What's New in Maven 3.0 with Matthew McCullough
Last night, I attended the Denver JUG meeting to hear some excellent talks by Matthew McCullough and Tim Berglund. I took notes during Matthew's talk, but my battery ran out before Tim's talk started. Below are my notes.
Commits to Maven 3 have been happening for the last 3 years. Matthew is not an employee of Sonatype, but he mentioned their name quite a bit in his talk. Sonatype has hired several committers (7 that Matthew knows of by name) that now work on Maven 3 full-time. For compatibility with Maven 2, the project has 450 integration tests and they test it against 100s of Maven 2 projects. Maven 3 has plugin classloader partitioning and a legacy simulation layer for old plugins.
The main improvement in Maven 3 is speed. It's been performance tuned to be 50% to 400% faster. Benchmarks (guaranteed by integration tests) include better: Disk I/O, Network I/O, CPU and Memory. Another new feature is extensibility so Maven is a better library rather than just a command-line tool. Now there's a library and APIs that you can use to do the things that Maven does. Plexus has been replaced with Guice and it's now much easier to embed Maven (Polyglot Maven and Maven Shell are examples of this).
Below are a number of other changes between Maven 2 and Maven 3.
- Syntax: pom.xml still uses <modelVersion>4.0.0</modelVersion> so it can be a drop-in replacement for Maven 2 projects.
- Validations: poms are heavily validated against common mistakes, warns when plugin versions are not specified (use mvn validate to see issues), blocks duplicate dependencies (examined in same POM only, conflict resolution used otherwise).
- Help URLs: wiki page URLs now shown for all error messages. One of the first Apache projects to do this.
- Removals: profiles.xml external file support removed, Maven 1.0 repository support removed <layout>legacy</layout> (it's been 5 years since any commits to Maven 1).
- Behavior: SNAPSHOTs always deployed with date-stamps, artifact resolution caching has been improved to do less checking (override with mvn <phase> -U).
- Plugins: version auto-selection favors RELEASEs over SNAPSHOTs (opposite for Maven 2), versions cannot be specified as RELEASE or LATEST, plugins only resolved from <pluginRepository> locations.
- See the Plugin Compatibility Matrix to see if your favorite plugins are compatible.
Maven 3 hopes to be a drop-in replacement for Maven 2, but non backwards-compatible changes will be happening in Maven 3.1. It's anticipated release is Q1 of 2011 and will likely contain the following features.
- "Mixins" for direct dependencies
- Site plugin takes over <reporting>
- Backwards compatibility by <modelVersion
- There's a good chance 3.1 breaks compatibility with legacy POMs
Another new thing in Maven 3 is Toolchain. Toolchain a common way to configure your JDK for multiple plugins. There are only a handful of plugins that are toolchain-enabled. User tool chain definitions are defined in ~/.m2/toolchains.xml. To use different toolchains (JDKs), you specify a vendor and version as part of your plugin configuration.
Maven Shell is a high performance console that's a Maven 3 add-on. It's hosted at GitHub to make community contributions easier. It goes on your command line and it offers syntax highlighting and context-sensitive help (by typing ? at the command prompt).
Another major improvement in Maven 3 is Polyglot Maven. Tools like Gant and Buildr have made Maven look ancient, but they've also given it a good challenge. Maven 3 is likely to leapfrog these tools because of its ability to use different languages for your build configuration. Currently, 6 languages are supported. Polyglot Maven is a super-set distribution of Maven 3. It's not shipped with Maven 3 core because it contains all the other language implementations and is quite large. Polyglot Maven also contains a translate tool that allows you to convert any-to-any language. It has a DSL framework with Macros and Lifecycle Hooks. Macros allows for more concise syntax.
After talking about Polyglot Maven a bit, Matthew shows us a demo translating pom.xml to pom.yaml and then running the build. After that, he showed us examples of what a pom looks like when defined in Clojure, Scala and Groovy. Someone asked about file parsing performance and Matthew said different languages would cause a single-digit performance difference as part of your build process. Personally, I can't help but think any non-XML parser would be faster than the XML parser.
In regards to m2eclipse, a new drop (0.10) occurred a few weeks ago and it's one of the highest quality releases to date. It has major refactoring and many performance improvements.
For sample Maven projects see Matthew's Maven Samples.Tim's talk on Decision Making was also excellent. The biggest things I learned were that conflict is good (idea-wise, not personal) and things to look out for between teams (fault lines). Hopefully both Tim and Matthew post their slides so I can link to them here.