Maven parent pom vs modules pom

Question

There seem to be several ways to structure parent poms in a multiproject build and I wondering if anyone had any thoughts on what the advantages   drawbacks are in each way   The simplest method of having a parent pom would be putting it in the root of a project i e   myproject    myproject-core    myproject-api    myproject-app    pom xml   where the pom xml is both the parent project as well as describes the -core -api and -app modules  The next method is to separate out the parent into its own subdirectory as in  myproject    mypoject-parent      pom xml   myproject-core    myproject-api    myproject-app    Where the parent pom still contains the modules but they re relative  e g     myproject-core  Finally  there s the option where the module definition and the parent are separated as in  myproject    mypoject-parent      pom xml   myproject-core    myproject-api    myproject-app    pom xml   Where the parent pom contains any  shared  configuration  dependencyManagement  properties etc   and the myproject pom xml contains the list of modules   The intention is to be scalable to a large scale build so should be scalable to a large number of projects and artifacts   A few 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   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     Edit  Each of the sub projects have their own pom xml  I ve left it out to keep it terse

User · Answer

From my experience and Maven best practices there are two kinds of "parent poms"

"company" parent pom - this pom contains your company specific information and configuration that inherit every pom and doesn't need to be copied. These informations are:
- repositories
- distribution managment sections
- common plugins configurations (like maven-compiler-plugin source and target versions)
- organization, developers, etc
Preparing this parent pom need to be done with caution, because all your company poms will inherit from it, so this pom have to be mature and stable (releasing a version of parent pom should not affect to release all your company projects!)
second kind of parent pom is a multimodule parent. I prefer your first solution - this is a default maven convention for multi module projects, very often represents VCS code structure

The intention is to be scalable to a large scale build so should be scalable to a large number of projects and artifacts.

Mutliprojects have structure of trees - so you aren't arrown down to one level of parent pom. Try to find a suitable project struture for your needs - a classic exmample is how to disrtibute mutimodule projects

distibution/
documentation/
myproject/
  myproject-core/
  myproject-api/
  myproject-app/
  pom.xml
pom.xml

A few 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).

This configuration has to be wisely splitted into a "company" parent pom and project parent pom(s). Things related to all you project go to "company" parent and this related to current project go to project one's.

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)?

Company parent pom have to be released first. For multiprojects standard rules applies. CI server need to know all to build the project correctly.

User · Answer

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  lt parent gt  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

User · Answer

There is one little catch with the third approach  Since aggregate POMs  myproject pom xml  usually don t have parent at all  they do not share configuration  That means all those aggregate POMs will have only default repositories   That is not a problem if you only use plugins from Central  however  this will fail if you run plugin using the plugin goal format from your internal repository  For example  you can have foo-maven-plugin with the groupId of org example providing goal generate-foo  If you try to run it from the project root using command like mvn org example foo-maven-plugin generate-foo  it will fail to run on the aggregate modules  see compatibility note    Several solutions are possible    Deploy plugin to the Maven Central  not always possible   Specify repository section in all of your aggregate POMs  breaks DRY principle   Have this internal repository configured in the settings xml  either in local settings at    m2 settings xml or in the global settings at  conf settings xml   Will make build fail without those settings xml  could be OK for large in-house projects that are never supposed to be built outside of the company   Use the parent with repositories settings in your aggregate POMs  could be too many parent POMs

User · Answer

An independent parent is the best practice for sharing configuration and options across otherwise uncoupled components  Apache has a parent pom project to share legal notices and some common packaging options  If your top-level project has real work in it  such as aggregating javadoc or packaging a release  then you will have conflicts between the settings needed to do that work and the settings you want to share out via parent  A parent-only project avoids that  A common pattern  ignoring  1 for the moment  is have the projects-with-code use a parent project as their parent  and have it use the top-level as a parent  This allows core things to  be shared by all  but avoids the problem described in  2  The site plugin will get very confused if the parent structure is not the same as the directory structure  If you want to build an aggregate site  you ll need to do some fiddling to get around this  Apache CXF is an example the pattern in  2

[java] Maven parent pom vs modules pom

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