We've been working on the java port of ruby-debug for awhile and it's getting perilously close to usable. And with the latest builds of NetBeans, you can actually use the NB ruby debugger with it. If you're very brave, you can try it for your yourself. Be warned, it's still a work in progress, but here's how to get started:
Check out jruby from trunk and build it. You'll need java and ant, but if you're interested in jruby chances are good you have those already. It's super easy to build, check it out from http://svn.codehaus.org/jruby/trunk and do ant dist and you're good to go.
Check out jruby-debug from http://debug-commons.rubyforge.org/svn/jruby-debug/trunk. In this directory, do rake install_gem. This will compile and build a jruby version ruby-debug-base, which ruby-debug (command line rdebug debugger) and ruby-debug-ide (which NetBeans and Eclipse use) both depend on.
Check out ruby-debug-ide from http://debug-commons.rubyforge.org/svn/trunk/ruby-debug-ide. Then do a rake gem to build the gem and gem install pkg/ruby-debug-ide-0.1.8.gem to install it.
Grab a copy of NetBeans daily builds ruby ide from here: http://deadlock.netbeans.org/hudson/job/ruby/
Fire up NetBeans like with some extra command line switches: ./netbeans -J-Dorg.netbeans.modules.ruby.debugger.force.rdebug=true -J-Dorg.netbeans.modules.ruby.debugger.fast.not.required=true The first tells NetBeans to always use the fast debugger even tho the classic debugger is what appears selected in the UI. The second tells NB to ignore the fact that the classic debugger is selected and allow us to debug a rails app anways. These flags were just added to allow us to play with jruby-debug in NetBeans for now, they won't be necessary once jruby-debug matures a little and full support for it comes into NB.
In NetBeans Tools | Options be sure and have your ruby interpreter pointing at the jruby you built from trunk in Step 1.
You should now be able to debug Ruby and ROR apps using the fast ruby debugger inside of jruby. Like I said, this is all very experimental. All these steps will go away in the next few weeks or months and jruby-debug, jruby, NetBeans, ruby-debug-ide do releases.
Things which seem to work ok:
setting breakpoints
looking at variables (global and local)
watches
A little flaky (tho it may already be fixed by the time you read this):
Stepping over some code in Rails
Stepping out of a method
Finally, big time kudos to Martin for kicking this whole thing off and Peter for knocking out bugs and implementing more features every hour just about. It's definitely looking close now.
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