Tuesday, March 9, 2021

Jackson Kotlin extension and reified types

Jackson Kotlin module library is a pleasure to use. It greatly simplifies gnarly code, specifically one's involving TypeReference

Consider a sample json which looks like this:
    "a" : ["b", "c"],
    "b" : ["a", "c"],
    "c" : ["a", "b"]
This content can be represented as a "Map<List<String>>" type in Java. 

So now, if I were to use straight Java to convert the string to the appropriate type, the code would look like this:
Map<String, List<String>> result = objectMapper.readValue(json, new TypeReference<>() {
What exactly is that "TypeReference" doing there... think of it as a way of making the type parameters of the generic type "Map" which are "String" and "List" available at runtime, without that the types are erased at runtime and Java would not know that it has to create a "Map<String, List<String>>".

Kotlin can hide this detail behind an extension function which if defined from scratch would look like this:
inline fun <reified T> ObjectMapper.readValue(src: String): T = readValue(src, object : TypeReference<T>() {})
and a code using such an extension function:
val result: Map<String, List<String>> = objectMapper.readValue(json)
See how all the TypeReference related code is well hidden in the extension function. 

This is the kind of capability that is provided by the Jackson Kotlin Module With the right packages imported a sample code with this module looks like this:
import com.fasterxml.jackson.module.kotlin.jacksonObjectMapper
import com.fasterxml.jackson.module.kotlin.readValue
val objectMapper = jacksonObjectMapper()
val result: Map<String, List<String>> = objectMapper.readValue(json)
This is just one of the simplifications offered by the use of the module. It also supports features like Kotlin Data classes, Kotlin built-in types like Pair, Triple etc out of the box.

Highly recommended if you are using Jackson with Kotlin.

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