title: How to perform JSF validation in actionListener or action method?
body: <p>I have Bean validation working nicely in my application. Now I want to check that a new user does not choose a username that has already been chosen.</p>
  
  <p>In the actionlistener I have the code that checks the database but how do I force the user to be sent back to the page they were on if they choose an already existing username?</p>