How does Jackson marshalling work with Kotlin data class? - kotlin

Question

I have a simple data class being returned from a REST endpoint.
data class SummarizedReturn(
val NET_CASH_FLOW: BigDecimal,
val ROI_PERCENTAGE: BigDecimal
)
When it is returned, the object looks like this:
{
summarizedReturn: {
net_CASH_FLOW: -194703.12028723184,
roi_PERCENTAGE: -35,
}
}
This is not what I need. I need all letters to be capitalized. So I added the JsonProperty annotation
data class SummarizedReturn(
#JsonProperty("NET_CASH_FLOW")
val NET_CASH_FLOW: BigDecimal,
#JsonProperty("ROI_PERCENTAGE")
val ROI_PERCENTAGE: BigDecimal,
)
This did not change anything. I still get the result the same as above.
I then changed the property names and kept the annotation
data class SummarizedReturn(
#JsonProperty("NET_CASH_FLOW")
val netCashFlow: BigDecimal,
#JsonProperty("ROI_PERCENTAGE")
val roiPercentage: BigDecimal,
)
and that returned what I wanted.
{
summarizedReturn: {
NET_CASH_FLOW: -194703.12028723184,
ROI_PERCENTAGE: -35,
}
}
Why did the annotation not work on the initial version of the class? How can I keep my property names all capitalized and have the Jackson value to be the same?

Answer 1

There is an issue with interoperability of Java annotations in Kotlin code. You can register Jackson's Kotlin module to get rid of this problems:
import com.fasterxml.jackson.module.kotlin.jacksonObjectMapper
fun main() {
val mapper = jacksonObjectMapper() // <= shortcut to ObjectMapper().registerKotlinModule()
println(mapper.writeValueAsString(SummarizedReturn(
BigDecimal("-194703.12028723184"),
BigDecimal("-35"))))
}
Output:
{"NET_CASH_FLOW":-194703.12028723184,"ROI_PERCENTAGE":-35}
This will also require you to add com.fasterxml.jackson.module:jackson-module-kotlin to your dependencies.
PS: Alternatively you can solve it by using slightly different target:
#JsonPropery => #get:JsonPropery (or field:#JsonPropery in case this data class will also be used for deserialization)