Understanding the "shape" of a codebase is something I've always been interested in and I started building a tool to help me understand and traverse code here:
However I don't think it would scale to 100M lines of code. I have run Linux through it and it was acceptable (both in run times and browse times). At 100M lines of code you need some way to see an overall "map" of the codebase and then drill in to the bits you are interested in. Just linking via symbols like SherlockCode does is too micro of a view.
There are a lot of interesting visualization tools out there both commercial and academic. I don't have any Java specific ones to recommend but a quick Google search for "java code visualization tools" shows a lot of promise.