Java多线程系列--“JUC集合”07之 ArrayBlockingQueue

2023-12-15 21:17:57

概要

本章对Java.util.concurrent包中的ArrayBlockingQueue类进行详细的介绍。内容包括：
ArrayBlockingQueue介绍
 ArrayBlockingQueue原理和数据结构
 ArrayBlockingQueue函数列表
 ArrayBlockingQueue源码分析(JDK1.7.0_40版本)
ArrayBlockingQueue示例

转载请注明出处：http://www.cnblogs.com/skywang12345/p/3498652.html

ArrayBlockingQueue介绍

ArrayBlockingQueue是数组实现的线程安全的有界的阻塞队列。
线程安全是指，ArrayBlockingQueue内部通过“互斥锁”保护竞争资源，实现了多线程对竞争资源的互斥访问。而有界，则是指ArrayBlockingQueue对应的数组是有界限的。阻塞队列，是指多线程访问竞争资源时，当竞争资源已被某线程获取时，其它要获取该资源的线程需要阻塞等待；而且，ArrayBlockingQueue是按 FIFO（先进先出）原则对元素进行排序，元素都是从尾部插入到队列，从头部开始返回。

注意：ArrayBlockingQueue不同于ConcurrentLinkedQueue，ArrayBlockingQueue是数组实现的，并且是有界限的；而ConcurrentLinkedQueue是链表实现的，是*限的。

ArrayBlockingQueue原理和数据结构

ArrayBlockingQueue的数据结构，如下图所示：

说明：
1. ArrayBlockingQueue继承于AbstractQueue，并且它实现了BlockingQueue接口。
2. ArrayBlockingQueue内部是通过Object[]数组保存数据的，也就是说ArrayBlockingQueue本质上是通过数组实现的。ArrayBlockingQueue的大小，即数组的容量是创建ArrayBlockingQueue时指定的。
3. ArrayBlockingQueue与ReentrantLock是组合关系，ArrayBlockingQueue中包含一个ReentrantLock对象(lock)。ReentrantLock是可重入的互斥锁，ArrayBlockingQueue就是根据该互斥锁实现“多线程对竞争资源的互斥访问”。而且，ReentrantLock分为公平锁和非公平锁，关于具体使用公平锁还是非公平锁，在创建ArrayBlockingQueue时可以指定；而且，ArrayBlockingQueue默认会使用非公平锁。
4. ArrayBlockingQueue与Condition是组合关系，ArrayBlockingQueue中包含两个Condition对象(notEmpty和notFull)。而且，Condition又依赖于ArrayBlockingQueue而存在，通过Condition可以实现对ArrayBlockingQueue的更精确的访问 -- (01)若某线程(线程A)要取数据时，数组正好为空，则该线程会执行notEmpty.await()进行等待；当其它某个线程(线程B)向数组中插入了数据之后，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。(02)若某线程(线程H)要插入数据时，数组已满，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。
关于ReentrantLock，公平锁，非公平锁，以及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

ArrayBlockingQueue函数列表

Java多线程系列--“JUC集合”07之 ArrayBlockingQueue

// 创建一个带有给定的（固定）容量和默认访问策略的 ArrayBlockingQueue。
ArrayBlockingQueue(int capacity)
// 创建一个具有给定的（固定）容量和指定访问策略的 ArrayBlockingQueue。
ArrayBlockingQueue(int capacity, boolean fair)
// 创建一个具有给定的（固定）容量和指定访问策略的 ArrayBlockingQueue，它最初包含给定 collection 的元素，并以 collection 迭代器的遍历顺序添加元素。
ArrayBlockingQueue(int capacity, boolean fair, Collection<? extends E> c)

// 将指定的元素插入到此队列的尾部（如果立即可行且不会超过该队列的容量），在成功时返回 true，如果此队列已满，则抛出 IllegalStateException。
boolean add(E e)
// 自动移除此队列中的所有元素。
void clear()
// 如果此队列包含指定的元素，则返回 true。
boolean contains(Object o)
// 移除此队列中所有可用的元素，并将它们添加到给定 collection 中。
int drainTo(Collection<? super E> c)
// 最多从此队列中移除给定数量的可用元素，并将这些元素添加到给定 collection 中。
int drainTo(Collection<? super E> c, int maxElements)
// 返回在此队列中的元素上按适当顺序进行迭代的迭代器。
Iterator<E> iterator()
// 将指定的元素插入到此队列的尾部（如果立即可行且不会超过该队列的容量），在成功时返回 true，如果此队列已满，则返回 false。
boolean offer(E e)
// 将指定的元素插入此队列的尾部，如果该队列已满，则在到达指定的等待时间之前等待可用的空间。
boolean offer(E e, long timeout, TimeUnit unit)
// 获取但不移除此队列的头；如果此队列为空，则返回 null。
E peek()
// 获取并移除此队列的头，如果此队列为空，则返回 null。
E poll()
// 获取并移除此队列的头部，在指定的等待时间前等待可用的元素（如果有必要）。
E poll(long timeout, TimeUnit unit)
// 将指定的元素插入此队列的尾部，如果该队列已满，则等待可用的空间。
void put(E e)
// 返回在无阻塞的理想情况下（不存在内存或资源约束）此队列能接受的其他元素数量。
int remainingCapacity()
// 从此队列中移除指定元素的单个实例（如果存在）。
boolean remove(Object o)
// 返回此队列中元素的数量。
int size()
// 获取并移除此队列的头部，在元素变得可用之前一直等待（如果有必要）。
E take()
// 返回一个按适当顺序包含此队列中所有元素的数组。
Object[] toArray()
// 返回一个按适当顺序包含此队列中所有元素的数组；返回数组的运行时类型是指定数组的运行时类型。
<T> T[] toArray(T[] a)
// 返回此 collection 的字符串表示形式。
String toString()

