Monday, July 30, 2012

Taming Maven: Version Properties

Getting started with Maven is easy, and once you use its dependency management feature, you'll wonder why you waited so long. For simple web-apps or single-module projects, it Just Works.

However, most software developers aren't working on simple, one-module projects. We work in organizations that manage many projects, often depending on one-another. And in this situation, the basic Maven project breaks down: you find that every project has a differing set of dependencies, some of which are incompatible. This is the first in a series of postings about taming a multi-project development environment.

To start, replace all of your hardcoded dependency versions with properties.

Projects accumulate dependencies over time: you might start out with a few of the core Spring packages, then add a few of the Apache Commons projects, then a few more Spring projects, then some libraries that another part of your organization maintains. Pretty soon you'll have dozens of dependencies, completely unordered. Just finding a dependency in the mess becomes difficult, even if you have a tool like m2eclipse. And it becomes very easy to have two related dependencies — or even duplicate dependencies — with different versions. Maven can resolve most of these problems automagically, but when it fails, you're in for a long and painful diagnosis session.

But, if you use properties for your dependencies, and adopt a consistent naming strategy for those properties, you may not be able to find your dependency references, but at least the versions will be defined in one place. Start by adding a <properties> section to your POM; I generally place it near the top of the POM, before the <build> and <dependencies> sections (both of these tend to be long).

    <!-- and so on, for all of your dependencies -->

Each property is its own element, and the element name is the property name. You can name your properties anything you want (as long as it's a legal XML element name), but for version properties I think that GROUPID.version makes the most sense. Or use GROUPID.ARTIFACTID.version if there are different artifacts for the same group that don't have a common version (for example, Spring Core and Spring Security).

Next, update the dependency to use that property, rather than a hardcoded version number.


Once all of your POMs use version properties, you can start to organize dependencies across projects. This can be as simple as running grep to find all projects that use a particular property. But Maven gives you a better solution, which will be the topic of tomorrow's post.

This series of posts was prompted by several recent projects where I worked with development organizations that had large project bases built on Maven. I was originally planning to show some of the Vim macros that I used to clean up POMs, but decided instead to start work on a tool to clean up POMs.


David Carver said...

Why not use the Dependency Management entries. You can control the version specified in Dependency Management, and then don't need to include the version at all in the POM when you specify a dependency on the artifact.

Keith Gregory said...

I have a deep dislike of the dependency management section, as noted in a later post. I have seen far too many teams that used dependency management, then had to work around it when updating dependencies on a single project (typically during development). I think version properties -- which can be overridden in the child pom -- are a far better approach.