The web deployment descriptor is loaded and parsed when the web container is first started. By default, changes to the descriptor are not detected and reloaded while the container is running. In fact, not many containers even provide this functionality it's typically not a wanted feature, due to the possible security issues involved. The Struts configuration files also are loaded and parsed when the container is launched, and changes to these configuration files also are not automatically detected; the same security issues are present.
Some applications might require the ability to reload the Struts configuration files without restarting the web container. If your application is one of them, there are ways you can do this. One approach is to create a Struts action that will reinitialize the ActionServlet (you may want to restrict which users can call this action). Once the ActionServlet is reinitialized, everything will be new and the application can again service requests, just like before.
A second strategy is to create a thread that monitors the lastModifiedTime of the configuration file. It can sleep for a few seconds and, upon awakening, compare the lastModifiedTime of the file against the one stored in a variable. If they are different, the file has changed and it's time to reload the application. This approach is nice because you don't have to worry about an unwelcome user reloading your application. However, the time that it gets reloaded is entirely up to the thread.