Clarify that configuration property default values are not generally available through the Environment

[wilkinsona]

See #2282 (comment) for background.

[mauromol]

There's no behavioral difference in terms of default values between your own code that uses @Value and your own code that uses @ConfigurationProperties. In either case, you won't get the default value that's defined in Spring Boot's own @ConfigurationProperties class. It's this that we need to clarify. I've opened #49654.

Thank you Andy for this.
The only part I don't understand in your last comment on the other discussion is the one quoted above: if I make my bean get an injected ServerProperties instance, in the above example, I do get the default value for that property through serverProperties.getTomat().getMaxConnections(). Substantially, if I want to know what's the currently used value for one of Spring Boot's own @ConfigurationProperties instances, I must get it through an injected instance of that @ConfigurationProperties annotated class: I can't use @Value there, unless I'm sure that the property I need is redefined somewhere in my externalized configuration.

[wilkinsona]

if I make my bean get an injected ServerProperties instance, in the above example, I do get the default value for that property

That's not your own code using @ConfigurationProperties. That's your own code using a Spring Boot class that happens to use @ConfigurationProperties. Ignoring relaxed binding for a moment, it would also have worked if ServerProperties used @Value and declared a default (@Value("${property:default}")).

[wilkinsona]

Closing in favor of #49835. Thanks for the PR, @kodama-kcc.