After over three years, the "ruby-debug" gem finally installs properly on JRuby.
~/projects/jruby ➔ gem install ruby-debug
Successfully installed ruby-debug-base-0.10.4-java
Successfully installed ruby-debug-0.10.4
2 gems installed
Back in 2007, folks working on NetBeans, Eclipse, and IntelliJ support for Ruby came together to build a new version of the ruby-debug backend that would work on JRuby. They shared the effort, we JRuby guys added features they needed to do a clean port of the ruby-debug C code, and the ultimate result was a ruby-debug-base gem that isolated the platform-specific bits.
For whatever reason, the JRuby version of ruby-debug-base never got pushed as a real, live "-java" gem. This meant that you had to download ruby-debug-base-VERSION-java.gem yourself to get ruby-debug to install. To make this process easier, we even shipped ruby-debug and ruby-debug-base preinstalled in JRuby 1.5.
Unfortunately, this was only a partial answer. Many libraries and applications want to install all their dependencies clean. If one of those dependencies was ruby-debug, it would fail to install. Rails even includes special JRuby-specific lines in its default Bundler Gemfile to exclude ruby-debug when bundling on JRuby.
All that nonsense ends today. Rocky Bernstein, one of the maintainers of the ruby-debug gem, agreed to push our ruby-debug-base to the canonical rubygems.org repository. As a result, ruby-debug now installs properly on JRuby. It only took three years to get that gem pushed (by nobody's fault...I think everyone expected everyone else to follow through on it).
Merry Christmas, Happy Chanukah, Joyous Kwanza, and enjoy your holiday season!