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.
The alternate is obviously the Multi-threaded model. Unfortunately, the Servlet container can't help you in this model as it did in case of Single threaded model. You need to make your application thread safe, by the traditional way - protect the shared resources (if any) yourself.