This week's book giveaway is in the OCMJEA forum. We're giving away four copies of OCM Java EE 6 Enterprise Architect Exam Guide and have Paul Allen & Joseph Bambara on-line! See this thread for details.
Yes, its thread safe, struts2 creates a new instance of the action class for every request that it processes. So unless you've written something special to make an instance of of an action class created by one thread, accessible by another thread, you don't need to worry about other threads modifying the values in your action class.