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Puck takes your app and packs it along with all your gems and a complete JRuby runtime in a standalone Jar file that can be run with just java -jar …


Add this to your Gemfile:

gem 'puck'

You will also need a JRuby runtime, so add this too:

gem 'jruby-jars', '= 1.7.4'

Make sure you specify a specific version of JRuby, and that it's the same as the one you're using locally. If you don't want to depend on jruby-jars for some reason there are ways to specify a path to jruby-complete.jar, see below for instructions.


Puck uses Bundler to figure out your gem dependencies, so if you're not using Bundler, Puck will not work.

Puck also requires you to specify the version of JRuby you want to bundle. The easiest way to do that is to add jruby-jars as a dependency, as described above, or provide your own jruby-complete.jar, as described below.


You can use Puck either from the command line, or from your own code, for example in a Rakefile. See below for examples, and configuration.

Once you have a Jar file you can run your application like this:

java -jar path/to/app.jar name-of-bin-script arg1 arg2

where name-of-bin-script is the name of a file from your app's bin directory. Any subsequent arguments will be passed to this script as if it was called directly from the command line. Everything your application needs to run will be included in the Jar, all gems and a complete JRuby runtime.

Creating a Jar from the command line

Just run puck and it build a Jar file from your app. The Jar will be placed in a directory called build in your application's root directory.

Creating a Jar with Rake

Probably the best way to use Puck is to create a Rake task:

task :dist do

As you can see the code to create a Jar file is tiny, you can easily integrate this with other tools like Thor or your custom build toolchain.


Puck has some sane defaults, and let's you override most of them. It will determine the name of your application from the current working directory (which will be the directory containing your Rakefile if you run it from a Rake task) and it will include all files in bin and lib automatically.

If you want to include files that are not in bin or lib you can pass in the :extra_files option:

Puck::Jar.new(extra_files: Dir['config/*.yml']).create!

or using the command line:

puck --extra-files config/*.yml

There are two more options that you can set:

  • :app_dir: your application's root directory, useful to set if it isn't the current working directory (and you're not using Rake).
  • :app_name: the name of your application, it defaults to the name of the current working directory (and if you change that you don't need to change this too, you only need this option if you want a different name than the base directory's).
  • :build_dir: defaults to build, but if you want the Jar file to end up somewhere else you can change it with this option.
  • :jruby_complete: if you don't want to depend on the jruby-jars gem for some reason you can provide the path to your own jruby-complete.jar

They can also be specified on the command line (e.g. puck --build-dir dist).


Don't do require 'bundler' or require 'bundler/setup' or similar. Even if Puck uses Bundler to determine which gems to pack into the Jar it doesn't include Bundler. The reason is that there is no need for it since the environment in the Jar is frozen (and the benefit is slighly faster startup times, especially if you're using git dependencies). Future versions of Puck may change this and make require 'bunder/setup' work.


Why not just use Warbler?

I've found Warbler to be opinionated in an unhelpful way. For example, if you have a config.ru in your application's root directory but don't want to create a War file you need to monkeypatch two classes, one of them seemingly unrelated to War files.

If Warbler works for you, you should continue using it.

The Jar file is huge, is there something I can do to slim it down?

Short answer: probably not.

The JRuby runtime with all its dependencies clocks in at 20 MiB, you could probably slim it down a little bit by removing the 1.8 standard library, but apart from that you should probably leave it. Your gems also take up quite a lot of space, but usually you don't notice because they're tucked away in some directory that you never see. The gems are bundled as-is and include everything that the gem author thought necessary to include, tests, documentation, etc. You can probably strip this, but it's included by default because many gems do weird things (I'm looking at you jruby-openssl).

Also, you're not going to put it on a floppy, you're going to send it over a network that handles megabytes per second, it's probably ok that you app is 50 MiB.


Copyright 2013 Theo Hultberg/Iconara

Licensed under the Apache License, Version 2.0 (the "License"); you may not use this file except in compliance with the License You may obtain a copy of the License at


Unless required by applicable law or agreed to in writing, software distributed under the License is distributed on an "AS IS" BASIS, WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied. See the License for the specific language governing permissions and limitations under the License.