ArrayBlockingQueue源码分析(JDK1.7.0_40版本)

ArrayBlockingQueue.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 import java.util.concurrent.locks.*;
 38 import java.util.*;
 39 
 40 /**
 41  * A bounded {@linkplain BlockingQueue blocking queue} backed by an
 42  * array.  This queue orders elements FIFO (first-in-first-out).  The
 43  * <em>head</em> of the queue is that element that has been on the
 44  * queue the longest time.  The <em>tail</em> of the queue is that
 45  * element that has been on the queue the shortest time. New elements
 46  * are inserted at the tail of the queue, and the queue retrieval
 47  * operations obtain elements at the head of the queue.
 48  *
 49  * <p>This is a classic &quot;bounded buffer&quot;, in which a
 50  * fixed-sized array holds elements inserted by producers and
 51  * extracted by consumers.  Once created, the capacity cannot be
 52  * changed.  Attempts to {@code put} an element into a full queue
 53  * will result in the operation blocking; attempts to {@code take} an
 54  * element from an empty queue will similarly block.
 55  *
 56  * <p>This class supports an optional fairness policy for ordering
 57  * waiting producer and consumer threads.  By default, this ordering
 58  * is not guaranteed. However, a queue constructed with fairness set
 59  * to {@code true} grants threads access in FIFO order. Fairness
 60  * generally decreases throughput but reduces variability and avoids
 61  * starvation.
 62  *
 63  * <p>This class and its iterator implement all of the
 64  * <em>optional</em> methods of the {@link Collection} and {@link
 65  * Iterator} interfaces.
 66  *
 67  * <p>This class is a member of the
 68  * <a href="{@docRoot}/../technotes/guides/collections/index.html">
 69  * Java Collections Framework</a>.
 70  *
 71  * @since 1.5
 72  * @author Doug Lea
 73  * @param <E> the type of elements held in this collection
 74  */
 75 public class ArrayBlockingQueue<E> extends AbstractQueue<E>
 76         implements BlockingQueue<E>, java.io.Serializable {
 77 
 78     /**
 79      * Serialization ID. This class relies on default serialization
 80      * even for the items array, which is default-serialized, even if
 81      * it is empty. Otherwise it could not be declared final, which is
 82      * necessary here.
 83      */
 84     private static final long serialVersionUID = -817911632652898426L;
 85 
 86     /** The queued items */
 87     final Object[] items;
 88 
 89     /** items index for next take, poll, peek or remove */
 90     int takeIndex;
 91 
 92     /** items index for next put, offer, or add */
 93     int putIndex;
 94 
 95     /** Number of elements in the queue */
 96     int count;
 97 
 98     /*
 99      * Concurrency control uses the classic two-condition algorithm
100      * found in any textbook.
101      */
102 
103     /** Main lock guarding all access */
104     final ReentrantLock lock;
105     /** Condition for waiting takes */
106     private final Condition notEmpty;
107     /** Condition for waiting puts */
108     private final Condition notFull;
109 
110     // Internal helper methods
111 
112     /**
113      * Circularly increment i.
114      */
115     final int inc(int i) {
116         return (++i == items.length) ? 0 : i;
117     }
118 
119     /**
120      * Circularly decrement i.
121      */
122     final int dec(int i) {
123         return ((i == 0) ? items.length : i) - 1;
124     }
125 
126     @SuppressWarnings("unchecked")
127     static <E> E cast(Object item) {
128         return (E) item;
129     }
130 
131     /**
132      * Returns item at index i.
133      */
134     final E itemAt(int i) {
135         return this.<E>cast(items[i]);
136     }
137 
138     /**
139      * Throws NullPointerException if argument is null.
140      *
141      * @param v the element
142      */
143     private static void checkNotNull(Object v) {
144         if (v == null)
145             throw new NullPointerException();
146     }
147 
148     /**
149      * Inserts element at current put position, advances, and signals.
150      * Call only when holding lock.
151      */
152     private void insert(E x) {
153         items[putIndex] = x;
154         putIndex = inc(putIndex);
155         ++count;
156         notEmpty.signal();
157     }
158 
159     /**
160      * Extracts element at current take position, advances, and signals.
161      * Call only when holding lock.
162      */
163     private E extract() {
164         final Object[] items = this.items;
165         E x = this.<E>cast(items[takeIndex]);
166         items[takeIndex] = null;
167         takeIndex = inc(takeIndex);
168         --count;
169         notFull.signal();
170         return x;
171     }
172 
173     /**
174      * Deletes item at position i.
175      * Utility for remove and iterator.remove.
176      * Call only when holding lock.
177      */
178     void removeAt(int i) {
179         final Object[] items = this.items;
180         // if removing front item, just advance
181         if (i == takeIndex) {
182             items[takeIndex] = null;
183             takeIndex = inc(takeIndex);
184         } else {
185             // slide over all others up through putIndex.
186             for (;;) {
187                 int nexti = inc(i);
188                 if (nexti != putIndex) {
189                     items[i] = items[nexti];
190                     i = nexti;
191                 } else {
192                     items[i] = null;
193                     putIndex = i;
194                     break;
195                 }
196             }
197         }
198         --count;
199         notFull.signal();
200     }
201 
202     /**
203      * Creates an {@code ArrayBlockingQueue} with the given (fixed)
204      * capacity and default access policy.
205      *
