Maven dependency resolution (conflicted)

Question

Let's say I have four projects:

• Project A (has a dependency on B and D)
• Project B (has a dependency on D)
• Project C (has a dependency on D)
• Project D

In this scenario if I run project A, Maven will correctly resolve the dependency to D. If I understand this correctly Maven always takes the dependency with the shortest path. Since D is a direct dependency of A it will be used rather then, the D which is specified within B.

But now assume this structure:

• Project A (has a dependency on B and C)
• Project B (has a dependency on D)
• Project C (has a dependency on D)
• Project D

In this case the paths to resolving D have the same depth. What happens is that Maven will have a conflict. I know that it is possible to tell Maven that he should exclude dependencies. But my question is how to address such kind of problems. I mean in a real world application you have a lot of dependencies and possibly a lot of conflicts as well.

Is the best practice solution really to exclude stuff or are there other possible solutions to this? I find it very hard to deal with when i suddenly get a ClassNotFound Exception because some versions have changed, which caused Maven to take a different dependency. Of course, knowing this fact makes it a little bit easier to guess that the problem is a dependency conflict.

I'm using maven 2.1-SNAPSHOT.

Answer 1

The maven way of resolving situations like this is to include a <dependencyManagement> section in your project's root pom, where you specify which version of which library will be used.

EDIT:

<dependencyManagement>
  <dependencies>
    <dependency>
        <groupId>foo</groupId>
        <artifactId>bar</artifactId>
        <version>1.2.3</version>
    </dependency>
   </dependencies>
</dependencyManagement>

Now no matter which version of library foo:bar is requested by a dependency, version 1.2.3 will always be used for this project and all sub-projects.

Reference:

• Dependency Management