Running a Clojure Uberjar inside Docker

For a sideproject I wanted to deploy a Clojure uberjar on a remote server using Docker. I imagined that to be fairly straight foward but there are some caveats you need to be aware of.

Naively my first attempt looked somewhat like this:

FROM dockerfile/java
ADD https://example.com/app-standalone.jar /
EXPOSE 8080
ENTRYPOINT [ "java", "-verbose", "-jar", "/app-standalone.jar" ]

I expected this to work. But it didn't. Instead it just printed the following:

[Opened /usr/lib/jvm/java-7-oracle/jre/lib/rt.jar]
# this can vary depending on what JRE you're using

And that has only been printed because I added -verbose when starting the jar. So if you're not running the jar verbosely it'll fail without any output. Took me quite some time to figure that out.

As it turns out the dockerfile/java image contains a WORKDIR command that somehow breaks my java invocation, even though it is using absolute paths everywhere.

What worked for me

I ended up splitting the procedure into two files in a way that allowed me to always get the most recent jar when starting the docker container.

The Dockerfile basically just adds a small script to the container that downloads and starts a jar it downloads from somewhere (S3 in my case).

FROM dockerfile/java
ADD fetch-and-run.sh /
EXPOSE 42042
EXPOSE 3000
CMD ["/bin/sh", "/fetch-and-run.sh"]

And here is fetch-and-run.sh:

#! /bin/sh
wget https://s3.amazonaws.com/example/yo-standalone.jar -O /yo-standalone.jar;
java -verbose -jar /yo-standalone.jar

Now when you build a new image from that Dockerfile it adds the fetch-and-run.sh script to the image's filesystem. Note that the jar is not part of the image but that it will be downloaded whenever a new container is being started from the image. That way a simple restart will always fetch the most recent version of the jar. In some scenarios it might become confusing to not have precise deployment tracking but in my case it turned out much more convenient than going through the process of destroying the container, deleting the image, creating a new image and starting up a new container.