Build executable jar in docker container using Maven

Question

How can I build an executable jar file in a docker container using Maven?

A simplified example of what I am trying to solve.

Local repo layout:

Main:

package org.example;

public class Main {
    public static void main(String[] args) {
        System.out.println("Hello world!");
    }
}

pom.xml:

<?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/xsd/maven-4.0.0.xsd">
    <modelVersion>4.0.0</modelVersion>

    <groupId>org.example</groupId>
    <artifactId>maven-troubleshooting</artifactId>
    <version>1.0-SNAPSHOT</version>
    <build>
        <plugins>
            <plugin>
                <groupId>org.apache.maven.plugins</groupId>
                <artifactId>maven-jar-plugin</artifactId>
                <version>3.3.0</version>
                <configuration>
                    <archive>
                        <manifest>
                            <mainClass>org.example.Main</mainClass>
                        </manifest>
                    </archive>
                </configuration>
            </plugin>
        </plugins>
    </build>

    <properties>
        <maven.compiler.source>11</maven.compiler.source>
        <maven.compiler.target>11</maven.compiler.target>
        <project.build.sourceEncoding>UTF-8</project.build.sourceEncoding>
    </properties>

</project>

Local machine workflow(working as expected):

mvn clean package

...[INFO] BUILD SUCCESS

java -jar target/maven-troubleshooting-1.0-SNAPSHOT.jar

Hello world!

Dockerfile:

FROM maven:3.8.6-openjdk-11
RUN mkdir -p /usr/src/app
WORKDIR /usr/src/app
COPY pom.xml /usr/src/app
COPY src /usr/src/app

Docker workflow:

docker build. -t simplified-mvn-troubleshooting
docker run -it simplified-mvn-troubleshooting bash

(now in the running container)

mvn clean package
...[INFO] BUILD SUCCESS

java -jar target/maven-troubleshooting-1.0-SNAPSHOT.jar

Error: Could not find or load main class org.example.Main
Caused by: java.lang.ClassNotFoundException: org.example.Main

What am I missing?

Answer 1

I solved this after a few hours of trial and error. It appears Maven is pretty strict with directory structure. I changed my dockerfile to the below which solved the issue.

FROM maven:3.8.6-openjdk-11 AS build
COPY pom.xml .
COPY src /src

I can now package and run the jar all in the docker container directly after attaching to the container.