本章对Java.util.concurrent包中的ArrayBlockingQueue类进行详细的介绍。
目录
1. ArrayBlockingQueue介绍
2. ArrayBlockingQueue原理和数据结构
3. ArrayBlockingQueue函数列表
4. ArrayBlockingQueue源码分析(JDK1.7.0_40版本)
4.1 创建
4.2 添加
4.3 取出
5. ArrayBlockingQueue示例
1. 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，是无界限的。
2. ArrayBlockingQueue原理和数据结构
ArrayBlockingQueue的数据结构，如下图所示：
说明：
(01). ArrayBlockingQueue继承于AbstractQueue，并且它实现了BlockingQueue接口。
(02). 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。
(03). 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。
(04). 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，可以参考前面的文章。
3. ArrayBlockingQueue函数列表
// 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()
4. ArrayBlockingQueue源码分析(JDK1.7.0_40版本)
ArrayBlockingQueue.java的完整源码如下：
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
* <em>head</em> of the queue is that element that has been on the
* queue the longest time. The <em>tail</em> of the queue is that
* element that has been on the queue the shortest time. New elements
* are inserted at the tail of the queue, and the queue retrieval
* operations obtain elements at the head of the queue.
*
* <p>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.
*
* <p>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.
*
* <p>This class and its iterator implement all of the
* <em>optional</em> methods of the {@link Collection} and {@link
* Iterator} interfaces.
*
* <p>This class is a member of the
* <a href="{@docRoot}/../technotes/guides/collections/index.html">
* Java Collections Framework</a>.
*
* @since 1.5
* @author Doug Lea
* @param <E> the type of elements held in this collection
*/
public class 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.
*
* <p>Note that you <em>cannot</em> always tell if an attempt to insert
* an element will succeed by inspecting {@code remainingCapacity}
* because it may be the case that another thread is about to
* insert or remove an element.
*/
public int remainingCapacity() {
final ReentrantLock lock = this.lock;
lock.lock();
try {
return 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).
*
* <p>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.
*
* <p>The returned array will be "safe" in that no references to it are
* maintained by this queue. (In other words, this method must allocate
* a new array). The caller is thus free to modify the returned array.
*
* <p>This method acts as bridge between array-based and collection-based
* APIs.
*
* @return an array containing all of the elements in this queue
*/
public Object[] toArray() {
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.
*
* <p>If this queue fits in the specified array with room to spare
* (i.e., the array has more elements than this queue), the element in
* the array immediately following the end of the queue is set to
* {@code null}.
*
* <p>Like the {@link #toArray()} method, this method acts as bridge between
* array-based and collection-based APIs. Further, this method allows
* precise control over the runtime type of the output array, and may,
* under certain circumstances, be used to save allocation costs.
*
* <p>Suppose {@code x} is a queue known to contain only strings.
* The following code can be used to dump the queue into a newly
* allocated array of {@code String}:
*
* <pre>
* String[] y = x.toArray(new String[0]);</pre>
*
* Note that {@code toArray(new Object[0])} is identical in function to
* {@code toArray()}.
*
* @param a the array into which the elements of the queue are to
* be stored, if it is big enough; otherwise, a new array of the
* same runtime type is allocated for this purpose
* @return an array containing all of the elements in this queue
* @throws ArrayStoreException if the runtime type of the specified array
* is not a supertype of the runtime type of every element in
* this queue
* @throws NullPointerException if the specified array is null
*/
@SuppressWarnings("unchecked")
public <T> T[] toArray(T[] a) {
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).
*
* <p>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();
}
}
}
}
下面从ArrayBlockingQueue的创建，添加，取出，遍历这几个方面对ArrayBlockingQueue进行分析。
4.1 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。
4.2 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。
4.3 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。
4.4 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();
}
}
}
5. 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。