merge data class with ObjectNode

mergeDataClass.kt

fun main(args: Array<String>) {
    val mapper = ObjectMapper().registerKotlinModule()
    val obj = Foo("foo", "bar", 3)

    val obj2 = merge<Foo>(obj, mapper.readValue<ObjectNode>("""{"x": "baz", "z": 5}"""))

    println("obj=${obj} obj2=${obj2}, obj === obj2 is ${obj === obj2}")
}

fun <T> merge(obj: Any, json: ObjectNode): T {

    require(obj::class.isData) { "Object is not an instance of a data class: $obj" }

    val mapper = ObjectMapper().registerKotlinModule()
    val copyMethod = obj::class.memberFunctions.find { method -> method.name == "copy" } ?:
            throw RuntimeException("Couldn't find copy method on instance: $obj")

    val args = mutableMapOf<KParameter, Any?>()

    for (param in copyMethod.parameters) {
        if (param.name == null) {
            args[param] = obj
            continue
        }

        if (!json.has(param.name)) {
            if (param.isOptional) {
                continue
            }
            throw RuntimeException("Missing required field: ${param.name}")
        }

        val node = json.get(param.name)

        if (node.isNull && !param.type.isMarkedNullable) {
            throw RuntimeException("Got null value for non-nullable field: ${param.name}")
        }

        val javaType = mapper.constructType(param.type.javaType)
        args[param] = mapper.readerFor(javaType).readValue(node)
    }

    return copyMethod.callBy(args) as T
}