There is a Java-based JRuby implementation of the fast ruby debugger rdebug.

Run jruby -S gem install ruby-debug

Now use it:

You can call it programmatically within the script

require 'ruby-debug'
debugger

and run jruby with the --debug flag:

jruby --debug my_script.rb

Starting the debugger on a Rails application:

cd my_rails_app
jruby --debug -S rdebug script/server

Starting the debugger to test a local ruby program and adding ''lib/'' to the load path:

ruby --debug -S rdebug -Ilib rubyprogram.rb 

just make sure to run it under a jruby with the --debug flags passed to it, or "next" will always act as if it were the "step" command. And the command line prompt won't even appear.

External Resources:

• trunk jruby-debug README
• Debugging with ruby-debug
• JRuby Debugger Troubleshooting - Forum Thread