Experimental Maven support for condensed POMs using attributes

As I’ve just posted to the Maven developers list, I’ve always wanted to see an attribute based POM, so based on Nicolas’ suggestion I killed some time after waking up early this morning to do it.

Here’s what you’ll need to try it:

• a build: http://people.apache.org/~brett/apache-maven-2.0.9-SNAPSHOT-terse-bin.tar.gz
• or if you prefer to roll your own, the source: http://svn.apache.org/repos/asf/maven/components/branches/maven-2.0.x-terse

The issue is being tracked under MNG-3397.

The result is that something like this:

...
<dependencies>
  <dependency>
    <groupId>junit</groupId>
    <artifactId>junit</artifactId>
    <version>3.8.1</version>
    <scope>test</scope>
  </dependency>
  <dependency>
    <groupId>easymock</groupId>
    <artifactId>easymock</artifactId>
    <version>1.2_Java1.3</version>
    <scope>test</scope>
  </dependency>
  ...

Halves in length to something like this:

<dependencies>
  <dependency groupId="junit" artifactId="junit" version="3.8.1" scope="test"/>
  <dependency groupId="easymock" artifactId="easymock" version="1.2_Java1.3" scope="test"/>
  ...

What I did is basically convert all the primitive types in the model to attributes. I think more could be done – for instance, lists could be flattened if the parent element is not really needed for grouping, and I think plugin configuration should get the same treatment as above – but this gets a big win for minimal work. Also, the settings and profiles files could conceivably get the same treatment.

Most importantly – the work is completely backwards compatible. Maven detects v4.0.0 POMs and reads it like it used to. Even, as in the case of Archiva above, if there are different ones within a project.

Now, it’s early days for this change, and there are a few issues to note:

• I switched from our bundled XPP3 based parser to StAX. This will likely introduce some quirks that will need to be ironed out through testing. I will likely in the future read the old models with the old parser, and the new ones with the new parser.
• Some plugins may not work with this version (I’m guessing at this stage the release plugin falls into this category though I haven’t checked) – as I removed the old parser altogether. This should be easy to make compatible.
• I haven’t tested performance with this new version – I expect that the v4.0.0 POMs will parse marginally slower than they used to and this may affect artifact resolution times. It also increased the size of the Maven download.

What comes next? Apart from correcting issues such as the above, I have in mind:

• A simple plugin to convert from v4.0.0 to v4.1.0 for a POM file so you can try this out on your own projects.
• Utilising the namespace for detecting the version so the modelVersion element is not required.
• Further improvements to the format

What do Maven users think of this alternative format?

(And apologies to Don, I got carried away doing this instead of finishing the integration of his patches into the main branch – but I’ll resume that again shortly!)