JUC回顾之-ArrayBlockingQueue底层实现和原理

曾经终败给现在 2022-05-26 06:29 79阅读 0赞

ArrayBlockingQueue的原理和底层实现的数据结构 ：

ArrayBlockingQueue是数组实现的线程安全的有界的阻塞队列，可以按照 FIFO（先进先出）原则对元素进行排序。

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；所谓公平的访问队列是指阻塞的线程，可以按照阻塞的先后顺序访问队列，先阻塞的线程先访问ArrayBlockingQueue队列。非公平性是对先等待的线程是非公平的，当队列可用时，阻塞的线程都可以争夺访问队列的资格，有可能先阻塞的线程最后才能够访问队列。然而为了保证公平性，通常会降低吞吐量。

1. ArrayBlockingQueue：基于数组实现的一个阻塞队列，在创建ArrayBlockingQueue对象时必须制定容量大小。 并且可以指定公平性与非公平性，默认情况下为非公平的，即不保证等待时间最长的队列最优先能够访问队列。

2.ArrayBlockingQueue内部通过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，即数组的容量是在创建创建ArrayBlockingQueue时候指定的。

3.如下图所示，ArrayBlockingQueue和ReentrantLock是组合关系，ArrayBlockingQueue中包含一个ReentrantLock对象。ReentrantLock是可重入的互斥锁。ArrayBlockingQueue就是根据ReentrantLock互斥锁实现"多线程对共享资源的访问"。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和Condition是组合关系，ArrayBlockingQueue中包含两个Condition对象(notEmpty和notFull)。使用通知模式实现:所谓通知模式，当生产者往满的队列里面添加元素的时候，会阻塞生产者（调用Condition notFull.await()进行等待）；当消费者消费了一个队列中的元素后，会通知（调用Condition notFull.signal()唤醒生产者)生产者当前队列可用。反之，当消费者消费的时候，发现队列是空的，则消费者会被阻塞（通过Condition的 notEmpty.await()进行等待），当生产者插入了队列中的一个元素后，则会调用notEmpty.signal()唤醒消费者继续消费。

ArrayBlockingQueue的数据结构如下：

![167213-20161024153218015-1753023750.png][]

ArrayBlockingQueue方法列表：

![复制代码][copycode.gif]

// 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()

![复制代码][copycode.gif]

总结下上面的方法：

![复制代码][copycode.gif]

*非阻塞队列中的方法：
     * 
     *  抛出异常的方法 Exception in thread "main" java.lang.IllegalStateException: Queue full
     
     *1. add(e) throw exception,将元素e插入到队列的末尾，如果插入成功，则返回true，如果插入失败 (队列已经满) 抛出异常
     *2. remove(e) throw exception，移除队首元素，若移除成功，则返回true；若移除失败（队列为空）则抛出异常
     *3. element() throw exception 获取队列首元素，若获取成功，则返回首元素，否则抛出异常 java.util.NoSuchElementException
     * 
     *  返回特定值的方法
     *
     * 1.offer(E e)，将元素e插入到队列末尾，如果插入成功，则返回true，如果插入失败（队列已满）,返回false
     * 2.poll(E e)，移除并获取队首元素，若成功，则返回队首元素，否则返回null
     * 3.peek(E e),获取队首元素，若成功，则返回队首元素，否则则返回null

*  可以指定TimeOut：
     * 
     * 3.offer(E e,long timeout, TimeUnit unit)：向队列尾部存入元素e，如果队列满，则等待一定的时间，当达到timeout时候，则返回false，否则返回true 　 4.poll(long timeout, TimeUnit unit)：从队首获取元素，如果队列为空，则等待一定时间，当达到timeout时，如果没有取到，则返回null，如果取到则返回取到的元素

