I don't see any issues with thread safety in this approach. However, I don't believe this is the best approach. A method called by
Struts is meant to do something and then return a forward that tells Struts what to do next. With this approach, if Method A finds an error and wants to redirect to a different forward, Method D is unaware of it.
The approach I would suggest would be to refactor and put the logic that Method A and Method D have in common into a new private method. Then have both Method A and Method D call the same method to do that portion of the work and then return the appropriate forward.