Mark, I see this feature in the new Tomcat 8.5 release.
- Add the ability for a UserDatabase to monitor the backing XML file for changes and reload the source file if a change in the last modified time is detected. This is enabled by default meaning that changes to $CATALINA_BASE/conf/tomcat-users.xml will now take effect a short time after the file is saved. I did some investigation and want to confirm this feature has been in 9.0, so that 9.0.20 already operates that way. Thanks Dave