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Java多线程系列--【JUC集合09】- LinkedBlockingDeque

2018-01-27 12:58 701 查看

参考：http://www.cnblogs.com/skywang12345/p/java_threads_category.html

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：

1 /*
2  * ORACLE PROPRIETARY/CONFIDENTIAL. Use is subject to license terms.
3  *
4  *
5  *
6  *
7  *
8  *
9  *
10  *
11  *
12  *
13  *
14  *
15  *
16  *
17  *
18  *
19  *
20  *
21  *
22  *
23  */
24
25 /*
26  *
27  *
28  *
29  *
30  *
31  * Written by Doug Lea with assistance from members of JCP JSR-166
32  * Expert Group and released to the public domain, as explained at
33  * http://creativecommons.org/publicdomain/zero/1.0/
34  */
35
36 package java.util.concurrent;
37
38 import java.util.AbstractQueue;
39 import java.util.Collection;
40 import java.util.Iterator;
41 import java.util.NoSuchElementException;
42 import java.util.concurrent.locks.Condition;
43 import java.util.concurrent.locks.ReentrantLock;
44
45 /**
46  * An optionally-bounded {@linkplain BlockingDeque blocking deque} based on
47  * linked nodes.
48  *
49  * <p> The optional capacity bound constructor argument serves as a
50  * way to prevent excessive expansion. The capacity, if unspecified,
51  * is equal to {@link Integer#MAX_VALUE}.  Linked nodes are
52  * dynamically created upon each insertion unless this would bring the
53  * deque above capacity.
54  *
55  * <p>Most operations run in constant time (ignoring time spent
56  * blocking).  Exceptions include {@link #remove(Object) remove},
57  * {@link #removeFirstOccurrence removeFirstOccurrence}, {@link
58  * #removeLastOccurrence removeLastOccurrence}, {@link #contains
59  * contains}, {@link #iterator iterator.remove()}, and the bulk
60  * operations, all of which run in linear time.
61  *
62  * <p>This class and its iterator implement all of the
63  * <em>optional</em> methods of the {@link Collection} and {@link
64  * Iterator} interfaces.
65  *
66  * <p>This class is a member of the
67  * <a href="{@docRoot}/../technotes/guides/collections/index.html">
68  * Java Collections Framework</a>.
69  *
70  * @since 1.6
71  * @author  Doug Lea
72  * @param <E> the type of elements held in this collection
73  */
74 public class LinkedBlockingDeque<E>
75     extends AbstractQueue<E>
76     implements BlockingDeque<E>,  java.io.Serializable {
77
78     /*
79      * Implemented as a simple doubly-linked list protected by a
80      * single lock and using conditions to manage blocking.
81      *
82      * To implement weakly consistent iterators, it appears we need to
83      * keep all Nodes GC-reachable from a predecessor dequeued Node.
84      * That would cause two problems:
85      * - allow a rogue Iterator to cause unbounded memory retention
86      * - cause cross-generational linking of old Nodes to new Nodes if
87      *   a Node was tenured while live, which generational GCs have a
88      *   hard time dealing with, causing repeated major collections.
89      * However, only non-deleted Nodes need to be reachable from
90      * dequeued Nodes, and reachability does not necessarily have to
91      * be of the kind understood by the GC.  We use the trick of
92      * linking a Node that has just been dequeued to itself.  Such a
93      * self-link implicitly means to jump to "first" (for next links)
94      * or "last" (for prev links).
95      */
96
97     /*
98      * We have "diamond" multiple interface/abstract class inheritance
99      * here, and that introduces ambiguities. Often we want the
100      * BlockingDeque javadoc combined with the AbstractQueue
101      * implementation, so a lot of method specs are duplicated here.
102      */
103
104     private static final long serialVersionUID = -387911632671998426L;
105
106     /** Doubly-linked list node class */
107     static final class Node<E> {
108         /**
109          * The item, or null if this node has been removed.
110          */
111         E item;
112
113         /**
114          * One of:
115          * - the real predecessor Node
116          * - this Node, meaning the predecessor is tail
117          * - null, meaning there is no predecessor
118          */
119         Node<E> prev;
120
121         /**
122          * One of:
123          * - the real successor Node
124          * - this Node, meaning the successor is head
125          * - null, meaning there is no successor
126          */
127         Node<E> next;
128
129         Node(E x) {
130             item = x;
131         }
132     }
133
134     /**
135      * Pointer to first node.
136      * Invariant: (first == null && last == null) ||
137      *            (first.prev == null && first.item != null)
138      */
139     transient Node<E> first;
140
141     /**
142      * Pointer to last node.
