protected function validateClassEntry($entry)
    {
        if (!is_string($entry) && !is_object($entry)) {
            $type = gettype($entry);
            $message = sprintf('Argument "entry" in "%s" must be a string or an object; "%s" was given.', __METHOD__, $type);
            throw new Exception\InvalidArgumentException($message);
        }

        if (is_string($entry) && !class_exists($entry)) {
            throw new Exception\ClassNotFoundException($entry);
        }
    }