Maven multi module projects

A multi-module project is built from an aggregator POM that manages a group of submodules. In most cases, the aggregator is located in the project's root directory and must have packaging of type pom.

Now, the submodules are regular Maven projects, and they can be built separately or through the aggregator POM.



By building the project through the aggregator POM, each project that has a packaging type different than pom will result in a built archive file.

Parent POM

Maven supports inheritance in a way that each pom.xml file has the implicit parent POM, it's called Super POM and can be located in the Maven binaries. These two files are merged by Maven and form the Effective POM.


Hence, we can create our own pom.xml file which will serve us as the parent project. Then, we can include there all configurations with dependencies and set this as the parent of our child modules, so they'll inherit from it.

Besides the inheritance, Maven provides the notion of aggregation. Parent POM that leverages this functionality is called an aggregate POM. Basically, this kind of POM declares its modules explicitly in its pom.xml file.

Submodule

Submodules or subprojects are regular Maven projects that inherit from parent POM. As we already know, inheritance lets us share the configuration and dependencies with submodules. However, if we'd like to build or release our project in one shot, we have to declare our submodules explicitly in parent POM. Ultimately, our parent POM will be the parent as well as the aggregate POM.

Creating parent pom

mvn archetype:generate -DgroupId=org.company -DartifactId=parent-project

<packaging>pom</packaging>

Creating submodule projects

navigate to the parent project directory

cd parent-project

mvn archetype:generate -DgroupId=org.company -DartifactId=core mvn archetype:generate -DgroupId=org.company -DartifactId=service mvn archetype:generate -DgroupId=org.company -DartifactId=webapp

 Add below tag into parent pom

<modules> <module>core</module> <module>service</module> <module>webapp</module> </modules>

Add below tag in each module

<parent> <groupId>org.company</groupId> <artifactId>parent-project</artifactId> <version>1.0-SNAPSHOT</version> </parent>

Build all the project from the parent pom using below command

mvn package

Source: https://www.baeldung.com/