Clarify precedence between yml and properties files on the same level

[leonard84]

In 4.2. Externalized Configuration (Spring Boot 2.4.2) it states that

Config data files are considered in the following order:

1. Application properties packaged inside your jar (application.properties and YAML variants).
2. Profile-specific application properties packaged inside your jar (application-{profile}.properties and YAML variants).
3. Application properties outside of your packaged jar (application.properties and YAML variants).
4. Profile-specific application properties outside of your packaged jar (application-{profile}.properties and YAML variants).

However, what is unclear is which config values wins if you have both application.yml and application.properties on the same level. We'd like to use Variant 1 (see below), which seems to work, but relies on undocumented behavior that could easily change, so it would be great if the precedence handling could be added to the docs and checked with tests, if that is not already the case.

Context:

In SB <2.4 we used:

• src/main/application.yml to store profiles
• src/main/application-default.properties to set spring.profiles.include=stage-workstation to activate a profile if nothing was activated via ENV/systemproperty. (We used the properties file as this key is invalid in yml, but using this file makes it also easy to override for test)
• src/test/application-default.properties to set spring.profiles.include=stage-workstation, test to activate the test profile
• src/main/application-test.yml with profile in the name to avoid overriding the application.yml

For SB 2.4.x

We are evaluating two solutions

Variant 1

• src/main/application.yml to store profiles and use spring.profiles.group.default=stage-workstation
• src/test/application.properties to set spring.profiles.group.default=stage-workstation,test to activate the test profile and use properties instead of yml to avoid overriding the application.yml
• src/main/application-test.yml with profile in the name to avoid overriding the application.yml

Variant 2

• src/main/application.yml to store profiles and use spring.profiles.group.default=stage-workstation
• src/test/application.yml that uses spring.config.import=file:${user.dir}/src/main/resources/application.yml and overrides the spring.profiles.group.default=stage-workstation,test (IMHO it is rather brittle to rely on ${user.dir} to be set to the correct location)

[philwebb]

See #24719 for a recent regression where the order did change between Boot 2.3 and 2.4. Although it's still undocumented, we do at least now have a test.

[mbhave]

The reason I think this isn't documented is because we thought people might stick with one format (either .yml or .properties) and not combine both. The use case described above is interesting because it lets you override spring.profiles.group since that is no longer allowed in profile-specific file. I think we should document that .properties will win in such a scenario.

[snicoll]

Closing in favor of PR #25300