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.
I wouldn't bother trying. It requires correctly catching browser events client-side (and firing off a request to the server) which is a black art, and never works the way you want it to. Easier is to timeout idle sessions.