MacOSX ~/.m2/toolchains.xml

toolchains.xml

<toolchains>
  <!-- JDK toolchains -->
  <toolchain>
    <type>jdk</type>
    <provides>
      <version>1.9</version>
      <vendor>oracle</vendor>
    </provides>
    <configuration>
      <jdkHome>/Library/Java/JavaVirtualMachines/jdk-9.jdk</jdkHome>
    </configuration>
  </toolchain>
  <toolchain>
    <type>jdk</type>
    <provides>
      <version>1.8</version>
      <vendor>oracle</vendor>
    </provides>
    <configuration>
      <jdkHome>/Library/Java/JavaVirtualMachines/jdk1.8.0_131.jdk</jdkHome>
    </configuration>
  </toolchain>
</toolchains>