![image](https://user-images.githubusercontent.com/14085645/35564102-fa158f28-05f3-11e8-92b4-6e552c96476d.png)
这里，如果invokers为空的话，创建loadbalancer也没有用吧？是否可以去掉？
}
    public URL getUrl() {
        return directory.getUrl();
    }
    public boolean isAvailable() {
        Invoker<T> invoker = stickyInvoker;
        if (invoker != null) {
            return invoker.isAvailable();
        }
        return directory.isAvailable();
    }
    public void destroy() {
        if (destroyed.compareAndSet(false, true)) {
            directory.destroy();
        }
    }
    /**
     * Select a invoker using loadbalance policy.</br>
     * a)Firstly, select an invoker using loadbalance. If this invoker is in previously selected list, or, if this invoker is unavailable, then continue step b (reselect), otherwise return the first selected invoker</br>
     * b)Reslection, the validation rule for reselection: selected > available. This rule guarantees that the selected invoker has the minimum chance to be one in the previously selected list, and also guarantees this invoker is available.
     *
     * @param loadbalance load balance policy
     * @param invocation
     * @param invokers invoker candidates
     * @param selected  exclude selected invokers or not
     * @return
     * @throws RpcExceptione
     */
    protected Invoker<T> select(LoadBalance loadbalance, Invocation invocation, List<Invoker<T>> invokers, List<Invoker<T>> selected) throws RpcException {
        if (invokers == null || invokers.isEmpty())