the build uses ruby-maven. note ruby-maven uses maven and maven is highly modular, i.e. it comes only with the core and the moment you need a plugin he first time its starts downloading it. with that in mind the first usage of (ruby-)maven involves a lot of downloading - so be prepared :)
first get all the development gems in place:
jruby -S bundle install
to build the (extension) jar for the lib directory (prepare the jar before packaging the gem)
rmvn prepare-package
this also runs all the test over a couple of jruy version each in 1.8 and 1.9 mode. so these tests take some time. in skip the tests when building the gem use:
rmvn prepare-package -DskipTests
to build the gem in target/jbundler-0.0.1.gem
rmvn package
or once the jar file is in place then
gem build jbundler.gemspec
will do as well.
once rmvn generated the Gemfile.pom you can use proper maven3 by setting a sybolic link from pom.xml to Gemfile.pom. in the end rmvn is just ruby wrapper around maven3. the Gemfile.pom is generated from the jbundler.gemspec, Gemfile, Gemfile.lock and Mavenfile.
your IDE might be able to use the pom.xml to manage the project and its java sources.