I’m trying to figure out how to create a two-stage Docker build using Maven that would properly utilise Docker layer caching. Containing the whole build in a Docker container without external dependencies on volume mounts (for .m2 cache) has a lot of advantages. The only missing piece of the puzzle I can’t figure out is how to force Maven to download ALL dependencies just by using the pom file and not executing any compilation. The idea behind is to: 1. copy the pom file into the image 2. use maven to download all dependencies 3. copy the sources to the image 4. use maven to build the final artifact This way Docker will reuse the layer containing the dependencies if the pom file didn’t change. So dependency caching would be implemented in a Docker-ish way that works anywhere the Docker daemon is present as opposed to relying on Maven being installed and artifacts getting stored in a shared .m2/repository directory.
Now the problem - Maven’s lazy downloading of dependencies. Even if I try to execute “mvn dependency:go-offline” Maven still doesn’t download plugins and other dependencies, which would only be required during the packaging phase. So the following Docker file doesn’t work as intended: FROM maven AS build WORKDIR /build ADD pom.xml /build RUN mvn dependency:go-offline ADD src /build/src RUN mvn -o package FROM openjdk:8-jdk WORKDIR /app COPY --from=build /build/target/app.jar /app CMD java -jar /app/app.jar Maven will still start downloading more dependencies on line 6. I found a workaround for this - execute “mvn package” with a single fake main class on line 4, but this is not a nice solution. Can anyone give me some option I’m missing or confirm that this is the way things are? In that case I can go ahead write a blog post showing my workaround to help other struggling with the same problem. Thank you! Adam Sandor from Container Solutions -- Ádám Sándor Senior Engineer / Consultant Container Solutions <http://container-solutions.com/> 0680126174 <https://twitter.com/adamsand0r> <https://www.linkedin.com/in/adamsandor/>
