并发队列之ArrayBlockingQueue

//数组用于存放队列元素
final Object[] items;
//出队索引
int takeIndex;
//入队索引
int putIndex;
//队列中元素数量
int count;
//独占锁
final ReentrantLock lock;
//如果数组中为空，还有线程取数据，就丢到这个条件变量中来阻塞
private final Condition notEmpty;
//队列满了，还有线程往数组中添加数据，就把线程丢到这里来阻塞
private final Condition notFull;

//指定容量，默认是非公平策略
public ArrayBlockingQueue(int capacity) {
    this(capacity, false);
}
//指定容量和独占锁的策略
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();
}
//可以指定容量，锁的策略，还有初始化数据
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;
        putIndex = (i == capacity) ? 0 : i;
    } finally {
        lock.unlock();
    }
}

public boolean offer(E e) {
    //非空检验
    checkNotNull(e);
    final ReentrantLock lock = this.lock;
    lock.lock();
    try {
        //如果数组中实际数量和最大容量相等，添加失败，返回false
        if (count == items.length)
            return false;
        else {
            //添加成功，方法实现在下面
            enqueue(e);
            return true;
        }
    } finally {
        //释放锁
        lock.unlock();
    }
}
private void enqueue(E x) {
   //拿到数组
    final Object[] items = this.items;
    //在putIndex这个位置放入数据x，然后把putIndex加一，说明这个参数表示的是下一个数据要放入的位置的索引
    items[putIndex] = x;
    //这里putIndex是先加一然后再比较是否相等，比如这里数组的最大容量是5，那么索引的最大值应该是4，而如果putIndex等于5了，说明数组
    //越界了，加把这个索引重置为0
    if (++putIndex == items.length)
        putIndex = 0;
    count++;
    //添加完成之后，说明了数组中有数据了，这里会唤醒之前因为去数组中取数据而阻塞的线程
    notEmpty.signal();
}

public void put(E e) throws InterruptedException {
    //非空检查
    checkNotNull(e);
    final ReentrantLock lock = this.lock;
    //注意该锁的获取方式
    lock.lockInterruptibly();
    try {
        //如果线程满了，就把当前线程放到notFull条件变量的阻塞队列中
        while (count == items.length)
            notFull.await();
        //没有满，就添加数据
        enqueue(e);
    } finally {
        //释放锁
        lock.unlock();
    }
}

public E poll() {
    final ReentrantLock lock = this.lock;
    lock.lock();
    try {
        //如果队列为空，就返回null
        //如果队列不为空，就调用dequeue方法获取并删除队列头部的元素
        return (count == 0) ? null : dequeue();
    } finally {
        lock.unlock();
    }
}
private E dequeue() {
    //获取数组
    final Object[] items = this.items;
    @SuppressWarnings("unchecked")
    //获取takeIndex位置的元素，最后会将这个返回
    E x = (E) items[takeIndex];
    //然后将takeInde位置置为空
    items[takeIndex] = null;
    //如果takeIndex已经是数组的最后一个位置了，就将takeIndex重置为0
    if (++takeIndex == items.length)
        takeIndex = 0;
    //实际数量减一
    count--;
    if (itrs != null)
        itrs.elementDequeued();
    //唤醒notFull中线程
    notFull.signal();
    return x;
}

 public E take() throws InterruptedException {
        final ReentrantLock lock = this.lock;
        //可中断的方式获取锁
        lock.lockInterruptibly();
        try {
            //如果数组为空，此时就唤醒notEmpty中条件队列里的线程
            while (count == 0)
                notEmpty.await();
            //获取并删除头节点
            return dequeue();
        } finally {
            lock.unlock();
        }
    }

public E peek() {
    final ReentrantLock lock = this.lock;
    lock.lock();
    try {
        return itemAt(takeIndex); // null when queue is empty
    } finally {
        lock.unlock();
    }
}

//获取到数组中索引为takeIndex中的数据
@SuppressWarnings("unchecked")
final E itemAt(int i) {
    return (E) items[i];
}