Last updated June 25, 2020

This document describes the general behavior of Heroku as it relates to the recognition and execution of Clojure applications. For a more detailed explanation of how to deploy an application, see:

• Getting Started with Clojure on Heroku
• Building a Database-Backed Clojure Web Application.

Activation

Heroku’s Clojure support is applied only when the application has a project.clj file in the root directory.

Clojure applications that use Maven can be deployed as well, but they will be treated as Java applications, so different documentation will apply.

Configuration

Leiningen 1.7.1 will be used by default, but if you have :min-lein-version "2.0.0" in project.clj (highly recommended) then the Leiningen 2.9.1 release will be used instead.

Your Procfile should declare what process types make your app run. Often in development Leiningen projects are launched using lein run -m my.project.namespace, but this is not recommended in production because it leaves Leiningen running in addition to your project’s process. It also uses profiles that are intended for development, which can let test libraries and test configuration sneak into production.

In order to ensure consistent builds, normally application config is not visible during compile time, with the exception of private repository credentials (LEIN_USERNAME, etc) if present. In order to change what is exposed, set the BUILD_CONFIG_ALLOWLIST config to a space-separated list of config var names. Note that this can result in unpredictable behavior since changing your app’s config does not result in a rebuild of your app.

Uberjar

If your project.clj contains an :uberjar-name setting, then lein uberjar will run during deploys. If you do this, your Procfile entries should consist of just java invocations.

If your main namespace doesn’t have a :gen-class then you can use clojure.main as your entry point and indicate your app’s main namespace using the -m argument in your Procfile:

web: java -cp target/myproject-standalone.jar clojure.main -m myproject.web

If you have custom settings you would like to only apply during build, you can place them in an :uberjar profile. This can be useful to use AOT-compiled classes in production but not during development where they can cause reloading issues:

:profiles {:uberjar {:main myproject.web, :aot :all}}

If you need Leiningen in a heroku run session, it will be downloaded on-demand.

Note that if you use Leiningen features which affect runtime like :jvm-opts, extraction of native dependencies, or :java-agents, then you’ll need to do a little extra work to ensure your Procfile’s java invocation includes these things. In these cases it might be simpler to use Leiningen at runtime instead.

Customizing the build

You can customize the Leiningen build by setting the following configuration variables:

• LEIN_BUILD_TASK: the Leinigen command to run.
• LEIN_INCLUDE_IN_SLUG: Set to yes to add lein to uberjar slug.

Leiningen at runtime

Instead of putting a direct java invocation into your Procfile, you can have Leiningen handle launching your app. If you do this, be sure to use the trampoline and with-profile tasks. Trampolining will cause Leiningen to calculate the classpath and code to run for your project, then exit and execute your project’s JVM, while with-profile will omit development profiles:

web: lein with-profile production trampoline run -m myapp.web

Including Leiningen in your slug will add about ten megabytes to its size and will add a second or two of overhead to your app’s boot time.

Overriding build behavior

If neither of these options get you quite what you need, you can check in your own executable bin/build script into your app’s repo and it will be run instead of compile or uberjar after setting up Leiningen.

Setting the Leiningen version

The buildpack will check for a bin/lein script in the repo, and if present will run it instead of the default lein command. This allows you to control the exact version of Leiningen used to build the app.

Setting Clojure CLI version

You can customize the version of the Clojure CLI installed with your app by setting the CLOJURE_CLI_VERSION config variable. The default version is 1.10.0.411.

Runtimes

Heroku makes a number of different runtimes available. You can configure your app to select a particular Clojure runtime, as well as the configure the JDK.

Supported JDK versions

Heroku currently uses OpenJDK 8 to run your application. OpenJDK version 7 is available. Depending on what major version you select the latest available update of that Java runtime will be used each time you deploy your app. For a list of current versions see the Java support article.

The JDK that your app uses will be included in the slug, which will affect your slug size.

Specifying a Java version

You can specify a Java version by adding a file called system.properties to your application.

Set a property java.runtime.version in the file:

java.runtime.version=1.8

1.8 is the default so if you’d like to use Java 8 you don’t need this file at all. You can specify JDK 7 by setting this property to “1.7”. For details on how to set specific update versions, see the Java support article.

Supported Clojure versions

Heroku supports apps on any production release of Clojure, running on a supported JDK version.

Add-ons

No add-ons are provisioned by default. If you need a SQL database for your app, add one explicitly:

$ heroku addons:create heroku-postgresql:hobby-dev