Project Object Model is the basic unit of Maven and defines the project structure, dependencies, etc.

The following are very minimal to create a POM:

• project root
• modelVersion – should be set to 4.0.0
• groupId – the ID of the project's group
• artifactId – the ID of the artifact (project)
• version – the version of the artifact under the specified group

groupId, artifactId and version are called Maven coordinates and sometimes abbreviated with GAV. They uniquely identify the resulting artifact of a project in a Maven repository (and should do so in the entire universe).

A minimal sample POM looks like:

<project>
  <modelVersion>4.0.0</modelVersion>

  <groupId>com.sample</groupId>
  <artifactId>sample-app</artifactId>
  <version>0.0.1-SNAPSHOT</version>
</project>

Inheritance is the biggest asset of the POM, where the following can be managed from super POM to child POM.

• dependencies
• developers and contributors
• plugin lists (including reports)
• plugin executions with matching ids
• plugin configuration

The following enables the inheritance

  <parent>
    <groupId>com.sample</groupId>
    <artifactId>sample-app-parent</artifactId>
    <version>1.0.0</version>
  </parent>

POM structure looks like

<project>
  <parent>
    <groupId>com.sample</groupId>
    <artifactId>sample-app-parent</artifactId>
    <version>1.0.0</version>
  </parent>
  <modelVersion>4.0.0</modelVersion>
  <groupId>com.sample</groupId>
  <artifactId>sample-app</artifactId>
  <version>0.0.1-SNAPSHOT</version>
</project>

The modules of a multi-module project are aggregated from a hierarchical structure.

The root pom packing should look like:

<packaging>pom</packaging>

The following will be the directory structure of the project:

 |-- sample-app
  \   `-- pom.xml
   |-- sample-module-1
   |   `-- pom.xml
   |-- sample-module-2
   |   `-- pom.xml

Root POM:

<project>
  <modelVersion>4.0.0</modelVersion>
  <groupId>com.sample</groupId>
  <artifactId>sample-app</artifactId>
  <version>0.0.1-SNAPSHOT</version>
  <packaging>pom</packaging>
 
  <modules>
    <module>sample-module-1</module>
    <module>sample-module-2</module>
  </modules>
  <dependencyManagement>
    ...
  </dependencyManagement>
</project>