LinkedBlockingDeque} with the given (fixed) capacity.
      *
      * @param capacity the capacity of this deque
      * @throws IllegalArgumentException if {@code capacity} is less than 1
      */
     public LinkedBlockingDeque(int capacity) {
         if (capacity <= 0) throw new IllegalArgumentException();
         this.capacity = capacity;
     }

     /**
      * Creates a {@code LinkedBlockingDeque} 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 LinkedBlockingDeque(Collection<? extends E> c) {
         this(Integer.MAX_VALUE);
         final ReentrantLock lock = this.lock;
         lock.lock(); // Never contended, but necessary for visibility
         try {
             for (E e : c) {
                 if (e == null)
                     throw new NullPointerException();
                 if (!linkLast(new Node<E>(e)))
                     throw new IllegalStateException("Deque full");
             }
         } finally {
             lock.unlock();
         }
     }

     // Basic linking and unlinking operations, called only while holding lock

     /**
      * Links node as first element, or returns false if full.
      */
     private boolean linkFirst(Node<E> node) {
         // assert lock.isHeldByCurrentThread();
         if (count >= capacity)
             return false;
         Node<E> f = first;
         node.next = f;
         first = node;
         if (last == null)
             last = node;
         else
             f.prev = node;
         ++count;
         notEmpty.signal();
         return true;
     }

     /**
      * Links node as last element, or returns false if full.
      */
     private boolean linkLast(Node<E> node) {
         // assert lock.isHeldByCurrentThread();
         if (count >= capacity)
             return false;
         Node<E> l = last;
         node.prev = l;
         last = node;
         if (first == null)
             first = node;
         else
             l.next = node;
         ++count;
         notEmpty.signal();
         return true;
     }

     /**
      * Removes and returns first element, or null if empty.
      */
     private E unlinkFirst() {
         // assert lock.isHeldByCurrentThread();
         Node<E> f = first;
         if (f == null)
             return null;
         Node<E> n = f.next;
         E item = f.item;
         f.item = null;
         f.next = f; // help GC
         first = n;
         if (n == null)
             last = null;
         else
             n.prev = null;
         --count;
         notFull.signal();
         return item;
     }

     /**
      * Removes and returns last element, or null if empty.
      */
     private E unlinkLast() {
         // assert lock.isHeldByCurrentThread();
         Node<E> l = last;
         if (l == null)
             return null;
         Node<E> p = l.prev;
         E item = l.item;
         l.item = null;
         l.prev = l; // help GC
         last = p;
         if (p == null)
             first = null;
         else
             p.next = null;
         --count;
         notFull.signal();
         return item;
     }

     /**
      * Unlinks x.
      */
     void unlink(Node<E> x) {
         // assert lock.isHeldByCurrentThread();
         Node<E> p = x.prev;
         Node<E> n = x.next;
         if (p == null) {
             unlinkFirst();
         } else if (n == null) {
             unlinkLast();
         } else {
             p.next = n;
             n.prev = p;
             x.item = null;
             // Don't mess with x's links.  They may still be in use by
             // an iterator.
             --count;
             notFull.signal();
         }
     }

     // BlockingDeque methods

     /**
      * @throws IllegalStateException {@inheritDoc}
      * @throws NullPointerException  {@inheritDoc}
      */
     public void addFirst(E e) {
         if (!offerFirst(e))
             throw new IllegalStateException("Deque full");
     }

     /**
      * @throws IllegalStateException {@inheritDoc}
      * @throws NullPointerException  {@inheritDoc}
      */
     public void addLast(E e) {
         if (!offerLast(e))
             throw new IllegalStateException("Deque full");
     }

     /**
      * @throws NullPointerException {@inheritDoc}
      */
     public boolean offerFirst(E e) {
         if (e == null) throw new NullPointerException();
         Node<E> node = new Node<E>(e);
         final ReentrantLock lock = this.lock;
         lock.lock();
         try {
             return linkFirst(node);
         } finally {
             lock.unlock();
         }
     }

     /**
      * @throws NullPointerException {@inheritDoc}
      */
     public boolean offerLast(E e) {
         if (e == null) throw new NullPointerException();
         Node<E> node = new Node<E>(e);
         final ReentrantLock lock = this.lock;
         lock.lock();
         try {
             return linkLast(node);
         } finally {
             lock.unlock();
         }
     }

