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.
My assumption is that you are using a form and did not specify any method in which case it is GET by default or either the method specified is GET. In which case the servlet serving the req has to have a doGet() method. Probably you have a doPost() method without a doGet().