Java LinkedBlockingQueue

描述

继承了AbstractQueue类，实现了BlockingQueue和Serializable接口

构造

/**
 * Creates a {@code LinkedBlockingQueue} with a capacity of
 * {@link Integer#MAX_VALUE}.
 */
// 如果没传capacity 则默认使用Integer.MAX_VALUE作为队列大小
public LinkedBlockingQueue() {
    this(Integer.MAX_VALUE);
}

/**
 * Creates a {@code LinkedBlockingQueue} with the given (fixed) capacity.
 *
 * @param capacity the capacity of this queue
 * @throws IllegalArgumentException if {@code capacity} is not greater
 *         than zero
 */
 //设置大小为capacity，并设置item为null的head和last辅助节点
public LinkedBlockingQueue(int capacity) {
    if (capacity <= 0) throw new IllegalArgumentException();
    this.capacity = capacity;
    last = head = new Node<E>(null);
}

/**
 * Creates a {@code LinkedBlockingQueue} with a capacity of
 * {@link Integer#MAX_VALUE}, initially containing the elements of the
 * given collection,
 * added in traversal order of the collection's iterator.
 *
 * @param c the collection of elements to initially contain
 * @throws NullPointerException if the specified collection or any
 *         of its elements are null
 */
public LinkedBlockingQueue(Collection<? extends E> c) {
    this(Integer.MAX_VALUE);
    //加入队锁
    final ReentrantLock putLock = this.putLock;
    putLock.lock(); // Never contended, but necessary for visibility
    try {
        int n = 0;
        //依次将Collection中的元素加入队列
        for (E e : c) {
            if (e == null)
                throw new NullPointerException();
            if (n == capacity)
                throw new IllegalStateException("Queue full");
            enqueue(new Node<E>(e));
            ++n;
        }
        //设置队列大小n
        count.set(n);
    } finally {
        //解锁
        putLock.unlock();
    }
}

从构造函数看出，LinkedBlockingQueue是一个有界的队列，队列最大值为capacity，如果初始化时不设置队列大小，则默认大小为Integer.MAX_VALUE

Api

put
将元素加入队列，如果队列满，则一直等待，直到线程被中断或被唤醒

/**
 * Inserts the specified element at the tail of this queue, waiting if
 * necessary for space to become available.
 *
 * @throws InterruptedException {@inheritDoc}
 * @throws NullPointerException {@inheritDoc}
 */
public void put(E e) throws InterruptedException {
    if (e == null) throw new NullPointerException();
    final int c;
    final Node<E> node = new Node<E>(e);
    
    //入队锁，如果收到中断信号，则抛出异常
    final ReentrantLock putLock = this.putLock;
    final AtomicInteger count = this.count;
    putLock.lockInterruptibly();
    try {
        /*
         * Note that count is used in wait guard even though it is
         * not protected by lock. This works because count can
         * only decrease at this point (all other puts are shut
         * out by lock), and we (or some other waiting put) are
         * signalled if it ever changes from capacity. Similarly
         * for all other uses of count in other wait guards.
         */
         //如果队列满了，则通知线程进入await状态。
        while (count.get() == capacity) {
            notFull.await();
        }
        //将node加入队列
        enqueue(node);
        //队列元素数加一
        c = count.getAndIncrement();
        //如果队列没满，则唤醒await的线程进行入队操作
        if (c + 1 < capacity)
            notFull.signal();
    } finally {
        //释放锁
        putLock.unlock();
    }
    //如果是第一次添加元素，则通知等待的读线程可以开始读数据了
    if (c == 0)
        signalNotEmpty();
}

offer

/**
 * 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.
 * When using a capacity-restricted queue, this method is generally
 * preferable to method {@link BlockingQueue#add add}, which can fail to
 * insert an element only by throwing an exception.
 *
 * @throws NullPointerException if the specified element is null
 */
 //将元素加入队列，如果队列满，则直接返回false
public boolean offer(E e) {
    if (e == null) throw new NullPointerException();
    final AtomicInteger count = this.count;
    //如果队列满了，则直接返回false。
    if (count.get() == capacity)
        return false;
    final int c;
    final Node<E> node = new Node<E>(e);
    //加入队锁
    final ReentrantLock putLock = this.putLock;
    putLock.lock();
    try {
        //再次判断， 队列是否满了，避免在第一次判断后和加锁前，队列被加满
        if (count.get() == capacity)
            return false;
        //将node添加到队列中
        enqueue(node);
        c = count.getAndIncrement();
        //如果队列没满，则唤醒await的线程进行入队操作
        if (c + 1 < capacity)
            notFull.signal();
    } finally {
        //释放锁
        putLock.unlock();
    }
    //如果是第一次添加元素，则通知等待的读线程可以开始读数据了
    if (c == 0)
        signalNotEmpty();
    return true;
}

