when you want this kind of functionality, it is better to use plain jsf or better yet, use seam.
thanks for your answer.
In case of using jsf: I am looking for the mechanism to compare both values and first of all how to tell the application to stay at the form.
How is it possible?
it's a pure jsf thing but I'll answer anyway... you just have to return an empty string as the value of your backingbean action method