206      * @param capacity the capacity of this queue
207      * @throws IllegalArgumentException if {@code capacity < 1}
208      */
209     public ArrayBlockingQueue(int capacity) {
210         this(capacity, false);
211     }
212 
213     /**
214      * Creates an {@code ArrayBlockingQueue} with the given (fixed)
215      * capacity and the specified access policy.
216      *
217      * @param capacity the capacity of this queue
218      * @param fair if {@code true} then queue accesses for threads blocked
219      *        on insertion or removal, are processed in FIFO order;
220      *        if {@code false} the access order is unspecified.
221      * @throws IllegalArgumentException if {@code capacity < 1}
222      */
223     public ArrayBlockingQueue(int capacity, boolean fair) {
224         if (capacity <= 0)
225             throw new IllegalArgumentException();
226         this.items = new Object[capacity];
227         lock = new ReentrantLock(fair);
228         notEmpty = lock.newCondition();
229         notFull =  lock.newCondition();
230     }
231 
232     /**
233      * Creates an {@code ArrayBlockingQueue} with the given (fixed)
234      * capacity, the specified access policy and initially containing the
235      * elements of the given collection,
236      * added in traversal order of the collection‘s iterator.
237      *
238      * @param capacity the capacity of this queue
239      * @param fair if {@code true} then queue accesses for threads blocked
240      *        on insertion or removal, are processed in FIFO order;
241      *        if {@code false} the access order is unspecified.
242      * @param c the collection of elements to initially contain
243      * @throws IllegalArgumentException if {@code capacity} is less than
244      *         {@code c.size()}, or less than 1.
245      * @throws NullPointerException if the specified collection or any
246      *         of its elements are null
247      */
248     public ArrayBlockingQueue(int capacity, boolean fair,
249                               Collection<? extends E> c) {
250         this(capacity, fair);
251 
252         final ReentrantLock lock = this.lock;
253         lock.lock(); // Lock only for visibility, not mutual exclusion
254         try {
255             int i = 0;
256             try {
257                 for (E e : c) {
258                     checkNotNull(e);
259                     items[i++] = e;
260                 }
261             } catch (ArrayIndexOutOfBoundsException ex) {
262                 throw new IllegalArgumentException();
263             }
264             count = i;
265             putIndex = (i == capacity) ? 0 : i;
266         } finally {
267             lock.unlock();
268         }
269     }
270 
271     /**
272      * Inserts the specified element at the tail of this queue if it is
273      * possible to do so immediately without exceeding the queue‘s capacity,
274      * returning {@code true} upon success and throwing an
275      * {@code IllegalStateException} if this queue is full.
276      *
277      * @param e the element to add
278      * @return {@code true} (as specified by {@link Collection#add})
279      * @throws IllegalStateException if this queue is full
280      * @throws NullPointerException if the specified element is null
281      */
282     public boolean add(E e) {
283         return super.add(e);
284     }
285 
286     /**
287      * Inserts the specified element at the tail of this queue if it is
288      * possible to do so immediately without exceeding the queue‘s capacity,
289      * returning {@code true} upon success and {@code false} if this queue
290      * is full.  This method is generally preferable to method {@link #add},
291      * which can fail to insert an element only by throwing an exception.
292      *
293      * @throws NullPointerException if the specified element is null
294      */
295     public boolean offer(E e) {
296         checkNotNull(e);
297         final ReentrantLock lock = this.lock;
298         lock.lock();
299         try {
300             if (count == items.length)
301                 return false;
302             else {
303                 insert(e);
304                 return true;
305             }
306         } finally {
307             lock.unlock();
308         }
309     }
310 
311     /**
312      * Inserts the specified element at the tail of this queue, waiting
313      * for space to become available if the queue is full.
314      *
315      * @throws InterruptedException {@inheritDoc}
316      * @throws NullPointerException {@inheritDoc}
317      */
318     public void put(E e) throws InterruptedException {
319         checkNotNull(e);
320         final ReentrantLock lock = this.lock;
321         lock.lockInterruptibly();
322         try {
323             while (count == items.length)
324                 notFull.await();
325             insert(e);
326         } finally {
327             lock.unlock();
328         }
329     }
330 
331     /**
332      * Inserts the specified element at the tail of this queue, waiting
333      * up to the specified wait time for space to become available if
334      * the queue is full.
335      *
336      * @throws InterruptedException {@inheritDoc}
337      * @throws NullPointerException {@inheritDoc}
338      */
339     public boolean offer(E e, long timeout, TimeUnit unit)
340         throws InterruptedException {
341 
342         checkNotNull(e);
343         long nanos = unit.toNanos(timeout);
344         final ReentrantLock lock = this.lock;
345         lock.lockInterruptibly();
346         try {
347             while (count == items.length) {
348                 if (nanos <= 0)
349                     return false;
350                 nanos = notFull.awaitNanos(nanos);
351             }
352             insert(e);
353             return true;
