Is this a struts action? If so, which struts feature are you using (or any other library you're using) to serialize objects in requests and responses?
If you're going to take your application to this level, I would advise you move to other framework: http://vraptor.caelum.com.br/ with @Consumes( "application/json"). But if you have a lot of code already written using struts, try to find something that allow you serialize JSON objects into the action context and you'll be able to get them already serialized. Something like this http://code.google.com/p/jsonplugin/.
Anyway, consider frameworks like these: VRaptor, RESTEasy, etc, they are more evolved in the "Java web development way of thinking," nothing against Struts but I've been seeing extremely good changes while using http-based technologies! REST with some powerful IoC container behind.
I hope this helps!
Feel free to ask me anything!
www.BlackBeltFactory.com/ui#!/ref=jmotta, SCJP 6, OCWCD JEE5, OCE EJB JEE6