You can use a ServletContextListener for this purpose and implement
its contextDestroyed method. For a thorough tutorial you may want to
consult the following OnJava article:

http://www.onjava.com/lpt/a/767

Catalina has a shutdown  hook which I have never happened to use but
you should be able to find it on the net. Google is our friend, right?

The other way is to use the
ServletContextListener.contextDestroyed(...) method. I like this one
more. Not I don't like Catalina but you don't want to depent on a
specific server.