I have this simple data class. What should I do to...

# getting-started

[Pihentagy]

I have this simple data class. What should I do to make it possible to use jackson to (de)serialize from-to json? I'd prefer not to specify property names explicitly.

data class MyClass  @JsonCreator constructor (
    var changedAt: Date,
    var from: Int,
    var to: Int,
    var percentChanged: Int,
    var levelUsed: Int,
)

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[Sam]

Just to check—have you installed the Jackson Kotlin module?

[Sam]

If you have, I would expect data classes to work out of the box. You shouldn't even need the @JsonCreator annotation.

[Sam]

If you're still having trouble, try #jackson-kotlin.

[Pihentagy]

You mean this one?

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[Pihentagy]

Without any annotation I get

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[Pihentagy]

These are the actual content of the classes

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[Sam]

As well as adding the jackson-module-kotlin dependency, it also needs to be registered when you create the object mapper. Maybe that's missing?

[Sam]

If you have something like val mapper = jacksonObjectMapper(), that's using the Kotlin module and you're good 👍. But if you just have val mapper = ObjectMapper(), with no extra Kotlin module being registered, that could definitely explain the issues you're seeing.

[Sam]

(Don't ask me why the functions are named that way, it's confusing to me too)

[Pihentagy]

Yesss, that was it.

[Sam]

🎉

[Pihentagy]

It's a spring boot project.

[Pihentagy]

And I'm actually mapping jsonb fields from-to Map<String,Any> to data classes.

[Sam]

Hm, that's interesting, I would expect Spring boot to configure the Kotlin module by default in its injected object mapper. Though that wouldn't help if you're creating your own separate instance I suppose.

[Pihentagy]

Is there a good pattern to do the conversion? Or is it possible to map to dataclasses the jsonb fields?

Now I map myself through an extension method, but that kind of painful and I cannot access the ObjectMapper bean, since using it from a companion object.

[Pihentagy]

I would do the conversion to Entity level, that's why I choose the extension method

[Sam]

Sure, if you're using a recent version of Spring Boot with Hibernate 6, you should be able to map JsonB fields directly to entity classes

[Sam]

I think you would use the @JdbcTypeCode annotation, but I've not done it myself

[Sam]

Although it seems like there might be an open issue that stops it working totally seamlessly https://github.com/spring-projects/spring-boot/issues/33870

[Pihentagy]

What would be the JdbcTypeCode of that field?

[Sam]

Again, I'm guessing, but: @JdbcTypeCode(SqlTypes.JSON)

[Pihentagy]

Oh, it works.
Btw started converting an old java spring project to kotlin.

Amazing how much boilerplate one can avoid 🤐

[Pihentagy]

On the other hand, if the json is big, it can take quite a bit of time to parse it. Can one make it lazy?

[thanksforallthefish]

you should (imo) almost never declared an object mapper bean yourself, spring boot configures it for you, check org.springframework.boot.autoconfigure.jackson.JacksonAutoConfiguration

you should need to define custom module and/or special configuration, eg if you want to configure the kotlin module:

@Bean
  fun kotlinModule(): KotlinModule = KotlinModule.Builder()
      .configure(KotlinFeature.NullIsSameAsDefault, enabled = true)
      .build()

[Pihentagy]

Huh, what does that Bean definition does? 🤔

[thanksforallthefish]

tl;dr spring boot provides a lot of pluggable configurations, in case of jackson there are modules that you can add to customize the object mapper that spring creates for you. so

@Bean
fun objectMapper() = jacksonObjectMapper()

completely overrides the whole logic spring gives you to configure the object mapper flexibly: https://docs.spring.io/spring-boot/docs/current/reference/html/howto.html#howto.spring-mvc.customize-jackson-objectmapper

[thanksforallthefish]

also, personal preference, jackson is very smart on its own, I would rather do ObjectMapper().findAndRegisterModules() over jacksonObjectMapper(), jackson will scan the classpath and preconfigure the object mapper accordingly