In Java, how to reload dynamically resources bundles in a web application?

If you're using JDK 1.6 you can use the callback methods getTimeToLive() and needsReload() in ResourceBundle.Control to control if the bundle cache needs to be loaded with new values from the database.

As others have pointed out in the comments, you might want to look into Spring - particularly the ReloadableResourceBundleMessageSource.