Allow @ConstructorBinding to be optional

[philwebb]

Currently ConfigurationPropertiesBindConstructorProvider requires that @ConstructorBinding be used to indicate that a @ConfigurationProperties instance should use the ValueObjectBinder. I think we could skip the need for this if there is a single constructor with one or more arguments.

Somewhat related to #23172

[Davio]

Would this mean that my @ConfigurationProperties classes which are currently annotated by Lombok's @Data (which creates getters and setters for my non-final fields) can be annotated by @Value instead (which creates final fields with an all args constructor)?

If so, I think that would be a reasonable improvement.

[snicoll]

I think we could skip the need for this if there is a single constructor with one or more arguments.

I don't think we can/should. I remember us having a very lengthy conversation that concluded with the current arrangement. One problem with this approach is that you can't opt-out if you happen to have a single constructor and you don't want it to be used that way (can be surprising for folks having an existing class that uses javabean conventions and a constructor for whatever reason).

[Davio]

Maybe it's an idea to consolidate @ConfigurationProperties and @ConstructorBinding by adding the binding mode as an optional property to @ConfigurationProperties or something like useConstructor = true which would work if there is a single constructor?

Would it work if the constructor only sets some of the properties, but others should still be set by setters?

[philwebb]

@snicoll I remember some discussions, but I couldn't remember exactly why we landed on the current setup.

One problem with this approach is that you can't opt-out if you happen to have a single constructor and you don't want it to be used that way

Ah yes, I remember now. The issue is that you may have a @ConfigurationProperties class that is either a @Component or is loaded via a @Import. In those situations you want Spring to inject beans into the constructor and getter/setter binding to be applied afterwards.