     /**
      * @throws NullPointerException {@inheritDoc}
      * @throws InterruptedException {@inheritDoc}
      */
     public void putFirst(E e) throws InterruptedException {
         if (e == null) throw new NullPointerException();
         Node<E> node = new Node<E>(e);
         final ReentrantLock lock = this.lock;
         lock.lock();
         try {
             while (!linkFirst(node))
                 notFull.await();
         } finally {
             lock.unlock();
         }
     }

     /**
      * @throws NullPointerException {@inheritDoc}
      * @throws InterruptedException {@inheritDoc}
      */
     public void putLast(E e) throws InterruptedException {
         if (e == null) throw new NullPointerException();
         Node<E> node = new Node<E>(e);
         final ReentrantLock lock = this.lock;
         lock.lock();
         try {
             while (!linkLast(node))
                 notFull.await();
         } finally {
             lock.unlock();
         }
     }

     /**
      * @throws NullPointerException {@inheritDoc}
      * @throws InterruptedException {@inheritDoc}
      */
     public boolean offerFirst(E e, long timeout, TimeUnit unit)
         throws InterruptedException {
         if (e == null) throw new NullPointerException();
         Node<E> node = new Node<E>(e);
         long nanos = unit.toNanos(timeout);
         final ReentrantLock lock = this.lock;
         lock.lockInterruptibly();
         try {
             while (!linkFirst(node)) {
                 if (nanos <= 0)
                     return false;
                 nanos = notFull.awaitNanos(nanos);
             }
             return true;
         } finally {
             lock.unlock();
         }
     }

     /**
      * @throws NullPointerException {@inheritDoc}
      * @throws InterruptedException {@inheritDoc}
      */
     public boolean offerLast(E e, long timeout, TimeUnit unit)
         throws InterruptedException {
         if (e == null) throw new NullPointerException();
         Node<E> node = new Node<E>(e);
         long nanos = unit.toNanos(timeout);
         final ReentrantLock lock = this.lock;
         lock.lockInterruptibly();
         try {
             while (!linkLast(node)) {
                 if (nanos <= 0)
                     return false;
                 nanos = notFull.awaitNanos(nanos);
             }
             return true;
         } finally {
             lock.unlock();
         }
     }

     /**
      * @throws NoSuchElementException {@inheritDoc}
      */
     public E removeFirst() {
         E x = pollFirst();
         if (x == null) throw new NoSuchElementException();
         return x;
     }

     /**
      * @throws NoSuchElementException {@inheritDoc}
      */
     public E removeLast() {
         E x = pollLast();
         if (x == null) throw new NoSuchElementException();
         return x;
     }

     public E pollFirst() {
         final ReentrantLock lock = this.lock;
         lock.lock();
         try {
             return unlinkFirst();
         } finally {
             lock.unlock();
         }
     }

     public E pollLast() {
         final ReentrantLock lock = this.lock;
         lock.lock();
         try {
             return unlinkLast();
         } finally {
             lock.unlock();
         }
     }

     public E takeFirst() throws InterruptedException {
         final ReentrantLock lock = this.lock;
         lock.lock();
         try {
             E x;
             while ( (x = unlinkFirst()) == null)
                 notEmpty.await();
             return x;
         } finally {
             lock.unlock();
         }
     }

     public E takeLast() throws InterruptedException {
         final ReentrantLock lock = this.lock;
         lock.lock();
         try {
             E x;
             while ( (x = unlinkLast()) == null)
                 notEmpty.await();
             return x;
         } finally {
             lock.unlock();
         }
     }

     public E pollFirst(long timeout, TimeUnit unit)
         throws InterruptedException {
         long nanos = unit.toNanos(timeout);
         final ReentrantLock lock = this.lock;
         lock.lockInterruptibly();
         try {
             E x;
             while ( (x = unlinkFirst()) == null) {
                 if (nanos <= 0)
                     return null;
                 nanos = notEmpty.awaitNanos(nanos);
             }
             return x;
         } finally {
             lock.unlock();
         }
     }

     public E pollLast(long timeout, TimeUnit unit)
         throws InterruptedException {
         long nanos = unit.toNanos(timeout);
         final ReentrantLock lock = this.lock;
         lock.lockInterruptibly();
         try {
             E x;
             while ( (x = unlinkLast()) == null) {
                 if (nanos <= 0)
                     return null;
                 nanos = notEmpty.awaitNanos(nanos);
             }
             return x;
         } finally {
             lock.unlock();
         }
     }

