Coordinates of Dependencies

Artifact coordinates for dependencies are calculated from a dependency element:

This also applies when the dependency element is a child of a dependencyManagement element.

Notice that the dependency element does not have an extension element. Instead it has a type element which is used to derive the extension and sometimes the classifier. Out of the box, Maven Core defines 11 “types” (handled by the same named ArtifactHandler components):

From the table above, you can see that if the dependency type is “war”, the extension is also war and the classifier is the value of the classifier element (if present) or the empty string if the classifier element is not present. If the type is “test-jar”, the extension is “jar” and the classifier is “tests”. If the type is not one of these 11 names, then the value of the “type” is used as the extension. For example, if the type element is <type>tar.gz</type>, the extension will be tar.gz, and the classifier will be set by the classifier element. This table may be extended by plugins and extensions used in the build.

This has “interesting” consequences. Consider the artifact org.project:reusable-test-support:1.0:tests:jar. Maybe surprisingly, a dependency on this artifact can be described in two ways:

<dependency>
  <groupId>org.project</groupId>
  <artifactId>reusable-test-support</artifactId>
  <version>1.0</version>
  <classifier>tests</classifier>
</dependency>

and the equivalent dependency would be:

<dependency>
  <groupId>org.project</groupId>
  <artifactId>reusable-test-support</artifactId>
  <version>1.0</version>
  <type>test-jar</type>
</dependency>

The obvious difference is that the first example has the <classifier>tests</classifier>, while the second has the <type>test-jar</type>. The type “test-jar” implies a classifier of “tests”. In both cases, the extension is “jar”. The first uses the default value for the extension, while the second derives it from the type.

Note: In this case, the first way is somewhat “explicit”, and is recommended. When the type handler carries important extra information such as custom packaging, using type is more appropriate. Simply put, in this example the type “test-jar” is like an alias for ordinary JARs with the “tests” classifier.

Coordinates of POM Files

The coordinates of a POM artifact that contain a project's pom.xml file are derived from the top-level groupId, artifactId, and version elements in that pom.xml file:

Coordinate values are computed after the POM is made into an effective POM; that is, after parent values have been inherited.

Coordinates of Build Plugins and Build Extensions

Build plugin and build extension artifacts are JARs. For build plugins, this is how the corresponding coordinates are computed from a plugin element:

This also applies when the plugin element is a child of a pluginManagement element.

Build extensions are similarly computed from an extension element (which is not the same as and should not be confused with the extension artifact coordinate):

Note: The extension artifact coordinate and a Maven build extension are two completely different things that unfortunately share the name “extension”. A Maven build extension is a JAR archive that is added to the project class loader's classpath. It is referenced by an extension element in pom.xml. An extension coordinate is usually the filename extension of an artifact's JAR file such as jar, zip, or txt. This extension is most often set to an implicit default value, but can be changed by the type child of a dependency element.