LinkedBlockingQueue源码讲解

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源码：\sources\Android-25

说到LinkedBlockingQueue，不得不提到LinkedBlockingDeque，他俩差不多，只不过LinkedBlockingDeque是双向的队列，而LinkedBlockingQueue是单向的队列，我们看一下他的节点就知道了

    /**     * Linked list node class.     */    static class Node<E> {        E item;        /**         * One of:         * - the real successor Node         * - this Node, meaning the successor is head.next         * - null, meaning there is no successor (this is the last node)         */        Node<E> next;        Node(E x) { item = x; }    }

Node只有后一个没有前一个，并且他是有容量限制的，容量大小是capacity，容量满了如果再加入就会阻塞。来看第一个方法dequeue

    /**     * Removes a node from head of queue.     *     * @return the node     */ //移除一个元素    private E dequeue() {        // assert takeLock.isHeldByCurrentThread();        // assert head.item == null;        Node<E> h = head;        Node<E> first = h.next;//让自己的下一个指向自己，相当于与链表断开了。        h.next = h; // help GC//然后让head的先一个成为head元素。        head = first;//head元素的item是空的。        E x = first.item;        first.item = null;        return x;    }

再来看其中的一个构造方法LinkedBlockingQueue(Collection<? extends E> c)，

    /**     * 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);        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();//如果满了就会报异常                if (n == capacity)                    throw new IllegalStateException("Queue full");//添加                enqueue(new Node<E>(e));                ++n;            }            count.set(n);        } finally {            putLock.unlock();        }    }

put(E e)方法调用的也是enqueue方法，如果满了就会等待。

            while (count.get() == capacity) {                notFull.await();            }            enqueue(node);

然后再看下一个方法peek，表示的是返回head节点的下一个节点值，因为head节点是没有值的，

    public E peek() {        if (count.get() == 0)            return null;        final ReentrantLock takeLock = this.takeLock;        takeLock.lock();        try {            return (count.get() > 0) ? head.next.item : null;        } finally {            takeLock.unlock();        }    }

接着看unlink(Node<E> p, Node<E> trail)方法，表示断开p到trail之间的节点，

    /**     * Unlinks interior Node p with predecessor trail.     */    void unlink(Node<E> p, Node<E> trail) {        // assert isFullyLocked();        // p.next is not changed, to allow iterators that are        // traversing p to maintain their weak-consistency guarantee.        p.item = null;        trail.next = p.next;        if (last == p)            last = trail;        if (count.getAndDecrement() == capacity)            notFull.signal();    }

剩下的基本上也没什么可说的。

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