在Maven多模块项目中，如何指定多个模块中所需的依赖关系？

Question

I know that the intent of a multi-module Maven project is that each module can be compiled and built separately as well as collectively from the parent.

Suppose two of my three modules need to use the Google Guava library. Do I specify that dependency in the POM of each of the two sub-modules? If so, what “scope”? Or do I specify the dependency in the parent module? I need Guava bundled in my final product, but I certainly don't need two copies.

Answer 1

I usually declare all dependencies in dependencyManagement of the parent project and then use them in depenencies of modules, but without version . Versions are only declared in the parent project.

As for scope , I normally don't declare scope in parent unless it is something very obvious like JUnit.

Do I specify that dependency in the POM of each of the two sub-modules?

If you use this dependency directly, then yes. Even if it would be included transitively.

If so, what “scope”?

This depends on what the dependency is and how you use it. Guava will normally be compile .

Or do I specify the dependency in the parent module?

You can declare it in dependencyManagement in parent so that you don't have to specify the version anywhere else.

But you normally will not declare the dependency in the parent module as it will be inherited by all the child modules. This is usually too much of an assumption.

I need Guava bundled in my final product, but I certainly don't need two copies.

Won't happen.

Answer 2

Suppose two of my three modules need to use the Google Guava library. Do I specify that dependency in the POM of each of the two sub-modules?

It depends what is the relation between those modules and what you do with them; let's call them moduleA , moduleB (the modules that share a dependency), and moduleC (the third module).

How do you deploy moduleA and moduleB ? Does it make sense that each have their own copy of the same library ?

• For example if they're both WARs that run independently, you'll want to give each, its own copy of the library.
• If they're both jars that end up in the same war (possibly represented by moduleC ), then you'll want to give each of them the dependency with scope=provided , and add a third dependency in the moduleC with the default scope.

Or do I specify the dependency in the parent module?

The problem with specifying the dependency in the parent module is that the dependency will also be picked up by moduleC which might not be what you want.

One possibility to solve this (for example if moduleA and moduleB share a lot of dependencies, or common properties or settings) is to add a common parent:

• An extra parent pom / reactor for moduleA and moduleB , but NOT for moduleC .
• This extra parent pom would be on the same level in the tree with moduleC (and so it would have, in turn, the same parent as moduleC ).
• It would contain whatever moduleA and moduleB have in common.