/**
 * Inserts the specified element at the tail of this queue, waiting if
 * necessary up to the specified wait time for space to become available.
 *
 * @return {@code true} if successful, or {@code false} if
 *         the specified waiting time elapses before space is available
 * @throws InterruptedException {@inheritDoc}
 * @throws NullPointerException {@inheritDoc}
 */
 //将元素加入队列，可以设置等待超时时间，如果队列满，则等待timeout毫秒，超时返回false
public boolean offer(E e, long timeout, TimeUnit unit)
    throws InterruptedException {

    if (e == null) throw new NullPointerException();
    long nanos = unit.toNanos(timeout);
    final int c;
    //加锁
    final ReentrantLock putLock = this.putLock;
    final AtomicInteger count = this.count;
    putLock.lockInterruptibly();
    try {
        //如果队列满了，则等待timeout毫秒，超时则返回false
        while (count.get() == capacity) {
            if (nanos <= 0L)
                return false;
            nanos = notFull.awaitNanos(nanos);
        }
        //入队
        enqueue(new Node<E>(e));
        c = count.getAndIncrement();
        //如果队列没满，则唤醒await的线程进行入队操作
        if (c + 1 < capacity)
            notFull.signal();
    } finally {
        //释放锁
        putLock.unlock();
    }
    //如果是第一次添加元素，则通知等待的读线程可以开始读数据了
    if (c == 0)
        signalNotEmpty();
    return true;
}

take
从队列中取出元素，如果队列为空，则一直等待，直到线程被中断或被唤醒

public E take() throws InterruptedException {
    final E x;
    final int c;
    final AtomicInteger count = this.count;
    //加出队锁
    final ReentrantLock takeLock = this.takeLock;
    takeLock.lockInterruptibly();
    try {
        //如果队列为空，则通知线程进入await状态。
        while (count.get() == 0) {
            notEmpty.await();
        }
        //从队列头部取出元素
        x = dequeue();
        //count减一
        c = count.getAndDecrement();
        //如果队列不为空，则唤醒出队等待线程
        if (c > 1)
            notEmpty.signal();
    } finally {
        //释放锁
        takeLock.unlock();
    }
    //如果从满的队列中出列，则唤醒入队线程，队列已经不满了，可以添加元素了
    if (c == capacity)
        signalNotFull();
    return x;
}

poll

public E poll() {
    final AtomicInteger count = this.count;
    //如果队列为空，直接返回null
    if (count.get() == 0)
        return null;
    final E x;
    final int c;
    //加出队锁
    final ReentrantLock takeLock = this.takeLock;
    takeLock.lock();
    try {
        //如果队列为空，直接返回null
        if (count.get() == 0)
            return null;
        //出队，移除第一个数据节点
        x = dequeue();
        c = count.getAndDecrement();
        //如果队列不为空，则唤醒出队等待线程
        if (c > 1)
            notEmpty.signal();
    } finally {
        takeLock.unlock();
    }
    //如果从满的队列中出列，则唤醒入队线程，队列已经不满了，可以添加元素了
    if (c == capacity)
        signalNotFull();
    return x;
}
public E poll(long timeout, TimeUnit unit) throws InterruptedException {
        final E x;
        final int c;
        long nanos = unit.toNanos(timeout);
        final AtomicInteger count = this.count;
        final ReentrantLock takeLock = this.takeLock;
        takeLock.lockInterruptibly();
        try {
            //如果队列为空，则等待timeout时间， 超时返回null
            while (count.get() == 0) {
                if (nanos <= 0L)
                    return null;
                nanos = notEmpty.awaitNanos(nanos);
            }
            //出队列
            x = dequeue();
            c = count.getAndDecrement();
            //如果队列不为空，则唤醒出队等待线程
            if (c > 1)
                notEmpty.signal();
        } finally {
            //释放锁
            takeLock.unlock();
        }
        //如果从满的队列中出列，则唤醒入队线程，队列已经不满了，可以添加元素了
        if (c == capacity)
            signalNotFull();
        return x;
    }

peek

public E peek() {
    final AtomicInteger count = this.count;
    //如果队列为空，返回null
    if (count.get() == 0)
        return null;
    final ReentrantLock takeLock = this.takeLock;
    takeLock.lock();
    try {
        //队列不为空返回第一个数据节点的元素，不移除节点，为空则返回null
        return (count.get() > 0) ? head.next.item : null;
    } finally {
        takeLock.unlock();
    }
}

remove

/**
 * Removes a single instance of the specified element from this queue,
 * if it is present.  More formally, removes an element {@code e} such
 * that {@code o.equals(e)}, if this queue contains one or more such
 * elements.
 * Returns {@code true} if this queue contained the specified element
 * (or equivalently, if this queue changed as a result of the call).
 *
 * @param o element to be removed from this queue, if present
 * @return {@code true} if this queue changed as a result of the call
 */
public boolean remove(Object o) {
    //如果移除的元素为null，在返回false
    if (o == null) return false;
    //加入队和出队锁
    fullyLock();
    try {
        //遍历队列，存在元素o则移除，返回true，否则返回false
        for (Node<E> pred = head, p = pred.next;
             p != null;
             pred = p, p = p.next) {
            if (o.equals(p.item)) {
                unlink(p, pred);
                return true;
            }
        }
        return false;
    } finally {
        //释放入队锁和出队锁
        fullyUnlock();
    }
}

总结

LinkedBlockingQueue是一个有界的阻塞队列，初始化时，需要设置队列大小， 在队列满时，入队操作会等待，队列为空时，出队操作会等待。
和ConcurrentLinkedQueue对比，LinkedBlockingQueue采用锁分离，比较适合生产和消费频率差不多的场景，并且锁同步更适合单消费者的任务队列，而ConcurrentLinkedQueue使用CAS，并发性能较高更适合消费者多的消息队列。