ArrayBlockingQueue
ArrayBlockingQueue
BlockingQueue定义的常用方法有：
放入数据：
-
offer(anObject):表示如果可能的话,将anObject加到BlockingQueue里,即如果BlockingQueue可以容纳,
则返回true,否则返回false.（本方法不阻塞当前执行方法的线程） -
offer(E o, long timeout, TimeUnit unit),可以设定等待的时间，如果在指定的时间内，还不能往队列中
加入BlockingQueue，则返回失败。 -
put(anObject):把anObject加到BlockingQueue里,如果BlockQueue没有空间,则调用此方法的线程被阻断
直到BlockingQueue里面有空间再继续. -
add(anObject):把anObject加到BlockingQueue里,如果BlockQueue没有空间，则抛出一个IIIegaISlabEepeplian异常
获取数据：
- poll(time):取走BlockingQueue里排在首位的对象,若不能立即取出,则可以等time参数规定的时间,
取不到时返回null; - poll(long timeout, TimeUnit unit)：从BlockingQueue取出一个队首的对象，如果在指定时间内，
队列一旦有数据可取，则立即返回队列中的数据。否则知道时间超时还没有数据可取，返回失败。 - take():取走BlockingQueue里排在首位的对象,若BlockingQueue为空,阻断进入等待状态直到
BlockingQueue有新的数据被加入; - remove(): 从BlockingQueue取出一个队首的对象,如果队列为空，则抛出一个NoSuchElementException异常
- peek(): 返回队列头部的元素.如果队列为空，则返回null
- drainTo():一次性从BlockingQueue获取所有可用的数据对象（还可以指定获取数据的个数），
通过该方法，可以提升获取数据效率；不需要多次分批加锁或释放锁。
ArrayBlockingQueue
ArrayBlockingQueue使用一个指定长度的数组来存放对应的数据，这个长度在初始化的时候被指定，并且不能够再改变。如果往一个满的队列里面插入就会阻塞，如果从一个空的队列里面取内容也会阻塞。
类定义：
public class ArrayBlockingQueue<E> extends AbstractQueue<E>
implements BlockingQueue<E>, java.io.Serializable {
从上面可以看出ArrayBlockingQueue继承了AbstractQueue ，它使用了父类的add方法。
变量：
/** The queued items */
//存放内容的数组
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;
/**
* Shared state for currently active iterators, or null if there
* are known not to be any. Allows queue operations to update
* iterator state.
*/
transient Itrs itrs = null;
解释都比较清楚，Itrs 实现比较复杂，以后仔细看下。
构造函数：
/**
* 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;
//如果集合的个数等于capacity，putIndex 就设置为0，否则设置为i
putIndex = (i == capacity) ? 0 : i;
} finally {
lock.unlock();
}
}
ArrayBlockingQueue 有三种构造函数，前两种都是定义一个空的queue，第三个是将一个集合转化为queue。
add()
public boolean add(E e) {
return super.add(e);
}
public boolean add(E e) {
if (offer(e))
return true;
else
throw new IllegalStateException("Queue full");
}
add 函数 是使用了父类的add 方法，但实际上还是调用了offer，如果offer 失败了，就抛出了异常。
offer()
public boolean offer(E e) {
checkNotNull(e);
final ReentrantLock lock = this.lock;
lock.lock();
try {
//判断是否满了，如果满了，就返回false
if (count == items.length)
return false;
else {
enqueue(e);
return true;
}
} finally {
lock.unlock();
}
}
offer 里面主要调用了enqueue。
private void enqueue(E x) {
// assert lock.getHoldCount() == 1;
// assert items[putIndex] == null;
//赋值，并将putIndex 的下标+1
final Object[] items = this.items;
items[putIndex] = x;
if (++putIndex == items.length)
putIndex = 0;
count++;
//释放信号
notEmpty.signal();
}
offer 还有另外的一种方式，就是设定等待的时间，这个等待的是用在notFull.awaitNanos()中
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 {
//这里增加了超时判断，如果超时了就返回false
while (count == items.length) {
if (nanos <= 0)
return false;
nanos = notFull.awaitNanos(nanos);
}
enqueue(e);
return true;
} finally {
lock.unlock();
}
}
put()
public void put(E e) throws InterruptedException {
checkNotNull(e);
final ReentrantLock lock = this.lock;
lock.lockInterruptibly();
//这里可以看出，如果队列满了，put一直在等待notFull condition，会造成阻塞
try {
while (count == items.length)
notFull.await();
enqueue(e);
} finally {
lock.unlock();
}
}
pull()
public E poll() {
final ReentrantLock lock = this.lock;
lock.lock();
try {
//通过count判断队列是否为空，如果为空返回null，否则调用dequeue
return (count == 0) ? null : dequeue();
} finally {
lock.unlock();
}
}
private E dequeue() {
// assert lock.getHoldCount() == 1;
// assert items[takeIndex] != null;
final Object[] items = this.items;
@SuppressWarnings("unchecked")
E x = (E) items[takeIndex];
items[takeIndex] = null;
if (++takeIndex == items.length)
takeIndex = 0;
count--;
if (itrs != null)
itrs.elementDequeued();
//dequeue会释放notFull.signal
notFull.signal();
return x;
}
poll也有等待指定时间的方式，具体和offer类似
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 dequeue();
} finally {
lock.unlock();
}
}
peek()
peek就是返回数组中takeIndex指向的值
public E peek() {
final ReentrantLock lock = this.lock;
lock.lock();
try {
return itemAt(takeIndex); // null when queue is empty
} finally {
lock.unlock();
}
}
remove(Object o)
remove比较有意思，从takeIndex开始到putIndex，找到第一个与o相等的元素，并删除,然后更新itrs
public boolean remove(Object o) {
if (o == null) return false;
final Object[] items = this.items;
final ReentrantLock lock = this.lock;
lock.lock();
try {
if (count > 0) {
final int putIndex = this.putIndex;
int i = takeIndex;
do {
if (o.equals(items[i])) {
removeAt(i);
return true;
}
if (++i == items.length)
i = 0;
} while (i != putIndex);
}
return false;
} finally {
lock.unlock();
}
}
take()
public E take() throws InterruptedException {
final ReentrantLock lock = this.lock;
lock.lockInterruptibly();
try {
//阻塞
while (count == 0)
notEmpty.await();
return dequeue();
} finally {
lock.unlock();
}
}
posted on 2016-06-26 23:32 walkwalkwalk 阅读(423) 评论(0) 编辑 收藏 举报
