How to extract subtitles from MKV files in pure Java?

does anybody know how to extract subtiles from MKV files using pure Java? (I know how to do it using tools such as mkvtoolnix. I know I could call them from Java but that's not what I want)

Thanks!

This looks like the file format specification:

http://www.matroska.org/technical/specs/index.html#track

I‘m not quite sure what exactly you expect as an answer. You can of course write your own Matroska parser (specification is available at matroska.org) and then simple extract the subtitle stream.

You might be able to use jebml https://github.com/Matroska-Org/jebml