I'm trying to convert a fork of a library from using Jackson to using Kotlinx.serialization. I've ne...

# serialization

[v79]

I'm trying to convert a fork of a library from using Jackson to using Kotlinx.serialization. I've never used Jackson before. Can anyone help me understand what this is doing:

objectMapper.readValue<Map<String, Any>>(it)

And how I might write it using Kotlinx.serialization instead? In the context, it's extracting a JWT claim from a Json Web Token. I don't know what the type of the 'Any' is, but I suspect it's just a String, or possibly a List of Strings.

[Adam S]

is it a string? My guess is that the KxS equivalent is decoding to JsonObject

https://github.com/Kotlin/kotlinx.serialization/blob/v1.4.1/docs/json.md#json-elements

you can try

val jwtObj: JsonObject = Json.decodeFromString(it)

And then you can treat JsonObject like a key-value Map

[v79]

Yes, it is a String. I'll try that, thanks.

[v79]

Next problem is:

response.body?.let { // type of it is T & Any, whatever that means!
  objectMapper.writeValueAsString(it)
}

I guess that Jackson is using reflection so can find out what the concrete class of T is, but kotlinx is not and I need to pass a SerializationStrategy - and since it's T, I don't know what type that is.

[Adam S]

I’d need a bit more context to figure that one out

[Adam S]

I think T & Any means definitely not null

[v79]

The entire function call is:

open fun <T> createResponse(contentType: MediaType, response: ResponseEntity<T>): APIGatewayProxyResponseEvent =
        when (response.body != null && serializationHandlerChain.supports(contentType, response.body)) {
            true -> contentType
            false -> MediaType.parse(router.defaultContentType)
        }.let { finalContentType ->
            APIGatewayProxyResponseEvent()
                .withStatusCode(response.statusCode)
                .withHeaders(response.headers.toMutableMap().apply { put("Content-Type", finalContentType.toString()) })
                .withBody(
                    response.body?.let {
//                        Json.encodeToString(it)
                        serializationHandlerChain.serialize(finalContentType, it as Any)
                    }
                )
        }

The call to serializationHandlerChain.serialize(finalContentType, it as Any) ultimately just ends with objectMapper.writeValueAsString(body)
I can't just write Json.encodeToString(it as Any)

[Adam S]

this is Spring Boot?

[v79]

No, not at all. It's https://github.com/moia-oss/lambda-kotlin-request-router - for AWS Lambda functions written in Kotlin. It's just a router.

[v79]

I had some issues with the library as it stands, so I'm experimenting with a fork for my own wee project.

[v79]

I have also experimented with writing my own router, but I'm not that clever.

[Adam S]

the first thought that comes to mind is to make the function inline, and T reified

inline fun <reified T> createResponse()

but I don’t think that will work because it’s open, and it’s probably too large/complicated to inline

[Adam S]

I think probably the best is to add the Serializer into the data class ResponseEntity

[Adam S]

maybe

data class ResponseEntity<T>(
    val statusCode: Int,
    val body: T? = null,
    val headers: Map<String, String> = emptyMap(),
    val serializer: KSerializer<T>,
)

[Adam S]

if you made the helper functions inline and reified T then I think you could get the KSerializer<T> from the `SerializerModule`… maybe

[v79]

A lot of changes required I fear.

[v79]

All because I've got some LocalDate values to deal with 😞.

[Adam S]

https://github.com/Kantis/ks3/blob/b745fc1cb42e79a4d419c30261aa119d88c6525e/ks3-jdk/src/jvmMain/kotlin/io/ks3/java/time/LocalDateAsStringSerializer.kt#L6

[v79]

Yeah, I've done it myself. Just a lot of changes to even get the tests to compile just now.

[Adam S]

yeah