Just to be clear with Spring 2.5
you should still be using the annotation approach. You would have to go back to Spring 2.0
Anyway you will see that BaseCommandController which the other form controllers extends has a setValidator. If you set your validator on the controller then it will be used to validate the form post. If a validation failure occurs the form view is returned so the user can correct the errors otherwise the onSubmit() will be called. Take a looks at DataBinder and its implementations for getting a handle to the BindingResult.
Good Luck