143      * Invariant: (first == null && last == null) ||
144      *            (last.next == null && last.item != null)
145      */
146     transient Node<E> last;
147
148     /** Number of items in the deque */
149     private transient int count;
150
151     /** Maximum number of items in the deque */
152     private final int capacity;
153
154     /** Main lock guarding all access */
155     final ReentrantLock lock = new ReentrantLock();
156
157     /** Condition for waiting takes */
158     private final Condition notEmpty = lock.newCondition();
159
160     /** Condition for waiting puts */
161     private final Condition notFull = lock.newCondition();
162
163     /**
164      * Creates a {@code LinkedBlockingDeque} with a capacity of
165      * {@link Integer#MAX_VALUE}.
166      */
167     public LinkedBlockingDeque() {
168         this(Integer.MAX_VALUE);
169     }
170
171     /**
172      * Creates a {@code LinkedBlockingDeque} with the given (fixed) capacity.
173      *
174      * @param capacity the capacity of this deque
175      * @throws IllegalArgumentException if {@code capacity} is less than 1
176      */
177     public LinkedBlockingDeque(int capacity) {
178         if (capacity <= 0) throw new IllegalArgumentException();
179         this.capacity = capacity;
180     }
181
182     /**
183      * Creates a {@code LinkedBlockingDeque} with a capacity of
184      * {@link Integer#MAX_VALUE}, initially containing the elements of
185      * the given collection, added in traversal order of the
186      * collection's iterator.
187      *
188      * @param c the collection of elements to initially contain
189      * @throws NullPointerException if the specified collection or any
190      *         of its elements are null
191      */
192     public LinkedBlockingDeque(Collection<? extends E> c) {
193         this(Integer.MAX_VALUE);
194         final ReentrantLock lock = this.lock;
195         lock.lock(); // Never contended, but necessary for visibility
196         try {
197             for (E e : c) {
198                 if (e == null)
199                     throw new NullPointerException();
200                 if (!linkLast(new Node<E>(e)))
201                     throw new IllegalStateException("Deque full");
202             }
203         } finally {
204             lock.unlock();
205         }
206     }
207
208
209     // Basic linking and unlinking operations, called only while holding lock
210
211     /**
212      * Links node as first element, or returns false if full.
213      */
214     private boolean linkFirst(Node<E> node) {
215         // assert lock.isHeldByCurrentThread();
216         if (count >= capacity)
217             return false;
218         Node<E> f = first;
219         node.next = f;
220         first = node;
221         if (last == null)
222             last = node;
223         else
224             f.prev = node;
225         ++count;
226         notEmpty.signal();
227         return true;
228     }
229
230     /**
231      * Links node as last element, or returns false if full.
232      */
233     private boolean linkLast(Node<E> node) {
234         // assert lock.isHeldByCurrentThread();
235         if (count >= capacity)
236             return false;
237         Node<E> l = last;
238         node.prev = l;
239         last = node;
240         if (first == null)
241             first = node;
242         else
243             l.next = node;
244         ++count;
245         notEmpty.signal();
246         return true;
247     }
248
249     /**
250      * Removes and returns first element, or null if empty.
251      */
252     private E unlinkFirst() {
253         // assert lock.isHeldByCurrentThread();
254         Node<E> f = first;
255         if (f == null)
256             return null;
257         Node<E> n = f.next;
258         E item = f.item;
259         f.item = null;
260         f.next = f; // help GC
261         first = n;
262         if (n == null)
263             last = null;
264         else
265             n.prev = null;
266         --count;
267         notFull.signal();
268         return item;
269     }
270
271     /**
272      * Removes and returns last element, or null if empty.
273      */
274     private E unlinkLast() {
275         // assert lock.isHeldByCurrentThread();
276         Node<E> l = last;
277         if (l == null)
278             return null;
279         Node<E> p = l.prev;
280         E item = l.item;
281         l.item = null;
282         l.prev = l; // help GC
283         last = p;
284         if (p == null)
285             first = null;
286         else
287             p.next = null;
288         --count;
289         notFull.signal();
290         return item;
291     }
292
293     /**
294      * Unlinks x.
295      */
296     void unlink(Node<E> x) {
297         // assert lock.isHeldByCurrentThread();
298         Node<E> p = x.prev;
299         Node<E> n = x.next;
300         if (p == null) {
301             unlinkFirst();
302         } else if (n == null) {
303             unlinkLast();
304         } else {
305             p.next = n;
306             n.prev = p;
307             x.item = null;
308             // Don't mess with x's links.  They may still be in use by
309             // an iterator.
