This week's book giveaway is in the OO, Patterns, UML and Refactoring forum. We're giving away four copies of Refactoring for Software Design Smells: Managing Technical Debt and have Girish Suryanarayana, Ganesh Samarthyam & Tushar Sharma on-line! See this thread for details.
Tomcat goes to great lengths to insure that sessions don't get invalidated. It serializes all the session data to file, shuts down, and then, when on startup, deserializes the data so the sessions can resume.