Add some guidance to the reference documentation about diagnosing unexpected property values

[mdeinum]

Lately I have been answering question on stackoverflow for people using "reserved" property names. Like env, environment, classpath and the likes. I think it would be beneficial to add a note/section to the reference guide mentioning those properties. This both as to add clarity and as a reference for answering questions.

See: https://stackoverflow.com/questions/58387443/spring-boot-reading-different-value-for-one-key-from-application-properties

[wilkinsona]

Thanks for the suggestion.

Lately I have been answering question on stackoverflow for people using "reserved" property names. Like env, environment, classpath and the likes.

I'm not sure that any of those are reserved. As far as I can tell, none of them is set by default as either a system property or an environment variable. We already document the property source ordering which should help people to diagnose why a property set in one source is being overridden.

The env and configprops endpoints can also be useful in determining why a property has a particular value. I think the best option here is probably to add a note to the docs about using those two endpoints to diagnose unexpected property values.

[mdeinum]

If I remember correctly the environment issue arose due to use an SpEL expression, #{environment} instead of a value expression, ${environment}. The SpEL expression resolves to the Environment instance configured, the value expression would indeed fail. So the first was due to a faulty use of SpEL to obtain a pre-configured property.

The classpath isn't indeed default set but when set through env or system properties it might be confusing.

Adding additional information on how to resolve/investigate this will certainly help. Another addition could be a best practice for naming properties, like to put them in a application/component namespace, so that they don't get mixed up with other properties.

[wilkinsona]

Closing in favour of #24336.