阻塞队列中的几个重要方法：
    
     * 1.put(E e) :用于队列尾部存入元素e，如果对满，则等待。
     * 2.take()：用于从对列首取出元素e，如果队列为空，则等待

注意：阻塞队列包括了非阻塞队列中的大部分方法，上面列举的5个方法在阻塞队列中都存在，但是要注意这5个方法在阻塞队列中都进行了同步措施，都是线程安全的。

![复制代码][copycode.gif]

### ArrayBlockingQueue源码分析(JDK1.7) ###

![ExpandedBlockStart.gif][]

![复制代码][copycode.gif]

/*
 * ORACLE PROPRIETARY/CONFIDENTIAL. Use is subject to license terms.
 *
 *
 *
 *
 *
 *
 *
 *
 *
 *
 *
 *
 *
 *
 *
 *
 *
 *
 *
 *
 */
 
 /*
 *
 *
 *
 *
 *
 * Written by Doug Lea with assistance from members of JCP JSR-166
 * Expert Group and released to the public domain, as explained at
 * http://creativecommons.org/publicdomain/zero/1.0/
 */
 
 package java.util.concurrent;
 import java.util.concurrent.locks.*;
 import java.util.*;
 
 /**
 * A bounded {
 @linkplain BlockingQueue blocking queue} backed by an
 * array. This queue orders elements FIFO (first-in-first-out). The
 * head of the queue is that element that has been on the
 * queue the longest time. The tail of the queue is that
 * element that has been on the queue the shortest time. New elements
 * are inserted at the tail of the queue, and the queue retrieval
 * operations obtain elements at the head of the queue.
 *
 * This is a classic "bounded buffer", in which a
 * fixed-sized array holds elements inserted by producers and
 * extracted by consumers. Once created, the capacity cannot be
 * changed. Attempts to {
 @code put} an element into a full queue
 * will result in the operation blocking; attempts to {
 @code take} an
 * element from an empty queue will similarly block.
 *
 * This class supports an optional fairness policy for ordering
 * waiting producer and consumer threads. By default, this ordering
 * is not guaranteed. However, a queue constructed with fairness set
 * to {
 @code true} grants threads access in FIFO order. Fairness
 * generally decreases throughput but reduces variability and avoids
 * starvation.
 *
 * This class and its iterator implement all of the
 * optional methods of the {
 @link Collection} and {
 @link
 * Iterator} interfaces.
 *
 * This class is a member of the
 * <a href="{@docRoot}/../technotes/guides/collections/index.html">
 * Java Collections Framework</a>.
 *
 * @since 1.5
 * @author Doug Lea
 * @param <E> the type of elements held in this collection
 */
 public class ArrayBlockingQueue<E> extends AbstractQueue<E>
 implements BlockingQueue<E>, java.io.Serializable {
 
 /**
 * Serialization ID. This class relies on default serialization
 * even for the items array, which is default-serialized, even if
 * it is empty. Otherwise it could not be declared final, which is
 * necessary here.
 */
 private static final long serialVersionUID = -817911632652898426L;
 
 /** The queued items */
 final Object[] items;
 
 /** items index for next take, poll, peek or remove */
 int takeIndex;
 
 /** items index for next put, offer, or add */
 int putIndex;
 
 /** Number of elements in the queue */
 int count;
 
 /*
 * Concurrency control uses the classic two-condition algorithm
 * found in any textbook.
 */
 
 /** Main lock guarding all access */
 final ReentrantLock lock;
 /** Condition for waiting takes */
 private final Condition notEmpty;
 /** Condition for waiting puts */
 private final Condition notFull;
 
 // Internal helper methods
 
 /**
 * Circularly increment i.
 */
 final int inc(int i) {
 return (++i == items.length) ? 0 : i;
 }
 
 /**
 * Circularly decrement i.
 */
 final int dec(int i) {
 return ((i == 0) ? items.length : i) - 1;
 }
 
 @SuppressWarnings("unchecked")
 static <E> E cast(Object item) {
 return (E) item;
 }
 
 /**
 * Returns item at index i.
 */
 final E itemAt(int i) {
 return this.<E>cast(items[i]);
 }
 
