This week's book giveaway is in the OO, Patterns, UML and Refactoring forum. We're giving away four copies of Refactoring for Software Design Smells: Managing Technical Debt and have Girish Suryanarayana, Ganesh Samarthyam & Tushar Sharma 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.