     /**
      * @throws NoSuchElementException {@inheritDoc}
      */
     public E getFirst() {
         E x = peekFirst();
         if (x == null) throw new NoSuchElementException();
         return x;
     }

     /**
      * @throws NoSuchElementException {@inheritDoc}
      */
     public E getLast() {
         E x = peekLast();
         if (x == null) throw new NoSuchElementException();
         return x;
     }

     public E peekFirst() {
         final ReentrantLock lock = this.lock;
         lock.lock();
         try {
             return (first == null) ? null : first.item;
         } finally {
             lock.unlock();
         }
     }

     public E peekLast() {
         final ReentrantLock lock = this.lock;
         lock.lock();
         try {
             return (last == null) ? null : last.item;
         } finally {
             lock.unlock();
         }
     }

     public boolean removeFirstOccurrence(Object o) {
         if (o == null) return false;
         final ReentrantLock lock = this.lock;
         lock.lock();
         try {
             for (Node<E> p = first; p != null; p = p.next) {
                 if (o.equals(p.item)) {
                     unlink(p);
                     return true;
                 }
             }
             return false;
         } finally {
             lock.unlock();
         }
     }

     public boolean removeLastOccurrence(Object o) {
         if (o == null) return false;
         final ReentrantLock lock = this.lock;
         lock.lock();
         try {
             for (Node<E> p = last; p != null; p = p.prev) {
                 if (o.equals(p.item)) {
                     unlink(p);
                     return true;
                 }
             }
             return false;
         } finally {
             lock.unlock();
         }
     }

     // BlockingQueue methods

     /**
      * Inserts the specified element at the end of this deque unless it would
      * violate capacity restrictions.  When using a capacity-restricted deque,
      * it is generally preferable to use method {@link #offer(Object) offer}.
      *
      * <p>This method is equivalent to {@link #addLast}.
      *
      * @throws IllegalStateException if the element cannot be added at this
      *         time due to capacity restrictions
      * @throws NullPointerException if the specified element is null
      */
     public boolean add(E e) {
         addLast(e);
         return true;
     }

     /**
      * @throws NullPointerException if the specified element is null
      */
     public boolean offer(E e) {
         return offerLast(e);
     }

     /**
      * @throws NullPointerException {@inheritDoc}
      * @throws InterruptedException {@inheritDoc}
      */
     public void put(E e) throws InterruptedException {
         putLast(e);
     }

     /**
      * @throws NullPointerException {@inheritDoc}
      * @throws InterruptedException {@inheritDoc}
      */
     public boolean offer(E e, long timeout, TimeUnit unit)
         throws InterruptedException {
         return offerLast(e, timeout, unit);
     }

     /**
      * Retrieves and removes the head of the queue represented by this deque.
      * This method differs from {@link #poll poll} only in that it throws an
      * exception if this deque is empty.
      *
      * <p>This method is equivalent to {@link #removeFirst() removeFirst}.
      *
      * @return the head of the queue represented by this deque
      * @throws NoSuchElementException if this deque is empty
      */
     public E remove() {
         return removeFirst();
     }

     public E poll() {
         return pollFirst();
     }

     public E take() throws InterruptedException {
         return takeFirst();
     }

     public E poll(long timeout, TimeUnit unit) throws InterruptedException {
         return pollFirst(timeout, unit);
     }

     /**
      * Retrieves, but does not remove, the head of the queue represented by
      * this deque.  This method differs from {@link #peek peek} only in that
      * it throws an exception if this deque is empty.
      *
      * <p>This method is equivalent to {@link #getFirst() getFirst}.
      *
      * @return the head of the queue represented by this deque
      * @throws NoSuchElementException if this deque is empty
      */
     public E element() {
         return getFirst();
     }

     public E peek() {
         return peekFirst();
     }

     /**
      * Returns the number of additional elements that this deque can ideally
      * (in the absence of memory or resource constraints) accept without
      * blocking. This is always equal to the initial capacity of this deque
      * less the current {@code size} of this deque.
      *
      * <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() {
         final ReentrantLock lock = this.lock;
         lock.lock();
         try {
             return capacity - count;
         } finally {
             lock.unlock();
         }
     }