 /**
 * Throws NullPointerException if argument is null.
 *
 * @param v the element
 */
 private static void checkNotNull(Object v) {
 if (v == null)
 throw new NullPointerException();
 }
 
 /**
 * Inserts element at current put position, advances, and signals.
 * Call only when holding lock.
 */
 private void insert(E x) {
 items[putIndex] = x;
 putIndex = inc(putIndex);
 ++count;
 notEmpty.signal();
 }
 
 /**
 * Extracts element at current take position, advances, and signals.
 * Call only when holding lock.
 */
 private E extract() {
 final Object[] items = this.items;
 E x = this.<E>cast(items[takeIndex]);
 items[takeIndex] = null;
 takeIndex = inc(takeIndex);
 --count;
 notFull.signal();
 return x;
 }
 
 /**
 * Deletes item at position i.
 * Utility for remove and iterator.remove.
 * Call only when holding lock.
 */
 void removeAt(int i) {
 final Object[] items = this.items;
 // if removing front item, just advance
 if (i == takeIndex) {
 items[takeIndex] = null;
 takeIndex = inc(takeIndex);
 } else {
 // slide over all others up through putIndex.
 for (;;) {
 int nexti = inc(i);
 if (nexti != putIndex) {
 items[i] = items[nexti];
 i = nexti;
 } else {
 items[i] = null;
 putIndex = i;
 break;
 }
 }
 }
 --count;
 notFull.signal();
 }
 
 /**
 * Creates an {
 @code ArrayBlockingQueue} with the given (fixed)
 * capacity and default access policy.
 *
 * @param capacity the capacity of this queue
 * @throws IllegalArgumentException if {
 @code capacity < 1}
 */
 public ArrayBlockingQueue(int capacity) {
 this(capacity, false);
 }
 
 /**
 * Creates an {
 @code ArrayBlockingQueue} with the given (fixed)
 * capacity and the specified access policy.
 *
 * @param capacity the capacity of this queue
 * @param fair if {
 @code true} then queue accesses for threads blocked
 * on insertion or removal, are processed in FIFO order;
 * if {
 @code false} the access order is unspecified.
 * @throws IllegalArgumentException if {
 @code capacity < 1}
 */
 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();
 }
 
 /**
 * Creates an {
 @code ArrayBlockingQueue} with the given (fixed)
 * capacity, the specified access policy and initially containing the
 * elements of the given collection,
 * added in traversal order of the collection's iterator.
 *
 * @param capacity the capacity of this queue
 * @param fair if {
 @code true} then queue accesses for threads blocked
 * on insertion or removal, are processed in FIFO order;
 * if {
 @code false} the access order is unspecified.
 * @param c the collection of elements to initially contain
 * @throws IllegalArgumentException if {
 @code capacity} is less than
 * {
 @code c.size()}, or less than 1.
 * @throws NullPointerException if the specified collection or any
 * of its elements are null
 */
 public ArrayBlockingQueue(int capacity, boolean fair,
 Collection<? extends E> c) {
 this(capacity, fair);
 
 final ReentrantLock lock = this.lock;
 lock.lock(); // Lock only for visibility, not mutual exclusion
 try {
 int i = 0;
 try {
 for (E e : c) {
 checkNotNull(e);
 items[i++] = e;
 }
 } catch (ArrayIndexOutOfBoundsException ex) {
 throw new IllegalArgumentException();
 }
 count = i;
 putIndex = (i == capacity) ? 0 : i;
 } finally {
 lock.unlock();
 }
 }
 
 /**
 * Inserts the specified element at the tail of this queue if it is
 * possible to do so immediately without exceeding the queue's capacity,
 * returning {
 @code true} upon success and throwing an
 * {
 @code IllegalStateException} if this queue is full.
 *
 * @param e the element to add
 * @return {
 @code true} (as specified by {
 @link Collection#add})
 * @throws IllegalStateException if this queue is full
 * @throws NullPointerException if the specified element is null
 */
 public boolean add(E e) {
 return super.add(e);
 }
 
