lein-jlink

A Leiningen plugin for custom Java environment.

Motivation

JDK 9 ships a new utility called jlink, which creates custom JRE based on modules you use. As of 1.9, Clojure has no special support for modular Java. However, we can still use customized JRE to run our Clojure application. If you care about the size of your application package(includes JRE), for example, is docker environment, this feature is a pretty good news for you: The minimal JRE is only 29MB. And it's enough to run a hello world Ring application, using jetty adapter. The overall size (app + dependency jars + custom JRE) is only 37MB, and 22MB in gzipped tarball.

Usage

First thing first, you need Java 9+ for jlink.

Put [lein-jlink "current-version"] into the :plugins vector of your project.clj.

Create a default Java environment:

$ lein jlink init

By default, we create a basic Java environment with only base module (the java.base), which is only 29MB. That's could enough for your clojure application.

To add more modules to your jlink environment, add them to :jlink-modules vector, like ["java.base" "java.sql"].

To add more module path, use: :jlink-module-path [(str (System/getProperty "java.home") "/jmods") "lib/"].

Run and test your app

You can use jlink generated JRE to run and test your clojure app, by which you can verify functionality of your app under customized JRE.

lein jlink run
lein jlink test

A more flexible way is to create a profile in your project:

:profiles {
  :jlink {:java-cmd "./target/jlink/bin/java"}
}

Assemble and package

By running lein jlink assemble, we create a uberjar and put it into custom JRE directory. A launcher script is also generated in target/jlink/bin that can run your application with this JRE.

lein jlink package creates a tarball for previously generated JRE and can be distributed without dependencies.

Create docker image

The jlink target directory can be put into a docker image for app distribution. I have created a minimal base image alpine-jlink-base, which is only 12.3MB.

Assume your application is called jlinktest, you can create a Dockerfile like:

FROM sunng/alpine-jlink-base

ADD target/default/jlink /opt/jlinktest
ENTRYPOINT /opt/jlinktest/bin/jlinktest

The result image size can be less than 50MB.

License

Copyright © 2018 Ning Sun

Distributed under the Eclipse Public License either version 1.0 or (at your option) any later version.