Maven dependency resolution (conflicted)

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.

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

Maven can handle both situations without any conflict. Conflicts will exist when two versions of a transitive dependency are required. The ClassNotFoundException you describe results from the app (or a dependency) attempting to use a class not available in the version of the conflicted dependency that actually gets used. There are multiple ways to fix the problem.

1. Update the versions of the libraries you are using that depend on the conflicted dependency, so that they all depend on the same version version of that dependency
2. Declare the conflicted dependency as a direct dependency of your project with the version you want to be included (in the example, the one with the missing class included in it)
3. Specify which version of the conflicted dependency that transitive dependencies should use, via the <dependencyManagement> section of the POM
4. Explicitly exclude the unwanted versions of the conflicted dependency from being included with the dependencies that rely on them using an <exclusion>

This is fundamentally not a maven issue, but a java issue. If Project B and Project C needs two incompatible versions of project D, then you can't use them both in Project A.

The Maven way of resolving conflicts like these is unfortunately, as you already know, to choose which ones to exclude.

Using mvn dependency:analyze and mvn dependency:tree helps in finding what conflicts you have.

You can enforce consistent dependencies in whole project with rule Dependency Convergence.

 <plugin>
     <groupId>org.apache.maven.plugins</groupId>
     <artifactId>maven-enforcer-plugin</artifactId>
     <version>1.3.1</version>
     <executions>
        <execution>
           <id>enforce</id>
           <configuration>
              <rules>
                 <DependencyConvergence/>
              </rules>
           </configuration>
           <goals>
              <goal>enforce</goal>
           </goals>
        </execution>
     </executions>
  </plugin>

One possible strategy is to specify for main project, what version of D to use (the newest one f.g.). However, if library D is not backward-compatible, you have a problem as stated by kukudas - it's impossible to use both libaries in your project.

In such situation there may be necessary to use either B or C in older version, so that both would depend on compatibile versions of D.