protected function setMappingClass($className) {
        if ($this->configured)                                         throw new IllegalStateException('Configuration is frozen');
        if (!is_class($className))                                     throw new StrutsConfigException('Class '.$className.' not found');
        if (!is_subclass_of($className, DEFAULT_ACTION_MAPPING_CLASS)) throw new StrutsConfigException('Not a subclass of '.DEFAULT_ACTION_MAPPING_CLASS.': '.$className);

        $this->mappingClass = $className;
    }