public function runApplication($appPath = '')
    {
        if ($appPath != '') {
            $rootPath = $appPath;
        } else {
            $rootPath = realpath(dirname(debug_backtrace()[0]['file']) . '/../');
        }

        // save the root path
        $this->absolutePath = realpath($rootPath) . DIRECTORY_SEPARATOR;
        $this->initializeEnvironment($this->absolutePath);
        $this->initializeRouter();
    }