     /**
      * @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();
         final ReentrantLock lock = this.lock;
         lock.lock();
         try {
             int n = Math.min(maxElements, count);
             for (int i = 0; i < n; i++) {
                 c.add(first.item);   // In this order, in case add() throws.
                 unlinkFirst();
             }
             return n;
         } finally {
             lock.unlock();
         }
     }

     // Stack methods

     /**
      * @throws IllegalStateException {@inheritDoc}
      * @throws NullPointerException  {@inheritDoc}
      */
     public void push(E e) {
         addFirst(e);
     }

     /**
      * @throws NoSuchElementException {@inheritDoc}
      */
     public E pop() {
         return removeFirst();
     }

     // Collection methods

     /**
      * Removes the first occurrence of the specified element from this deque.
      * If the deque does not contain the element, it is unchanged.
      * More formally, removes the first element {@code e} such that
      * {@code o.equals(e)} (if such an element exists).
      * Returns {@code true} if this deque contained the specified element
      * (or equivalently, if this deque changed as a result of the call).
      *
      * <p>This method is equivalent to
      * {@link #removeFirstOccurrence(Object) removeFirstOccurrence}.
      *
      * @param o element to be removed from this deque, if present
      * @return {@code true} if this deque changed as a result of the call
      */
     public boolean remove(Object o) {
         return removeFirstOccurrence(o);
     }

     /**
      * Returns the number of elements in this deque.
      *
      * @return the number of elements in this deque
      */
     public int size() {
         final ReentrantLock lock = this.lock;
         lock.lock();
         try {
             return count;
         } finally {
             lock.unlock();
         }
     }

     /**
      * Returns {@code true} if this deque contains the specified element.
      * More formally, returns {@code true} if and only if this deque contains
      * at least one element {@code e} such that {@code o.equals(e)}.
      *
      * @param o object to be checked for containment in this deque
      * @return {@code true} if this deque contains the specified element
      */
     public boolean contains(Object o) {
         if (o == null) return false;
         final ReentrantLock lock = this.lock;
         lock.lock();
         try {
             for (Node<E> p = first; p != null; p = p.next)
                 if (o.equals(p.item))
                     return true;
             return false;
         } finally {
             lock.unlock();
         }
     }

     /*
      * TODO: Add support for more efficient bulk operations.
      *
      * We don't want to acquire the lock for every iteration, but we
      * also want other threads a chance to interact with the
      * collection, especially when count is close to capacity.
      */

 //     /**
 //      * Adds all of the elements in the specified collection to this
 //      * queue.  Attempts to addAll of a queue to itself result in
 //      * {@code IllegalArgumentException}. Further, the behavior of
 //      * this operation is undefined if the specified collection is
 //      * modified while the operation is in progress.
 //      *
 //      * @param c collection containing elements to be added to this queue
 //      * @return {@code true} if this queue changed as a result of the call
 //      * @throws ClassCastException            {@inheritDoc}
 //      * @throws NullPointerException          {@inheritDoc}
 //      * @throws IllegalArgumentException      {@inheritDoc}
 //      * @throws IllegalStateException         {@inheritDoc}
 //      * @see #add(Object)
 //      */
 //     public boolean addAll(Collection<? extends E> c) {
 //         if (c == null)
 //             throw new NullPointerException();
 //         if (c == this)
 //             throw new IllegalArgumentException();
 //         final ReentrantLock lock = this.lock;
 //         lock.lock();
 //         try {
 //             boolean modified = false;
 //             for (E e : c)
 //                 if (linkLast(e))
 //                     modified = true;
 //             return modified;
 //         } finally {
 //             lock.unlock();
 //         }
 //     }

