What version of JRuby should plugins be built with?

Elastic Stack Logstash
1

I see a lot of mentions of "1.9" but that doesn't seem to be a real version.

2

Since the plugins will be run by Logstash core, matching the Logstash Core Prerequisites is going to be your best bet:

Prerequisites

  • Install JDK version 8. Make sure to set the JAVA_HOME environment variable to the path to your JDK installation directory. For example set JAVA_HOME=<JDK_PATH>
  • Install JRuby 9.1.x It is recommended to use a Ruby version manager such as RVM or rbenv.