Figuring Out Maven: Group/Artifact Names and Repositories

Dec 8, 2014 4:34 PM

Tags: maven

As I fiddle with Maven, I figure it may be useful to share my growing understanding of it - or at least preliminary assumptions. Any of these posts should not be taken as a true guide to learning Maven, since I'm just muddling through myself, but I suspect that my path will be similar to a lot of other Domino developers'.

The first thing I feel I grokked about Maven is its concept of repositories, mostly because it's the easiest concept I've run across. Repositories in Maven seem to match up nicely to their analogues in other environments, such as Eclipse Update Sites or Debian/Ubuntu apt repositories. There's the default "Maven Central" repository, which is similar to the main apt repositories: it contains a very large collection of software projects, available by group+artifact name. This is what you see on the pages for popular software projects: they mention the group/artifact pair and that's enough to use it.

For projects that aren't in Central, it's similar to adding a repo to Debian or an Update Site to Eclipse. You add some repository information to your project or the your user environment's settings.xml and then refer to the plugin similar to how you would with Central ones; Hibernate OGM is one such plugin.

In addition to remote repositories, there is also your local repository, stored in ~/.m2/repository. This contains any Maven projects where you built and ran install locally, and are then available to other Maven projects. This is how I handled my dependencies on the ExtLib and ODA: I ran Maven installs for each to add them to my local repository.

You can also download and store repositories of pre-built plugins locally, and the IBM Domino Update Site for Build Management is an example of this. The way to use this is to extract the ZIP file and then point to the updateSite directory in the same way that you would a remote repository, albeit with a file:// URL (in this case, ideally stored in a Maven environment variable).

The final aspect of this is the way bits of software are designated within a repository: by "group ID" and "artifact ID". The group ID seems like it should be globally unique, and tends to follow the reverse-DNS convention of Java package names. So a group ID might be something like "com.google.guava" or "com.igm.xsp.extlib". These don't have a specific analogue with OSGi development, but are effectively similar to the naming scheme for update site projects (even though Maven groups may contain OSGi update sites). Within a repository, individual projects, called "artifacts", are identified in a way that just needs to be unique in the repository, and it looks like conventions differ here. Sometimes, the artifacts have simple base names, like "guava" or "el", while other times they have OSGi-style full reverse-DNS names. I gather that the convention falls along OSGi lines: for generic projects, short names rule the day, while for OSGi-plugin projects, the name matches the plugin ID.

So... that's the easiest part! I'm slowly getting more of a grasp of other aspects of Maven, but at least repositories seem to make sense so far.

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