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Heroku buildpack: Clojure

This is a Heroku buildpack for Clojure apps. It uses Leiningen.

Note that you don't have to do anything special to use this buildpack with Clojure apps on Heroku; it will be used by default for all projects containing a project.clj file, though it may be an older revision than current master. This repository is made available so users can fork for their own needs and contribute patches back.


Example usage for an app already stored in git:

$ tree
|-- Procfile
|-- project.clj
`-- src
    `-- sample
        `-- core.clj

$ heroku create --stack cedar --buildpack

$ git push heroku master
-----> Heroku receiving push
-----> Fetching custom buildpack
-----> Clojure app detected
-----> Installing Leiningen
       Downloading: leiningen-1.7.1-standalone.jar
       Downloading: rlwrap-0.3.7
       Writing: lein script
-----> Installing dependencies with Leiningen
       Running: LEIN_NO_DEV=y lein deps
       Downloading: org/clojure/clojure/1.2.1/clojure-1.2.1.pom from central
       Downloading: org/clojure/clojure/1.2.1/clojure-1.2.1.jar from central
       Copying 1 file to /tmp/build_2e5yol0778bcw/lib
-----> Discovering process types
       Procfile declares types -> core
-----> Compiled slug size is 10.0MB
-----> Launching... done, v4 deployed to Heroku

The buildpack will detect your app as Clojure if it has a project.clj file in the root. If you use the clojure-maven-plugin, the standard Java buildpack should work instead.


Currently most of the build-level configurations require turning on the user_env_compile functionality so the build step will have access to environment variables.

By default your project is built by running lein deps, which copies all the dependencies into the lib directory. You may wish to perform a full AOT compile during build; this is done by setting LEIN_BUILD_TASK=compile :all. This has the benefit of both speeding up dyno launch times and catching certain classes of error during push.

By default your project will run in "no dev" mode, which means :dev-dependencies will not be available and the test and test-resources directories will not be on the classpath. You can set LEIN_DEV=y to disable this if you need access to a plugin at runtime.

Finally, you can reduce memory consumption by using the trampoline task in your Procfile. This will cause Leiningen to calculate the classpath and code to run for your project, then exit and execute your project's JVM:

web: lein trampoline run -m myapp.web


To change this buildpack, fork it on GitHub. Push up changes to your fork, then create a test app with --buildpack YOUR_GITHUB_URL and push to it. If you already have an existing app you may use heroku config:add BUILDPACK_URL=YOUR_GITHUB_URL instead.

For example, you could adapt it to generate an uberjar at build time.

Open bin/compile in your editor, and replace the block labeled "fetch deps with lein" with something like this:

echo "-----> Generating uberjar with Leiningen:"
echo "       Running: LEIN_NO_DEV=y lein uberjar"
PATH=.lein/bin:/usr/local/bin:/usr/bin:/bin JAVA_OPTS="-Xmx500m -Duser.home=$BUILD_DIR" LEIN_NO_DEV=y lein uberjar 2>&1 | sed -u 's/^/       /'
if [ "${PIPESTATUS[*]}" != "0 0" ]; then
  echo " !     Failed to create uberjar with Leiningen"
  exit 1

The LEIN_NO_DEV environment variable will cause Leiningen to keep the test directories and dev dependencies off the classpath, so be sure to set it for every lein invocation.

Commit and push the changes to your buildpack to your GitHub fork, then push your sample app to Heroku to test. You should see:

-----> Generating uberjar with Leiningen:


To see what the buildpack has produced, do heroku run bash and you will be logged into an environment with your compiled app available. From there you can explore the filesystem and run lein commands.

Note that projects with the :local-repo-classpath option set in project.clj will cause dependencies to be re-fetched when each process is started, which is highly undesirable. It's recommended you disable this option.

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