App Engine Ruby Cupcake
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App Engine Ruby Cupcake

A minimal instalation of JRuby and JRuby-Rack for the Google App Engine.


Because I tried to set one JRuby sample up on App Engine and found it a little tough.

For instance, there's a well-known incompatibility between the Ruby libraries for the App Engine and the more recent versions of JRuby-Rack.

I spent a few days getting to know all of the technologies better and spotted a bug when trying to use the most recent version of JRuby-Rack when I tried to avoid using some of the Java classes of the Ruby libraries for the App Engine.

I've also seen many other folks having issues getting started with it because of said incompatibilies. Even the more recent JRuby-Jars files (1.6x series) seem to have gotten in the way.

Laundry List

While trying to use the Ruby libraries for the App Engine I learned that they worked better with Ruby 1.8.7 when getting things like the older version of the Bundler (bundler08-0.8.4) working.

After I got those versions sorted I could issue commands like appcfg.rb and dev_appserver.rb.

The App Engine Ruby libraries developers found it better to lock the versions of JRuby-Rack (1.0.4) and JRuby-Jars (1.5.6) to the versions that worked.

Like many others I had a hard time getting it all in my head. Not really used to the Bundler tool, even! :-)

Getting Better At It

Getting App Engine errors was no fun. To me it was like a blackbox. It was a little maddening when it worked locally during development/testing and when I uploaded it it stopped working with no meaningful feedback.

I had a break-through when I could see the logs from the Dashboard:

I was able to cut some dependency on a few Java classes of the App Engine Ruby libraries, namely those 5 classes created with a Mirah file. It was only a matter of replacing the Servlet Event Listener with the one provided by JRuby-Rack.


Depending on the version of the JRuby-Rack or App Engine Ruby libraries you may need to place the file accordingly, either to /WEB-INF/ or to its parent directory /

Using the Java Ant tool can be handy at the build.xml directory, like so:

ant runserver

ant update

Those are shorter versions to these commands:

/long/path/to/appengine-java-sdk-1.5.5/bin/ www

/long/path/to/appengine-java-sdk-1.5.5/bin/ update www

Edit the WEB-INF/appengine-web.xml file and add your application id.

Edit the build.xml file and update the location to the "appengine.sdk", like so:


For now, only JRuby-Jars-1.5.6 work. There's an issue with the latest JRuby-Jars-1.6.5 versions. To keep it simple I'll just include the files myself, in the WEB-INF/lib directory.

Related Projects

JRuby-Rack -

App Engine Ruby libraries -


Thanks to all the wonderful folks who have worked on these open source projects.