ArrayBlockingQueue代码阅读

public class ArrayBlockingQueue extends AbstractQueue
        implements BlockingQueue, java.io.Serializable {

//用于存储元素的数组
final Object[] items;

// 下一个取出元素的坐标
int takeIndex;

// 可以添加元素的坐标
int putIndex;

// 队列中元素的数量
int count;

//队列插入和删除元素锁
final ReentrantLock lock;

//生产者信号量，如果取的时候为空，等待
private final Condition notEmpty;
    
//消费者信号量，如果放入元素队列满，等待
private final Condition notFull;

public ArrayBlockingQueue(int capacity) {
    this(capacity, false); //默认非公平锁
}

public ArrayBlockingQueue(int capacity, boolean fair) {
    if (capacity <= 0)
        throw new IllegalArgumentException();
    this.items = new Object[capacity];
    lock = new ReentrantLock(fair); //公平锁或者非公平锁
    notEmpty = lock.newCondition();
    notFull =  lock.newCondition();
}
    
    //循环加1 
    final int inc(int i) {
        return (++i == items.length) ? 0 : i;
    }

    //循环减1
    final int inc(int i) {
        return (++i == items.length) ? 0 : i;
    }

    //插入元素
    private void insert(E x) {
        items[putIndex] = x; 
        putIndex = inc(putIndex); //索引位置加1
        ++count; //总数量加1
        notEmpty.signal(); //唤醒，等待数据出队的线程
    }

    //元素出队
    private E extract() {
        final Object[] items = this.items;
        E x = this.cast(items[takeIndex]);
        items[takeIndex] = null;  
        takeIndex = inc(takeIndex); //取索引加1
        --count; //总数量减1
        notFull.signal(); //唤醒，等待数据入队的线程
        return x;
    }


//入队，非阻塞，失败抛异常
public boolean add(E e) {
    return super.add(e); //最后调用还是offer(E e),如果失败抛异常
}

//入队，非阻塞，失败不抛异常
public boolean offer(E e) {
    checkNotNull(e); //元素不能为空
    final ReentrantLock lock = this.lock;
    lock.lock(); //加锁
    try {
        if (count == items.length) //队列已满，返回失败
            return false;
        else {
            insert(e);  //插入元素
            return true;
        }
    } finally {
        lock.unlock();
    }
}

//入队，阻塞并响应中断
public void put(E e) throws InterruptedException {
    checkNotNull(e); //元素不能为空
    final ReentrantLock lock = this.lock;
    lock.lockInterruptibly(); //加锁，响应中断
    try {
        while (count == items.length) //如果队列已满
            notFull.await(); //阻塞等待
        insert(e); //队列有多余空间，插入元素
    } finally {
        lock.unlock();
    }
}

//入队，阻塞并响应中断
public boolean offer(E e, long timeout, TimeUnit unit)
        throws InterruptedException {
        checkNotNull(e);
        long nanos = unit.toNanos(timeout);
        final ReentrantLock lock = this.lock;
        lock.lockInterruptibly(); //加锁，响应中断
        try {
            while (count == items.length) { //如果队列已满
                if (nanos <= 0)  //直接返回失败
                    return false;
                nanos = notFull.awaitNanos(nanos);  //阻塞等待一段时间
            }
            insert(e); //插入元素
            return true;
        } finally {
            lock.unlock();
        }
}

//出队,非阻塞
public E poll() {
    final ReentrantLock lock = this.lock;
    lock.lock(); //加锁
    try {
        return (count == 0) ? null : extract(); //队列为空返回，否则出队
    } finally {
        lock.unlock();
    }
}

//出对，阻塞并响应中断
public E take() throws InterruptedException {
    final ReentrantLock lock = this.lock;
    lock.lockInterruptibly(); //加锁,响应中断
    try {
        while (count == 0)  //队列为空
            notEmpty.await();  //出队等待
        return extract(); //出队
    } finally {
        lock.unlock();
    }
}

//出队，阻塞一定时间
public E poll(long timeout, TimeUnit unit) throws InterruptedException {
    long nanos = unit.toNanos(timeout);
    final ReentrantLock lock = this.lock;
    lock.lockInterruptibly(); //加锁,响应中断
    try {
        while (count == 0) { //队列为空
            if (nanos <= 0)  //直接返回null
                return null;
            nanos = notEmpty.awaitNanos(nanos); //阻塞等待一段时间
        }
        return extract(); //出队
    } finally {
        lock.unlock();
    }
}

//返回队头元素，不从队列中删除
public E peek() {
    final ReentrantLock lock = this.lock;
    lock.lock(); //加锁
    try {
        return (count == 0) ? null : itemAt(takeIndex); //队列为空返回null,否则直接下1个元素
    } finally {
        lock.unlock();
    }
}

//删除指定索引位置的元素
void removeAt(int i) {
    final Object[] items = this.items;    
    if (i == takeIndex) { //删除索引和takeIndex相同，直接从当前位置删除
        items[takeIndex] = null;
        takeIndex = inc(takeIndex);
    } else {  //否则遍历      
        for (;;) { // 否则将删除位置之后的元素都向前移一位
            int nexti = inc(i);
            if (nexti != putIndex) {
                items[i] = items[nexti];
                i = nexti;
            } else {
                items[i] = null;
                putIndex = i;
                break;
            }
        }
    }
    --count; //总数量减1
    notFull.signal(); //唤醒因为队列满，插入数据阻塞的线程
}

//删除某个指定的元素
public boolean remove(Object o) {
    if (o == null) return false;
    final Object[] items = this.items;
    final ReentrantLock lock = this.lock;
    lock.lock();
    try {
        for (int i = takeIndex, k = count; k > 0; i = inc(i), k--) {
            if (o.equals(items[i])) { //找到指定元素
                removeAt(i); //删除指定位置元素
                return true;
            }
        }
        return false;
    } finally {
        lock.unlock();
    }
}

//获取剩余容量
public int remainingCapacity() {
    final ReentrantLock lock = this.lock;
    lock.lock();
    try {
        return items.length - count; //返回剩余容量
    } finally {
        lock.unlock();
    }
}

}