I have 3rd party library that is configured by placing properties file on the root of the classpath. That library is using getClass().getResourceAsStream("/file.properties") to load that file. As it is 3rd party, it is unmodifiable. I have placed that configuration file into external resources directory (not to be mistaken with resources from eg. Maven's or Gradle's directory structure.
Directory structure is like this.
How to run/configure Spring boot to include content of resources directory to the classpath so getResourceAsStream wil work?
On SE application I would simply do java -jar myApp.jar with classpath in MANIFEST and that would work.
EDIT:
Just a word of clarification - putting configuration file inside project resources (along sources) is missing the whole point. I want to keep configuration externalized.