Inheritance in Maven

In order the understand how inheritance works in Maven there are a few notions that you must be familiar with:

• The maven super model
• how parent poms are processed
• the order in which elements in the POM are overridden
• minimum element-set for a valid project pom

Maven super model

Inheritance is recursive in Maven but there is a special model which is the implicit super parent in the lineage of models you may specify:

all of the models that you specify are collected to produce a lineage and then the super model is place at the top of that lineage to provide default values.

The super model is where we place all the values which we believe to be standard, values that can be shared and utilized across all your maven projects.

 m0 <- m1 <- m2

which is transformed into

 super model <- m0 <- m1 <- m2

<project>
 <modelVersion>4.0.0</modelVersion>
 <parent>
   <groupId></groupId>
   <artifactId></artifactId>
   <version></version>
 </parent>
</project>

Extending the POM in the future

In order to anticipate changes in the structure of the POM as new releases of maven occur, the maven inheritance model must be pinned against a particular modelVersion. This is required so that a change from modelVersion 4.0.0 to 4.1.0 doesn't cause compatibility problems with users who have not upgraded their projects. It also allows for a certain amount of legacy support into the future. Therefore, we should rename this file from pom.xml in the maven-core jarfile to pom-4.0.0.xml (and successive revisions to the base pom made to accommodate changes in the model will be named accordingly - 4.0.1, 4.0.2, ...).

Minimum Element-Set for a Project POM

In order to furnish a basic set of information which we need to distinguish one project from another, maven requires a minimum set of elements to be specified in each project's pom.xml.

- [modelVersion] tells maven which version of the object model this project is compliant with (to support future legacy concerns)

- [artifactId] tells maven what the artifact name is for this particular project. Each project should have a distinct artifactId.

- [version] tells maven what release of this artifact we're trying to produce. The fact that a project has a distinct pom.xml should indicate a separate release cycle that is also distinct to that project, so a concrete version declaration is required.

- [name] tells maven what the user-friendly name of this project is. The name element is similar to artifactId in that it is a unique descriptor for this and only this project, and therefore should be concretely defined.