protected 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;
}