     /**
      * Returns an array containing all of the elements in this deque, in
      * proper sequence (from first to last element).
      *
      * <p>The returned array will be "safe" in that no references to it are
      * maintained by this deque.  (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 deque
      */
     @SuppressWarnings("unchecked")
     public Object[] toArray() {
         final ReentrantLock lock = this.lock;
         lock.lock();
         try {
             Object[] a = new Object[count];
             int k = 0;
             for (Node<E> p = first; p != null; p = p.next)
                 a[k++] = p.item;
             return a;
         } finally {
             lock.unlock();
         }
     }

     /**
      * Returns an array containing all of the elements in this deque, in
      * proper sequence; the runtime type of the returned array is that of
      * the specified array.  If the deque 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 deque.
      *
      * <p>If this deque fits in the specified array with room to spare
      * (i.e., the array has more elements than this deque), the element in
      * the array immediately following the end of the deque 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 deque known to contain only strings.
      * The following code can be used to dump the deque 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 deque 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 deque
      * @throws ArrayStoreException if the runtime type of the specified array
      *         is not a supertype of the runtime type of every element in
      *         this deque
      * @throws NullPointerException if the specified array is null
      */
     @SuppressWarnings("unchecked")
     public <T> T[] toArray(T[] a) {
         final ReentrantLock lock = this.lock;
         lock.lock();
         try {
             if (a.length < count)
                 a = (T[])java.lang.reflect.Array.newInstance
                     (a.getClass().getComponentType(), count);

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

     public String toString() {
         final ReentrantLock lock = this.lock;
         lock.lock();
         try {
             Node<E> p = first;
             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 {
             lock.unlock();
         }
     }

     /**
      * Atomically removes all of the elements from this deque.
      * The deque will be empty after this call returns.
      */
     public void clear() {
         final ReentrantLock lock = this.lock;
         lock.lock();
         try {
             for (Node<E> f = first; f != null; ) {
                 f.item = null;
                 Node<E> n = f.next;
                 f.prev = null;
                 f.next = null;
                 f = n;
             }
             first = last = null;
             count = 0;
             notFull.signalAll();
         } finally {
             lock.unlock();
         }
     }

     /**
      * Returns an iterator over the elements in this deque 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 deque in proper sequence
      */
     public Iterator<E> iterator() {
         return new Itr();
     }

     /**
      * Returns an iterator over the elements in this deque in reverse
      * sequential order.  The elements will be returned in order from
      * last (tail) to first (head).
      *
      * <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 deque in reverse order
      */
     public Iterator<E> descendingIterator() {
         return new DescendingItr();
     }

     /**
      * Base class for Iterators for LinkedBlockingDeque
      */
     private abstract class AbstractItr implements Iterator<E> {
         /**
          * The next node to return in next()
          */
          Node<E> next;

         /**
          * nextItem holds on to item fields because once we claim that
          * an element exists in hasNext(), we must return item read
          * under lock (in advance()) even if it was in the process of
          * being removed when hasNext() was called.
          */
         E nextItem;

         /**
          * Node returned by most recent call to next. Needed by remove.
          * Reset to null if this element is deleted by a call to remove.
          */
         private Node<E> lastRet;

         abstract Node<E> firstNode();
         abstract Node<E> nextNode(Node<E> n);

         AbstractItr() {
             // set to initial position
             final ReentrantLock lock = LinkedBlockingDeque.this.lock;
             lock.lock();
             try {
                 next = firstNode();
                 nextItem = (next == null) ? null : next.item;
             } finally {
                 lock.unlock();
             }
         }

         /**
          * Returns the successor node of the given non-null, but
          * possibly previously deleted, node.
          */
         private Node<E> succ(Node<E> n) {
             // Chains of deleted nodes ending in null or self-links
             // are possible if multiple interior nodes are removed.
             for (;;) {
                 Node<E> s = nextNode(n);
                 if (s == null)
                     return null;
                 else if (s.item != null)
                     return s;
                 else if (s == n)
                     return firstNode();
                 else
                     n = s;
             }
         }

         /**
          * Advances next.
          */
         void advance() {
             final ReentrantLock lock = LinkedBlockingDeque.this.lock;
             lock.lock();
             try {
                 // assert next != null;
                 next = succ(next);
                 nextItem = (next == null) ? null : next.item;
             } finally {
                 lock.unlock();
             }
         }

