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.
Your servlet ServletUser doesnt implement doGet method, and hence when you enter the URL to servlet in browser addressbar and hit enter, you get this error.
Implement doGet in your servlet and the error will go away.