protected static function getPluginArchivers()
    {
        if (empty(static::$archivers)) {
            $pluginNames = \Piwik\Plugin\Manager::getInstance()->getActivatedPlugins();
            $archivers = array();
            foreach ($pluginNames as $pluginName) {
                $archivers[$pluginName] = self::getPluginArchiverClass($pluginName);
            }
            static::$archivers = array_filter($archivers);
        }
        return static::$archivers;
    }