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JRuby is a ruby interpreter that gives the best of the JVM -- as battle-tested in the field for many years -- together with the awesomeness that is Ruby syntax.
$ rvm install jruby
Choosing the Ruby API
Currently, as of this writing, JRuby loads the Ruby 1.8 API and runs as 1.8.7 version. As of JRuby version 1.7 the default will become 1.9.
If you need to run JRuby with the 1.9 Ruby API, you can do so in RVM using after_use hooks. You must record the JRUBY configuration
Be sure to add this to your jruby environment file(s).
For your console environment, I recommend detecting JRuby in an after_use hook and adding the export.
So for example, your after_use hook file might look something like this:
case "$GEM_HOME" in *jruby*) JRUBY_OPTS="--1.9" ; export JRUBY_OPTS ;; esac
Choosing the Ruby API on Install
You can choose which JRuby API to use by default on install or even reinstall it with the desired option:
rvm install jruby --1.9 # OR rvm install jruby --1.8