Convert JsonFactory object into a stringified JSON field in Kotlin - json

Question

The posts I've seen are reversed but I need to create a JSON string from an object. I'm brand new to Kotlin but in JS I'm essentially looking for stringify.
For example:
JSON.stringify({foo: 'bar'})
That output is what I want.
I built an object like this (which, if this is wrong it doesn't need to be built this way, I'm just trying to figure this out)
val factory = JsonFactory()
val generator = factory.createGenerator(output, JsonEncoding.UTF8)
generator.writeStartObject()
generator.writeNumberField("statusCode", 200)
generator.writeObjectFieldStart("body") // The value inside specifically needs to be a JSON string!
generator.writeStringField("userId", user.id.toString())
generator.writeStringField("amount", user.amount.toString())
generator.writeStringField("percent", user.percent.toString())
generator.writeEndObject()
generator.writeEndObject()
generator.close()
This will create an object like:
{"statusCode": 200, body: {userId: ..., amount: ..., percent: ...}}
However, this won't work (working with a Lambda + API Gateway) and the body needs to be a stringified JSON blob like this:
{"statusCode": 200, body: "{\"userId\": ..., \"amount:\" ..., \"percent:\" ...}"}
I'm just not sure how to do it. Currently (so I can get something to work) I'm just concat'ing in a stringField like:
generator.writeStringField("body", "{" +
Which is very gross and hard to manage so I'd like to do it the "right" way and I have a feeling there's an easy way to do this but can't seem to find it.

Answer 1

kotlinx.serialization provides solution for that:
import kotlinx.serialization.*
import kotlinx.serialization.json.*
#UseExperimental(ImplicitReflectionSerializer::class)
fun main() {
val json = Json(JsonConfiguration.Default)
println(json.stringify(Response(status = 200,
body = json.stringify(Payment("abc", 100, 20)))))
}
#Serializable
data class Payment(val userId: String, val amount: Int, val percent: Int)
#Serializable
data class Response(val status: Int, val body: String)
But let's say you cannot use kotlinx.serialization, since it's experimental.
You can still use ObjectMapper from jackson library (as I see you're using their JsonFactory, which is low level):
import com.fasterxml.jackson.databind.ObjectMapper
fun main() {
val json = ObjectMapper()
println(json.writeValueAsString(Response(status = 200,
body = json.writeValueAsString(Payment("abc", 100, 20)))))
}
data class Payment(val userId: String, val amount: Int, val percent: Int)
data class Response(val status: Int, val body: String)
You can even have your own JSON.stringify:
object JSON {
fun stringify(e: Any) = ObjectMapper().writeValueAsString(e)
}
Or have it as an extension function:
fun main() {
println(Response(status = 200,
body = Payment("abc", 100, 20).asJson().toString()).asJson())
}
data class Payment(val userId: String, val amount: Int, val percent: Int)
data class Response(val status: Int, val body: String)
fun Any.asJson() = ObjectMapper().writeValueAsString(this)