时间：2020-12-16 10:10:37 | 栏目：JAVA代码 | 点击：次
本文基于jdk1.8进行分析
ArrayBlockingQueue的功能简介参考https://www.jb51.net/article/154211.htm。
首先看一下ArrayBlockingQueue的成员变量。如下图。最主要的成员变量是items，它是一个Object类型的数组用于保存阻塞队列中的元素。其次是takeIndex，putIndex，count，分别表示了从队列获取元素的位置，往队列里放元素的位置和队列中元素的个数。然后是lock，notEmpty和notFull三个和锁相关的成员变量。lock是一个可重入锁，而notEmpty和notFull是和lock绑定的2个Condition。对可重入锁不是很了解的同学，可以参考这篇文章https://www.jb51.net/article/154207.htm。对可重入锁的理解，是理解ArrayBlockingQueue的基础。也可以这么说，理解了可重入锁，那么在理解ArrayBlockingQueue就很顺利了。
/** 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; /** * 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;
接下来介绍ArrayBlockingQueue的主要方法。首先是入队方法。ArrayBlockingQueue的入队方法有好几个，功能略有差异，下面我们逐一介绍各个入队方法。首先看一下put方法，如下图。put方法的功能是，往队列尾部插入指定元素，如果队列已满，那么就等待可用空间。方法的实现过程是，首先判断元素是否非空。然后，进行加锁，加锁后判断队列是否已满。如果已满，则等待不满条件。当被唤醒后，进行入队操作。入队方法中，会唤醒在notEmpty条件上等待的线程。
/**
* 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();
enqueue(e);
} finally {
lock.unlock();
}
}
/**
* Inserts element at current put position, advances, and signals.
* Call only when holding lock.
**/
private void enqueue(E x) {
// assert lock.getHoldCount() == 1;
// assert items[putIndex] == null;
final Object[] items = this.items;
items[putIndex] = x;
if (++putIndex == items.length)
putIndex = 0;
count++;
notEmpty.signal();
}
另一个入队方法是offer，代码如下。这个方法与add方法的区别是，offer方法是立刻返回的，它并不像add方法那样，当队列满时会一直等待。
/**
* 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 {
enqueue(e);
return true;
}
} finally {
lock.unlock();
}
}
接下来看一下出队方法take，代码如下。首先对可重入锁加锁，然后判断元素个数是否为0.如果为0，则等待不空条件，否则进行出队操作。
public E take() throws InterruptedException {
final ReentrantLock lock = this.lock;
lock.lockInterruptibly();
try {
while (count == 0)
notEmpty.await();
return dequeue();
} finally {
lock.unlock();
}
}
ArrayListBlockingqueue中还有其他相关方法，这里就不一一介绍了。
总结