Maven parent pom vs modules pom

by

In my opinion, to answer this question, you need to think in terms of project life cycle and version control. In other words, does the parent pom have its own life cycle i.e. can it be released separately of the other modules or not?

If the answer is yes (and this is the case of most projects that have been mentioned in the question or in comments), then the parent pom needs his own module from a VCS and from a Maven point of view and you’ll end up with something like this at the VCS level:

root
|-- parent-pom
|   |-- branches
|   |-- tags
|   `-- trunk
|       `-- pom.xml
`-- projectA
    |-- branches
    |-- tags
    `-- trunk
        |-- module1
        |   `-- pom.xml
        |-- moduleN
        |   `-- pom.xml
        `-- pom.xml

This makes the checkout a bit painful and a common way to deal with that is to use svn:externals. For example, add a trunks directory:

root
|-- parent-pom
|   |-- branches
|   |-- tags
|   `-- trunk
|       `-- pom.xml
|-- projectA
|   |-- branches
|   |-- tags
|   `-- trunk
|       |-- module1
|       |   `-- pom.xml
|       |-- moduleN
|       |   `-- pom.xml
|       `-- pom.xml
`-- trunks

With the following externals definition:

parent-pom http://host/svn/parent-pom/trunk
projectA http://host/svn/projectA/trunk

A checkout of trunks would then result in the following local structure (pattern #2):

root/
  parent-pom/
    pom.xml
  projectA/

Optionally, you can even add a pom.xml in the trunks directory:

root
|-- parent-pom
|   |-- branches
|   |-- tags
|   `-- trunk
|       `-- pom.xml
|-- projectA
|   |-- branches
|   |-- tags
|   `-- trunk
|       |-- module1
|       |   `-- pom.xml
|       |-- moduleN
|       |   `-- pom.xml
|       `-- pom.xml
`-- trunks
    `-- pom.xml

This pom.xml is a kind of “fake” pom: it is never released, it doesn’t contain a real version since this file is never released, it only contains a list of modules. With this file, a checkout would result in this structure (pattern #3):

root/
  parent-pom/
    pom.xml
  projectA/
  pom.xml

This “hack” allows to launch of a reactor build from the root after a checkout and make things even more handy. Actually, this is how I like to setup maven projects and a VCS repository for large builds: it just works, it scales well, it gives all the flexibility you may need.

If the answer is no (back to the initial question), then I think you can live with pattern #1 (do the simplest thing that could possibly work).

Now, about the bonus questions:

  • Where is the best place to define the various shared configuration as in source control, deployment directories, common plugins etc. (I’m assuming the parent but I’ve often been bitten by this and they’ve ended up in each project rather than a common one).

Honestly, I don’t know how to not give a general answer here (like “use the level at which you think it makes sense to mutualize things”). And anyway, child poms can always override inherited settings.

  • How do the maven-release plugin, hudson and nexus deal with how you set up your multi-projects (possibly a giant question, it’s more if anyone has been caught out when by how a multi-project build has been set up)?

The setup I use works well, nothing particular to mention.

Actually, I wonder how the maven-release-plugin deals with pattern #1 (especially with the <parent> section since you can’t have SNAPSHOT dependencies at release time). This sounds like a chicken or egg problem but I just can’t remember if it works and was too lazy to test it.

More Related Contents:

  1. How can I create an executable JAR with dependencies using Maven?
  2. Maven: add a dependency to a jar by relative path
  3. Maven Modules + Building a Single Specific Module
  4. Different dependencies for different build profiles
  5. Building executable jar with maven?
  6. Is it possible to create an “uber” jar containing the project classes and the project dependencies as jars with a custom manifest file?
  7. Maven: How to include jars, which are not available in reps into a J2EE project?
  8. Building a runnable jar with Maven 2
  9. Why does Maven have such a bad rep? [closed]
  10. Is there a Maven alternative or port for the .NET world?
  11. Maven and the JOGL library?
  12. Maven for other languages? [closed]
  13. How do I tell Maven to use the latest version of a dependency?
  14. Make Maven to copy dependencies into target/lib
  15. Warning – Build path specifies execution environment J2SE-1.4
  16. Difference of Maven JAXB plugins
  17. How do I put all required JAR files in a library folder inside the final JAR file with Maven?
  18. How to configure JPA for testing in Maven
  19. Maven: How to change path to target directory from command line?
  20. What is the best way to avoid maven-jar?
  21. Is there anyway to exclude artifacts inherited from a parent POM?
  22. Maven dependency for Servlet 3.0 API?
  23. Unable to use Intellij with a generated sources folder
  24. Which maven dependencies to include for spring 3.0?
  25. Can a program depend on a library during compilation but not runtime?
  26. How to download SNAPSHOT version from maven SNAPSHOT repository?
  27. Changing packaging based on active profile in pom
  28. How can I get Maven to stop attempting to check for updates for artifacts from a certain group from maven-central-repo?
  29. Tool for creating a Java daemon service on Linux [closed]
  30. Why use Gradle instead of Ant or Maven? [closed]

Leave a Comment