code: public void shutdown() {
      if (mbeanName != null) {
        MBeanUtil.unregisterMBean(mbeanName);
      }
      if (versionBeanName != null) {
        MBeanUtil.unregisterMBean(versionBeanName);
      }
      if (namenodeMXBeanName != null) {
        MBeanUtil.unregisterMBean(namenodeMXBeanName);
      }
    }
docs: shutdown FSNamesystem