I think Kotlin support makes this topic pretty much moot. This should address all of OPs issues.
@Carsten_Schmied I agree strongly with you. I write almost all my build scripts in Java, using my own iwant build system, which you may find interesting. There is a simple example of using it, too: iwant-demo. (Sorry for advertising another tool here, but I think this is relevant to the topic.)
@mark_vieira Kotlin is progress compared to the Groovy black box insanity. But I am still not really satisfied.
@wipu thanks, I will look into it. At least this thread delivered some alternatives in pure Java. Somebody else suggested using http://jerkar.github.io
It is pretty good. I will some find out if your iwant is even better 
@Carsten_Schmied You are welcome. Donāt hesitate to ask for help, contact info can be found on the webpage.