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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 puck and jruby-jars to your Gemfile, like this:

group :development do
  gem 'puck'
  gem 'jruby-jars', '= 1.7.18'

Make sure you add them to a group, otherwise they will be included in the Jar, which isn't needed and only adds to its already considerable size.

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.

Puck also exposes all gem's bin files, as well as JRuby's, so if you have Pry among your (included) gems you can do this

java -jar path/to/app.jar pry

to get a Pry session that can access your application's code.

Creating a Jar from the command line

Just run puck and it will 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 (for an example of this see Jarå).


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

The more advanced :extra_files feature, where the destination paths within the JAR file is specified as described below, is not available as a command line option.

These are the options that you can set (check the API documentation for Puck::Jar#initialize for full documentation):

  • :gem_groups: the groups from your Gemfile to include, defaults to :default (which is all gems that aren't in an explicit group). Don't include the group that contains puck and jruby-jars.
  • :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
  • :extra_files: a list of files to include in the Jar. The option can be either an Array, in which case paths must be below the :app_dir, or a Hash, in which case the file specified by the key is included at the path specified by the corresponding value.

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



You can't have require 'bundler' or require 'bundler/setup' or anything else that references Bundler in your code.

Even if Puck uses Bundler to determine which gems to pack into the Jar it doesn't pack Bundler. Bundler is extremely opinionated, and assumes that it has full control, so creating an environment where it can run is not easy (just see the tests for this project to get an idea of what lengths you need to go to to get a clean environment to load Bundler). Bundler is also not need since the environment in the Jar is frozen – there are no dependencies to resolve or Gem paths to set up. Bundler, just like Puck, is a build time tool that shouldn't be required at run time.

Future versions of Puck may change this and make require 'bunder/setup' work, even though Bundler is not included, just to make it easier to run your code outside of the Jar – in JRuby using require 'bundler/setup' is many, many times faster than doing bundle exec … since it doesn't start an extra process.

I get “Cannot run program "ant"”

You need to install Ant, most systems with a JDK come with Ant already installed, so the ant integration in JRuby seems to assume that it's always installed. brew install ant, yum install ant or apt-get install ant should all work.

Puck uses Ant primarily for its easy-to-use ZIP file merging abilities. Just like Puck, Ant is only a build-time dependency and is not included in artifact.


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?

First off make sure that the jruby-jars gem is not in a group that gets included in the Jar. Puck uses one of the Jars from this gem as the basis for the Jar it creates, but it's purely a development dependency, just like Puck itself. Including it in the Jar is essentially including itself.

But if that is not the issue, the short answer is unfortunately: 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.

Will it work with Rails?

I have no idea. Puck should be able to package anything that has a bin file that starts it, but Rails makes a lot of assumptions. Try it (and report back), but if it doesn't work you're probably better off with Warbler. Puck wasn't designed with Rails in mind, it was designed primarily for headless services, but it runs Rack applications just fine, the integration tests package a Rack app, launches it with Puma and throws a request at it to make sure it works.

Copyright 2013-2015 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.