Java 9 in JAVA_HOME

$ export JAVA_HOME=/path/to/jdk9
$ mvn test

The plugin will automatically add --add-modules java.se.ee on JVM argument in CLI (unless already specified by user) and all Java 9 API is provided to run your tests.

Java 9 in configuration of plugin

The plugin provides you with configuration parameter jvm which can point to path of executable Java in JDK, e.g.:

<configuration>
    <jvm>/path/to/jdk9/bin/java</jvm>
</configuration>

Now you can run the build with tests on the top of Java 9.

Maven Toolchains with JDK 9

This is an example on Windows to run unit tests with custom path to Toolchain (-t …).

$ mvn -t D:\.m2\toolchains.xml test

Without (-t …) the Toolchain should be located in ${user.home}/.m2/toolchains.xml.

The content of toolchains.xml would become as follows however multiple different JDKs can be specified.

<toolchains xmlns="http://maven.apache.org/POM/4.0.0"
            xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance"
            xsi:schemaLocation="http://maven.apache.org/TOOLCHAINS/1.1.0 https://maven.apache.org/xsd/toolchains-1.1.0.xsd">
  <toolchain>
    <type>jdk</type>
    <provides>
      <version>9</version>
      <vendor>oracle</vendor>
      <id>jdk9</id>
    </provides>
    <configuration>
      <jdkHome>/path/to/jdk9</jdkHome>
    </configuration>
  </toolchain>
</toolchains>

Your POM should specify the plugin which activates only particular JDK in toolchains.xml which specifies version 9:

<plugin>
    <groupId>org.apache.maven.plugins</groupId>
    <artifactId>maven-toolchains-plugin</artifactId>
    <version>1.1</version>
    <executions>
      <execution>
        <phase>validate</phase>
        <goals>
          <goal>toolchain</goal>
        </goals>
      </execution>
    </executions>
    <configuration>
      <toolchains>
        <jdk>
          <version>9</version>
        </jdk>
      </toolchains>
    </configuration>
</plugin>

Now you can run the build with tests on the top of Java 9.