Maven 3 and Plugin Mysteries

You probably know that Maven 3 has landed. Before testing it with our projects, I was curious about the plugins that are defined in the Maven master POM's pluginManagement section and hence are locked-down with respect to their version. Since all projects inherit from this master POM, they will use the respective version of the plugins if not explicitely overwritten anywhere in the project's POM hierarchy.

Maven 3 is a bit more strict concerning automatic version resolution of invoked plugins. Other than Maven 2, it will always use the latest release (i.e. non-SNAPSHOT) version of a plugin if there was no explicit version specified in the POM or on the command line. Moreover, it will issue a warning when missing plugin versions are detected "to encourage the addition of plugin versions to the POM or one of its parent POMs". This is to increase reproducability of builds.

Thus, in Maven 3 the desired build stability is ensured by urging the POM author to give explicit plugin versions, and doesn't any more rely on a full list of plugins (with versions) defined in the master POM. That's why I expected to find a small or even empty pluginManagement section. Well, let's see.

To find out what's in the pluginManagement of master POM, you just have to create a minimal POM and show the effective POM (that results from the application of interpolation and inheritance, including master POM and active profiles) by calling help:effective-pom for this simple project.

So, what do we get? The following list shows the plugin versions that are defined in the Maven 2.2.1 master POM, the Maven 3 master POM, as well as the most recent version of those plugins.

Well, we can see some interesting details here:

• The number of plugins defined in the master POMs pluginManagement section is drastically less for Maven 3 than for Maven 2.2.1 – that's what we expected. However, there are still a few.


• Which pluings are listed and which are not? It seems like the plugins for the most basic lifecycle phases (like clean, install, deploy) are predefined, but others are not (like compile or jar). Is there any policy?


• What is really odd: for some of the plugins that are predefined, there is a newer version available than is listed in the Maven 3 master POM (colored red). Why could that be? I have not checked, but Maven 3 is out for a few days now, so I suspect for most of those plugins the new version has been available before. Is that intentionally? Are the new versions not considered "good" or "stable" by the Maven guys? Or did they just forgot to upgrade? Or did not found it important in any kind?


• Another thing I can't explain: when you look on the Maven 3 Project Plugin Management site, there are listed a lot more plugins, and some are even of other version than what we got by showing the effective POM for a minimal project POM. How could this be? I have no clue...

In a previous post, I have listed the plugins predefined by Maven 3.0-alpha5. Interestingly, there have been a lot more of them (like for Maven 2.2.1), but the "stale version" question was the same...