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.
If the server is down, you obviously can't show anything. What most of my clients do when there's a planned outage is to route the requests to another server (usually a static Apache server) that shows a "We'll be back soon" page.
It has nothing at all to do with Java or servlets. Just server administration.