354         } finally {
355             lock.unlock();
356         }
357     }
358 
359     public E poll() {
360         final ReentrantLock lock = this.lock;
361         lock.lock();
362         try {
363             return (count == 0) ? null : extract();
364         } finally {
365             lock.unlock();
366         }
367     }
368 
369     public E take() throws InterruptedException {
370         final ReentrantLock lock = this.lock;
371         lock.lockInterruptibly();
372         try {
373             while (count == 0)
374                 notEmpty.await();
375             return extract();
376         } finally {
377             lock.unlock();
378         }
379     }
380 
381     public E poll(long timeout, TimeUnit unit) throws InterruptedException {
382         long nanos = unit.toNanos(timeout);
383         final ReentrantLock lock = this.lock;
384         lock.lockInterruptibly();
385         try {
386             while (count == 0) {
387                 if (nanos <= 0)
388                     return null;
389                 nanos = notEmpty.awaitNanos(nanos);
390             }
391             return extract();
392         } finally {
393             lock.unlock();
394         }
395     }
396 
397     public E peek() {
398         final ReentrantLock lock = this.lock;
399         lock.lock();
400         try {
401             return (count == 0) ? null : itemAt(takeIndex);
402         } finally {
403             lock.unlock();
404         }
405     }
406 
407     // this doc comment is overridden to remove the reference to collections
408     // greater in size than Integer.MAX_VALUE
409     /**
410      * Returns the number of elements in this queue.
411      *
412      * @return the number of elements in this queue
413      */
414     public int size() {
415         final ReentrantLock lock = this.lock;
416         lock.lock();
417         try {
418             return count;
419         } finally {
420             lock.unlock();
421         }
422     }
423 
424     // this doc comment is a modified copy of the inherited doc comment,
425     // without the reference to unlimited queues.
426     /**
427      * Returns the number of additional elements that this queue can ideally
428      * (in the absence of memory or resource constraints) accept without
429      * blocking. This is always equal to the initial capacity of this queue
430      * less the current {@code size} of this queue.
431      *
432      * <p>Note that you <em>cannot</em> always tell if an attempt to insert
433      * an element will succeed by inspecting {@code remainingCapacity}
434      * because it may be the case that another thread is about to
435      * insert or remove an element.
436      */
437     public int remainingCapacity() {
438         final ReentrantLock lock = this.lock;
439         lock.lock();
440         try {
441             return items.length - count;
442         } finally {
443             lock.unlock();
444         }
445     }
446 
447     /**
448      * Removes a single instance of the specified element from this queue,
449      * if it is present.  More formally, removes an element {@code e} such
450      * that {@code o.equals(e)}, if this queue contains one or more such
451      * elements.
452      * Returns {@code true} if this queue contained the specified element
453      * (or equivalently, if this queue changed as a result of the call).
454      *
455      * <p>Removal of interior elements in circular array based queues
456      * is an intrinsically slow and disruptive operation, so should
457      * be undertaken only in exceptional circumstances, ideally
458      * only when the queue is known not to be accessible by other
459      * threads.
460      *
461      * @param o element to be removed from this queue, if present
462      * @return {@code true} if this queue changed as a result of the call
463      */
464     public boolean remove(Object o) {
465         if (o == null) return false;
466         final Object[] items = this.items;
467         final ReentrantLock lock = this.lock;
468         lock.lock();
469         try {
470             for (int i = takeIndex, k = count; k > 0; i = inc(i), k--) {
471                 if (o.equals(items[i])) {
472                     removeAt(i);
473                     return true;
474                 }
475             }
476             return false;
477         } finally {
478             lock.unlock();
479         }
480     }
481 
482     /**
483      * Returns {@code true} if this queue contains the specified element.
484      * More formally, returns {@code true} if and only if this queue contains
485      * at least one element {@code e} such that {@code o.equals(e)}.
486      *
487      * @param o object to be checked for containment in this queue
488      * @return {@code true} if this queue contains the specified element
489      */
490     public boolean contains(Object o) {
491         if (o == null) return false;
492         final Object[] items = this.items;
493         final ReentrantLock lock = this.lock;
494         lock.lock();
495         try {
496             for (int i = takeIndex, k = count; k > 0; i = inc(i), k--)
497                 if (o.equals(items[i]))
498                     return true;
499             return false;
500         } finally {
501             lock.unlock();
502         }
503     }
504 
505     /**
506      * Returns an array containing all of the elements in this queue, in
507      * proper sequence.
508      *
509      * <p>The returned array will be "safe" in that no references to it are
510      * maintained by this queue.  (In other words, this method must allocate
511      * a new array).  The caller is thus free to modify the returned array.
512      *
513      * <p>This method acts as bridge between array-based and collection-based
