Heroku installs wrong JDK version

Question

I added system.properties file with the content java.runtime.version=11 to my application like described here, committed the changes and pushed afterwards with git push heroku master

unfortunately, Heroku still installs JDK 1.8:

...
remote: -----> Java app detected
remote: -----> Installing JDK 1.8... done
remote: -----> Installing Maven 3.3.9... done
...
...
...
remote:        [ERROR] Failed to execute goal org.apache.maven.plugins:maven-compiler-plugin:3.8.0:compile (default-compile) on project my-module-persistence: Fatal error compiling: invalid target release: 11 -> [Help 1]

What further/other steps do I have to do?

Answer 1

I had the same problem and want to post my solution here.

Make sure your system.properties file is in your root directory. Do not confuse this file with application.properties inside src/main/resources.