Add configuration property for preferred json mapper in webflux

[vpelikh]

Jackson sometimes used as transitive dependency (looking at spring-actuator), so it can casually end up in your classpath without your knowledge. This complicates json codec setup if you want to use something else. Spring-webmvc already have good solution to this problem, so I ported it to webflux. I don't think this is the best implementation, but it's definitely the simplest.

I will be glad to hear any suggestions.

[hojooo]

To extend this concept to WebFlux, would it be more consistent to use the existing key "spring.http.converters.preferred-json-mapper" as the common entry point rather than introducing a new one "spring.http.codecs.preferred-json-mapper"?

[vpelikh]

To extend this concept to WebFlux, would it be more consistent to use the existing key "spring.http.converters.preferred-json-mapper" as the common entry point rather than introducing a new one "spring.http.codecs.preferred-json-mapper"?

Thanks for suggestion.

As I understood spring-http-codecs module is webflux alternative to webmvc's spring-http-conveters and it would not be very good to mix these concepts, even at the configuration level. Also spring already have some properties under spring.http.codecs so it can be confusing to users.

I also think that merging http-codecs with http-converters might be a good idea, but it is out of scope of this MR.

[wilkinsona]

You're correct, @vpelikh, that property should not be used to influence both HTTP codecs and HTTP message converters. Please leave things as they are and we'll review your proposal more fully in due course.