Leiningen: Split an uberjar into dependencies.jar and app.jar (to optimize Docker layers and AWS Lambda functions)

November 19, 2019

project.clj
Dockerfile
Building
Alternatives

I want to split my application uberjar into a separate JAR with only the dependencies and a JAR with only the application code so that I can upload them as separate “layers” and thus leverage layer caching. While my code changes frequently and is tiny, the dependencies change rarely and are much bigger. If I can add them as a separate Docker layer or AWS Lambda layer then this can be cached on the server and reused when I upload a new version - saving time, bandwidth, and money.

I was able to figure how how to do that with Leiningen thanks to the invaluable help of mikerod at the #leiningen channel:

`project.clj`

(defproject myapp "0.1.0-SNAPSHOT"
  :dependencies [...]
  :target-path "target/%s/"                                     ;; (1)
  :auto-clean false                                             ;; (2)
  :profiles {:jar     {:jar-name "myapp.jar"                    ;; (3)
                       :main minbedrift-uav.core ; actually not needed
                       :aot [minbedrift-uav.core]}
             :uberjar {:uberjar-name "myapp-dependencies.jar"   ;; (4)
                       :uberjar-exclusions [#"README.md" "project.clj"]
                       :source-paths ^:replace []
                       :resource-paths ^:replace []}}
  :aliases {"jar" ["with-profile" "jar" "jar"]})                ;; (5)

Highlights

Store jar and uberjar build artefacts separately so that they do not mess up with each other
Disable :auto-clean so that building the uberjar won’t delete the jar
Move the :main and :aot from the top level into the new :jar profile so that they are not run during uberjar-ing (it seems that :main cannot be overriden to nil in :uberjar; and with nil we cannot apply ^:replace)
Configure the uberjar to have empty (re)source paths and thus not include the application code; here ^:replace is crucial
Add a convenience alias so that lein jar will automatically activate the jar profile

`Dockerfile`

FROM azul/zulu-openjdk-alpine:11-jre

WORKDIR /app
COPY /target/uberjar/myapp-dependencies.jar /app/
COPY /target/jar/myapp.jar /app/

CMD java -Dfile.encoding=UTF-8 -cp myapp-dependencies.jar:myapp.jar myapp.core

The crucial thing here is to include the dependencies before the application code.

Building

lein do clean, jar, uberjar
docker build -t myapp .

(Notice that we have to clean manually now.)

Alternatives

Alternatively we could use e.g. the lein-libdir plugin to copy all dependencies into a directory and include their jars individually (e.g. COPY /lib/*.jar /app/)

Tags: clojure DevOps Docker