Maven 2.2.1 project inheritance: necessary to use packaging 'pom' even without the use of project aggregation (multimodule)?

Question

I want to inherit the dependencies of a (parent) pom.xml in a child project i.e. use Project Inheritance. It seems it is necessary to change the default packaging type from 'jar' to 'pom' in this case. But doesn't tell the Maven2 documentation that the packaging type 'pom' is necessary for Project Aggregation i.e. multimodule projects which use submodules but not for Project Inheritance (see http://maven.apache.org/guides/introduction/introduction-to-the-pom.html#Project_Inheritance)?

<project>
 <modelVersion>4.0.0</modelVersion>
 <groupId>example</groupId>
 <artifactId>example-parent</artifactId>
 <version>1</version>

 <dependencies>
   <dependency>
     <groupId>log4j</groupId>
     <artifactId>log4j</artifactId>
     <version>1.2.14</version>
   </dependency>
 </dependencies>
</project>

<project>     
 <parent>
   <groupId>example</groupId>
   <artifactId>example-parent</artifactId>
   <version>1</version>
 </parent>

 <modelVersion>4.0.0</modelVersion>
 <groupId>example</groupId> 
 <artifactId>example-child</artifactId>
</project>

But if you call maven (e.g. mvn clean) with this configuration you get an error:

Project ID: example:example-child

Reason: Parent: example:example-parent:jar:1 
 of project: example:example-child has wrong packaging: jar.
Must be 'pom'. for project example:example-child

With the entry

<project> 
 ... 
 <packaging>pom</packaging>
 ... 
</project> 

in the parent pom.xml maven can be executed without any error.

Is this behavior of maven correct?

Answer 1

As documented in the Inheritance section of the POM Reference:

The packaging type required to be pom for parent and aggregation (multi-module) projects.

So Maven's behavior seems correct to me (and the error message is nicely self explaining).

Answer 2

As noted by Pascal, the behaviour is correct.

If you're still looking for means to share dependencies between modules, you can consider bundling up the dependencies in question into a pom, and then having your modules both depend on that new "dependencies" pom.

See Maven Book Section 3.6.1 for more details.