I believe signatures will be different every time and that is expected, since we're dealing with non-deterministic signatures here.

Anyways, xeroc'x post over there is how I managed to solve this in graphenej. The relevant part of the code is in here: https://github.com/kenCode-de/graphenej/blob/master/graphenej/src/main/java/de/bitsharesmunich/graphenej/Transaction.java#L142