See https://github.com/google/google-java-format#eclipse

1. Uncomment <module>eclipse_plugin</module> in the parent pom.xml

2. Run mvn install, which will copy the dependences of the plugin to eclipse_plugin/lib.

3. If you are using Java 9, add

   -vm
   /Library/Java/JavaVirtualMachines/jdk1.8.0_91.jdk/Contents/Home/bin/java
   

   to /Applications/Eclipse.app/Contents/Eclipse/eclipse.ini.

4. Open the eclipse_plugin project in a recent Eclipse SDK build.

5. From File > Export select Plugin-in Development > Deployable plugin-ins and fragments and follow the wizard to export a plugin jar.