Create re-usable Docker layered images from java fat-jar apps.

Deploying fat-jar scala/java applications to kubernetes requires creating a docker image that normally endups being very big since it has to include at least a JVM runtime and your fat-jar file.

However, most of the time, your application code is only a tiny fraction of what makes up the fat-jar. Most likely all other content just comes from the libraries and transitive dependencies your project has. Nevertheless, even if you only make a simple change on your codebase, the whole fat-jar files gets generated again, which means the docker image also has to be generated again.

Re-creating the image and pushing to a container registry becomes problematic since each time you are building and sending another >700M image. Even if your image includes only a distroless JVM runtime plus your application fat-jar, you endup re-using almost nothing.

This project is intended to help you split the fat-jar into reusable docker layers, so that building and pushing to a Container Registry will reuse all other layers that had not changed.

This program will expand a fatjar and use the du utility to find directories that are over a size threshold (default 2M) and create a separate jar for each of those directories.

It can also be given a list of explicit directories to separate into a layer using the --add-layer option. See --help.

It's also possible to create a layer containing only the fatjar toplevel files which will change only if you edit /application.conf for example.

• If you already have Nix installed, you can run it directly using:

# Run directly using this project flake.
nix run github:vic/docker-layered-fatjar -- --help
# Or install it locally if you prefer. Installed binary is named: layers-from-fatjar
nix profile install github:vic/docker-layered-fatjar
layers-from-fatjar --help

• If you dont have Nix, you can still download the bin/layers-from-fatjar script and make sure your system has this dependencies installed

The following will split fatjar.jar file and build a docker image named myapp:layered

nix run github:vic/docker-layered-fatjar -- --docker-build fatjar.jar -- --tag myapp:layered