Smarter @ConfigurationProperties binding for nested namespaces

[snicoll]

ignoreUnknownFields on @ConfigurationProperties is handled locally while it should probably be validated against a given application.

Consider the following case:

@ConfigurationProperties(prefix= "foo", ignoreUnknownFields=true)
public class FooProperties { ...}

Then somewhere else in the project we have the following

@ConfigurationProperties("foo.bar")
public class FooBarProperties { ...}

The binder might fail if we provide a valid property, let's say foo.bar.name because when it is processed against FooProperties that class does not have such field. In practice, this setup is perfectly OK, that property is valid, it's just that it's managed by a different class.

Maybe we should build that knowledge since we have the list @ConfigurationProperties annotated POJO for a given app.

[dsyer]

I like the idea of a global validation. It would be a bit hairy to implement though (as anything global is wont to be).

[snicoll]

This enhancement is there for quite a while and there isn't a obvious way to replace it so we're considering removing that feature altogether instead (#12601) rather than keeping it with false promise.

Looking at properties unbound on startup and generating a report would be a nice alternative, see #10030 for more details.