本文基于jdk1.8进行分析

ArrayBlockingQueue的功能简介参考。

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。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

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