Here is the problem. From the system administrators perspective Java means something entirely different than the developers perspective (though I might argue this is an excellent spot for the DevOps philosophy to rear it's head...but I digress).
You as an SA are concerned with a narrower scope than the developers. Essentially, you need to familiarize yourself with the intricacies of the JVM (namely garbage collection and tuning). You will also likely want to gain an understanding of whatever Application Server your particular company uses(JBoss, Tomcat (which is not an app server per se, but a servlet container), Web Sphere etc).
Also spend some time with the developers and find out the 30K feet view of how the app works. You don't need to be able to read Java, but you should understand how the app operates and the flow of data through it.