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.
you can check out the POI project from jakarta. With that you are able to create an excel sheet which you can send back to the client. POI is only responsible for creating an excel file, then simply put it into the response stream of the servlet or whatever you use. (Don't forget to set the response type)