Tools for rapidly understanding large java projects

What are the best tools for rapidly gaining understanding of large java projects? For C stuff, I've used Gnu Global/htags to produce hyperlinked sources. In addition, I've used some static call graph analyzers and also written some perl code to insert meaningful stuff into the source to produce logs for dynamic analysis.

What, if anything, do people often use? Are there good automated UML/class hierarchy/etc diagram makers? Static call graph analyzers, etc?

Thanks,
Nate