Caching Maven parent in offline build

Question

TL/DR: How can I cache the pom.xml file's <parent> so my build can be run in offline mode?

I'm using Docker to build a maven project. My goal is to add two steps to the build: one to download all of the dependencies, and another to build the project. Here's what my Dockerfile looks like so far:

FROM maven:3.3-jdk-8

# Download the project dependencies (so they can be cached by Docker)
ADD pom.xml /runtime/
WORKDIR /runtime
RUN mvn dependency:go-offline
RUN mvn dependency:resolve-plugins

# Mount the local repository
ADD . /runtime

# Build the service
RUN mvn clean package -o -DskipTests

This seems to work fine for the plugins. I checked the /root/.m2/repository and everything seems to be in order.

Edit: When double checking for the /root/.m2/repository directory, it's no longer there. For some reason, Maven isn't saving any of the dependencies to this location.

Edit 2: After building the Docker image, there's no /root/.m2/repository directory. However, if I run mvn dependency:go-offline from within a shell inside the Docker container, the directory is created without a problem.

When I attempt build my application, I get the following error:

[ERROR] [ERROR] Some problems were encountered while processing the POMs:
[FATAL] Non-resolvable parent POM for com.example:service:1.2: Cannot access central (http://jcenter.bintray.com) in offline mode and the artifact org.springframework.boot:spring-boot-starter-parent:pom:1.4.0.M3 has not been downloaded from it before. and 'parent.relativePath' points at wrong local POM @ line 14, column 13
 @
[ERROR] The build could not read 1 project -> [Help 1]
[ERROR]
[ERROR]   The project com.sample:service:1.2 (/runtime/pom.xml) has 1 error
[ERROR]     Non-resolvable parent POM for com.oe:graph-service:1.2: Cannot access central (http://jcenter.bintray.com) in offline mode and the artifact org.springframework.boot:spring-boot-starter-parent:pom:1.4.0.M3 has not been downloaded from it before. and 'parent.relativePath' points at wrong local POM @ line 14, column 13 -> [Help 2]
[ERROR]
[ERROR] To see the full stack trace of the errors, re-run Maven with the -e switch.
[ERROR] Re-run Maven using the -X switch to enable full debug logging.
[ERROR]
[ERROR] For more information about the errors and possible solutions, please read the following articles:
[ERROR] [Help 1] http://cwiki.apache.org/confluence/display/MAVEN/ProjectBuildingException
[ERROR] [Help 2] http://cwiki.apache.org/confluence/display/MAVEN/UnresolvableModelException

The problem seems to be that mvn dependency:go-offline isn't resolving the parent. When I run the build in offline mode, it breaks.

Here are the relevant portions of my pom.xml file:

<?xml version="1.0" encoding="UTF-8"?>

<project xmlns="http://maven.apache.org/POM/4.0.0" xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance"
         xsi:schemaLocation="http://maven.apache.org/POM/4.0.0 http://maven.apache.org/maven-v4_0_0.xsd">
    <modelVersion>4.0.0</modelVersion>

    ...

    <parent>
        <groupId>org.springframework.boot</groupId>
        <artifactId>spring-boot-starter-parent</artifactId>
        <version>1.4.0.M3</version>
    </parent>

    ...

    <repositories>
        <repository>
            <snapshots>
                <enabled>false</enabled>
            </snapshots>
            <id>central</id>
            <name>bintray</name>
            <url>http://jcenter.bintray.com</url>
        </repository>

        <repository>
            <id>repository.springsource.snapshot</id>
            <name>SpringSource Snapshot Repository</name>
            <url>http://repo.springsource.org/snapshot</url>
        </repository>

        <repository>
            <id>spring-milestones</id>
            <url>http://repo.spring.io/milestone</url>
        </repository>
    </repositories>

    <dependencies>
       ...