 /**
 * Inserts the specified element at the tail of this queue if it is
 * possible to do so immediately without exceeding the queue's capacity,
 * returning {
 @code true} upon success and {
 @code false} if this queue
 * is full. This method is generally preferable to method {
 @link #add},
 * which can fail to insert an element only by throwing an exception.
 *
 * @throws NullPointerException if the specified element is null
 */
 public boolean offer(E e) {
 checkNotNull(e);
 final ReentrantLock lock = this.lock;
 lock.lock();
 try {
 if (count == items.length)
 return false;
 else {
 insert(e);
 return true;
 }
 } finally {
 lock.unlock();
 }
 }
 
 /**
 * Inserts the specified element at the tail of this queue, waiting
 * for space to become available if the queue is full.
 *
 * @throws InterruptedException {
 @inheritDoc}
 * @throws NullPointerException {
 @inheritDoc}
 */
 public void put(E e) throws InterruptedException {
 checkNotNull(e);
 final ReentrantLock lock = this.lock;
 lock.lockInterruptibly();
 try {
 while (count == items.length)
 notFull.await();
 insert(e);
 } finally {
 lock.unlock();
 }
 }
 
 /**
 * Inserts the specified element at the tail of this queue, waiting
 * up to the specified wait time for space to become available if
 * the queue is full.
 *
 * @throws InterruptedException {
 @inheritDoc}
 * @throws NullPointerException {
 @inheritDoc}
 */
 public boolean offer(E e, long timeout, TimeUnit unit)
 throws InterruptedException {
 
 checkNotNull(e);
 long nanos = unit.toNanos(timeout);
 final ReentrantLock lock = this.lock;
 lock.lockInterruptibly();
 try {
 while (count == items.length) {
 if (nanos <= 0)
 return false;
 nanos = notFull.awaitNanos(nanos);
 }
 insert(e);
 return true;
 } finally {
 lock.unlock();
 }
 }
 
 public E poll() {
 final ReentrantLock lock = this.lock;
 lock.lock();
 try {
 return (count == 0) ? null : extract();
 } finally {
 lock.unlock();
 }
 }
 
 public E take() throws InterruptedException {
 final ReentrantLock lock = this.lock;
 lock.lockInterruptibly();
 try {
 while (count == 0)
 notEmpty.await();
 return extract();
 } finally {
 lock.unlock();
 }
 }
 
 public E poll(long timeout, TimeUnit unit) throws InterruptedException {
 long nanos = unit.toNanos(timeout);
 final ReentrantLock lock = this.lock;
 lock.lockInterruptibly();
 try {
 while (count == 0) {
 if (nanos <= 0)
 return null;
 nanos = notEmpty.awaitNanos(nanos);
 }
 return extract();
 } finally {
 lock.unlock();
 }
 }
 
 public E peek() {
 final ReentrantLock lock = this.lock;
 lock.lock();
 try {
 return (count == 0) ? null : itemAt(takeIndex);
 } finally {
 lock.unlock();
 }
 }
 
 // this doc comment is overridden to remove the reference to collections
 // greater in size than Integer.MAX_VALUE
 /**
 * Returns the number of elements in this queue.
 *
 * @return the number of elements in this queue
 */
 public int size() {
 final ReentrantLock lock = this.lock;
 lock.lock();
 try {
 return count;
 } finally {
 lock.unlock();
 }
 }
 
 // this doc comment is a modified copy of the inherited doc comment,
 // without the reference to unlimited queues.
 /**
 * Returns the number of additional elements that this queue can ideally
 * (in the absence of memory or resource constraints) accept without
 * blocking. This is always equal to the initial capacity of this queue
 * less the current {
 @code size} of this queue.
 *
 * Note that you cannot always tell if an attempt to insert
 * an element will succeed by inspecting {
 @code remainingCapacity}
 * because it may be the case that another thread is about to
 * insert or remove an element.
 */
 public int remainingCapacity() {
 final ReentrantLock lock = this.lock;
 lock.lock();
 try {
 return items.length - count;
 } finally {
 lock.unlock();
 }
 }
 
