How to embed maven modules as .class dependencies as opposed to jars?

Question

I have a multi-module maven project that is used to produce a single spring boot fat jar. My project looks something like this.

 - Parent Module Aggergator 
    - A 
    - B
    - C 
    - app <-- app.jar is the only thing I want to publish 

In my case module A, B, C are only ever used by app and should not be published into maven repo. I have split up the app into multi-module project because it's a lot of code in the app and it's to work with that way.

Currently the app.jar will contain inside it a.jar, b.jar c.jar.

Is there a way to tell maven that the compiled classes from module A, B, C should just be inserted into app.jar classes folder without ever producing A.JAR, B.JAR, C.JAR?

Answer 1

I use the Maven Shade Plugin for my multi-module project; it creates a single JAR and extracts each module into it rather than creating multiple JAR files:

Parent 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>

    <properties>
        <maven.compiler.source>1.8</maven.compiler.source>
        <maven.compiler.target>1.8</maven.compiler.target>
    </properties>

    <groupId>...</groupId>
    <artifactId>pipeline</artifactId>
    <version>1.0-SNAPSHOT</version>
    <packaging>pom</packaging>

    <modules>
        <module>firehose</module>
        <module>gson</module>
        <module>lambda</module>
        <module>mapper</module>
        <module>model</module>
        <module>receiver</module>
        <module>redshift</module>
        <module>reloader</module>
        <module>s3</module>
        <module>sns</module>
        <module>sqs</module>
        <module>systemstests</module>
        <module>transaction</module>
        <module>utility</module>
    </modules>

    <dependencies>
        ...
    </dependencies>

    <build>
        <plugins>
            <plugin>
                <groupId>org.apache.maven.plugins</groupId>
                <artifactId>maven-shade-plugin</artifactId>
                <version>3.1.1</version>
                <executions>
                    <execution>
                        <phase>package</phase>
                        <goals>
                            <goal>shade</goal>
                        </goals>
                        <configuration>
                            <filters>
                                <filter>
                                    <artifact>*:*</artifact>
                                    <excludes>
                                        <exclude>META-INF/*.SF</exclude>
                                        <exclude>META-INF/*.DSA</exclude>
                                        <exclude>META-INF/*.RSA</exclude>
                                    </excludes>
                                </filter>
                            </filters>
                        </configuration>
                    </execution>
                </executions>
            </plugin>
        </plugins>
    </build>
</project>

Child pom.xml (the JAR):

<?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>

    <parent>
        <groupId>...</groupId>
        <artifactId>pipeline</artifactId>
        <version>1.0-SNAPSHOT</version>
    </parent>

    <artifactId>lambda</artifactId>

    <dependencies>
        ...
    </dependencies>

    <build>
        <plugins>
            <plugin>
                <groupId>org.apache.maven.plugins</groupId>
                <artifactId>maven-jar-plugin</artifactId>
                <version>3.1.0</version>
                <configuration>
                   <finalName>MyJar</finalName>
                </configuration>
            </plugin>
        </plugins>
    </build>
</project>

Answer 2

Here is a non answer.

Is there a way to tell maven that the compiled classes from module A, B, C should just be inserted into app.jar classes folder without ever producing A.JAR, B.JAR, C.JAR?

You could use the repackage goal of the spring boot maven plugin that flatten the dependencies into classes in the uber jar.

In my case module A, B, C are only ever used by app and should not be published into maven repo.

Adding the modules in a local repository is really wanted to have a efficient and standard build.
Without that, you will need to compile systematically each module at each time you want to run your spring boot app.
While actually sometimes you need to build dependencies, but other times you don't need because these are already updated.
Or else you will be constraint to twist the default Maven way of work by adding manual tasks to compile from the spring boot module the other modules and to move the compiled classes into the spring boot module. Really not a gift for the people that will have to read/maintain this configuration.