protected function getClassParameter()
    {
        if (! $this->hasRequestParameter(self::CLASS_PARAMETER)) {
            throw new \common_exception_MissingParameter(self::CLASS_PARAMETER, __CLASS__);
        }

        $uri = $this->getRequestParameter(self::CLASS_PARAMETER);
        if (empty($uri) || !common_Utils::isUri($uri)) {
            throw new \common_exception_MissingParameter(self::CLASS_PARAMETER, __CLASS__);
        }

        return $this->getClass($uri);
    }