        <dependency>
            <groupId>org.springframework.boot</groupId>
            <artifactId>spring-boot-starter-web</artifactId>
            <exclusions>
                <exclusion>
                    <groupId>org.springframework.boot</groupId>
                    <artifactId>spring-boot-starter-tomcat</artifactId>
                </exclusion>
            </exclusions>
        </dependency>
        <dependency>
            <groupId>org.springframework.boot</groupId>
            <artifactId>spring-boot-starter-jetty</artifactId>
        </dependency>
        <dependency>
            <groupId>org.springframework.boot</groupId>
            <artifactId>spring-boot-starter-test</artifactId>
            <scope>test</scope>
        </dependency>
        <dependency>
            <groupId>org.springframework.scala</groupId>
            <artifactId>spring-scala_2.11</artifactId>
            <version>1.0.0.BUILD-SNAPSHOT</version>
        </dependency>
        <dependency>
            <groupId>org.springframework.boot</groupId>
            <artifactId>spring-boot-starter-security</artifactId>
        </dependency>
        <dependency>
            <groupId>org.springframework.boot</groupId>
            <artifactId>spring-boot-starter-actuator</artifactId>
        </dependency>
        <dependency>
            <groupId>org.springframework.boot</groupId>
            <artifactId>spring-boot-starter-jdbc</artifactId>
        </dependency>
        <!-- disabling Spring cloud AWS until proper testing harnesses can be set up -->
        <dependency>
            <groupId>org.apache.httpcomponents</groupId>
            <artifactId>httpcore</artifactId>
            <version>LATEST</version>
        </dependency>
        <dependency>
            <groupId>org.springframework.cloud</groupId>
            <artifactId>spring-cloud-aws-autoconfigure</artifactId>
            <version>1.1.0.RELEASE</version>
        </dependency>

        ...
    </dependencies>

    ...
</project>

Answer 1

If you customize maven's settings.xml file you can save your repository files on image.

Create a custom version of settings.xml, with the localRepository setting modified, like this:

<settings xmlns="http://maven.apache.org/SETTINGS/1.0.0"
          xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance"
          xsi:schemaLocation="http://maven.apache.org/SETTINGS/1.0.0 http://maven.apache.org/xsd/settings-1.0.0.xsd">
  <!-- localRepository
   | The path to the local repository maven will use to store artifacts.
   |
   | Default: ${user.home}/.m2/repository -->
  <localRepository>/usr/share/maven/repo</localRepository>
...

Then override the default configuration when building your image:

FROM maven:3-jdk-8

COPY settings.xml /usr/share/maven/conf/settings.xml

RUN mkdir -p /usr/src/app
WORKDIR /usr/src/app

ADD . /usr/src/app

RUN mvn dependency:go-offline

RUN mvn clean package

Now your repository is stored in /usr/share/maven/repo.

Using onbuild

You can also create a base image using ONBUILD, this will allow to have custom configured images for every maven project.

Like this:

FROM maven:3-jdk-8

COPY settings.xml /usr/share/maven/conf/settings.xml

RUN mkdir -p /usr/src/app
WORKDIR /usr/src/app

ONBUILD ADD . /usr/src/app

ONBUILD RUN mvn dependency:go-offline

ONBUILD RUN mvn clean package

Then build the image:

docker build -t mvn_bldr .

This will create a template for other maven images. Then you can create your custom downstream image with:

FROM mvn_bldr

If you want to customize your image further you can add more instructions, every instruction of the template will be triggered after the FROM mvn_bldr command, as in the docs:

The trigger will be executed in the context of the downstream build, as if it had been inserted immediately after the FROM instruction in the downstream Dockerfile.

Answer 2

It may seem like using RUN with Docker is like executing commands in a shell script, but it's not. Every instance of RUN gets applied to a new container that results from the changes created by the previous command. So each command is executing inside of a new container context.

Dockerifles can contain a VOLUME reference, which mounts an external directory inside the Docker container. If there were any files inside the volume, those files are wiped out. If you don't explicitly specify a volume, Docker is happy to instead create an empty folder.

While my Dockerfile doesn't contain an explicit reference to a VOLUME, its parent does. So, even though my mvn dependency:go-offline command was running, those files were being wiped out in the next step by the VOLUME specified in the docker-maven Dockerfile.

In the end, I couldn't find a good way to make the maven Docker image work, so I switched to the openjdk image and installed maven via apt-get instead.