本文基于jdk1.8进行分析

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

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