 /**
 * Removes a single instance of the specified element from this queue,
 * if it is present. More formally, removes an element {
 @code e} such
 * that {
 @code o.equals(e)}, if this queue contains one or more such
 * elements.
 * Returns {
 @code true} if this queue contained the specified element
 * (or equivalently, if this queue changed as a result of the call).
 *
 * Removal of interior elements in circular array based queues
 * is an intrinsically slow and disruptive operation, so should
 * be undertaken only in exceptional circumstances, ideally
 * only when the queue is known not to be accessible by other
 * threads.
 *
 * @param o element to be removed from this queue, if present
 * @return {
 @code true} if this queue changed as a result of the call
 */
 public boolean remove(Object o) {
 if (o == null) return false;
 final Object[] items = this.items;
 final ReentrantLock lock = this.lock;
 lock.lock();
 try {
 for (int i = takeIndex, k = count; k > 0; i = inc(i), k--) {
 if (o.equals(items[i])) {
 removeAt(i);
 return true;
 }
 }
 return false;
 } finally {
 lock.unlock();
 }
 }
 
 /**
 * Returns {
 @code true} if this queue contains the specified element.
 * More formally, returns {
 @code true} if and only if this queue contains
 * at least one element {
 @code e} such that {
 @code o.equals(e)}.
 *
 * @param o object to be checked for containment in this queue
 * @return {
 @code true} if this queue contains the specified element
 */
 public boolean contains(Object o) {
 if (o == null) return false;
 final Object[] items = this.items;
 final ReentrantLock lock = this.lock;
 lock.lock();
 try {
 for (int i = takeIndex, k = count; k > 0; i = inc(i), k--)
 if (o.equals(items[i]))
 return true;
 return false;
 } finally {
 lock.unlock();
 }
 }
 
 /**
 * Returns an array containing all of the elements in this queue, in
 * proper sequence.
 *
 * The returned array will be "safe" in that no references to it are
 * maintained by this queue. (In other words, this method must allocate
 * a new array). The caller is thus free to modify the returned array.
 *
 * This method acts as bridge between array-based and collection-based
 * APIs.
 *
 * @return an array containing all of the elements in this queue
 */
 public Object[] toArray() {
 final Object[] items = this.items;
 final ReentrantLock lock = this.lock;
 lock.lock();
 try {
 final int count = this.count;
 Object[] a = new Object[count];
 for (int i = takeIndex, k = 0; k < count; i = inc(i), k++)
 a[k] = items[i];
 return a;
 } finally {
 lock.unlock();
 }
 }
 
 /**
 * Returns an array containing all of the elements in this queue, in
 * proper sequence; the runtime type of the returned array is that of
 * the specified array. If the queue fits in the specified array, it
 * is returned therein. Otherwise, a new array is allocated with the
 * runtime type of the specified array and the size of this queue.
 *
 * If this queue fits in the specified array with room to spare
 * (i.e., the array has more elements than this queue), the element in
 * the array immediately following the end of the queue is set to
 * {
 @code null}.
 *
 * Like the {
 @link #toArray()} method, this method acts as bridge between
 * array-based and collection-based APIs. Further, this method allows
 * precise control over the runtime type of the output array, and may,
 * under certain circumstances, be used to save allocation costs.
 *
 * Suppose {
 @code x} is a queue known to contain only strings.
 * The following code can be used to dump the queue into a newly
 * allocated array of {
 @code String}:
 *
 * <pre>
 * String[] y = x.toArray(new String[0]);</pre>
 *
 * Note that {
 @code toArray(new Object[0])} is identical in function to
 * {
 @code toArray()}.
 *
 * @param a the array into which the elements of the queue are to
 * be stored, if it is big enough; otherwise, a new array of the
 * same runtime type is allocated for this purpose
 * @return an array containing all of the elements in this queue
 * @throws ArrayStoreException if the runtime type of the specified array
 * is not a supertype of the runtime type of every element in
 * this queue
 * @throws NullPointerException if the specified array is null
 */
 @SuppressWarnings("unchecked")
 public <T> T[] toArray(T[] a) {
 final Object[] items = this.items;
 final ReentrantLock lock = this.lock;
 lock.lock();
 try {
 final int count = this.count;
 final int len = a.length;
 if (len < count)
 a = (T[])java.lang.reflect.Array.newInstance(
 a.getClass().getComponentType(), count);
 for (int i = takeIndex, k = 0; k < count; i = inc(i), k++)
 a[k] = (T) items[i];
 if (len > count)
 a[count] = null;
 return a;
 } finally {
 lock.unlock();
 }
 }
 
