I'll post here my question. 🙂 I'm very new to Java/Kotlin - I've mostly been a php dev throughout my carrier, so just learning here. First question: How do I control JSON serialization when sending a POST to a controller action?

@PostMapping("/")
fun addAnnouncement(@RequestBody dto: AnnouncementDTO) {
    println(dto);
    manager.addAnnouncement(dto.toAnnouncement())
}

This is my body:

{
  "text": "Hello",
  "occupant": "35A",
  "date": "2020-10-09"
}

It works. Second question: Why? I'm assuming this might not be even a kotlin-specific question, rather Spring, but as I said, I'm quite new at this so excuse some confusion. 🙂

the easiest answer (not because I am jerk) is read the docs. I assume you are using Spring boot, there is a lot of stuff that is done by “magic” in Spring Boot (which is sort of the point, removing mundane work from the developer). if you care about understanding the nitty gritty details (which you should, otherwise good luck when things go wrong) docs and blogs online are your best resource

Honestly, I tried Googling the docs and can't say I had much luck with that. But I think I've found my answer.

this being said, Spring MVC starter comes preconfiguring Jackson, which is one of the main libraries to convert from json to java/kotlin classes

There's a class

RequestResponseBodyMethodProcessor

which processes all the requests. In in there's a method called

readWithMessageConverters

which calls it's parent (from the abstract class) that contains a bunch of converters (

this.messageConverters

) - an array of various converters that have a

canRead

method. At some point

AbstractJackson2HttpMessageConverter

is invoked and

canDeserialize

method is checked. After that a converter is invoked and Jackson takes over and converts it to the class.

This spring-boot doc should help https://docs.spring.io/spring-boot/docs/current/reference/html/howto.html#howto-spring-mvc And more generally, you can read about spring mvc here https://docs.spring.io/spring-framework/docs/current/spring-framework-reference/web.html#mvc