Anyone who is making instance variables in Action classes a "requirement" is asking for an application that doesn't work -- or even worse -- sometimes works and sometimes doesn't. If you need data for a single user to psersist between requests, put that data in the user's HTTPSession. For example:
String myData = "hello world";
request.getSession.setAttribute("myData", myData);
Then, in the next request, you can retrieve it with:
String myData = (String) request.getSession().getAttribute("myData");