you don't have anything to display any error messages.
but since you're doing all the Ajax/JavaScript outside of the Struts 2 framework you'd have to build up that object in some way. You haven't even said how you're doing the Ajax submission.
You haven't even said how you're doing the Ajax submission.
Why not take a look at how "stock" Struts 2 does its validation, learn where the error messages are stored, and figure out how to craft an Ajax response containing those messages?
With WHAT are you making your Ajax call?
Struts 2 has Ajax validation built in