In case you haven't seen it, we've added experimental support for pure Java plugins in Logstash. We're promoting it to a beta feature in the upcoming Logstash 6.7 release with plans to move to GA shortly after that.
I know a few folks have been trying out this feature and have encountered a bug or two as well as some deficiencies in the documentation for Java plugins. We're working hard to get those all sorted out, but if you have any questions about Java plugins, feel free to ask them here and we'll investigate them.