CODEWITHSUNDEEP
mavenmaven-2
rk2010
rk2010

Reputation: 3529

maven sub projects - should they have their own depedencies

If a project has many sub-projects, and all the sub projects have a common parent pom.xml, shouldn't all the the dependencies be listed in parent pom.xml ?

What's the point of allowing sub project to have their own dependencies.. ? It only opens up the possibility, that one sub-project will use apache_xyzlibrary_1.0.jar and another sub project might use _2.0.jar ?

Note: All the maven sub projects combine to form a single webapp WAR.

Upvotes: 0

Views: 627

Answers (2)

user454043
user454043

Reputation:

It will be very inefficient to include all the projects dependency into every sub project as by doing that, you are effectively bring in unnecessary dependencies into every of the sub projects' build. And one other issue is if you have inter sub project dependency, i.e. sub project a depending on sub project b, then you can easily end up a cycle dependency which maven unable to resolve.

To keep the consistence of dependency versions across the project, maven's approach is to make use of the dependency management section in the project's parent pom. Hence, only set the version of every dependency in the parent pom's dependency management section. In the sub project's pom, only the group Id and artefact Id is stated. No version tag should be used in the sub project's pom unless that is needed (i.e. when a sub project required a particular version of a dependency that is different from the rest of the project)

Upvotes: 3

Jesse Webb
Jesse Webb

Reputation: 45303

If all the children declare the same dependency, it is a good idea to declare that dependency in the parent instead. You are right about this ensuring all projects use consistent versions of dependencies.

As for why did the Maven developers decide to allow dependencies to be declared in child projects if this makes it possible to use different versions of the same dependency? I imagine they were thinking the advantage of locally declared dependencies in projects, when they are different from sibling projects, was nicer to have than restrict the parent to declare all dependencies for all its children.

Think of it like variables declared at different scopes in a class. If you have a private variable in a method, you know that it is only used in the one method and you can change it as you see fit without needing to worry about your entire code base. If you have a private field in a class, you have to aware of all it's usages in that class but not in other files. If you have a public variable, well you get the idea. Dependencies are the same, if you have one declared for a single project, you can be certain it is only used/needed by the single project. Furthermore, it doesn't need to be present if you are building a different child project of the same parent.

As for you building a WAR, you are right, using different versions of the same dependency in different child projects will probably cause you grief with the final artifact. Just be aware that there are other project types and build cycles where have mixed version numbers between projects may not be a problem and is actually desired.

Remember too that maven parent pom files can also have parents. This allows for more tiers of a hierarchy than just 2 levels. I think it would be ridiculous if all children projects had to declare its dependencies in the top level parent project pom file.

Upvotes: 0

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