docker-maven-sample

This is a simple spring boot application that is dockerized with maven docker build plugin.

You will need all of the following components to successfully build and run this application locally following the the steps in this README:

| Java 8 | Maven 3 | Git | Docker |

In addition, to push the image to a private registry either have your own registry running somewhere or create a registry in Azure. For that you will need an Azure subscription; if you don't already have an Azure subscription, you can sign up for a free Azure account and deploy your Azure Container Registry. The registry references in the pom.xml in this project uses an Azure Container Registry.

It uses the following Dockerfile

FROM frolvlad/alpine-oraclejdk8:slim
VOLUME /tmp
ARG JAR_FILE_NAME
COPY $JAR_FILE_NAME app.jar
RUN sh -c 'touch /app.jar'
ENV JAVA_OPTS=""
ENTRYPOINT [ "sh", "-c", "java $JAVA_OPTS -Djava.security.egd=file:/dev/./urandom -jar /app.jar" ]

Build and Push Image

Since the Docker build process integrated with the Maven build process and it binds the default phases, we can type mvn package and get a Docker image. When use mvn deploy, the image gets pushed to the registry. Also we can just say mvn dockerfile:build dockerfile:push to build and push the image to the registry.

This sample demonstrates the following two features

Authentication with private Docker registry

Authenticating with maven settings.xml

Since version 1.3.0 (we use the latest 1.3.6 version in this project), private registry authentication can be done using maven settings.xml instead of docker configuration in ~/.dockercfg or ~/.docker/config.json file when pulling, pushing, or building images to private registries. This is configured in the pom.xml with the following:

<configuration>
  <serverId>${docker.image.prefix}</serverId>
  <registryUrl>https://${docker.image.prefix}</registryUrl>
  <useMavenSettingsForAuth>true</useMavenSettingsForAuth>
  <repository>${docker.image.prefix}/${project.artifactId}</repository>
  <tag>${project.version}</tag>
</configuration>

Here we provide the serverId and registryUrl also set useMavenSettingsForAuth tag value to true. This allows the maven docker plugin to read authentication information like usename and password from the maven settings.xml file.

Then, in the maven settings file, add configuration for the server:

<servers>
  <server>
    <id>registryName.example.com</id>
      <username>myUserName</username>
      <password>myPassword</password>
  </server>
</servers>

Dynamically provide arguments to the Dockerfile

Since in most cases our jar file name would change form build to build i.e. adding different version number to the file name, we can not use a static name in the Dockerfile for the jar file name. The following pom.xml maven configuration setting helps us with providing build arguments for the Dockerfile

<configuration>
  <buildArgs>
    <JAR_FILE_NAME>${project.build.finalName}.jar</JAR_FILE_NAME>
  </buildArgs>
</configuration>

The JAR_FILE_NAME is referenced in the Dockerfile where the value of JAR_FILE_NAME is ${project.build.finalName}.jar.

The Dockerfile argument passing feature comes with the docker client version 8.8.4 that's why we had to override the docker client version in the dockerfile-maven-plugin setup within the plugin section.