514      * APIs.
515      *
516      * @return an array containing all of the elements in this queue
517      */
518     public Object[] toArray() {
519         final Object[] items = this.items;
520         final ReentrantLock lock = this.lock;
521         lock.lock();
522         try {
523             final int count = this.count;
524             Object[] a = new Object[count];
525             for (int i = takeIndex, k = 0; k < count; i = inc(i), k++)
526                 a[k] = items[i];
527             return a;
528         } finally {
529             lock.unlock();
530         }
531     }
532 
533     /**
534      * Returns an array containing all of the elements in this queue, in
535      * proper sequence; the runtime type of the returned array is that of
536      * the specified array.  If the queue fits in the specified array, it
537      * is returned therein.  Otherwise, a new array is allocated with the
538      * runtime type of the specified array and the size of this queue.
539      *
540      * <p>If this queue fits in the specified array with room to spare
541      * (i.e., the array has more elements than this queue), the element in
542      * the array immediately following the end of the queue is set to
543      * {@code null}.
544      *
545      * <p>Like the {@link #toArray()} method, this method acts as bridge between
546      * array-based and collection-based APIs.  Further, this method allows
547      * precise control over the runtime type of the output array, and may,
548      * under certain circumstances, be used to save allocation costs.
549      *
550      * <p>Suppose {@code x} is a queue known to contain only strings.
551      * The following code can be used to dump the queue into a newly
552      * allocated array of {@code String}:
553      *
554      * <pre>
555      *     String[] y = x.toArray(new String[0]);</pre>
556      *
557      * Note that {@code toArray(new Object[0])} is identical in function to
558      * {@code toArray()}.
559      *
560      * @param a the array into which the elements of the queue are to
561      *          be stored, if it is big enough; otherwise, a new array of the
562      *          same runtime type is allocated for this purpose
563      * @return an array containing all of the elements in this queue
564      * @throws ArrayStoreException if the runtime type of the specified array
565      *         is not a supertype of the runtime type of every element in
566      *         this queue
567      * @throws NullPointerException if the specified array is null
568      */
569     @SuppressWarnings("unchecked")
570     public <T> T[] toArray(T[] a) {
571         final Object[] items = this.items;
572         final ReentrantLock lock = this.lock;
573         lock.lock();
574         try {
575             final int count = this.count;
576             final int len = a.length;
577             if (len < count)
578                 a = (T[])java.lang.reflect.Array.newInstance(
579                     a.getClass().getComponentType(), count);
580             for (int i = takeIndex, k = 0; k < count; i = inc(i), k++)
581                 a[k] = (T) items[i];
582             if (len > count)
583                 a[count] = null;
584             return a;
585         } finally {
586             lock.unlock();
587         }
588     }
589 
590     public String toString() {
591         final ReentrantLock lock = this.lock;
592         lock.lock();
593         try {
594             int k = count;
595             if (k == 0)
596                 return "[]";
597 
598             StringBuilder sb = new StringBuilder();
599             sb.append(‘[‘);
600             for (int i = takeIndex; ; i = inc(i)) {
601                 Object e = items[i];
602                 sb.append(e == this ? "(this Collection)" : e);
603                 if (--k == 0)
604                     return sb.append(‘]‘).toString();
605                 sb.append(‘,‘).append(‘ ‘);
606             }
607         } finally {
608             lock.unlock();
609         }
610     }
611 
612     /**
613      * Atomically removes all of the elements from this queue.
614      * The queue will be empty after this call returns.
615      */
616     public void clear() {
617         final Object[] items = this.items;
618         final ReentrantLock lock = this.lock;
619         lock.lock();
620         try {
621             for (int i = takeIndex, k = count; k > 0; i = inc(i), k--)
622                 items[i] = null;
623             count = 0;
624             putIndex = 0;
625             takeIndex = 0;
626             notFull.signalAll();
627         } finally {
628             lock.unlock();
629         }
630     }
631 
632     /**
633      * @throws UnsupportedOperationException {@inheritDoc}
634      * @throws ClassCastException            {@inheritDoc}
635      * @throws NullPointerException          {@inheritDoc}
636      * @throws IllegalArgumentException      {@inheritDoc}
637      */
638     public int drainTo(Collection<? super E> c) {
639         checkNotNull(c);
640         if (c == this)
641             throw new IllegalArgumentException();
642         final Object[] items = this.items;
643         final ReentrantLock lock = this.lock;
644         lock.lock();
645         try {
646             int i = takeIndex;
647             int n = 0;
648             int max = count;
649             while (n < max) {
650                 c.add(this.<E>cast(items[i]));
651                 items[i] = null;
652                 i = inc(i);
653                 ++n;
654             }
655             if (n > 0) {
656                 count = 0;
657                 putIndex = 0;
658                 takeIndex = 0;
659                 notFull.signalAll();
660             }
661             return n;
662         } finally {
