Java Servlets — Session tracking

The series on servlet technology is growing nicely. Marek Branický explains
 in his article 
 Java Servlets — session tracking
 how to use sessions in a servlet environment. Of particular note is a very useful list and description
 of the interfaces called when a servlet 
 container works with an HttpSession. This lets us respond conveniently
 from objects stored in the session to events such as the session being destroyed (either because it expires 
 or because the method invalidate is called manually), other objects being added to the session, and so on.