protected function loadJsonConfigFile(string $fileKey, string $filePath)
    {
        $json   = file_get_contents($filePath);
        $config = json_decode($json);

        if ($config === null) {
            throw new Exception(
                json_last_error_msg(),
                $this::ERR_JSON_PARSE
            );
        }

        $this->config[$fileKey] = $config;
    }