 public String toString() {
 final ReentrantLock lock = this.lock;
 lock.lock();
 try {
 int k = count;
 if (k == 0)
 return "[]";
 
 StringBuilder sb = new StringBuilder();
 sb.append('[');
 for (int i = takeIndex; ; i = inc(i)) {
 Object e = items[i];
 sb.append(e == this ? "(this Collection)" : e);
 if (--k == 0)
 return sb.append(']').toString();
 sb.append(',').append(' ');
 }
 } finally {
 lock.unlock();
 }
 }
 
 /**
 * Atomically removes all of the elements from this queue.
 * The queue will be empty after this call returns.
 */
 public void clear() {
 final Object[] items = this.items;
 final ReentrantLock lock = this.lock;
 lock.lock();
 try {
 for (int i = takeIndex, k = count; k > 0; i = inc(i), k--)
 items[i] = null;
 count = 0;
 putIndex = 0;
 takeIndex = 0;
 notFull.signalAll();
 } finally {
 lock.unlock();
 }
 }
 
 /**
 * @throws UnsupportedOperationException {
 @inheritDoc}
 * @throws ClassCastException {
 @inheritDoc}
 * @throws NullPointerException {
 @inheritDoc}
 * @throws IllegalArgumentException {
 @inheritDoc}
 */
 public int drainTo(Collection<? super E> c) {
 checkNotNull(c);
 if (c == this)
 throw new IllegalArgumentException();
 final Object[] items = this.items;
 final ReentrantLock lock = this.lock;
 lock.lock();
 try {
 int i = takeIndex;
 int n = 0;
 int max = count;
 while (n < max) {
 c.add(this.<E>cast(items[i]));
 items[i] = null;
 i = inc(i);
 ++n;
 }
 if (n > 0) {
 count = 0;
 putIndex = 0;
 takeIndex = 0;
 notFull.signalAll();
 }
 return n;
 } finally {
 lock.unlock();
 }
 }
 
 /**
 * @throws UnsupportedOperationException {
 @inheritDoc}
 * @throws ClassCastException {
 @inheritDoc}
 * @throws NullPointerException {
 @inheritDoc}
 * @throws IllegalArgumentException {
 @inheritDoc}
 */
 public int drainTo(Collection<? super E> c, int maxElements) {
 checkNotNull(c);
 if (c == this)
 throw new IllegalArgumentException();
 if (maxElements <= 0)
 return 0;
 final Object[] items = this.items;
 final ReentrantLock lock = this.lock;
 lock.lock();
 try {
 int i = takeIndex;
 int n = 0;
 int max = (maxElements < count) ? maxElements : count;
 while (n < max) {
 c.add(this.<E>cast(items[i]));
 items[i] = null;
 i = inc(i);
 ++n;
 }
 if (n > 0) {
 count -= n;
 takeIndex = i;
 notFull.signalAll();
 }
 return n;
 } finally {
 lock.unlock();
 }
 }
 
