CODEWITHSUNDEEP
maven
Lencalot
Lencalot

Reputation: 395

Do maven plugins need dependencies?

Does Maven install the necessary dependencies when adding a plugin to the pom file?

For example, when adding the following code to a pom file, does maven download the necessary dependencies for that plugin? 

      <plugin>
        <groupId>org.codehaus.mojo</groupId>
        <artifactId>sonar-maven-plugin</artifactId>
        <version>2.7</version>
      </plugin>

Upvotes: 3

Views: 121

Answers (2)

JavaSheriff
JavaSheriff

Reputation: 7677

If you take a look at the plugin pom

http://repo1.maven.org/maven2/org/codehaus/sonar/sonar-maven-plugin/2.7/sonar-maven-plugin-2.7.pom

Maven will pull all relevant dependencies for the plugin according to 

<dependencies>

section of the plugin pom

Upvotes: 1

Don Scott
Don Scott

Reputation: 3467

Yes to both questions. Sub-dependencies are called Transitive Dependencies and are handled natively in Maven 2.0. You can read about it more on the Maven website.

Specifically:

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.

This feature is facilitated by reading the project files of your dependencies from the remote repositories specified. In general, all dependencies of those projects are used in your project, as are any that the project inherits from its parents, or from its dependencies, and so on.

There is no limit to the number of levels that dependencies can be gathered from, and will only cause a problem if a cyclic dependency is discovered.

With transitive dependencies, the graph of included libraries can quickly grow quite large. For this reason, there are some additional features that will limit which dependencies are included:

  • Dependency mediation - this determines what version of a dependency will be used when multiple versions of an artifact are encountered. Currently, Maven 2.0 only supports using the "nearest definition" which means that it will use the version of the closest dependency to your project in the tree of dependencies. You can always guarantee a version by declaring it explicitly in your project's POM. Note that if two dependency versions are at the same depth in the dependency tree, until Maven 2.0.8 it was not defined which one would win, but since Maven 2.0.9 it's the order in the declaration that counts: the first declaration wins.
    • "nearest definition" means that the version used will be the closest one to your project in the tree of dependencies, eg. if dependencies for A, B, and C are defined as A -> B -> C -> D 2.0 and A -> E -> D 1.0, then D 1.0 will be used when building A because the path from A to D through E is shorter. You could explicitly add a dependency to D 2.0 in A to force the use of D 2.0
  • Dependency management - this allows project authors to directly specify the versions of artifacts to be used when they are encountered in transitive dependencies or in dependencies where no version has been specified. In the example in the preceding section a dependency was directly added to A even though it is not directly used by A. Instead, A can include D as a dependency in its dependencyManagement section and directly control which version of D is used when, or if, it is ever referenced.
  • Dependency scope - this allows you to only include dependencies appropriate for the current stage of the build. This is described in more detail below.
  • Excluded dependencies - If project X depends on project Y, and project Y depends on project Z, the owner of project X can explicitly exclude project Z as a dependency, using the "exclusion" element. Optional dependencies - If project Y depends on project Z, the owner of project Y can mark project Z as an optional dependency, using the "optional" element. When project X depends on project Y, X will depend only on Y and not on Y's optional dependency Z. The owner of project X may then explicitly add a dependency on Z, at her option. (It may be helpful to think of optional dependencies as "excluded by default.")

Upvotes: 2

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