Jackson <-> Kotlin with a custom Introspector

build.gradle

buildscript {
  ext.kotlin_version = '1.3.10'
  ext.jacksonVersion = '2.9.0'
  repositories {
    mavenCentral()
  }
  dependencies {
    classpath "org.jetbrains.kotlin:kotlin-gradle-plugin:$kotlin_version"
  }
}

apply plugin: 'kotlin'
apply plugin: 'application'

mainClassName = 'demo.HelloWorldKt'

defaultTasks 'run'

repositories {
  mavenCentral()
}

dependencies {
  compile "org.jetbrains.kotlin:kotlin-stdlib:$kotlin_version"
  testCompile 'junit:junit:4.11'
  testCompile "org.jetbrains.kotlin:kotlin-test-junit:$kotlin_version"
  compile "com.fasterxml.jackson.module:jackson-module-kotlin:$jacksonVersion"
}

jar {
  manifest { attributes 'Main-Class': 'demo.HelloWorldKt' }
  from { configurations.compile.collect { it.isDirectory() ? it : zipTree(it) } }
}

task wrapper(type: Wrapper) {
  gradleVersion = "4.10.2"
}

CustomAnnotation.kt

package demo


import com.fasterxml.jackson.annotation.JsonProperty
import com.fasterxml.jackson.databind.PropertyName
import com.fasterxml.jackson.databind.introspect.Annotated
import com.fasterxml.jackson.databind.introspect.JacksonAnnotationIntrospector

class CustomAnotationInspector() :
        JacksonAnnotationIntrospector() {

    override fun findNameForSerialization(annotation: Annotated): PropertyName {

        var pn = super.findNameForSerialization(annotation)
        if (annotation.hasAnnotation(JsonProperty::class.java)) {
            return pn
        }

        // if not annotated, value may be set dynamically
        pn = if(annotation.name == "average") {
            PropertyName("f_average")
        } else {
            PropertyName(annotation.name)
        }

        return pn
    }
}

helloWorld.kt

package demo

import com.fasterxml.jackson.databind.DeserializationFeature
import com.fasterxml.jackson.module.kotlin.jacksonObjectMapper

fun getGreeting(): String {
    val mapper = jacksonObjectMapper()
            .disable(DeserializationFeature.FAIL_ON_UNKNOWN_PROPERTIES)
            .setAnnotationIntrospector(CustomAnotationInspector())
    val helloWorldJson = """{"salutation": "Hello","name": "World", "f_average": "92"}"""

    val greeting = mapper.readValue(helloWorldJson, Greeting::class.java)

    return greeting.salutation +  " " + greeting.name + "!"
}

fun main(args: Array<String>) {
    println(getGreeting())
}

data class Greeting(val salutation: String, val name: String, val foo: String?, val average: Int)