JUC-LinkedBlockingDeque

和LinkedBlockQueue类似，LinkedBlockingDeque也是一个基于链表实现的队列，不过是双端队列，队列双端都可以做插入和移除操作，而且实现的是BlockingDeque接口而不是BlockingQeque接口，当然BlockingDeque还是继承自BlockingQeque。

1. 主要成员

transient Node<E> first;// 头节点
    transient Node<E> last;// 尾节点
    private transient int count;// 双端队列数量
    private final int capacity;// 双端队列最大容量
    final ReentrantLock lock = new ReentrantLock();// 一把锁
    private final Condition notEmpty = lock.newCondition();// 两个队列
    private final Condition notFull = lock.newCondition();

节点类：

static final class Node<E> {
        E item;// 节点值
        Node<E> prev;// 前驱节点
        Node<E> next;// 后继节点
        Node(E x) {
            item = x;
        }
    }

2. 基本操作

2.1构造方法

public LinkedBlockingDeque() {
        this(Integer.MAX_VALUE);// 默认容量是整型最大值，链表，无界队列
    }
        
    public LinkedBlockingDeque(int capacity) {
        if (capacity <= 0) throw new IllegalArgumentException();
        this.capacity = capacity;
    }

2.2 入队

​​offer()方法​​

public boolean offer(E e) {
        return offerLast(e);// 默认是队尾入队，直接返回入队结果（true/false）
    }

    public boolean offerLast(E e) {
        // offer(e):入队元素e不能是null
        if (e == null) throw new NullPointerException();
        Node<E> node = new Node<E>(e);// 构造节点
        final ReentrantLock lock = this.lock;
        lock.lock();
        try {
            // 尾部入队
            return linkLast(node);
        } finally {
            lock.unlock();
        }
    }

        // 尾部入队，直接返回入队结果（true/false）
    private boolean linkLast(Node<E> node) {
        if (count >= capacity)
            return false;
        Node<E> l = last;
        node.prev = l;
        last = node;
        if (first == null)
            first = node;
        else
            l.next = node;
        ++count;
        notEmpty.signal();
        return true;
    }

​​put()方法​​

public void put(E e) throws InterruptedException {
        putLast(e);
    }

    public void putLast(E e) throws InterruptedException {
        // 入队元素同样不能为null
        if (e == null) throw new NullPointerException();
        Node<E> node = new Node<E>(e);
        final ReentrantLock lock = this.lock;
        lock.lock();
        try {
            // 和put不同的地方在于，只要添加失败就会一直阻塞
            while (!linkLast(node))
                notFull.await();
        } finally {
            lock.unlock();
        }
    }

​​add()​​

public boolean add(E e) {
        addLast(e);// 入队失败，直接抛出异常
        return true;
    }

    public void addLast(E e) {
        // 入队失败，直接抛出异常
        if (!offerLast(e))
            throw new IllegalStateException("Deque full");
    }

2.3 出队

​​poll()​​

public E poll() {
    return pollFirst();// 头部出队
}

    public E pollFirst() {
        final ReentrantLock lock = this.lock;
        lock.lock();
        try {
            // 实际出队方法：unlinkFirst
            return unlinkFirst();
        } finally {
            lock.unlock();
        }
    }

    private E unlinkFirst() {
        // assert lock.isHeldByCurrentThread();
        Node<E> f = first;
        if (f == null)
            return null;
        Node<E> n = f.next;
        E item = f.item;
        f.item = null;
        f.next = f; // help GC
        first = n;
        if (n == null)
            last = null;
        else
            n.prev = null;
        --count;
        notFull.signal();
        return item;
    }

​​remove()​​

public E remove() {
        return removeFirst();
    }

    public E removeFirst() {
        E x = pollFirst();
        if (x == null) throw new NoSuchElementException();
        return x;
    }

    public E pollFirst() {
        final ReentrantLock lock = this.lock;
        lock.lock();
        try {
            return unlinkFirst();
        } finally {
            lock.unlock();
        }
    }