Maven multi-module: aggregate common dependencies in a single one?

Question

I have searched for such a question without finding anything, so here I go.

I have a multi-module maven project. Multiple modules all inherit the same parent, where common dependencies are defined. Among them, there is one my own modules, a 'common' one, where some common functionality is implemented.

My question is: What would be a better practice for common dependencies: Define them all explicitly in the parent, like I currently do? Or define them in a 'common' module, which other modules reference, and then rely on transitivity (like a single-entry-point for common dependencies)?

score 18 · Accepted Answer · edited Sep 25 '15 at 13:36

18

It's best to use the dependencyManagement tag in your parent pom to define your dependencies and their versions then reference these dependencies in your sub modules where needed. When you require other sub modules in your project (ie your common submodule from another submodule) then the dependencies will be found transitively. For example:

In your parent pom:

<dependencyManagement>
  <dependencies>
    <dependency>
      <groupId>junit</groupId>
      <artifactId>junit</artifactId>
      <version>4.7</version>
      <scope>test</scope>
    </dependency>
  </dependencies>
</dependencyManagement>

In your common pom (notice there is no version or scope):

<dependencies>
  <dependency>
    <groupId>junit</groupId>
    <artifactId>junit</artifactId>
  </dependency>
</dependencies>

And then you can just reference your common submodule from other submodules are you are already.

edited Sep 25 '15 at 13:36

Andrei Petrenko

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answered Jul 26 '11 at 16:57

Caps

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Be aware that dependency management is viral, though. I.e. if A -> B, A -> C:1.0, and B -> C:2.0 by way of dependency management, then in building A, you'll get C:2.0 instead of C:1.0, as you would expect from typical transitive dependency rules. To get the version you want, A must use dependency management to force it to C:1.0. Hence: viral. – Ryan Stewart Jul 26 '11 at 17:25
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Yes, I know the dependencyManagement tag, but I see it more as a mean of enforcing dependencies versions across a project, without error-prone redundancy. However, you still need to declare the dependencies themselves in each module. I was rather thinking of a situation where **all** (or nearly) modules share some dependency: regardless of versioning, is it a better practice to define those _global_ dependencies in the parent (like I currently do) or create a module for that specific purpose, on which all other ones depend? – Eleaar Jul 30 '11 at 10:39
2

I use dependencyManagagement as described above and put each module's dependencies within the module level POM unless the dependency is for 100% of the modules always...then I put it in the parent POM. Do NOT put a dependency in the parent POM that is not needed in every child POM and definitely do not create a project just to pull in dependencies. It is not a big deal to repeat these dependencies across child modules because you do not need to supply the version as the dependencyManagement stanza in the parent does that for you. – Dave Sep 20 '11 at 19:49

score 0 · Answer 2 · answered Jul 26 '11 at 17:16

0

I can see two reasonable options:

Declare common dependencies in a common module
Use import scope

Second option is useful when you have multiple modules depend on same large stack of components with number of their own dependencies.

answered Jul 26 '11 at 17:16

Eugene Kuleshov

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Maven multi-module: aggregate common dependencies in a single one?

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