Migrate Rails to JRuby on Heroku

13 Nov 2012

A “Why Jruby” talk by Charles Nutter recently inspired me to move one of my personal projects to JRuby. The app is fairly small, running Rails 3.2 using Postgresql, activemodel-serializers and oAuth to name a few.

Update 2013-09-10

Heroku now has a much better and updated guide for running your Rails app with JRuby on Heroku

Using JRuby on Heroku

In order for Heroku to use JRuby instead of MRI, I’ve added the following to my Gemfile leveraging Bundler’s ruby directive.

ruby '1.9.3', engine: 'jruby', engine_version: '1.7.4'

To get JRuby 1.7.4 locally I used rbenv install jruby-1.7.4.

Applying JRuby specific buildpack

For Heroku to get all the plumbing needed for JRuby setup, add this buildpack.

heroku config:add BUILDPACK_URL=git://github.com/jruby/heroku-buildpack-jruby.git

Note: I’m not sure this buildpack is still required for JRuby Rails apps.

JRuby specific gems

Because the pg gem (postgres driver) includes c-extensions, it will not work nicely with JRuby. An alternative for JRuby is jdbc-postgres. Hence, I added the following to my Gemfile replacing the pg gem.

gem 'activerecord-jdbc-adapter'
gem 'activerecord-jdbcpostgresql-adapter'
gem 'jdbc-postgres'

I previously used Thin as the webserver for my app, but given my move to JRuby I wanted a webserver which utilized threads better. I chose Trinidad for this task as it appears to perform quite well according to this benchmark.

Make the following changes to use Tinidad.

# Gemfils
gem 'trinidad', require: nil

# Procfile
web: bin/trinidad -t -r -p $PORT -e $RACK_ENV

Compiling assets

One final change I had to make was ensuring assets would compile properly using JRuby. This require me to replace therubyracer with therubyrhino.

# Gemfile
gem 'therubyrhino'

# config/application.rb
config.assets.initialize_on_precompile = false

# config/environments/production.rb
config.serve_static_assets = true
STDOUT.sync = true
config.logger = Logger.new(STDOUT)

For fast asset pre-compilation I’ve also added the Google Closure Compiler.

gem 'closure-compiler', group: :assets

# config/environments/production.rb
config.assets.js_compressor = :closure

Going threadsafe!

One final thing I wanted to change in order to leverage the JVM’s concurrency better, was to enable Rails to use multiple threads for requests by setting config.treadsafe! in config/environments/production.rb'


As I encounter issues I will update this section with a corresponding solution.

/usr/bin/env: jruby: No such file or directory

Ensure that your PATH includes jruby/bin. To inspect whether it does you can run heroku run export and to set it run heroku config:add PATH="bin:jruby/bin:/usr/bin:/bin".


Given my project only has two users I’ve yet to see any noticable differences between MRI and JRuby for my site. One downside of the move however is that the slug size has increased quite dramatically from around 20MB to 67MB. This only affects app startup/scaling times and is most likely because the slug now contains JRuby (JVM, etc).

I plan to write a follow up post once I’ve collected some data in NewRelic as to how JRuby performs compared to MRI.