This is an example of visualising a couple of random java files
> I am trying this out on a simple open source case. I haven't seen any output, only errors.
Please tell me the files that are generating the failures, and I will look at why the Java source code parser is falling over on certain syntax.
More general tips on using GitUML
Because of the integration with GitHub, it is very easy for people to click the root of a project and thereby attempt to parse every single file in a repository - which involves downloading each file and parsing it - the traffic would be intense, time taken would be long, and the API limits GitHub imposes would be eaten up pretty quickly. On top of that, generating a diagram that huge would just be a huge tangled mess, and create a .svg miles wide etc. and probably overload the memory of the Plant UML rendering server - see https://plantuml.com/server.
The total number of diagram elements limit is currently 250.
UML-like diagrams are typically used in a more focussed way, to tell a story about something, with comments etc. Generating huge diagrams is unlikely to be intelligible.