310             --count;
311             notFull.signal();
312         }
313     }
314
315     // BlockingDeque methods
316
317     /**
318      * @throws IllegalStateException {@inheritDoc}
319      * @throws NullPointerException  {@inheritDoc}
320      */
321     public void addFirst(E e) {
322         if (!offerFirst(e))
323             throw new IllegalStateException("Deque full");
324     }
325
326     /**
327      * @throws IllegalStateException {@inheritDoc}
328      * @throws NullPointerException  {@inheritDoc}
329      */
330     public void addLast(E e) {
331         if (!offerLast(e))
332             throw new IllegalStateException("Deque full");
333     }
334
335     /**
336      * @throws NullPointerException {@inheritDoc}
337      */
338     public boolean offerFirst(E e) {
339         if (e == null) throw new NullPointerException();
340         Node<E> node = new Node<E>(e);
341         final ReentrantLock lock = this.lock;
342         lock.lock();
343         try {
344             return linkFirst(node);
345         } finally {
346             lock.unlock();
347         }
348     }
349
350     /**
351      * @throws NullPointerException {@inheritDoc}
352      */
353     public boolean offerLast(E e) {
354         if (e == null) throw new NullPointerException();
355         Node<E> node = new Node<E>(e);
356         final ReentrantLock lock = this.lock;
357         lock.lock();
358         try {
359             return linkLast(node);
360         } finally {
361             lock.unlock();
362         }
363     }
364
365     /**
366      * @throws NullPointerException {@inheritDoc}
367      * @throws InterruptedException {@inheritDoc}
368      */
369     public void putFirst(E e) throws InterruptedException {
370         if (e == null) throw new NullPointerException();
371         Node<E> node = new Node<E>(e);
372         final ReentrantLock lock = this.lock;
373         lock.lock();
374         try {
375             while (!linkFirst(node))
376                 notFull.await();
377         } finally {
378             lock.unlock();
379         }
380     }
381
382     /**
383      * @throws NullPointerException {@inheritDoc}
384      * @throws InterruptedException {@inheritDoc}
385      */
386     public void putLast(E e) throws InterruptedException {
387         if (e == null) throw new NullPointerException();
388         Node<E> node = new Node<E>(e);
389         final ReentrantLock lock = this.lock;
390         lock.lock();
391         try {
392             while (!linkLast(node))
393                 notFull.await();
394         } finally {
395             lock.unlock();
396         }
397     }
398
399     /**
400      * @throws NullPointerException {@inheritDoc}
401      * @throws InterruptedException {@inheritDoc}
402      */
403     public boolean offerFirst(E e, long timeout, TimeUnit unit)
404         throws InterruptedException {
405         if (e == null) throw new NullPointerException();
406         Node<E> node = new Node<E>(e);
407         long nanos = unit.toNanos(timeout);
408         final ReentrantLock lock = this.lock;
409         lock.lockInterruptibly();
410         try {
411             while (!linkFirst(node)) {
412                 if (nanos <= 0)
413                     return false;
414                 nanos = notFull.awaitNanos(nanos);
415             }
416             return true;
417         } finally {
418             lock.unlock();
419         }
420     }
421
422     /**
423      * @throws NullPointerException {@inheritDoc}
424      * @throws InterruptedException {@inheritDoc}
425      */
426     public boolean offerLast(E e, long timeout, TimeUnit unit)
427         throws InterruptedException {
428         if (e == null) throw new NullPointerException();
429         Node<E> node = new Node<E>(e);
430         long nanos = unit.toNanos(timeout);
431         final ReentrantLock lock = this.lock;
432         lock.lockInterruptibly();
433         try {
434             while (!linkLast(node)) {
435                 if (nanos <= 0)
436                     return false;
437                 nanos = notFull.awaitNanos(nanos);
438             }
439             return true;
440         } finally {
441             lock.unlock();
442         }
443     }
444
445     /**
446      * @throws NoSuchElementException {@inheritDoc}
447      */
448     public E removeFirst() {
449         E x = pollFirst();
450         if (x == null) throw new NoSuchElementException();
451         return x;
452     }
453
454     /**
455      * @throws NoSuchElementException {@inheritDoc}
456      */
457     public E removeLast() {
458         E x = pollLast();
459         if (x == null) throw new NoSuchElementException();
460         return x;
461     }
462
463     public E pollFirst() {
464         final ReentrantLock lock = this.lock;
465         lock.lock();
466         try {
467             return unlinkFirst();
468         } finally {
469             lock.unlock();
470         }
471     }
472
473     public E pollLast() {
474         final ReentrantLock lock = this.lock;
475         lock.lock();
476         try {
477             return unlinkLast();
478         } finally {
479             lock.unlock();
480         }
481     }
482
483     public E takeFirst() throws InterruptedException {
484         final ReentrantLock lock = this.lock;
485         lock.lock();
486         try {
487             E x;
