Maven multi module projects are not shiny. They are not new. They are the plumbing that lets a codebase grow without turning into spaghetti. If your repo already has a few folders with random build files, or you are shipping more than one app that shares the same core classes, you are a good candidate. Today the tools are solid, the plugins are mature, and Jenkins just split from Hudson, which means build servers are getting a lot of attention. The big question is not can Maven do this. The big question is how to structure it so it lasts.
Problem framing
Projects age. Teams add a web app, a batch job, a shared library, and a couple of command line tools. Copy and paste shows up. Version mismatches show up. Releases get scary. Maven gives you a parent POM with a reactor that builds modules in the right order. With a parent you lock down versions of dependencies and plugins in one place. With modules you keep clear borders. The goal is one place to manage versions and many small artifacts that move together.
Three case walkthrough
Case 1: Core library used by several apps
Picture a shared domain library and two consumers. One is a WAR, one is a REST service that outputs JSON. Create a parent POM named company-parent. Under it add modules: domain, webapp, service. The web and service depend on domain. Keep all versions in the parent with dependencyManagement. Put plugin versions there too. You now tag and release from the top. The Maven reactor builds domain first, then the apps. Your build server like Jenkins or Hudson points at the parent and you get one green light for everything.
Case 2: Product with extensions
You ship a platform with optional add ons for payments, search, and import. Use a parent with modules per extension: platform, ext-payments, ext-search, ext-import. Keep the platform as a clean API. Extensions depend on the API but not on each other. Add an assembly module that creates bundles for customers with the right set of JARs. You can run one pipeline per extension on your build server and another aggregator job at the top for release time. This gives you steady daily builds and a simple story when you tag a release.
Case 3: Layers split for testability
Your app started as a single module and test time is painful. Split into api, persistence, service, web. Keep contracts in api. Put database code in persistence. Move business rules to service. Keep controllers and views in web. Unit tests run fast in api and service. Slower tests run in persistence and web. The parent POM wires it all and a top level failsafe setup runs slower tests only when needed. IDE support is ready out of the box in Eclipse m2e, IntelliJ IDEA, and NetBeans.
Objections and replies
It is slower. A top level build can be heavier than a single module. Use -pl with a module list or -am to build what changed. Developers run module builds locally. The server runs the full cycle on a schedule and on pull requests.
It is hard to navigate. Only if modules are random. Keep names clear and keep boundaries strict. Put docs in the parent README that map module names to purpose. Add a dependency graph report with maven project info and share it on your server.
IDE support is flaky. A few years ago, yes. Today m2e is stable, IntelliJ handles reactors well, and NetBeans has Maven by default. Import the root POM and let the IDE manage classpaths per module. If something looks off, reimport the project. That fixes most sync issues.
Releases are scary. They stop being scary when the parent owns versions. Use maven release plugin to bump versions and tag. Use versions maven plugin to track updates. Do not let modules declare their own plugin versions unless there is a clear reason.
Action oriented close
Pick one repo and try this:
- Create a parent POM with packaging pom and a short groupId and artifactId.
- Move common dependency and plugin versions into dependencyManagement and pluginManagement.
- Split code into small modules with single focus. Start with three.
- Point your build server at the parent. Keep a fast job for quick feedback and a nightly job for the full run.
- Add quality gates in the parent: surefire for unit tests, failsafe for slow tests, checkstyle, PMD, and FindBugs.
- Adopt a simple version rule. Release from the root with the release plugin and tag in Git or Subversion.
Keep the parent small, keep module borders obvious, and let Maven do the boring work. When your app grows or the team splits, you will be glad you set the structure early. That is the quiet power of a good multi module project: steady builds, clear ownership, and stress free releases.