663             lock.unlock();
664         }
665     }
666 
667     /**
668      * @throws UnsupportedOperationException {@inheritDoc}
669      * @throws ClassCastException            {@inheritDoc}
670      * @throws NullPointerException          {@inheritDoc}
671      * @throws IllegalArgumentException      {@inheritDoc}
672      */
673     public int drainTo(Collection<? super E> c, int maxElements) {
674         checkNotNull(c);
675         if (c == this)
676             throw new IllegalArgumentException();
677         if (maxElements <= 0)
678             return 0;
679         final Object[] items = this.items;
680         final ReentrantLock lock = this.lock;
681         lock.lock();
682         try {
683             int i = takeIndex;
684             int n = 0;
685             int max = (maxElements < count) ? maxElements : count;
686             while (n < max) {
687                 c.add(this.<E>cast(items[i]));
688                 items[i] = null;
689                 i = inc(i);
690                 ++n;
691             }
692             if (n > 0) {
693                 count -= n;
694                 takeIndex = i;
695                 notFull.signalAll();
696             }
697             return n;
698         } finally {
699             lock.unlock();
700         }
701     }
702 
703     /**
704      * Returns an iterator over the elements in this queue in proper sequence.
705      * The elements will be returned in order from first (head) to last (tail).
706      *
707      * <p>The returned {@code Iterator} is a "weakly consistent" iterator that
708      * will never throw {@link java.util.ConcurrentModificationException
709      * ConcurrentModificationException},
710      * and guarantees to traverse elements as they existed upon
711      * construction of the iterator, and may (but is not guaranteed to)
712      * reflect any modifications subsequent to construction.
713      *
714      * @return an iterator over the elements in this queue in proper sequence
715      */
716     public Iterator<E> iterator() {
717         return new Itr();
718     }
719 
720     /**
721      * Iterator for ArrayBlockingQueue. To maintain weak consistency
722      * with respect to puts and takes, we (1) read ahead one slot, so
723      * as to not report hasNext true but then not have an element to
724      * return -- however we later recheck this slot to use the most
725      * current value; (2) ensure that each array slot is traversed at
726      * most once (by tracking "remaining" elements); (3) skip over
727      * null slots, which can occur if takes race ahead of iterators.
728      * However, for circular array-based queues, we cannot rely on any
729      * well established definition of what it means to be weakly
730      * consistent with respect to interior removes since these may
731      * require slot overwrites in the process of sliding elements to
732      * cover gaps. So we settle for resiliency, operating on
733      * established apparent nexts, which may miss some elements that
734      * have moved between calls to next.
735      */
736     private class Itr implements Iterator<E> {
737         private int remaining; // Number of elements yet to be returned
738         private int nextIndex; // Index of element to be returned by next
739         private E nextItem;    // Element to be returned by next call to next
740         private E lastItem;    // Element returned by last call to next
741         private int lastRet;   // Index of last element returned, or -1 if none
742 
743         Itr() {
744             final ReentrantLock lock = ArrayBlockingQueue.this.lock;
745             lock.lock();
746             try {
747                 lastRet = -1;
748                 if ((remaining = count) > 0)
749                     nextItem = itemAt(nextIndex = takeIndex);
750             } finally {
751                 lock.unlock();
752             }
753         }
754 
755         public boolean hasNext() {
756             return remaining > 0;
757         }
758 
759         public E next() {
760             final ReentrantLock lock = ArrayBlockingQueue.this.lock;
761             lock.lock();
762             try {
763                 if (remaining <= 0)
764                     throw new NoSuchElementException();
765                 lastRet = nextIndex;
766                 E x = itemAt(nextIndex);  // check for fresher value
767                 if (x == null) {
768                     x = nextItem;         // we are forced to report old value
769                     lastItem = null;      // but ensure remove fails
770                 }
771                 else
772                     lastItem = x;
773                 while (--remaining > 0 && // skip over nulls
774                        (nextItem = itemAt(nextIndex = inc(nextIndex))) == null)
775                     ;
776                 return x;
777             } finally {
778                 lock.unlock();
779             }
780         }
781 
782         public void remove() {
783             final ReentrantLock lock = ArrayBlockingQueue.this.lock;
784             lock.lock();
785             try {
786                 int i = lastRet;
787                 if (i == -1)
788                     throw new IllegalStateException();
789                 lastRet = -1;
790                 E x = lastItem;
791                 lastItem = null;
792                 // only remove if item still at index
793                 if (x != null && x == items[i]) {
794                     boolean removingHead = (i == takeIndex);
795                     removeAt(i);
796                     if (!removingHead)
797                         nextIndex = dec(nextIndex);
798                 }
799             } finally {
800                 lock.unlock();
801             }
802         }
803     }
804 
805 }

