Hello all. I'm using the lib, `jackson-module-kotlin,` to serialize but I'm having an error:

# jackson-kotlin

[Rodrigo Silva]

Hello all. I'm using the lib, jackson-module-kotlin, to serialize but I'm having an error:

[Rodrigo Silva]

I have the following methods, to serilize to JsonNode and deserialize.

Sem título

[Rodrigo Silva]

Deserializing a class:

Sem título

[Rodrigo Silva]

But I always get the following error:

java.lang.ClassCastException: class java.util.LinkedHashMap cannot be cast to class com.rjdesenvolvimento.aries.access.consumer.consumer.Consumer (java.util.LinkedHashMap is in module java.base of loader 'bootstrap'; com.rjdesenvolvimento.aries.access.consumer.consumer.Consumer is in unnamed module of loader 'app')

[Rodrigo Silva]

Could anyone help?

[araqnid]

My guess is that you have to tell Jackson what the “T” type is when deserializing- if you don’t, it assumes “T” is “Any” and just uses the default serialization - which will convert a JSON object into a LinkedHashMap. Are you expecting an Event<Consumer> ?

[araqnid]

You tell it what type “T” is by passing a TypeReference - the Kotlin module has a helper function to create one called jacksonTypeRef()

[Rodrigo Silva]

Yes, I'm waiting for Event <Consumer>
But I also expect other classes, like
Event <Person>
Event <Address>....
How do I solve this problem?

[Joe]

https://github.com/FasterXML/jackson-docs/wiki/JacksonPolymorphicDeserialization

[Joe]

one way to go about it is to serialize a "type" property that jackson can key off of when deserializing based on polymorphic type annotations

[araqnid]

sth like

jacksonObjectMapper().readValue("{}", jacksonTypeRef<Event<Consumer>>())

[Joe]

https://github.com/trib3/leakycauldron/blob/master/graphql/src/main/kotlin/com/trib3/graphql/websocket/GraphQLWebSocketProtocol.kt has a working example

[araqnid]

yes, the polymorphic deserialisation works too, provided the JSON has a clear indication which type is there

[araqnid]

if that type is more from context, it might be easier to specify the type when reading

[Joe]

agreed, if you know what you're going to get then Steve's way is easier to understand what's going on, if you're listening for messages and don't know the order they'll arrive in, then the JSON needs some way to tell which type is being read

[Rodrigo Silva]

Thank you @araqnid, @Joe. I will read about it and try to implement it.