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:
Edit: Each of the sub projects have their own pom.xml, I've left it out to keep it terse.
This question is related to
java
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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.
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:
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!)
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.
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:
Source: Stackoverflow.com