View Code

下面从ArrayBlockingQueue的创建，添加，取出，遍历这几个方面对ArrayBlockingQueue进行分析。

1. 创建

下面以ArrayBlockingQueue(int capacity, boolean fair)来进行说明。

public ArrayBlockingQueue(int capacity, boolean fair) {
    if (capacity <= 0)
        throw new IllegalArgumentException();
    this.items = new Object[capacity];
    lock = new ReentrantLock(fair);
    notEmpty = lock.newCondition();
    notFull =  lock.newCondition();
}

说明：
(01) items是保存“阻塞队列”数据的数组。它的定义如下：

final Object[] items;

(02) fair是“可重入的独占锁(ReentrantLock)”的类型。fair为true，表示是公平锁；fair为false，表示是非公平锁。
notEmpty和notFull是锁的两个Condition条件。它们的定义如下：

final ReentrantLock lock;
private final Condition notEmpty;
private final Condition notFull;

简单对Condition和Lock的用法进行说明，更多内容请参考“Java多线程系列--“JUC锁”06之 Condition条件”。
Lock的作用是提供独占锁机制，来保护竞争资源；而Condition是为了更加精细的对锁进行控制，它依赖于Lock，通过某个条件对多线程进行控制。
notEmpty表示“锁的非空条件”。当某线程想从队列中取数据时，而此时又没有数据，则该线程通过notEmpty.await()进行等待；当其它线程向队列中插入了元素之后，就调用notEmpty.signal()唤醒“之前通过notEmpty.await()进入等待状态的线程”。
同理，notFull表示“锁的满条件”。当某线程想向队列中插入元素，而此时队列已满时，该线程等待；当其它线程从队列中取出元素之后，就唤醒该等待的线程。

2. 添加

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

public boolean offer(E e) {
    // 创建插入的元素是否为null，是的话抛出NullPointerException异常
    checkNotNull(e);
    // 获取“该阻塞队列的独占锁”
    final ReentrantLock lock = this.lock;
    lock.lock();
    try {
        // 如果队列已满，则返回false。
        if (count == items.length)
            return false;
        else {
        // 如果队列未满，则插入e，并返回true。
            insert(e);
            return true;
        }
    } finally {
        // 释放锁
        lock.unlock();
    }
}

说明：offer(E e)的作用是将e插入阻塞队列的尾部。如果队列已满，则返回false，表示插入失败；否则，插入元素，并返回true。
(01) count表示”队列中的元素个数“。除此之外，队列中还有另外两个遍历takeIndex和putIndex。takeIndex表示下一个被取出元素的索引，putIndex表示下一个被添加元素的索引。它们的定义如下：

// 队列中的元素个数
int takeIndex;
// 下一个被取出元素的索引
int putIndex;
// 下一个被添加元素的索引
int count;

(02) insert()的源码如下：

private void insert(E x) {
    // 将x添加到”队列“中
    items[putIndex] = x;
    // 设置”下一个被取出元素的索引“
    putIndex = inc(putIndex);
    // 将”队列中的元素个数”+1
    ++count;
    // 唤醒notEmpty上的等待线程
    notEmpty.signal();
}

insert()在插入元素之后，会唤醒notEmpty上面的等待线程。

inc()的源码如下：

final int inc(int i) {
    return (++i == items.length) ? 0 : i;
}

若i+1的值等于“队列的长度”，即添加元素之后，队列满；则设置“下一个被添加元素的索引”为0。

3. 取出

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

public E take() throws InterruptedException {
    // 获取“队列的独占锁”
    final ReentrantLock lock = this.lock;
    // 获取“锁”，若当前线程是中断状态，则抛出InterruptedException异常
    lock.lockInterruptibly();
    try {
        // 若“队列为空”，则一直等待。
        while (count == 0)
            notEmpty.await();
        // 取出元素
        return extract();
    } finally {
        // 释放“锁”
        lock.unlock();
    }
}