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

         public E next() {
             if (next == null)
                 throw new NoSuchElementException();
             lastRet = next;
             E x = nextItem;
             advance();
             return x;
         }

         public void remove() {
             Node<E> n = lastRet;
             if (n == null)
                 throw new IllegalStateException();
             lastRet = null;
             final ReentrantLock lock = LinkedBlockingDeque.this.lock;
             lock.lock();
             try {
                 if (n.item != null)
                     unlink(n);
             } finally {
                 lock.unlock();
             }
         }
     }

     /** Forward iterator */
     private class Itr extends AbstractItr {
         Node<E> firstNode() { return first; }
         Node<E> nextNode(Node<E> n) { return n.next; }
     }

     /** Descending iterator */
     private class DescendingItr extends AbstractItr {
         Node<E> firstNode() { return last; }
         Node<E> nextNode(Node<E> n) { return n.prev; }
     }

     /**
      * Save the state of this deque to a stream (that is, serialize it).
      *
      * @serialData The capacity (int), followed by 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 {
         final ReentrantLock lock = this.lock;
         lock.lock();
         try {
             // Write out capacity and any hidden stuff
             s.defaultWriteObject();
             // Write out all elements in the proper order.
             for (Node<E> p = first; p != null; p = p.next)
                 s.writeObject(p.item);
             // Use trailing null as sentinel
             s.writeObject(null);
         } finally {
             lock.unlock();
         }
     }

     /**
      * Reconstitute this deque from a stream (that is,
      * deserialize it).
      * @param s the stream
      */
     private void readObject(java.io.ObjectInputStream s)
         throws java.io.IOException, ClassNotFoundException {
         s.defaultReadObject();
         count = 0;
         first = null;
         last = 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ArrayBlockingQueue的创建，添加，取出，遍历这几个方面对LinkedBlockingDeque进行分析

1. 创建

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

public LinkedBlockingDeque(int capacity) {
    if (capacity <= 0) throw new IllegalArgumentException();
    this.capacity = capacity;
}

说明：capacity是“链式阻塞队列”的容量。

LinkedBlockingDeque中相关的数据结果定义如下：

// “双向队列”的表头
transient Node<E> first;
// “双向队列”的表尾
transient Node<E> last;
// 节点数量
private transient int count;
// 容量
private final int capacity;
// 互斥锁 , 互斥锁对应的“非空条件notEmpty”, 互斥锁对应的“未满条件notFull”
final ReentrantLock lock = new ReentrantLock();
private final Condition notEmpty = lock.newCondition();
private final Condition notFull = lock.newCondition();

说明：lock是互斥锁，用于控制多线程对LinkedBlockingDeque中元素的互斥访问；而notEmpty和notFull是与lock绑定的条件，它们用于实现对多线程更精确的控制。

双向链表的节点Node的定义如下：

static final class Node<E> {
    E item;       // 数据
    Node<E> prev; // 前一节点
    Node<E> next; // 后一节点

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

2. 添加

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

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

offer()实际上是调用offerLast()将元素添加到队列的末尾。

offerLast()的源码如下：

public boolean offerLast(E e) {
    if (e == null) throw new NullPointerException();
    // 新建节点
    Node<E> node = new Node<E>(e);
    final ReentrantLock lock = this.lock;
    // 获取锁
    lock.lock();
    try {
        // 将“新节点”添加到双向链表的末尾
        return linkLast(node);
    } finally {
        // 释放锁
        lock.unlock();
    }
}

说明：offerLast()的作用，是新建节点并将该节点插入到双向链表的末尾。它在插入节点前，会获取锁；操作完毕，再释放锁。

linkLast()的源码如下：

private boolean linkLast(Node<E> node) {
    // 如果“双向链表的节点数量” > “容量”，则返回false，表示插入失败。
    if (count >= capacity)
        return false;
    // 将“node添加到链表末尾”，并设置node为新的尾节点
    Node<E> l = last;
    node.prev = l;
    last = node;
    if (first == null)
        first = node;
    else
        l.next = node;
    // 将“节点数量”+1
    ++count;
    // 插入节点之后，唤醒notEmpty上的等待线程。
    notEmpty.signal();
    return true;
}

