Shameless self-plug: Me and a friend developed a Java debugger [1] that visualizes (Java) programs with UML class and sequence diagrams [2]. It isn't actively developed anymore, but the demo version [3] (along with Help -> Tutorial) can still be used to give it a try.
It was quite an involved & fun project as we used Java VM Tool Interface directly instead of using JDI and the layout algorithms for the diagrams are quite fancy too :)
It was quite an involved & fun project as we used Java VM Tool Interface directly instead of using JDI and the layout algorithms for the diagrams are quite fancy too :)
[1] http://www.jbixbe.com [2] http://www.jbixbe.com/VisualTour.html [3] http://www.jbixbe.com/download/jar/jbixbe.jar