说明：take()的作用是取出并返回队列的头。若队列为空，则一直等待。

extract()的源码如下：

private E extract() {
    final Object[] items = this.items;
    // 强制将元素转换为“泛型E”
    E x = this.<E>cast(items[takeIndex]);
    // 将第takeIndex元素设为null，即删除。同时，帮助GC回收。
    items[takeIndex] = null;
    // 设置“下一个被取出元素的索引”
    takeIndex = inc(takeIndex);
    // 将“队列中元素数量”-1
    --count;
    // 唤醒notFull上的等待线程。
    notFull.signal();
    return x;
}

说明：extract()在删除元素之后，会唤醒notFull上的等待线程。

4. 遍历

下面对ArrayBlockingQueue的遍历方法进行说明。

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

Itr是实现了Iterator接口的类，它的源码如下：

private class Itr implements Iterator<E> {
    // 队列中剩余元素的个数
    private int remaining; // Number of elements yet to be returned
    // 下一次调用next()返回的元素的索引
    private int nextIndex; // Index of element to be returned by next
    // 下一次调用next()返回的元素
    private E nextItem;    // Element to be returned by next call to next
    // 上一次调用next()返回的元素
    private E lastItem;    // Element returned by last call to next
    // 上一次调用next()返回的元素的索引
    private int lastRet;   // Index of last element returned, or -1 if none

    Itr() {
        // 获取“阻塞队列”的锁
        final ReentrantLock lock = ArrayBlockingQueue.this.lock;
        lock.lock();
        try {
            lastRet = -1;
            if ((remaining = count) > 0)
                nextItem = itemAt(nextIndex = takeIndex);
        } finally {
            // 释放“锁”
            lock.unlock();
        }
    }

    public boolean hasNext() {
        return remaining > 0;
    }

    public E next() {
        // 获取“阻塞队列”的锁
        final ReentrantLock lock = ArrayBlockingQueue.this.lock;
        lock.lock();
        try {
            // 若“剩余元素<=0”，则抛出异常。
            if (remaining <= 0)
                throw new NoSuchElementException();
            lastRet = nextIndex;
            // 获取第nextIndex位置的元素
            E x = itemAt(nextIndex);  // check for fresher value
            if (x == null) {
                x = nextItem;         // we are forced to report old value
                lastItem = null;      // but ensure remove fails
            }
            else
                lastItem = x;
            while (--remaining > 0 && // skip over nulls
                   (nextItem = itemAt(nextIndex = inc(nextIndex))) == null)
                ;
            return x;
        } finally {
            lock.unlock();
        }
    }

    public void remove() {
        final ReentrantLock lock = ArrayBlockingQueue.this.lock;
        lock.lock();
        try {
            int i = lastRet;
            if (i == -1)
                throw new IllegalStateException();
            lastRet = -1;
            E x = lastItem;
            lastItem = null;
            // only remove if item still at index
            if (x != null && x == items[i]) {
                boolean removingHead = (i == takeIndex);
                removeAt(i);
                if (!removingHead)
                    nextIndex = dec(nextIndex);
            }
        } finally {
            lock.unlock();
        }
    }
}

ArrayBlockingQueue示例

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

/*
 *   ArrayBlockingQueue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ArrayBlockingQueue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 ArrayBlockingQueueDemo1{

    // 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 ArrayBlockingQueue<String>(20);
    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, ta1, 
tb1, ta1, ta2, 
tb1, ta1, ta2, tb1, tb2, 
ta2, ta1, tb2, tb1, ta3, 
ta2, ta1, tb2, tb1, ta3, ta2, tb3, 
tb2, ta1, ta3, tb1, tb3, ta2, ta4, 
tb2, ta1, ta3, tb1, tb3, ta2, ta4, tb2, tb4, 
ta3, ta1, tb3, tb1, ta4, ta2, tb4, tb2, ta5, 
ta3, ta1, tb3, tb1, ta4, ta2, tb4, tb2, ta5, ta3, tb5, 
tb3, ta1, ta4, tb1, tb4, ta2, ta5, tb2, tb5, ta3, ta6, 
tb3, ta4, tb4, ta5, tb5, ta6, tb6,

结果说明：如果将源码中的queue改成LinkedList对象时，程序会产生ConcurrentModificationException异常。

更多内容

1. Java多线程系列--“JUC集合”01之框架

2. Java多线程系列目录(共xx篇)