This week's book giveaway is in the OCAJP 8 forum. We're giving away four copies of OCA Java SE 8 Programmer I Study Guide and have Edward Finegan & Robert Liguori on-line! See this thread for details.
In your action declaration, just include an additional result element of type "input". In the action code, if you notice validation errors on the input values, just return Action.INPUT ("input") and the framework will forward to the input page.