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A Leiningen plugin for working with Java modules

Clojars Project


The Java Platform Modules System (JPMS), commonly referred to as just "modules", have been a part of the platform since Java 9. Modules are bigger than packages and it's likely that some of your favorite Java packages (like "java.sql") have been broken out into modules. Many modules are bundled with the standard runtime and development kits for Java (like "java.sql"), you don't have to do much of anything special to use the. Others are now distributed separately and need to be managed specially if you want to use them, for instance, JavaFX.

This plugin provides some additional tasks and middleware for Leiningen that makes it easier to interact with the Java module system. It makes it easier to include modules in your project and to interact with the module-specific tool like jlink.

In addition to making it easier to interact with external modules, jlink also provides an way to build a customized Java runtime that includes only the modules your project needs to run. You can then package this smaller runtime with your application to provide self-contained distribution package.

If you are packaging your application for use in a Docker image, 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 runtime) is only 37MB, and 22MB in gzipped tarball.


First you need to migrate to Java 9 or newer. You can get the latest development kit from Adopt OpenJDK:

This plugin assumes that you have set a valid JAVA_HOME environment variable and that it will be available when Leiningen runs. If you don't have it set, go ahead and set JAVA_HOME now. The plugin uses this variable to find the Java modules distributed with your JDK as well as to locate tools like jlink and jpackage. If you prefer to use another JDK, you can provide it with the jlink-jdk-path on your project.clj file.

Next, add this plugin to your project.clj file in the :plugins section.

[lein-jlink "0.3.0"]

Then add the middleware into the :middleware section.


Your project should look something like this...

{defproject myorganization/myproject
  :plugins    [...
                [lein-jlink "0.3.0"]]
  :middleware [...

The middleware will alter the way Leiningen calls out to java and javac such that it calls out to the correct ones and that these calls include references to the modules that your project needs. By default only the java.base module is included, if you need other modules you can simply add them to the :jlink-modules key of your project.clj file.

:jlink-modules ["java.sql"]

There are many modules distributed with the JDK, you can ask java to list them all.

$ java --list-modules

If you only use modules packaged with your JDK, then you can use all of the regular Leiningen commands without issue. When you compile a build a JAR file with lein build or execute it with lein run, the middleware will make sure that the modules are on the path.

External Modules

If you are using modules that are not distributed with the JDK, you can download and add them to your project with the :jlink-modules-paths keyword in your project.clj. For instance, if you downloaded the JavaFX modules from Gluon's website (referred to as "jmods"), you would unpack them somewhere on your machine and then add a reference to them like so...

:jlink-module-paths ["/opt/java/javafx-jmods-14.0.1"]

If you are running on Windows, you will want to escape the \ character in your paths with \\. For example:

:jlink-module-paths ["C:\\Program Files\\Java\\javafx-jmods-14.0.1"]

External modules only work at compile time, they cannot be passed to your default java command at runtime. There are two solutions:

  • You could download and install the SDK for the module. This plugin supports this but not all modules provide an SDK.
  • Create a runtime image and use the java command from the image to execute your project; this is the recommended solution.

Adding a Module SDK

If you happen to have the SDK for a module installed and you don't want to build a custom runtime image, you can add the path to the SDK to your project.clj file on with the :jlink-sdk-paths key. For example...

:jlink-sdk-paths ["C:\\Program Files\\Java\\javafx-sdk-14\\lib"]

That being said, building a custom image might be easier since you don't need to install anything except the modules.

Buidling a Custom Runtime Environment

The plugin will build up a custom runtime environment (Java runtime) for your project, including only the modules your project uses. If you have external modules but lack access to an SDK, you will need to create a custom runtime in order to actually run your project.

Create a custom Java environment with the plugin's init task. It will call out to the jlink tool that is bundled with your JDK to create a new "runtime image". This image will be in the "image" directory at the root of your project. If you would like to store the runtime image in a different location, you can provide that location with the :jlink-image-path key in your project.clj file.

lein jlink init

By default, the plugin will create a basic Java environment with only base module (the java.base), which is only 29MB. This represents the minimum and it might be enough for your Clojure application, if you don't use any other modules. If you do use other modules, you can add them through the :jlink-modules key in your project.clj. You don't need to include java.base, that one is automatically pulled in by the plugin.

With your custom runtime created, you can now build and run your project with the regular Leiningen commands.


If you aren't using any external modules then you can continue to use Leiningen's regular clean task. If you are using a image, you will have to ask the plugin to perform its clean task.

$ lein jlink clean

The plugin will remove the image directory and then call out to Leiningen to perform it's regular clean task.


The plugin's middleware will take care of correctly setting the path to java and providing the module options. You can continue to run your project with Leiningen's run task. The only thing to keep in mind is that while the plugin can call Leiningen's task, we can't alter Leiningen's task to call the plugin. That is, if you are using an image then you need to make sure you create the image before you try to run your project.

$ lein jlink init
$ lein run


By running lein jlink assemble, we call out to Leiningen to create an uberjar and then move it into the custom runtime image directory and then create scripts to launch your project. Once this step is complete, your image will have everything it needs to run your application. You can test it out from the console.

$ cd image
$ bin\java -jar my-project-uberjar.jar

Or use the appropriate launcher script.

$ cd image
$ .\

Your application will launch and it will have access to all of the required modules. If you need to set specific options for the JVM, you may use the JAVA_TOOL_OPTIONS environment variable


Lastly you may package your custom runtime, uberjar and launcher scripts into one archive for distribution.

$ leink jlink package

The plugin will create a GZIPped TARball of the image by default, if you need a ZIP archive you can set the :jlink-archive key to "zip".

Create a Docker Image

The custom image directory can be copied into a docker image for distribution. We have created a minimal base image using Alpine Linux, which is only 12.3MB and is suitable for running many applications. ;-)

Assume your application is called jlinktest, you can create a Dockerfile that looks like this...

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!


Copyright © 2018 Ning Sun

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

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