HOW-TO: Merge sub-items from parent POM to child POM in a Maven plugin configuration
UPDATE: This has been cross-posted to my work blog: http://blogs.sonatype.com/john.
I just ran into a bug report about this, so I thought it might be a good idea to post about it. The bug report detailed a case where the following configurations were not being merged correctly:
Parent POM:
<configuration>
<items>
<item>one</item>
<item>two</item>
</items>
</configuration>
Child POM:
<configuration>
<items>
<item>three</item>
</items>
</configuration>
Expected result, according to MNG-2591:
<configuration>
<items>
<item>one</item>
<item>two</item>
<item>three</item>
</items>
</configuration>
By default, Maven would simply replace the parent's <items> sub-elements with those from the child, resulting in only one item: three. This is mainly because Maven treats all plugin configuration as XML DOM content, without any idea of what makes up a list of related items, and what is just complex configuration. Moreover, sometimes you want to override the entire list instead of merely appending to it...it's a difficult balance to strike. However, Maven does support a special attribute to control this behavior. Using the combine.children attribute with a value of append, you can tell Maven to treat the sub-items as a list, and combine them into a single, larger list during inheritance calculations.
As of Revision 545315 in Maven's Subversion repository (in the latest trunk), things have changed subtly in this functionality, to render a more consistent and intuitive result. Given the same parent POM from above, and this child POM:
<configuration>
<items combine.children="append">
<item>three</item>
</items>
</configuration>
Maven versions prior to this new revision will render the following configuration:
<configuration>
<items>
<item>three</item>
<item>one</item>
<item>two</item>
</items>
</configuration>
...while the newest revision in Maven's SVN repository will render this configuration:
<configuration>
<items>
<item>one</item>
<item>two</item>
<item>three</item>
</items>
</configuration>
Admittedly, this is a fairly arcane configuration trick. Hopefully, during the course of our 2.1.x work, we can find a simpler way to change configuration-merging behavior for plugins in Maven's POM.