读源码之:ArrayBlockingQueue

    ArrayBlockingQueue是concurrent包提供的一个线程安全的队列,由一个数组来保存队列元素.通过 takeIndex和 putIndex来分别记录出队列和入队列的下标,以保证在出队列时 不进行元素移动.

//在出队列或者入队列的时候对takeIndex或者putIndex进行累加,如果已经到了数组末尾就又从0开始,保证数组的循环使用.
final int inc(int i) {
        return (++i == items.length) ? 0 : i;
    }

//入队列操作
private void insert(E x) {
        items[putIndex] = x;
        putIndex = inc(putIndex);
        ++count;
        notEmpty.signal();//唤醒等待的出队线程
    }

//使用offer入队列,如果队列已满就立即返回false
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();
        }
    }

//使用put入队列的话,如果队列已满当前线程就等待然后释放锁,直到被notFull唤醒,再重新检查,直到成功插入队列
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();
        }
    }

//出队列操作
private E extract() {
        final Object[] items = this.items;
        E x = this.<E>cast(items[takeIndex]);
        items[takeIndex] = null;
        takeIndex = inc(takeIndex);
        --count;
        notFull.signal();//唤醒等待入队的线程
        return x;
    }

//poll出队是不需要等待的,如果当前队列是空就直接返回null
public E poll() {
        final ReentrantLock lock = this.lock;
        lock.lock();
        try {
            return (count == 0) ? null : extract();
        } finally {
            lock.unlock();
        }
    }

//take就跟put一样,如果队列是空的就等待直到被notEmpty唤醒
public E take() throws InterruptedException {
        final ReentrantLock lock = this.lock;
        lock.lockInterruptibly();
        try {
            while (count == 0)
                notEmpty.await();
            return extract();
        } finally {
            lock.unlock();
        }
    }

最后需要注意的就是带 超时唤醒的是 offer和 poll而不是put和take