Tag Archives: tips

Apache Maven 3.0 Released: a Few Important Tips

Just short of the 5 year anniversary of the Maven 2.0 release (Oct 19, 2005), Maven 3.0 has shipped today. You can download it from the Maven website. I’ve now been using it for about 6 months for all but one project, by which point it was already quite stable. Luckily it also arrives just in time for ApacheCon, since I have updated my training to cover it! All-in-all it’s a great release – quite a bit faster and in places more predictable.

New Features

For the most part, this is not a feature release, but a performance and architectural release. Much has already been written about underlying technology changes (like switching from Plexus to Guice). The main points of interest for me are:

  • parallel builds – build modules in parallel when enabled to utilise multiple cores and get even more performance gains. It’s optional, so take some time to try it out
  • improved performance & predictability – while maintaining and documenting compatibility
  • improved reactor – behaves more consistently between building multi-module projects and subsets of them
  • validation and error reporting – unrecommended and deprecated behaviour is now pointed out and error reporting improved
  • improved classloading – extensions and plugins are loaded in a more self-contained fashion to allow more flexibility

There are still some “gotchas”, tips and tricks to take note of, however.

Check the Compatibility Notes

There is a page dedicated to compatibility notes between Maven 2 and Maven 3. This should be considered required reading for anyone making the switch, as it highlights some changes that you may need to adjust your projects or environment for. In most cases there’ll be no major issues, and only some quick fixes. In my opinion, the key ones to keep an eye on:

  • Stricter POM validation – many projects will need to quickly tighten up their POMs to get running
  • Site plugin – if you’re using this for reporting, you may have work to do to get the same results
  • Metadata updates – Maven 3 checks remote repositories less often in most cases. Intermittent remote failures can be cached for a period of time – so check error messages carefully
  • Plugin compatibility matrix – check this for any plugins you’re using that might not be updated yet

Watch the Start of the Build

As mentioned above, POM validation is stricter and you may see a few failures on some projects that need updating. A much larger number of projects will probably see warnings about unrecommended behaviour or deprecated features. Watch the start of your builds carefully on the first run of a project for any warnings, and take care of the reported issues as soon as you can.

Make Switching Easy

I highly recommend a script such as the one attached to MNG-2730 to make it easy to switch between Maven versions. This has always been quite useful for adopting new releases, but even more so in this case as you may on rare occasions need to drop back to Maven 2.2.1 for a particular project.

A Few Words

I’ll admit there were long periods of time where I thought this release would never happen. Particular congratulations go to Benjamin for his effort over the last year and a bit to pick it up and methodically drive it home – I’ve been there before and I know that it is full of both fun and frustration!

Implementation of parallel builds was also a big job. I recall Dan hacking away at it last ApacheCon, and from there Kristian put in a huge effort to get a production-ready implementation and work through nasty thread safety issues in some plugins and components. I hope this is something that gains more traction going forward.

It’s also worth acknowledging the guys plugging away at getting the Site plugin infrastructure back in place – Olivier, Hervé, and Dennis in particular.

And congrats to all the Maven developers and contributors that had a hand in this release, and all those that got Maven where it is today. Hopefully more great things to come, and perhaps a little faster next time! :)

Reactor: My New Favourite Maven Plugin

Dan recently put together a great new plugin to manipulate the Maven reactor, currently being voted on for release.

It has a few goals:

  • reactor:resume – pick up where a previous build left off
  • reactor:make / make-dependents – make a project and its dependencies or those that depend on it
  • reactor:make-scm-changes – make any projects that have uncommitted changes and their dependencies

This is particularly useful for building a subset of modules during development to reduce the time of a build. While it is a good practice to break down your build into a structure that might reflect common build scenarios, this will be useful in plenty of circumstances.

Think about the 10 minute build that fails with a missing dependency about 9 minutes in, but still with 5 modules to build. You don’t want to build it all again, and you don’t want to build 5 modules manually. Now, you just fix the dependency problem (say, by adding a new proxy/artifact to your locked down repository manager), then run mvn reactor:resume -Dfrom=module-that-failed. Simple!

As I mentioned in an earlier post, this same functionality is intended to be built natively into the final Maven 2.1.0 release (with some exceptions) – keep an eye out for it!