Can maven projects have multiple parents?

Question

We have Java and Flex projects. We currently have 1 base pom that contains the configurations we want to use for both projects. Problem with this is: Flex projects inherit configuration, for example, for javadoc and pmd plugins, which is not desirable.

I want to clean it up and have a real base pom, and then a java-base-pom and a flex-base-pom. But how does this work in a multi-module that has both a Flex part and a Java part?

We have plugins to our own application where we use the following structure:

• my-plugin
  • my-plugin-client (flex)
  • my-plugin-server (java)

my-plugin just contains a pom.xml with <modules/> section. I would use my-plugin pom.xml as a parent for both, but then I cannot also use the java base-pom or the flex base-pom as parent. What would be the best approach for this?

Answer 1

Even though maven projects have single parent, they can import any number of other pom's like this:

<dependencyManagement>     <dependencies>         <dependency>             <groupId>org.example</groupId>             <artifactId>my-shared-dependencies</artifactId>             <version>0.0.1-SNAPSHOT</version>             <type>pom</type>             <scope>import</scope>         </dependency>     </dependencies> </dependencyManagement> 

This has two important differences compared to a parent:

1. Plugins defined in the imported pom won't be imported
2. Dependencies defined in the imported pom won't be added to the current pom, it will only import dependencies into the dependency management section

However, if your parent pom has a <dependencies> section and you want to include those into your dependencies, then you can add the parent to your <dependencies> section just like a regular dependency:

<dependency>     <groupId>org.example</groupId>     <artifactId>my-shared-dependencies</artifactId>     <version>0.0.1-SNAPSHOT</version> </dependency> 

Even though the same dependency is already imported, the version tag has to be specified again. To reduce duplication, it can be stored in a property

Answer 2

A project can have only one parent (unlike multiple inheritance in C++) but this parent can be part of a bigger parent hierarchy. As pointed out by others, you could thus have something like this:

 base-pom/ |-- flex-base-pom |   |-- my-plugin-client |   |   `-- pom.xml |   `-- pom.xml |-- java-base-pom |   |-- my-plugin-server |   |   `-- pom.xml |   `-- pom.xml  `-- pom.xml 

That said, I noticed you wrote that your actual problem is that:

flex projects inherit configuration for javadoc and pmd for example, which they do not want.

You should use the pluginManagement element to avoid this situation:

pluginManagement is an element that is seen along side plugins. Plugin Management contains plugin elements in much the same way, except that rather than configuring plugin information for this particular project build, it is intended to configure project builds that inherit from this one. However, this only configures plugins that are actually referenced within the plugins element in the children. The children have every right to override pluginManagement definitions.

So, in the parent pom, configure your plugins in pluginManagement (javadoc and pmd for example), and reference them within the plugins element in the desired children (only in my-plugin-server here). This would solve your current issue.