What is an Action Class used for? | Java Structs

An Action is an adapter between the contents of an incoming HTTP request and the corresponding business logic that should be executed to process this request. The controller (ActionServlet) will select an appropriate Action for each request, create an instance (if necessary), and call the perform method.
Actions must be programmed in a thread-safe manner, because the controller will share the same instance for multiple simultaneous requests. In this means you should design with the following items in mind:

•  Instance and static variables MUST NOT be used to store information related to the state of a particular request. They MAY be used to share global resources across requests for the same action.
• Access to other resources (JavaBeans, session variables, etc.) MUST be synchronized if those resources require protection. (Generally, however, resource classes should be designed to provide their own protection where necessary.

When an Action instance is first created, the controller servlet will call setServlet() with a non-null argument to identify the controller servlet instance to which this Action is attached. When the controller servlet is to be shut down (or restarted), the setServlet() method will be called with a null argument, which can be used to clean up any allocated resources in use by this Action.