How can i detect maven 2 jar dependencies ?

Question

I added some jars as dependencies in pom.xml, but it seems that some of them are useless because those jars were already downloaded using dependencies mechanism...

Is there a way to see those "built-in" dependencies, so that I could add only the needed dependencies in my pom.xml?

For example if I add a hibernate dependency in pom.xml one for cglib is not needed.

Answer 1

Don't do that - list every dependency of your code, but not of the libraries you use; Maven will do its transitive dependency thing and take care of them.

Answer 2

you can run mvn dependency:tree to get the whole tree including the transient dependencies that get included in your project. There you can start looking

Hope that helped

Answer 3

If you are using Eclipse then you are see a visual dependency tree which is automatically generated. While adding a dependency if you do not want it to automatically pull some transitive dependencies use the exclusions tag like so

<dependency>
  <groupId>org.hibernate</groupId>
  <artifactId>hibernate</artifactId>
  <version>${hibernate.version}</version>
  <exclusions>
    <exclusion>
      <groupId>asm</groupId>
      <artifactId>asm</artifactId>
    </exclusion>
    <exclusion>
      <groupId>asm</groupId>
      <artifactId>asm-attrs</artifactId>
    </exclusion>
    <exclusion>
      <groupId>cglib</groupId>
      <artifactId>cglib</artifactId>
    </exclusion>
  </exclusions>
</dependency>

There are some known conflicts when using certain versions of hibernate and AOP due to cglib.

Answer 4

First, check out Transitive Dependencies:

Transitive dependencies are a new feature in Maven 2.0. This allows you to avoid needing to discover and specify the libraries that your own dependencies require, and including them automatically.

Then, a good dependency analyzer will help...

The mvn command-line is your first aid:

mvn dependency:tree

Sometimes you have to figure out where the version numbers came from (See also: Dependency Management). Here you'll have to unveil the parent relationships, and the 'effective-pom' command can help with that:

mvn help:effective-pom

Tool support is helpful as well...

m2eclipse has a Dependency Tree tab that shows how the different hierarchies collapse::

IntelliJ has another interesting view that lets you detect the conflicts:

Answer 5

Is there a way to see those "built-in" dependencies, so that I could add only the needed dependencies in my pom.xml?

There are no built-in dependencies. However, when declaring a dependency on a given artifact, Maven will also retrieve the dependencies of this dependency, transitively. Such dependencies are called 3.4.4. Transitive Dependencies:

A transitive dependency is a dependency of a dependency. If project-a depends on project-b, which in turn depends on project-c, then project-c is considered a transitive dependency of project-a. If project-c depended on project-d, then project-d would also be considered a transitive dependency of project-a.

So if you need a dependency in your project, just declare it (and the dependencies of this dependency will come transitively).

To visualize the dependency tree of a project, the best tool is mvn dependency:tree (or any fronted offered by your favorite IDE). This is a must use tool to analyze your dependencies and check them for proper convergence and potential conflicts resulting in expected version being used.

For example if I add a hibernate dependency in pom.xml one for cglib is not needed.

Actually, this is a bad example, cglib is an optional dependency of Hibernate Core which declares in its pom.xml:

<dependency>
  <groupId>org.hibernate</groupId>
  <artifactId>hibernate-cglib-repack</artifactId>
  <version>2.1_3</version>
  <optional>true</optional><!-- think of it as "excluded by default" -->
</dependency>

Hibernate gives you the choice between javassist and cglib, it's up to you to decide which one to use and to declare it explicitly, hence the optional status.

See also

• Introduction to the Dependency Mechanism
• The whole section 3.4. Project Dependencies in the Maven Reference Guide