There are tools that "reverse engineer" Java code into UML diagrams. They mostly do "static" diagrams that just tell you which classes refer to each other through parameters, variables, extends, implements. The ones I've tried show every detail of every class used, even
String and such, so the diagrams are so cluttered you can't really see what's going on. The real value of models is leaving out the non-essential stuff to reveal the essential stuff, and tool-generated models don't do that well.
I think there are some that attempt to follow execution paths in a more "dynamic" way. I haven't tried em.
Search through the OO and UML, etc forum down the page a ways for prior discussions of UML tools. There may be some recommendations already there.