JSON Processing in Kotlin: Libraries, Parsing, and Serialization

JSON Processing in Kotlin: Complete Guide

JSON (JavaScript Object Notation) remains one of the most popular data exchange formats in web development, mobile applications, and microservices. Kotlin, being a modern language for JVM, Android, and multiplatform projects, offers several powerful tools for working with JSON. In this article, we will thoroughly examine the main libraries, their capabilities, and best practices.

Why do you need a JSON library in Kotlin?

Although Kotlin runs on JVM and can use Java libraries, native Kotlin solutions (e.g., kotlinx.serialization) provide:

• Type safety at compile time
• Support for data classes and sealed classes
• Multiplatform capability (KMP)
• Minimal boilerplate code

Main libraries for working with JSON in Kotlin

1. kotlinx.serialization (recommended)

This is the official JetBrains library, built into the Kotlin ecosystem. It supports multiplatform and does not require reflection.

Setup (Gradle):

plugins {    kotlin("jvm") version "1.9.22"    kotlin("plugin.serialization") version "1.9.22"}

dependencies {    implementation("org.jetbrains.kotlinx:kotlinx-serialization-json:1.6.2")}

JSON parsing example:

import kotlinx.serialization.*import kotlinx.serialization.json.*

@Serializabledata class User(    val id: Int,    val name: String,    val email: String,    @SerialName("is_active") val isActive: Boolean = false)

fun main() {    val jsonString = """        {            "id": 1,            "name": "Ivan Petrov",            "email": "ivan@example.com",            "is_active": true        }    """.trimIndent()

    val user = Json.decodeFromString<User>(jsonString)    println("User: ${user.name}, active: ${user.isActive}")

    // Serialization back to JSON    val jsonOutput = Json.encodeToString(user)    println(jsonOutput)}

2. Gson (by Google)

A popular Java library that easily adapts to Kotlin. Suitable for simple projects without multiplatform support.

Example:

import com.google.gson.Gsonimport com.google.gson.annotations.SerializedName

data class Product(    @SerializedName("product_id") val id: Int,    val title: String,    val price: Double)

fun main() {    val gson = Gson()    val json = """{"product_id": 101, "title": "Laptop", "price": 79999.99}"""        val product = gson.fromJson(json, Product::class.java)    println("Product: ${product.title}, price: ${product.price} rub.")        // Reverse serialization    val jsonOut = gson.toJson(product)    println(jsonOut)}

3. Moshi (by Square)

A modern alternative to Gson with better performance and support for Kotlin code generation.

Example:

import com.squareup.moshi.Moshiimport com.squareup.moshi.kotlin.reflect.KotlinJsonAdapterFactoryimport com.squareup.moshi.adapters.PolymorphicJsonAdapterFactory

data class Book(    val isbn: String,    val author: String,    val year: Int)

fun main() {    val moshi = Moshi.Builder()        .add(KotlinJsonAdapterFactory())        .build()        val adapter = moshi.adapter(Book::class.java)        val json = """{"isbn": "978-5-17-123456-7", "author": "Leo Tolstoy", "year": 1877}"""    val book = adapter.fromJson(json)    println("Book: ${book?.author} - ISBN: ${book?.isbn}")}

Library comparison

• kotlinx.serialization — the best choice for new projects, multiplatform, and Android. Requires a compiler plugin.
• Gson — easy to use, suitable for quick prototypes