This week's book giveaway is in the OCAJP 8 forum. We're giving away four copies of OCA Java SE 8 Programmer I Study Guide and have Edward Finegan & Robert Liguori on-line! See this thread for details.
The ActionServlet class is not final, so the answer is yes, you can override the service() method. The real question is why you'd ever want to. If you're making that radical a change to the ActionServlet, why even use the Struts framework at all? If there is something you want to change about the way Struts handles things, I suspect there are easier and less radical ways to do so than overriding the service() method of ActionServlet.