.mvn/extensions.xml file:

The old way (up to Maven 3.2.5) was to create a jar (must be shaded if you have other dependencies) which contains the extension and put it manually into the ${MAVEN_HOME}/lib/ext directory. This means you had to change the Maven installation. The consequence was that everyone who likes to use this needed to change it’s installation and makes the on-boarding for a developer much more inconvenient. The other option was to give the path to the jar on command line via mvn -Dmaven.ext.class.path=extension.jar. This has the drawback giving those options to your Maven build every time you are calling Maven. Not very convenient as well.

From now on this can be done much more simpler and in a more Maven like way. So you can define an .mvn/extensions.xml file which looks like the following:

<extensions xmlns="http://maven.apache.org/EXTENSIONS/1.1.0" xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance"
  xsi:schemaLocation="http://maven.apache.org/EXTENSIONS/1.1.0 https://maven.apache.org/xsd/core-extensions-1.0.0.xsd">
  <extension>
    <groupId/>
    <artifactId/>
    <version/>
  </extension>
</extensions>

Now you can simply use an extension by defining the usual maven coordinates groupId, artifactId, version as any other artifact. Furthermore all transitive dependencies of those extensions will automatically being downloaded from your repository. So no need to create a shaded artifact anymore.

.mvn/maven.config file:

It’s really hard to define a general set of options for calling the maven command line. Starting with Maven 3.3.1+, this can be solved by putting this options to a script but this can now simple being done by defining .mvn/maven.config file which contains the configuration options for the mvn command line.

For example things like -T3 -U --fail-at-end. So you only have to call Maven just by using mvn clean package instead of mvn -T3 -U --fail-at-end clean package and not to miss the -T3 -U --fail-at-end options on every call. The .mvn/maven.config is located in the project's top level .mvn directory also works if in the root of a multi module build.

NOTICE starting with Maven 3.9.0 each single argument must be put in new line, so for the mentioned example your file will have content like:

-T3
-U 
--fail-at-end

.mvn/jvm.config file:

Starting with Maven 3.3.1+ you can define JVM configuration via .mvn/jvm.config file which means you can define the options for your build on a per project base. This file will become part of your project and will be checked in along with your project. So no need anymore for MAVEN_OPTS, .mavenrc files.

So for example if you put the following JVM options into the .mvn/jvm.config file

-Xmx2048m -Xms1024m -XX:MaxPermSize=512m -Djava.awt.headless=true

You don't need to use these options in MAVEN_OPTS or switch between different configurations.