488             while ( (x = unlinkFirst()) == null)
489                 notEmpty.await();
490             return x;
491         } finally {
492             lock.unlock();
493         }
494     }
495
496     public E takeLast() throws InterruptedException {
497         final ReentrantLock lock = this.lock;
498         lock.lock();
499         try {
500             E x;
501             while ( (x = unlinkLast()) == null)
502                 notEmpty.await();
503             return x;
504         } finally {
505             lock.unlock();
506         }
507     }
508
509     public E pollFirst(long timeout, TimeUnit unit)
510         throws InterruptedException {
511         long nanos = unit.toNanos(timeout);
512         final ReentrantLock lock = this.lock;
513         lock.lockInterruptibly();
514         try {
515             E x;
516             while ( (x = unlinkFirst()) == null) {
517                 if (nanos <= 0)
518                     return null;
519                 nanos = notEmpty.awaitNanos(nanos);
520             }
521             return x;
522         } finally {
523             lock.unlock();
524         }
525     }
526
527     public E pollLast(long timeout, TimeUnit unit)
528         throws InterruptedException {
529         long nanos = unit.toNanos(timeout);
530         final ReentrantLock lock = this.lock;
531         lock.lockInterruptibly();
532         try {
533             E x;
534             while ( (x = unlinkLast()) == null) {
535                 if (nanos <= 0)
536                     return null;
537                 nanos = notEmpty.awaitNanos(nanos);
538             }
539             return x;
540         } finally {
541             lock.unlock();
542         }
543     }
544
545     /**
546      * @throws NoSuchElementException {@inheritDoc}
547      */
548     public E getFirst() {
549         E x = peekFirst();
550         if (x == null) throw new NoSuchElementException();
551         return x;
552     }
553
554     /**
555      * @throws NoSuchElementException {@inheritDoc}
556      */
557     public E getLast() {
558         E x = peekLast();
559         if (x == null) throw new NoSuchElementException();
560         return x;
561     }
562
563     public E peekFirst() {
564         final ReentrantLock lock = this.lock;
565         lock.lock();
566         try {
567             return (first == null) ? null : first.item;
568         } finally {
569             lock.unlock();
570         }
571     }
572
573     public E peekLast() {
574         final ReentrantLock lock = this.lock;
575         lock.lock();
576         try {
577             return (last == null) ? null : last.item;
578         } finally {
579             lock.unlock();
580         }
581     }
582
583     public boolean removeFirstOccurrence(Object o) {
584         if (o == null) return false;
585         final ReentrantLock lock = this.lock;
586         lock.lock();
587         try {
588             for (Node<E> p = first; p != null; p = p.next) {
589                 if (o.equals(p.item)) {
590                     unlink(p);
591                     return true;
592                 }
593             }
594             return false;
595         } finally {
596             lock.unlock();
597         }
598     }
599
600     public boolean removeLastOccurrence(Object o) {
601         if (o == null) return false;
602         final ReentrantLock lock = this.lock;
603         lock.lock();
604         try {
605             for (Node<E> p = last; p != null; p = p.prev) {
606                 if (o.equals(p.item)) {
607                     unlink(p);
608                     return true;
609                 }
610             }
611             return false;
612         } finally {
613             lock.unlock();
614         }
615     }
616
617     // BlockingQueue methods
618
619     /**
620      * Inserts the specified element at the end of this deque unless it would
621      * violate capacity restrictions.  When using a capacity-restricted deque,
622      * it is generally preferable to use method {@link #offer(Object) offer}.
623      *
624      * <p>This method is equivalent to {@link #addLast}.
625      *
626      * @throws IllegalStateException if the element cannot be added at this
627      *         time due to capacity restrictions
628      * @throws NullPointerException if the specified element is null
629      */
630     public boolean add(E e) {
631         addLast(e);
632         return true;
633     }
634
635     /**
636      * @throws NullPointerException if the specified element is null
637      */
638     public boolean offer(E e) {
639         return offerLast(e);
640     }
641
642     /**
643      * @throws NullPointerException {@inheritDoc}
644      * @throws InterruptedException {@inheritDoc}
645      */
646     public void put(E e) throws InterruptedException {
647         putLast(e);
648     }
649
650     /**
651      * @throws NullPointerException {@inheritDoc}
652      * @throws InterruptedException {@inheritDoc}
653      */
654     public boolean offer(E e, long timeout, TimeUnit unit)
655         throws InterruptedException {
656         return offerLast(e, timeout, unit);
657     }
658
659     /**
660      * Retrieves and removes the head of the queue represented by this deque.
661      * This method differs from {@link #poll poll} only in that it throws an
662      * exception if this deque is empty.
