Support adding and overriding report plugins in new maven-site-plugin 3.x reportPlugins configuration format

What you have to do is to add to indicate how to merge site plugin configuration when maven build the model
in the child pom, you can change to ( attribute combine.children="append" )

<plugin>
  <groupId>org.apache.maven.plugins</groupId>
  <artifactId>maven-site-plugin</artifactId>
  <version>version</version>
  <configuration>
    <reportPlugins combine.children="append">

btw this doesn't fix overriding configuration, with this trick some plugins can be executed more than one time

so this will probably need a hack in maven core for site plugin regarding plugin configuration inheritance.

yes, a new mecanism is to be defined in Maven core, since plugin configuration inheritance works at DOM level, without any interpretation of any content.

Just a note that this affects the 3.0 release of the plugin as well. Perhaps until the Maven core modifications are done the logic in the maven-site-plugin that handles the reportPlugins could be made configurable in the situation where the same reportPlugin groupId & artifactId exist.

Perhaps something like:

<plugin>
  <groupId>org.apache.maven.plugins</groupId>
  <artifactId>maven-site-plugin</artifactId>
  <version>version</version>
  <configuration>
    <duplicatePluginPolicy>USE_LAST</duplicatePluginPolicy>
    <reportPlugins combine.children="append">

The duplicatePluginPolicy option could be an enum with USE_FIRST, USE_LAST, USE_BOTH options, allowing the site plugin to know what to do if there are multiple plugins with the same groupId:artifactId in the list. This would at least allow for overrides of a single plugin without having to list every plugin the parent defines.

This patch (against r1170684) provides robust support for merging PlexusConfiguration objects. I've included a unit test which demonstrates the functionality and have tested it locally with various plugin overlays with great success.

For example if the parent pom's maven-site-plugin is:

<plugin>
  <artifactId>maven-site-plugin</artifactId>
  <configuration>
    <reportPlugins>
      <plugin>
        <artifactId>maven-javadoc-plugin</artifactId>
        <version>2.8</version>
        <configuration>
          <links>
            <link>http://java.sun.com/javase/6/docs/api/</link>
            <link>http://java.sun.com/javaee/5/docs/api/</link>
          </links>
          <source>1.6</source>
        </configuration>
      </plugin>
    </reportPlugins>
  </configuration>
</plugin>

And the child project pom's maven-site-plugin is:

<plugin>
  <artifactId>maven-site-plugin</artifactId>
  <configuration>
    <defaultMergePolicy>MERGE</defaultMergePolicy>
    <reportPlugins combine.children="append">
      <plugin>
        <artifactId>maven-javadoc-plugin</artifactId>
        <configuration>
          <links mergePolicy.children="BOTH" mergePolicy.removeDuplicates="true">
            <link>http://static.springsource.org/spring/docs/3.0.x/api/</link>
            <link>http://ehcache.org/apidocs/</link>
            <link>http://aopalliance.sourceforge.net/doc/</link>
            <link>http://java.sun.com/javase/6/docs/api/</link>
          </links>
          <maxmemory>768m</maxmemory>
        </configuration>
      </plugin>
    </reportPlugins>
  </configuration>
</plugin>

The runtime effective result is:

<plugin>
  <artifactId>maven-site-plugin</artifactId>
  <configuration>
    <defaultMergePolicy>MERGE</defaultMergePolicy>
    <reportPlugins combine.children="append">
      <plugin>
        <artifactId>maven-javadoc-plugin</artifactId>
        <configuration>
          <links mergePolicy.children="BOTH" mergePolicy.removeDuplicates="true">
            <link>http://static.springsource.org/spring/docs/3.0.x/api/</link>
            <link>http://ehcache.org/apidocs/</link>
            <link>http://aopalliance.sourceforge.net/doc/</link>
            <link>http://java.sun.com/javase/6/docs/api/</link>
            <link>http://java.sun.com/javaee/5/docs/api/</link>
          </links>
          <source>1.6</source>
          <maxmemory>768m</maxmemory>
        </configuration>
      </plugin>
    </reportPlugins>
  </configuration>
</plugin>

Note that I tried to implement this feature in a way such that it could be used by any plugin. It operates on the PlexusConfiguration interface and the merging logic has no direct ties to the maven-site-plugin

looks interesting patch. Could you please fix license header and use correct ASF attribution (I see :"....Licensed to Jasig...." )

I just realized the patch is incomplete, I'll get the final version posted tomorrow.

FWIW, it would be great if the mergePolicy.children and mergePolicy.removeDuplicates attributes (and also combine.children) could finally become part of the POM's XML Schema: http://maven.apache.org/xsd/maven-4.0.0.xsd. It's quite annoying to see validation errors on elements like developers, which would benefit from this feature and are specified by the XSD (unlike, e.g., the plugins' configuration which is xsd:any), because these attributes are not part of the XSD.

On a related note: Why not use XML Namespaces for mergePolicy.*? Verbosity can't be the only reason; after all, it's one xmlns declaration + a short prefix like mp vs. the longer mergePolicy on every attribute.

Updated patch, replaces mergePolicy. prefix with merge: xml namespace. In my testing you can specify whatever you want for the namespace URI since I don't actually have an XSD to go with it, I used xmlns:merge="http://maven.apache.org/plugin-merge" in my test pom.

This patch should be complete, all integration tests pass and the copyright headers are correct. I do have an Apache ICLA on file and am an active committer in the portals project: http://people.apache.org/committer-index.html

I would love this to become a core feature of Maven itself for merging plugin configurations. I'm not sure where to go with getting that requested as a feature. I'm hoping that this this seems to affect the site plugin more than others it would be faster to get the fix in here so it could be used as soon as possible.

I didn't have time to check for the moment, but yes, this seems interesting at Maven core level: there have already been discussion about having more flexible merge policy for some pom content
see MNG-3124 and MNG-2807