BlockingQueue的作用以及实现的几个常用阻塞队列原理

1、BlockingQueue

BlockingQueue为阻塞队列，其与普通队列不同的是，以put和get为例，阻塞队列在put时，若列表满了，则会等待直到队列可以加入元素；而阻塞队列在get时候，若列表为空，则会等待到队列非空。可以用来解决 生产者消费者问题。

2、ArrayBlockingQueue

阻塞队列在add时，若列表满了，则抛出异常；

阻塞队列在offer时根据不同参数决定；

阻塞队列在put时，若列表满了，则会等待直到队列可以加入元素；

阻塞队列在get时候，若列表为空，则会等待到队列非空。

特点：通过一个lock进行实现，存取共用一个锁，并且是先进先出FIFO；

源码中该阻塞队列提供了3个构造函数：

    // 通过指定队列的大小
    public ArrayBlockingQueue(int capacity) {
        this(capacity, false);
    }

    // 队列大小 以及 是否创建公平锁 true为公平锁  false为非公平锁
    public ArrayBlockingQueue(int capacity, boolean fair) {
        if (capacity <= 0)
            throw new IllegalArgumentException();
        this.items = new Object[capacity];
        lock = new ReentrantLock(fair);
        notEmpty = lock.newCondition();
        notFull =  lock.newCondition();
    }

    // 将队列c元素都加入初始化的队列中
    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();
        }
    }

从上面可以了解到，ArrayBlockQueue是指定大小的，并且是通过ReentrantLock（实现公平锁和非公平锁）来进行锁定

ArrayBlockQueue的add（e）实际就是调用了offer（e）
// 根据代码就是获取到ReentranLock 通过加锁和解锁 来实现
    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();
        }
    }
// 加入到队列中，putIndex 在每次执行完后都回加一，即表示下次放入数组中的元素
    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++;
        // 标记表示该队列非空，等待中的消费者可以进行执行
        notEmpty.signal();
    }

// offer中有一个方法为在指定时间内，若队列满则等等
    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();
        try {
            while (count == items.length) {
                if (nanos <= 0)
                    return false;
                nanos = notFull.awaitNanos(nanos);
            }
            enqueue(e);
            return true;
        } finally {
            lock.unlock();
        }
    }

// put则为一直等待 知道队列非满
    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();
        }
    }

阻塞队列的take

// 从阻塞队列中获取元素
    public E take() throws InterruptedException {
        final ReentrantLock lock = this.lock;
        // 获取锁，若中断则获取 数据失败
        lock.lockInterruptibly();
        try {
            // 若没有元素则等待 则到notEmpty条件被清除notEmpty.signal();
            while (count == 0)
                notEmpty.await();
            return dequeue();
        } finally {
            lock.unlock();
        }
    }

// 从队列中取出元素takeIndex 与putIndex类似初始值为0，遍历完一次take后从0从新开始，遵循先进先出
    private E dequeue() {
        // assert lock.getHoldCount() == 1;
        // assert items[takeIndex] != null;
        final Object[] items = this.items;
        @SuppressWarnings("unchecked")
        E x = (E) items[takeIndex];
        items[takeIndex] = null;
        if (++takeIndex == items.length)
            takeIndex = 0;
        count--;
        if (itrs != null)
            itrs.elementDequeued();//同时更新迭代器中的元素数据
        // 置为非满队列
        notFull.signal();
        return x;
    }

3、LinkedBlockingQueue

private final ReentrantLock putLock = new ReentrantLock();写入元素的锁指定为非公平锁

/** Wait queue for waiting puts */
private final Condition notFull = putLock.newCondition();

private final ReentrantLock takeLock = new ReentrantLock(); 取出元素的锁，被指定为非公平锁。

/** Wait queue for waiting takes */
private final Condition notEmpty = takeLock.newCondition();

特点：通过存取锁分别用两个锁，都为非公平锁，（可以有效的防止共用一个锁时，一直被写锁lock而读锁一直得不到锁的情况）文中以读锁和写锁相称：

同样通过构造器查看：

    // 生成一个没有大小限制的链表
    public LinkedBlockingQueue() {
        this(Integer.MAX_VALUE);
    }

    // 指定大小的链表
    public LinkedBlockingQueue(int capacity) {
        if (capacity <= 0) throw new IllegalArgumentException();
        this.capacity = capacity;
        last = head = new Node<E>(null);
    }
    // 将其他队列中的元素加入该链表
    public LinkedBlockingQueue(Collection<? extends E> c) {
        this(Integer.MAX_VALUE);
        final ReentrantLock putLock = this.putLock;
        putLock.lock(); // Never contended, but necessary for visibility
        try {
            int n = 0;
            for (E e : c) {
                if (e == null)
                    throw new NullPointerException();
                if (n == capacity)
                    throw new IllegalStateException("Queue full");
                enqueue(new Node<E>(e));
                ++n;
            }
            count.set(n);
        } finally {
            putLock.unlock();
        }
    }

加入元素的方法：

    // 加入元素 
    public boolean offer(E e) {
        if (e == null) throw new NullPointerException();
        final AtomicInteger count = this.count;//判断是否为指定容器大小，使用原子包装，这样可以获取实施的count，防止被take了这里却还未减少数量
        if (count.get() == capacity) // 若等于容器则返回false
            return false;
        int c = -1;
        Node<E> node = new Node<E>(e);
        final ReentrantLock putLock = this.putLock;// 获取写锁并进行锁定
        putLock.lock();
        try {
            if (count.get() < capacity) {
                enqueue(node);
                c = count.getAndIncrement();
                if (c + 1 < capacity)
                    notFull.signal();
            }
        } finally {
            putLock.unlock();
        }
        if (c == 0)
            signalNotEmpty();
        return c >= 0;
    }

    // 加入队列只需要使其下一个节点设值
    private void enqueue(Node<E> node) {
        // assert putLock.isHeldByCurrentThread();
        // assert last.next == null;
        last = last.next = node;
    }

    // 获取读锁，并将非空条件去除
    private void signalNotEmpty() {
        final ReentrantLock takeLock = this.takeLock;
        takeLock.lock();
        try {
            notEmpty.signal();
        } finally {
            takeLock.unlock();
        }
    }
    

// put方法
    public void put(E e) throws InterruptedException {
        if (e == null) throw new NullPointerException();
        // Note: convention in all put/take/etc is to preset local var
        // holding count negative to indicate failure unless set.
        int c = -1;
        Node<E> node = new Node<E>(e);
        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.
             */
            while (count.get() == capacity) {
                notFull.await();//若满则释放写锁，并等待
            }
            enqueue(node);
            c = count.getAndIncrement();
            if (c + 1 < capacity)
                notFull.signal();
        } finally {
            putLock.unlock();
        }
        if (c == 0)
            signalNotEmpty();
    }

取出元素：

// 取出元素 锁定读锁，
    public E take() throws InterruptedException {
        E x;
        int c = -1;
        final AtomicInteger count = this.count;
        final ReentrantLock takeLock = this.takeLock;
        takeLock.lockInterruptibly();
        try {
            while (count.get() == 0) {
                notEmpty.await(); //若为空 则释放读锁并等待
            }
            x = dequeue();
            c = count.getAndDecrement();
            if (c > 1)
                notEmpty.signal();
        } finally {
            takeLock.unlock();
        }
        if (c == capacity)
            signalNotFull();
        return x;
    }
// 标记为队列非满状态
    private void signalNotFull() {
        final ReentrantLock putLock = this.putLock;
        putLock.lock();
        try {
            notFull.signal();
        } finally {
            putLock.unlock();
        }
    }

4、待续。。。