 /**
 * Returns an iterator over the elements in this queue in proper sequence.
 * The elements will be returned in order from first (head) to last (tail).
 *
 * The returned {
 @code Iterator} is a "weakly consistent" iterator that
 * will never throw {
 @link java.util.ConcurrentModificationException
 * ConcurrentModificationException},
 * and guarantees to traverse elements as they existed upon
 * construction of the iterator, and may (but is not guaranteed to)
 * reflect any modifications subsequent to construction.
 *
 * @return an iterator over the elements in this queue in proper sequence
 */
 public Iterator<E> iterator() {
 return new Itr();
 }
 
 /**
 * Iterator for ArrayBlockingQueue. To maintain weak consistency
 * with respect to puts and takes, we (1) read ahead one slot, so
 * as to not report hasNext true but then not have an element to
 * return -- however we later recheck this slot to use the most
 * current value; (2) ensure that each array slot is traversed at
 * most once (by tracking "remaining" elements); (3) skip over
 * null slots, which can occur if takes race ahead of iterators.
 * However, for circular array-based queues, we cannot rely on any
 * well established definition of what it means to be weakly
 * consistent with respect to interior removes since these may
 * require slot overwrites in the process of sliding elements to
 * cover gaps. So we settle for resiliency, operating on
 * established apparent nexts, which may miss some elements that
 * have moved between calls to next.
 */
 private class Itr implements Iterator<E> {
 private int remaining; // Number of elements yet to be returned
 private int nextIndex; // Index of element to be returned by next
 private E nextItem; // Element to be returned by next call to next
 private E lastItem; // Element returned by last call to next
 private int lastRet; // Index of last element returned, or -1 if none
 
 Itr() {
 final ReentrantLock lock = ArrayBlockingQueue.this.lock;
 lock.lock();
 try {
 lastRet = -1;
 if ((remaining = count) > 0)
 nextItem = itemAt(nextIndex = takeIndex);
 } finally {
 lock.unlock();
 }
 }
 
 public boolean hasNext() {
 return remaining > 0;
 }
 
 public E next() {
 final ReentrantLock lock = ArrayBlockingQueue.this.lock;
 lock.lock();
 try {
 if (remaining <= 0)
 throw new NoSuchElementException();
 lastRet = nextIndex;
 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();
 }
 }
 }
 
 }

![复制代码][copycode.gif]

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：

![复制代码][copycode.gif]

/**
 * Creates an {
 @code ArrayBlockingQueue} with the given (fixed)
 * capacity and default access policy.
 *
 * @param capacity the capacity of this queue
 * @throws IllegalArgumentException if {
 @code capacity < 1}
 只是指定ArrayBlockingQueue的容量，默认采用非公平互斥锁

*/ public ArrayBlockingQueue(int capacity) { this(capacity, false); }

![复制代码][copycode.gif]

/**
 * Creates an {
 @code ArrayBlockingQueue} with the given (fixed)
 * capacity and the specified access policy.
 *
 * @param capacity the capacity of this queue
 * @param fair if {
 @code true} then queue accesses for threads blocked
 * on insertion or removal, are processed in FIFO order;
 * if {
 @code false} the access order is unspecified.
 * @throws IllegalArgumentException if {
 @code capacity < 1}
 指定容量和ReetrantLock的类型是否为公平锁创建阻塞队列
 */
 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();
 }

![复制代码][copycode.gif]

上面源码进行说明：

items是保存“阻塞队列”数据的数组。它的定义如下：

/** The queued items */
        final Object[] items;

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：

/** Main lock guarding all access */
        final ReentrantLock lock;
        /** Condition for waiting takes */
        private final Condition notEmpty;
        /** Condition for waiting puts */
        private final Condition notFull;

说明：Lock的作用是提供独占锁机制，来保护竞争的资源;而Condition是为了更精细的对锁进行控制，但是依赖于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获取数据的时候，而此时队列中的数据为空，则该线程通过notEmpty.await()方法进行等待；

