We do a lot of Java hosting for our customers. As with any technology, there are a lot of different deployment scenarios to choose from. With Java we tend to see mostly Tomcat hosting because most folks don't require a full fledged application server (and the associated memory costs that come along with it).
JBoss is the most commonly used Java application server that we see, but we're seeing more and more Glassfish usage as time marches on. It's not the easiest thing to install so Paul volunteered to put together an installation script to automate the process. :)
Give it a try and let us know if you run into any issues. We'll pass those along to Paul whilst we administer a beating on your behalf. Just kidding...it should be good for general use now (but let us know - just in case). :)