说明：linkLast()的作用，是将节点插入到双向队列的末尾；插入节点之后，唤醒notEmpty上的等待线程。

3. 删除

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

public E take() throws InterruptedException {
    return takeFirst();
}

take()实际上是调用takeFirst()队列的第一个元素。

takeFirst()的源码如下：

public E takeFirst() throws InterruptedException {
    final ReentrantLock lock = this.lock;
    // 获取锁
    lock.lock();
    try {
        E x;
        // 若“队列为空”，则一直等待。否则，通过unlinkFirst()删除第一个节点。
        while ( (x = unlinkFirst()) == null)
            notEmpty.await();
        return x;
    } finally {
        // 释放锁
        lock.unlock();
    }
}

说明：takeFirst()的作用，是删除双向链表的第一个节点，并返回节点对应的值。它在插入节点前，会获取锁；操作完毕，再释放锁。

unlinkFirst()的源码如下：

private E unlinkFirst() {
    // assert lock.isHeldByCurrentThread();
    Node<E> f = first;
    if (f == null)
        return null;
    // 删除并更新“第一个节点”
    Node<E> n = f.next;
    E item = f.item;
    f.item = null;
    f.next = f; // help GC
    first = n;
    if (n == null)
        last = null;
    else
        n.prev = null;
    // 将“节点数量”-1
    --count;
    // 删除节点之后，唤醒notFull上的等待线程。
    notFull.signal();
    return item;
}

说明：unlinkFirst()的作用，是将双向队列的第一个节点删除；删除节点之后，唤醒notFull上的等待线程。

4. 遍历

下面对LinkedBlockingDeque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 extends AbstractItr {
    // “双向队列”的表头
    Node<E> firstNode() { return first; }
    // 获取“节点n的下一个节点”
    Node<E> nextNode(Node<E> n) { return n.next; }
}

Itr继承于AbstractItr，而AbstractItr的定义如下：

private abstract class AbstractItr implements Iterator<E> {
    // next是下一次调用next()会返回的节点。
    Node<E> next;
    // nextItem是next()返回节点对应的数据。
    E nextItem;
    // 上一次next()返回的节点。
    private Node<E> lastRet;
    // 返回第一个节点
    abstract Node<E> firstNode();
    // 返回下一个节点
    abstract Node<E> nextNode(Node<E> n);

    AbstractItr() {
        final ReentrantLock lock = LinkedBlockingDeque.this.lock;
        // 获取“LinkedBlockingDeque的互斥锁”
        lock.lock();
        try {
            // 获取“双向队列”的表头
            next = firstNode();
            // 获取表头对应的数据
            nextItem = (next == null) ? null : next.item;
        } finally {
            // 释放“LinkedBlockingDeque的互斥锁”
            lock.unlock();
        }
    }

    // 获取n的后继节点
    private Node<E> succ(Node<E> n) {
        // Chains of deleted nodes ending in null or self-links
        // are possible if multiple interior nodes are removed.
        for (;;) {
            Node<E> s = nextNode(n);
            if (s == null)
                return null;
            else if (s.item != null)
                return s;
            else if (s == n)
                return firstNode();
            else
                n = s;
        }
    }

    // 更新next和nextItem。
    void advance() {
        final ReentrantLock lock = LinkedBlockingDeque.this.lock;
        lock.lock();
        try {
            // assert next != null;
            next = succ(next);
            nextItem = (next == null) ? null : next.item;
        } finally {
            lock.unlock();
        }
    }

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

    // 返回下一个节点
    public E next() {
        if (next == null)
            throw new NoSuchElementException();
        lastRet = next;
        E x = nextItem;
        advance();
        return x;
    }

    // 删除下一个节点
    public void remove() {
        Node<E> n = lastRet;
        if (n == null)
            throw new IllegalStateException();
        lastRet = null;
        final ReentrantLock lock = LinkedBlockingDeque.this.lock;
        lock.lock();
        try {
            if (n.item != null)
                unlink(n);
        } finally {
            lock.unlock();
        }
    }
}

LinkedBlockingDeque示例

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

 /*
  *   LinkedBlockingDeque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LinkedBlockingDeque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 LinkedBlockingDequeDemo1 {

     // 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 LinkedBlockingDeque<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：

ta1, ta1, tb1, tb1,

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

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LinkedBlockingDeque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LinkedBlockingDeque中；接着，遍历并输出LinkedBlockingDeque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LinkedBlockingDeque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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