当其他线程向队列中插入元素之后，就调用notEmpty.signal()方法进行唤醒之前等待的线程。同理，notFull表示“锁满的条件“。当某个线程向队列中插入元素

，而此时队列已满时，该线程等待，即阻塞通过notFull.wait()方法；其他线程从队列中取出元素之后，就唤醒该等待的线程，这个线程调用notFull.signal()方法。

2.添加：

![复制代码][copycode.gif]

/**
         * Inserts the specified element at the tail of this queue, waiting
         * for space to become available if the queue is full.
         *
         * @throws InterruptedException {
         @inheritDoc}
         * @throws NullPointerException {
         @inheritDoc}
           添加元素，当队列满的时候，该线程等待，即阻塞。
         */
        public void put(E e) throws InterruptedException {
         
    //校验插入的元素不能为null
            checkNotNull(e);
            final ReentrantLock lock = this.lock;
            lock.lockInterruptibly();
            try {
         
    //队列满的时候 while (count == items.length)
     //线程调用await方法阻塞
                    notFull.await();
                insert(e);
            } finally {
                lock.unlock();
            }
        }

![复制代码][copycode.gif]

/** items index for next take, poll, peek or remove
        下一个被取出元素的索引
    */
        int takeIndex;
    
        /** items index for next put, offer, or add 
           下一个被添加元素的索引
        */
        int putIndex;
    
        /** Number of elements in the queue
           队列中的元素的个数
    */
        int count;

![复制代码][copycode.gif]

/**
         * Inserts element at current put position, advances, and signals.
         * Call only when holding lock.
         */
        private void insert(E x) {
            items[putIndex] = x;
            putIndex = inc(putIndex);
     //队列中的元素个数 ++count;
    //唤醒notEmpty Condition锁上面等待的线程，告诉该线程队列不为空了，可以消费了
            notEmpty.signal();
        }

![复制代码][copycode.gif]

/**
         * Circularly increment i.
           队列中的元素个数==队列的长度的时候，队列满，则设置下一个被添加元素的索引为0
         */
        final int inc(int i) {
            return (++i == items.length) ? 0 : i;
        }

![复制代码][copycode.gif]

2.取出：

![复制代码][copycode.gif]

public E take() throws InterruptedException {
            final ReentrantLock lock = this.lock;
    //获取锁，如果当前线程是中断状态，则抛出interruptedException异常
            lock.lockInterruptibly();
            try {
         
    //队列为空的时候，则线程一直阻塞等待 while (count == 0)
                    notEmpty.await();
    //取元素 return extract();
            } finally {
                lock.unlock();
            }
        }

![复制代码][copycode.gif]

/**
 * Extracts element at current take position, advances, and signals.
 * Call only when holding lock.
 */
 private E extract() {
 final Object[] items = this.items;
 //强制将元素转化为"泛型E"
 E x = this.<E>cast(items[takeIndex]);
 items[takeIndex] = null;
 //设置下一个被取出元素的索引
 takeIndex = inc(takeIndex);
 //将队列中的元素-1 --count;
 //唤醒notFull条件上面等待的线程，告诉该线程队列不是满的了，可以添加元素了
 notFull.signal();
 return x;
 }

![复制代码][copycode.gif]

Face your past without regret. Handle your present with confidence.Prepare for future without fear. keep the faith and drop the fear. 面对过去无怨无悔，把握现在充满信心，备战未来无所畏惧。保持信念，克服恐惧！一点一滴的积累，一点一滴的沉淀，学技术需要不断的积淀！

转载于：https://www.cnblogs.com/200911/p/5994044.html

[167213-20161024153218015-1753023750.png]: /images/20220526/e2d219f95d6b4d36b5a73e8562101206.png
[copycode.gif]: /images/20220526/51fe1331497a4decabcaf88822fcb997.png
[ExpandedBlockStart.gif]: /images/20220526/e2b51019b98a45ea9503a243a5f5bacd.png