Docker with Maven and Tomcat

Question

i am trying to create a docket container with Java,Maven and Tomcat.

So far, i'm able to create a container with Maven and compile my java code and create the final WAR file.

But i want to copy this WAR file to a particular folder(webapp folder of tomcat) of another container which is of Tomcat.

Approach one, Dockerfile

FROM maven:3.6.1-jdk-8 as maven_builder

ENV HOME=/app

WORKDIR $HOME

ADD pom.xml $HOME

RUN ["/usr/local/bin/mvn-entrypoint.sh", "mvn", "verify", "clean", "--fail-never"]

ADD . $HOME

RUN ["mvn","clean","install","-T","2C","-DskipTests=true"]

FROM tomcat:8.5.43-jdk8

COPY --from=maven_builder $HOME/wc_admin/target/wc-admin.war /usr/local/tomcat/webapps

i tried to simply copy it, but the intermediate container is deleted before copying.

Approach 2, i am trying to write a YAML for docker-compose and use shared volume concept.

version: '3'
services:
    maven-build:
        build: .
        volumes:
            - 'myshare:/shared'

    tomcat-build:
        image: 'tomcat:8.5.43-jdk8'
        ports:
            - '8080:8080'
        volumes:
            - 'myshare:/shared'

volumes:
  myshare:
    driver: local '

i'm not able to figure out, how do i copy a particular file from one container(i.e maven_build) to another container (i.e tomcat's webapp folder).

Answer 1

Your first (multi-stage build) approach is better practice.

In the multi-stage build, every time you have a new FROM command to start a new image, the execution environment resets. In particular, when the first stage sets a HOME environment variable, that gets reset in the second stage, so the final COPY command is copying out of /wc_admin/... and not the directory where the application got built.

It's typical to treat Docker filesystem layouts as fixed, and to not set variables like HOME that are meaningful in other contexts but not really in Docker. I'd just hard-code /app throughout:

FROM maven:3.6.1-jdk-8 as maven_builder
WORKDIR /app
ADD pom.xml .
...
FROM tomcat:8.5.43-jdk8
COPY --from=maven_builder /app/wc_admin/target/wc-admin.war /usr/local/tomcat/webapps

The volumes approach has two big problems. People try to take advantage of Docker populating a named volume with content from the image, but this only works the first time you run it. If you rebuild the image, Docker refuses to touch the volume (it has important user data that must be preserved, that's what volumes are for) and so your volume will keep your old .war file. If you'll ever run your application in Kubernetes, it will not auto-populate volumes from images and you'll need to do more work to make a volumes-for-sharing setup work.