663      *
664      * <p>This method is equivalent to {@link #removeFirst() removeFirst}.
665      *
666      * @return the head of the queue represented by this deque
667      * @throws NoSuchElementException if this deque is empty
668      */
669     public E remove() {
670         return removeFirst();
671     }
672
673     public E poll() {
674         return pollFirst();
675     }
676
677     public E take() throws InterruptedException {
678         return takeFirst();
679     }
680
681     public E poll(long timeout, TimeUnit unit) throws InterruptedException {
682         return pollFirst(timeout, unit);
683     }
684
685     /**
686      * Retrieves, but does not remove, the head of the queue represented by
687      * this deque.  This method differs from {@link #peek peek} only in that
688      * it throws an exception if this deque is empty.
689      *
690      * <p>This method is equivalent to {@link #getFirst() getFirst}.
691      *
692      * @return the head of the queue represented by this deque
693      * @throws NoSuchElementException if this deque is empty
694      */
695     public E element() {
696         return getFirst();
697     }
698
699     public E peek() {
700         return peekFirst();
701     }
702
703     /**
704      * Returns the number of additional elements that this deque can ideally
705      * (in the absence of memory or resource constraints) accept without
706      * blocking. This is always equal to the initial capacity of this deque
707      * less the current {@code size} of this deque.
708      *
709      * <p>Note that you <em>cannot</em> always tell if an attempt to insert
710      * an element will succeed by inspecting {@code remainingCapacity}
711      * because it may be the case that another thread is about to
712      * insert or remove an element.
713      */
714     public int remainingCapacity() {
715         final ReentrantLock lock = this.lock;
716         lock.lock();
717         try {
718             return capacity - count;
719         } finally {
720             lock.unlock();
721         }
722     }
723
724     /**
725      * @throws UnsupportedOperationException {@inheritDoc}
726      * @throws ClassCastException            {@inheritDoc}
727      * @throws NullPointerException          {@inheritDoc}
728      * @throws IllegalArgumentException      {@inheritDoc}
729      */
730     public int drainTo(Collection<? super E> c) {
731         return drainTo(c, Integer.MAX_VALUE);
732     }
733
734     /**
735      * @throws UnsupportedOperationException {@inheritDoc}
736      * @throws ClassCastException            {@inheritDoc}
737      * @throws NullPointerException          {@inheritDoc}
738      * @throws IllegalArgumentException      {@inheritDoc}
739      */
740     public int drainTo(Collection<? super E> c, int maxElements) {
741         if (c == null)
742             throw new NullPointerException();
743         if (c == this)
744             throw new IllegalArgumentException();
745         final ReentrantLock lock = this.lock;
746         lock.lock();
747         try {
748             int n = Math.min(maxElements, count);
749             for (int i = 0; i < n; i++) {
750                 c.add(first.item);   // In this order, in case add() throws.
751                 unlinkFirst();
752             }
753             return n;
754         } finally {
755             lock.unlock();
756         }
757     }
758
759     // Stack methods
760
761     /**
762      * @throws IllegalStateException {@inheritDoc}
763      * @throws NullPointerException  {@inheritDoc}
764      */
765     public void push(E e) {
766         addFirst(e);
767     }
768
769     /**
770      * @throws NoSuchElementException {@inheritDoc}
771      */
772     public E pop() {
773         return removeFirst();
774     }
775
776     // Collection methods
777
778     /**
779      * Removes the first occurrence of the specified element from this deque.
780      * If the deque does not contain the element, it is unchanged.
781      * More formally, removes the first element {@code e} such that
782      * {@code o.equals(e)} (if such an element exists).
783      * Returns {@code true} if this deque contained the specified element
784      * (or equivalently, if this deque changed as a result of the call).
785      *
786      * <p>This method is equivalent to
787      * {@link #removeFirstOccurrence(Object) removeFirstOccurrence}.
788      *
789      * @param o element to be removed from this deque, if present
790      * @return {@code true} if this deque changed as a result of the call
791      */
792     public boolean remove(Object o) {
793         return removeFirstOccurrence(o);
794     }
795
796     /**
797      * Returns the number of elements in this deque.
798      *
799      * @return the number of elements in this deque
800      */
801     public int size() {
802         final ReentrantLock lock = this.lock;
803         lock.lock();
804         try {
805             return count;
806         } finally {
807             lock.unlock();
808         }
809     }
810
811     /**
812      * Returns {@code true} if this deque contains the specified element.
813      * More formally, returns {@code true} if and only if this deque contains
814      * at least one element {@code e} such that {@code o.equals(e)}.
815      *
816      * @param o object to be checked for containment in this deque
817      * @return {@code true} if this deque contains the specified element
818      */
819     public boolean contains(Object o) {
820         if (o == null) return false;
821         final ReentrantLock lock = this.lock;
822         lock.lock();
823         try {
824             for (Node<E> p = first; p != null; p = p.next)
825                 if (o.equals(p.item))
826                     return true;
827             return false;
828         } finally {
829             lock.unlock();
830         }
831     }
832
833     /*
834      * TODO: Add support for more efficient bulk operations.
835      *
836      * We don't want to acquire the lock for every iteration, but we
837      * also want other threads a chance to interact with the
838      * collection, especially when count is close to capacity.
839      */
840
841 //     /**
842 //      * Adds all of the elements in the specified collection to this
843 //      * queue.  Attempts to addAll of a queue to itself result in
844 //      * {@code IllegalArgumentException}. Further, the behavior of
845 //      * this operation is undefined if the specified collection is
846 //      * modified while the operation is in progress.
847 //      *
848 //      * @param c collection containing elements to be added to this queue
849 //      * @return {@code true} if this queue changed as a result of the call
850 //      * @throws ClassCastException            {@inheritDoc}
851 //      * @throws NullPointerException          {@inheritDoc}
852 //      * @throws IllegalArgumentException      {@inheritDoc}
853 //      * @throws IllegalStateException         {@inheritDoc}
854 //      * @see #add(Object)
855 //      */
856 //     public boolean addAll(Collection<? extends E> c) {
857 //         if (c == null)
858 //             throw new NullPointerException();
859 //         if (c == this)
860 //             throw new IllegalArgumentException();
861 //         final ReentrantLock lock = this.lock;
862 //         lock.lock();
863 //         try {
864 //             boolean modified = false;
865 //             for (E e : c)
866 //                 if (linkLast(e))
867 //                     modified = true;
868 //             return modified;
869 //         } finally {
870 //             lock.unlock();
871 //         }
872 //     }
873
874     /**
875      * Returns an array containing all of the elements in this deque, in
876      * proper sequence (from first to last element).
877      *
878      * <p>The returned array will be "safe" in that no references to it are
879      * maintained by this deque.  (In other words, this method must allocate
880      * a new array).  The caller is thus free to modify the returned array.
881      *
882      * <p>This method acts as bridge between array-based and collection-based
883      * APIs.
884      *
885      * @return an array containing all of the elements in this deque
886      */
887     @SuppressWarnings("unchecked")
888     public Object[] toArray() {
889         final ReentrantLock lock = this.lock;
890         lock.lock();
891         try {
892             Object[] a = new Object[count];
893             int k = 0;
894             for (Node<E> p = first; p != null; p = p.next)
895                 a[k++] = p.item;
896             return a;
897         } finally {
898             lock.unlock();
899         }
900     }
901
902     /**
903      * Returns an array containing all of the elements in this deque, in
904      * proper sequence; the runtime type of the returned array is that of
905      * the specified array.  If the deque fits in the specified array, it
906      * is returned therein.  Otherwise, a new array is allocated with the
907      * runtime type of the specified array and the size of this deque.
908      *
909      * <p>If this deque fits in the specified array with room to spare
910      * (i.e., the array has more elements than this deque), the element in
911      * the array immediately following the end of the deque is set to
912      * {@code null}.
913      *
914      * <p>Like the {@link #toArray()} method, this method acts as bridge between
915      * array-based and collection-based APIs.  Further, this method allows
916      * precise control over the runtime type of the output array, and may,
917      * under certain circumstances, be used to save allocation costs.
918      *
919      * <p>Suppose {@code x} is a deque known to contain only strings.
920      * The following code can be used to dump the deque into a newly
921      * allocated array of {@code String}:
922      *
923      * <pre>
924      *     String[] y = x.toArray(new String[0]);</pre>
925      *
926      * Note that {@code toArray(new Object[0])} is identical in function to
927      * {@code toArray()}.
