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 Supplementing Missing POM Information - Maven Remote Resources Plugin
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 John Casey
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 14 September 2009
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Supplementing Missing POM Information - Maven Remote Resources Plugin

 (<Since 1.0-alpha-5>)
 
 One of Maven's great strengths is that it allows you, the user, to build directly on the work of
 others with minimal effort. For instance, by adding five lines (or less) to your POM, you can 
 declare a dependency on someone else's library, and instruct Maven to use that library during
 the build for your own project. Since the library's jar is usually accompanied by a POM of its
 own, your project doesn't need to provide an exhaustive listing of all libraries used by all
 of your direct dependencies. 
 
 However, this approach of reusing POM metadata from your dependencies can be inhibited by a
 dependency POM that provides less-than-complete information about its project. While most POMs
 provide reasonably accurate information about their dependencies, they sometimes leave out some
 information critical to assembling licensing and dependency notice files.
 
 For example, the templates used by many Apache distributions assemble a listing of project
 dependencies according to their organization name (and URL), along with the URL each project's
 website. When dependency POMs are missing this information, the dependency notice file
 that the remote-resources plugin renders can be invalid. 
 
 To compensate for incomplete dependency POMs, you can use the supplemental models support, introduced 
 to the remote-resources plugin in version 1.0-alpha-5, and improved in version 1.1.
 
* Fixing Incomplete POMs: Using Supplemental Models

 For those cases where your project's dependencies don't list organization name, organization URL,
 project URL, or whatever other metadata you require for you legal notice files, the remote-resources
 plugin allows you to configure a series of supplemental models. These models consist of one or more
 model files, each of which contains one or more POM fragments that can be merged into existing 
 dependency POMs to supplement the metadata provided there. The plugin uses the key <<<groupId:artifactId>>>
 to match each supplemental model with the dependency POM into which it should be merged.
 
 For example, imagine your project's POM declared the following two dependencies:
 
+---+
    <dependency>
      <groupId>org.foo</groupId>
      <artifactId>missing-org-info</artifactId>
      <version>1</version>
    </dependency>
    <dependency>
      <groupId>org.foo</groupId>
      <artifactId>missing-project-url</artifactId>
      <version>1</version>
    </dependency>
+---+

  Now imagine that your project needs to include a file in its distribution that lists organization and
  project URLs for each dependency, in a format like this:
  
+---+
From: Apache Software Foundation (http://www.apache.org/)

 - Commons FOO (http://commons.apache.org/foo/) org.apache.commons.foo:foo:1
 - Commons BAR (http://commons.apache.org/bar/) org.apache.commons.bar:bar:1
+---+

 However, as you try generating these licensing resources for your project, you quickly learn that your dependencies
 provide POMs that don't always include all the information needed by this format. The first dependency is missing
 all of the information provided by the <<<organization>>> element of the POM, and the second is missing a project-
 specific URL (the <<<url>>> element of the POM).
 
 To fix these omissions for the purposes of resource generation, we can simply provide a supplemental-model file that
 contains the missing information. The file, located in <<<src/main/appended-resources/supplemental-models.xml>>>,
 would look something like this:
 
+---+
<?xml version="1.0" encoding="UTF-8"?>
<supplementalDataModels>
  <supplement>
    <project>
      <groupId>org.foo</groupId>
      <artifactId>missing-org-info</artifactId>
      
      <organization>
        <name>FOO, Inc.</name>
        <url>http://www.foo.org/</url>
      </organization>
    </project>
  </supplement>
  <supplement>
    <project>
      <groupId>org.foo</groupId>
      <artifactId>missing-project-url</artifactId>
      
      <url>http://www.foo.org/projects/missing-project-url/</url>
    </project>
  </supplement>
</supplementalDataModels>
+---+

 Finally, we tell the remote-resources plugin to use the new supplemental-model file:
 
+---+
<plugin>
  <artifactId>maven-remote-resources-plugin</artifactId>
  <version>${project.version}</version>
  [...]
  
  <executions>
    <execution>
      <id>process-remote-resources</id>
      <goals>
        <goal>process</goal>
      </goals>
      <configuration>
        <supplementalModels>
          <supplementalModel>supplemental-models.xml</supplementalModel>
        </supplementalModels>
        [...]
      </configuration>
    </execution>
  </executions>
</plugin>
+---+

 After re-running the project build, the supplemental information we provided will be merged with the metadata 
 from the dependency POMs, providing enough information to complete the dependencies listing.

* Publishing and Reusing Supplemental Models (<Since 1.1>)

 The configuration above is fine for single projects that need to address deficiencies in their dependencies'
 metadata. But what happens when your organization wants to use these same deficient dependencies across multiple
 projects? The configuration we just examined cannot handle reuse of supplemental-model files.
 
 To address this shortcoming, version 1.1 of the remote-resources plugin introduces a new parameter: 
 <<<supplementalModelArtifacts>>>. When combined with the <<<supplementalModels>>> parameter used above, this new
 parameter allows the remote-resources plugin to resolve artifacts containing supplemental model information, then
 search those artifacts for the paths given in the <<<supplementalModels>>> listing.
 
 To make the supplemental models above reusable using this mechanism, we first publish them in their own project:
 
+---+
<?xml version="1.0" encoding="UTF-8"?>
<project xsi:schemaLocation="http://maven.apache.org/POM/4.0.0 http://maven.apache.org/xsd/maven-4.0.0.xsd" 
    xmlns="http://maven.apache.org/POM/4.0.0" xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance">
    
  <modelVersion>4.0.0</modelVersion>
  
  <groupId>org.myco</groupId>
  <artifactId>dependency-resource-supplement</artifactId>
  <version>1</version>
</project>
+---+

 Note that this is just a simple, unadorned jar artifact. We then move the above POM and supplemental-models.xml file
 into a separate project directory structure:
 
+---+
|-- pom.xml
`-- src
    `-- main
        `-- resources
            `-- supplemental-models.xml
+---+

 Once we install this new project, we can reference it from the configuration for the remote-resources plugin, 
 like this:
 
+---+
<plugin>
  <artifactId>maven-remote-resources-plugin</artifactId>
  <version>${project.version}</version>
  [...]
  
  <executions>
    <execution>
      <id>process-remote-resources</id>
      <goals>
        <goal>process</goal>
      </goals>
      <configuration>
        <!-- reference the supplemental-model artifact from above. -->
        <supplementalModelArtifacts>
          <supplementalModelArtifact>org.myco:dependency-resource-supplement:1</supplementalModelArtifact>
        </supplementalModelArtifacts>
        
        <!-- specify the path, relative to the jar root, where the supplemental model file is located -->
        <supplementalModels>
          <supplementalModel>supplemental-models.xml</supplementalModel>
        </supplementalModels>
        [...]
      </configuration>
    </execution>
  </executions>
</plugin>
+---+

 Once the supplemental-model project is released and deployed, any number of projects can then make use of the
 supplemental information it provides.