I’ve been exploring kotlin for new projects on the JVM and it’s been lovely so far. I’ve enjoyed the mostly seamless interop with the Java I know and love. I say mostly because there are a few hurdles to jump through.

One I issue I recently encountered involved using Jackson’s CSV dataformat with Kotlin data classes. This was obvious when I found the solution and tracked it back to the documentation, but not so obvious when I first encountered it.

Running the following Kotlin program:

import com.fasterxml.jackson.dataformat.csv.CsvMapper
import com.fasterxml.jackson.dataformat.csv.CsvSchema
import com.fasterxml.jackson.module.kotlin.KotlinModule

data class Customer(val id: Int)

fun main () {
    val mapper = CsvMapper()
    val csv = "id\n1234"
    val customers = mapper.readerFor(Customer::class.java)

Results in this lovely exception:

com.fasterxml.jackson.databind.exc.MismatchedInputException: Cannot construct instance of `Line_2$Customer` (although at least one Creator exists): cannot deserialize from Object value (no delegate- or property-based Creator)

This is because even though we’ve included jackson-module-kotlin, we still need to register it with the CSVMapper in order to properly handle Kotlin classes. Luckily, registering is simple:

fun main () {
    val mapper = CsvMapper().registerModule(KotlinModule())
    // ...

We can now properly deserialize our CSV:


Hopefully that saves you some time spent Googling and scratching your head!