Keycloak Userstorage SPI: how to use external application.properties?

So I have a Userstorage SPI that connects to a DB. At the moment it is hard coded (db url, username, password, etc.) into the class and I would like to use an application.properties file near the jar.

So the keycloak folder structure should look something like:

keycloak:
  bin
  conf
  data
  lib
  providers
    my-userstorage-spi.jar
    application.properties  

So when the keycloak starts: bin/kc.bat start-dev, the my-userstorage-spi.jar should read the values from the application.properties.

I am up for any solution. Autoconfiguration, file handling...

For example if I try to use it like this, it would say the file is not found:

Properties mainProperties = new Properties();
String path = "./application.properties";
FileInputStream  file = new FileInputStream(path);
mainProperties.load(file);

System.out.println(mainProperties.get("my.value"));
file.close();

The exception:

java.io.FileNotFoundException: .\application.properties (The system cannot find the file specified)

Thanks in advance.