I don't know how about including ready compiled java files into an own azw (don't know about spotify's app nor java advanced features).

My Experience: I was porting a java printserver (had the sourcecode).
- All the static-variables killed me (somehow kindle could not handle some)
- everything that needs files makes problems at first
- the +-Operator for Strings is not usable (so almost all debug/error/whatever-Messages will just make the kindle stop the programm without telling)

Maybe you encounter some of these errors.
Otherwise I'd advice you to buy some smartphone with an app. Probably more timesaving...


greets