I am looking for a way to launch a background thread when a Tomcat server starts
I think you are looking for a way to launch a background thread when your web application is started by Tomcat.
This can be done using a ServletContextListener. It is registered in web.xml and will be called when your app is started or stopped. You can then created (and later stop) your Thread, using the normal Java ways to create a Thread (or ExecutionService).