928      *
929      * @param a the array into which the elements of the deque are to
930      *          be stored, if it is big enough; otherwise, a new array of the
931      *          same runtime type is allocated for this purpose
932      * @return an array containing all of the elements in this deque
933      * @throws ArrayStoreException if the runtime type of the specified array
934      *         is not a supertype of the runtime type of every element in
935      *         this deque
936      * @throws NullPointerException if the specified array is null
937      */
938     @SuppressWarnings("unchecked")
939     public <T> T[] toArray(T[] a) {
940         final ReentrantLock lock = this.lock;
941         lock.lock();
942         try {
943             if (a.length < count)
944                 a = (T[])java.lang.reflect.Array.newInstance
945                     (a.getClass().getComponentType(), count);
946
947             int k = 0;
948             for (Node<E> p = first; p != null; p = p.next)
949                 a[k++] = (T)p.item;
950             if (a.length > k)
951                 a[k] = null;
952             return a;
953         } finally {
954             lock.unlock();
955         }
956     }
957
958     public String toString() {
959         final ReentrantLock lock = this.lock;
960         lock.lock();
961         try {
962             Node<E> p = first;
963             if (p == null)
964                 return "[]";
965
966             StringBuilder sb = new StringBuilder();
967             sb.append('[');
968             for (;;) {
969                 E e = p.item;
970                 sb.append(e == this ? "(this Collection)" : e);
971                 p = p.next;
972                 if (p == null)
973                     return sb.append(']').toString();
974                 sb.append(',').append(' ');
975             }
976         } finally {
977             lock.unlock();
978         }
979     }
980
981     /**
982      * Atomically removes all of the elements from this deque.
983      * The deque will be empty after this call returns.
984      */
985     public void clear() {
986         final ReentrantLock lock = this.lock;
987         lock.lock();
988         try {
989             for (Node<E> f = first; f != null; ) {
990                 f.item = null;
991                 Node<E> n = f.next;
992                 f.prev = null;
993                 f.next = null;
994                 f = n;
995             }
996             first = last = null;
997             count = 0;
998             notFull.signalAll();
999         } finally {
1000             lock.unlock();
1001         }
1002     }
1003
1004     /**
1005      * Returns an iterator over the elements in this deque in proper sequence.
1006      * The elements will be returned in order from first (head) to last (tail).
1007      *
1008      * <p>The returned iterator is a "weakly consistent" iterator that
1009      * will never throw {@link java.util.ConcurrentModificationException
1010      * ConcurrentModificationException}, and guarantees to traverse
1011      * elements as they existed upon construction of the iterator, and
1012      * may (but is not guaranteed to) reflect any modifications
1013      * subsequent to construction.
1014      *
1015      * @return an iterator over the elements in this deque in proper sequence
1016      */
1017     public Iterator<E> iterator() {
1018         return new Itr();
1019     }
1020
1021     /**
1022      * Returns an iterator over the elements in this deque in reverse
1023      * sequential order.  The elements will be returned in order from
1024      * last (tail) to first (head).
1025      *
1026      * <p>The returned iterator is a "weakly consistent" iterator that
1027      * will never throw {@link java.util.ConcurrentModificationException
1028      * ConcurrentModificationException}, and guarantees to traverse
1029      * elements as they existed upon construction of the iterator, and
1030      * may (but is not guaranteed to) reflect any modifications
1031      * subsequent to construction.
1032      *
1033      * @return an iterator over the elements in this deque in reverse order
1034      */
1035     public Iterator<E> descendingIterator() {
1036         return new DescendingItr();
1037     }
1038
1039     /**
1040      * Base class for Iterators for LinkedBlockingDeque
1041      */
1042     private abstract class AbstractItr implements Iterator<E> {
1043         /**
1044          * The next node to return in next()
1045          */
1046          Node<E> next;
1047
1048         /**
1049          * nextItem holds on to item fields because once we claim that
1050          * an element exists in hasNext(), we must return item read
1051          * under lock (in advance()) even if it was in the process of
1052          * being removed when hasNext() was called.
1053          */
1054         E nextItem;
1055
1056         /**
1057          * Node returned by most recent call to next. Needed by remove.
1058          * Reset to null if this element is deleted by a call to remove.
1059          */
1060         private Node<E> lastRet;
1061
1062         abstract Node<E> firstNode();
1063         abstract Node<E> nextNode(Node<E> n);
1064
1065         AbstractItr() {
1066             // set to initial position
1067             final ReentrantLock lock = LinkedBlockingDeque.this.lock;
1068             lock.lock();
1069             try {
1070                 next = firstNode();
1071                 nextItem = (next == null) ? null : next.item;
1072             } finally {
1073                 lock.unlock();
1074             }
1075         }
1076
1077         /**
1078          * Returns the successor node of the given non-null, but
1079          * possibly previously deleted, node.
1080          */
1081         private Node<E> succ(Node<E> n) {
1082             // Chains of deleted nodes ending in null or self-links
1083             // are possible if multiple interior nodes are removed.
1084             for (;;) {
1085                 Node<E> s = nextNode(n);
1086                 if (s == null)
1087                     return null;
1088                 else if (s.item != null)
1089                     return s;
1090                 else if (s == n)
1091                     return firstNode();
1092                 else
1093                     n = s;
1094             }
1095         }
1096
1097         /**
1098          * Advances next.
