A .mvn directory in the root of our project can contains some useful extras. For example we can set default Maven options or Java VM options when we run a Maven command. We can also define Maven extensions we want to add to the Maven classpath using the file extensions.xml in the .mvn directory. The Maven extension we want to add can be referenced using a groupId, artifactId and version inside an <extension> element. We can define one or more extensions within the parent element extensions. Once we have defined the extension and run Maven the extension is added to classpath of Maven itself.

In the following example we apply the Maven Enforcer extension to our project. We add the file extensions.xml in the .mvn directory that we first have to create:

<!-- File: .mvn/extensions.xml -->
<extensions xmlns="http://maven.apache.org/EXTENSIONS/1.1.0"
    xsi:schemaLocation="http://maven.apache.org/EXTENSIONS/1.1.0 http://maven.apache.org/xsd/core-extensions-1.1.0.xsd">
    <!-- Add Maven Enforcer extension -->

This extension requires an extra configuration file with rules we want to apply to our project. The filename must be enforcer-extension.xml and placed in the .mvn directory as well. In our example we add a rule that checks that the Java version we use to run Maven is at least version 17. And a rule to check that our Maven version is at least 3.9.1:

<!-- File: .mvn/enforcer-extension.xml -->
                    <!-- Our build requires Java 17 or higher -->
                    <!-- Use Maven 3.9.1 or higher -->

Next we run the package command with Java version 11.0.17 and we see in the output that the Enforcer extension failed because of the required Java version rule:

$ mvn package
[INFO] --- enforcer:3.3.0:enforce (maven-enforcer-extension) @ sample ---
[INFO] Rule 1: org.apache.maven.enforcer.rules.version.RequireMavenVersion passed
[INFO] ------------------------------------------------------------------------
[INFO] ------------------------------------------------------------------------
[INFO] Total time:  0.777 s (Wall Clock)
[INFO] Finished at: 2023-04-06T17:19:16+02:00
[INFO] ------------------------------------------------------------------------
[ERROR] Failed to execute goal org.apache.maven.plugins:maven-enforcer-plugin:3.3.0:enforce (maven-enforcer-extension) on project sample:
[ERROR] Rule 0: org.apache.maven.enforcer.rules.version.RequireJavaVersion failed with message:
[ERROR] Detected JDK version 11.0.17 (JAVA_HOME=/Users/mrhaki/.sdkman/candidates/java/11.0.17-tem) is not in the allowed range [17,).
[ERROR] -> [Help 1]
[ERROR] To see the full stack trace of the errors, re-run Maven with the -e switch.
[ERROR] Re-run Maven using the -X switch to enable full debug logging.
[ERROR] For more information about the errors and possible solutions, please read the following articles:
[ERROR] [Help 1] http://cwiki.apache.org/confluence/display/MAVEN/MojoExecutionException

Written with Maven 3.9.1.