Heroku java version selection not working

[Jf Okeeffe]

I want to deploy my Spring Boot REST API to Heroku.

My app is java 11, but Heroku is incorrectly installing 1.8 on deploy.

This is how it happens.

I go to the STS terminal / heroku CLI and execute the following command:

git push heroku main

It starts running saying:

Installing jdk 1.8

and then generates a build error at the end.

Fatal error compiling: invalid target release: 11 -> [Help 1]

I tried the following:

1) Pom.xml:

<java.version>11</java.version>

2) Added system.properties file at the root with the following entry:

java.runtime.version=11

3) Project > Properties > Java Compiler > Version 11

4) Preferences > Java > Installed JRE's > jdk 11

Still not working. Heroku continues to install 1.8.

When I build the application locally, it does generate the .war file successfully.