1099          */
1100         void advance() {
1101             final ReentrantLock lock = LinkedBlockingDeque.this.lock;
1102             lock.lock();
1103             try {
1104                 // assert next != null;
1105                 next = succ(next);
1106                 nextItem = (next == null) ? null : next.item;
1107             } finally {
1108                 lock.unlock();
1109             }
1110         }
1111
1112         public boolean hasNext() {
1113             return next != null;
1114         }
1115
1116         public E next() {
1117             if (next == null)
1118                 throw new NoSuchElementException();
1119             lastRet = next;
1120             E x = nextItem;
1121             advance();
1122             return x;
1123         }
1124
1125         public void remove() {
1126             Node<E> n = lastRet;
1127             if (n == null)
1128                 throw new IllegalStateException();
1129             lastRet = null;
1130             final ReentrantLock lock = LinkedBlockingDeque.this.lock;
1131             lock.lock();
1132             try {
1133                 if (n.item != null)
1134                     unlink(n);
1135             } finally {
1136                 lock.unlock();
1137             }
1138         }
1139     }
1140
1141     /** Forward iterator */
1142     private class Itr extends AbstractItr {
1143         Node<E> firstNode() { return first; }
1144         Node<E> nextNode(Node<E> n) { return n.next; }
1145     }
1146
1147     /** Descending iterator */
1148     private class DescendingItr extends AbstractItr {
1149         Node<E> firstNode() { return last; }
1150         Node<E> nextNode(Node<E> n) { return n.prev; }
1151     }
1152
1153     /**
1154      * Save the state of this deque to a stream (that is, serialize it).
1155      *
1156      * @serialData The capacity (int), followed by elements (each an
1157      * {@code Object}) in the proper order, followed by a null
1158      * @param s the stream
1159      */
1160     private void writeObject(java.io.ObjectOutputStream s)
1161         throws java.io.IOException {
1162         final ReentrantLock lock = this.lock;
1163         lock.lock();
1164         try {
1165             // Write out capacity and any hidden stuff
1166             s.defaultWriteObject();
1167             // Write out all elements in the proper order.
1168             for (Node<E> p = first; p != null; p = p.next)
1169                 s.writeObject(p.item);
1170             // Use trailing null as sentinel
1171             s.writeObject(null);
1172         } finally {
1173             lock.unlock();
1174         }
1175     }
1176
1177     /**
1178      * Reconstitute this deque from a stream (that is,
1179      * deserialize it).
1180      * @param s the stream
1181      */
1182     private void readObject(java.io.ObjectInputStream s)
1183         throws java.io.IOException, ClassNotFoundException {
1184         s.defaultReadObject();
1185         count = 0;
1186         first = null;
1187         last = null;
1188         // Read in all elements and place in queue
1189         for (;;) {
1190             @SuppressWarnings("unchecked")
1191             E item = (E)s.readObject();
1192             if (item == null)
1193                 break;
1194             add(item);
1195         }
1196     }
1197
1198 }

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

1 import java.util.*;
2 import java.util.concurrent.*;
3
4 /*
5  *   LinkedBlockingDeque是“线程安全”的队列，而LinkedList是非线程安全的。
6  *
7  *   下面是“多个线程同时操作并且遍历queue”的示例
8  *   (01) 当queue是LinkedBlockingDeque对象时，程序能正常运行。
9  *   (02) 当queue是LinkedList对象时，程序会产生ConcurrentModificationException异常。
10  *
11  * @author skywang
12  */
13 public class LinkedBlockingDequeDemo1 {
14
15     // TODO: queue是LinkedList对象时，程序会出错。
16     //private static Queue<String> queue = new LinkedList<String>();
17     private static Queue<String> queue = new LinkedBlockingDeque<String>();
18     public static void main(String[] args) {
19
20         // 同时启动两个线程对queue进行操作！
21         new MyThread("ta").start();
22         new MyThread("tb").start();
23     }
24
25     private static void printAll() {
26         String value;
27         Iterator iter = queue.iterator();
28         while(iter.hasNext()) {
29             value = (String)iter.next();
30             System.out.print(value+", ");
31         }
32         System.out.println();
33     }
34
35     private static class MyThread extends Thread {
36         MyThread(String name) {
37             super(name);
38         }
39         @Override
40         public void run() {
41                 int i = 0;
42             while (i++ < 6) {
43                 // “线程名” + "-" + "序号"
44                 String val = Thread.currentThread().getName()+i;
45                 queue.add(val);
46                 // 通过“Iterator”遍历queue。
47                 printAll();
48             }
49         }
50     }
51 }

(某一次)运行结果：

ta1, ta1, tb1, tb1,

ta1, ta1, tb1, tb1, 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。

概要