Creating a Multi-Module Maven Project

A multi-module Maven project uses a root POM to orchestrate the build and share configuration across related modules. The key is understanding what the root POM does and what each child declares itself.

Root POM structure

• The root POM uses pom and serves two separate roles: the block lists child directories for Reactor build orchestration, and defines shared dependency versions that children inherit
• in the child's block points to the parent POM location on the filesystem — without it, Maven looks in ~/.m2/repository first, which fails on fresh checkouts before the parent has been installed locally
• Inter-module dependencies use ${project.version} to stay synchronized — hardcoding a version in a module reference means you must update it manually every release

Maven Reactor — build ordering

• The Reactor analyzes references between modules to determine build order automatically — service depending on core means core always compiles first
• mvn install -pl :service -am builds the target module and all its upstream dependencies, skipping unrelated modules — the most useful productivity command in large multi-module projects
• mvn install -pl :service,core -amd builds a module and everything that depends on it downstream — useful for validating a change ripples correctly through consumers

Shared configuration best practices

• Define all dependency versions in at the root — children declare what they need, the root controls which version; no version drift between modules
• Define plugin configuration in at the root and bind plugins in child POMs — this separates "what version of the plugin" from "which modules use it"
• Never commit Nexus credentials to the root POM — keep the URL in the POM and credentials in ~/.m2/settings.xml or CI environment variables