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.
Yes, you can override ActionServlet. Make sure to configure the same in web.xml. Note: I have seen people often overriding ActionServlet do 'common things', most of the time the 'common things' can be implemented in Servlet filters.