Java并发编程之ConcurrentLinkedQueue队列详情

public ConcurrentLinkedQueue() {
    head = tail = new Node<E>(null);
}

public boolean offer(E e) {
//（1）e为null这抛出空指针异常
    checkNotNull(e);
    //(2)构造Node节点，在构造函数内部调用unsafe.putObject
    final Node<E> newNode = new Node<E>(e);
    //(3) 从尾节点插入
    for (Node<E> t = tail, p = t;;) {
        Node<E> q = p.next;
 //(4) 如果q==null说明p是尾节点，则执行插入
        if (q == null) {
            // p is last node
            //（5）使用CAS设置p节点的next节点
            if (p.casNext(null, newNode)) {
                // Successful CAS is the linearization point
                // for e to become an element of this queue,
                // and for newNode to become "live".
                   //（6）CAS成功，则说明新增节点已经放入链表，然后设置当前尾巴节点

                if (p != t) // hop two nodes at a time
                    casTail(t, newNode);  // Failure is OK.
                return true;
            }
            // Lost CAS race to another thread; re-read next
        }
        else if (p == q)
            // We have fallen off list.  If tail is unchanged, it
            // will also be off-list, in which case we need to
            // jump to head, from which all live nodes are always
            // reachable.  Else the new tail is a better bet.
            p = (t != (t = tail)) ? t : head;
        else
            // Check for tail updates after two hops.
            p = (p != t && t != (t = tail)) ? t : q;
    }
}

public boolean add(E e) {
    return offer(e);
}

public E poll() {
    restartFromHead:
    for (;;) {
        for (Node<E> h = head, p = h, q;;) {
            E item = p.item;

            if (item != null && p.casItem(item, null)) {
                // Successful CAS is the linearization point
                // for item to be removed from this queue.
                if (p != h) // hop two nodes at a time
                    updateHead(h, ((q = p.next) != null) ? q : p);
                return item;
            }
            else if ((q = p.next) == null) {
                updateHead(h, p);
                return null;
            }
            else if (p == q)
                continue restartFromHead;
            else
                p = q;
        }
    }
}

public E peek() {
    restartFromHead:
    for (;;) {
        for (Node<E> h = head, p = h, q;;) {
            E item = p.item;
            
            //注释
            if (item != null || (q = p.next) == null) {
                updateHead(h, p);
                return item;
            }
            else if (p == q)
                continue restartFromHead;
            else
                p = q;
        }
    }
}