Change effective on 18 October 2016
Heroku now provides early access to prerelease versions of JDK 9. To use JDK 9 with your Java, Scala, Clojure, Groovy, or JRuby application, create a
system.properties file in the root directory of your repository with the following contents:
Then add the file to Git, and JDK 9 will be installed the next time you deploy. For a full list of supported versions, see the Dev Center documentation on JVM versions.