atsea-sample-shop-app | sample app that uses a Java Spring Boot backend | Continuous Deployment library

I came across the same question. I found out it's due to differences between Maven targets (e.g. dependency:resolve vs dependency:resolve-plugin). Basically, dependency:resolve is for application libraries, dependency:resolve-plugin is for plugin libraries. Hence, libraries are downloaded in both RUN steps.

dependency:resolve tells Maven to resolve all dependencies and displays the version. JAVA 9 NOTE: will display the module name when running with Java 9.

dependency:resolve-plugins tells Maven to resolve plugins and their dependencies.

https://maven.apache.org/plugins/maven-dependency-plugin/index.html

Even with dependency:resolve-plugins, Maven will not download all required libraries as package is a built-in target and requires additional libraries which dependency:resolve-plugin won't know to resolve in the first RUN. I also tried dependency:go-offline without success.

One solution is to run your build targets before and after adding your code to the build image. This will pull all the plugin dependencies into the lower layer allowing them to be re-used.

Applying this solution to your example above is as follows: