Working with JSON in Kotlin: Parsing and Serialization in Practice

Introduction: Why JSON is the Foundation of Modern Kotlin Development

JSON (JavaScript Object Notation) remains the de facto standard for data exchange between server and client, whether it's a mobile Android app or a Ktor backend. In the Kotlin ecosystem, working with JSON is especially convenient thanks to the language's strong typing and powerful serialization libraries. In this article, we'll cover two main approaches: using the official kotlinx.serialization library and the popular Gson. You'll learn how to parse complex structures, serialize objects, and handle exceptions in practice.

Project Setup and Adding Dependencies

Before writing code, you need to add the libraries to the build.gradle.kts file (for Gradle). Let's look at both options.

Option 1: kotlinx.serialization (recommended)

• Add the plugin to plugins: kotlin("plugin.serialization") version "1.9.22"
• In dependencies, add: implementation("org.jetbrains.kotlinx:kotlinx-serialization-json:1.6.2")

Option 2: Gson (for compatibility with Java projects)

• In dependencies, add: implementation("com.google.code.gson:gson:2.10.1")

// build.gradle.kts — full example for kotlinx.serializationplugins {    kotlin("jvm") version "1.9.22"    kotlin("plugin.serialization") version "1.9.22"}

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

Parsing JSON in Kotlin: From Simple to Complex

Parsing (deserialization) is the process of converting a JSON string into a Kotlin object. Let's look at an example using user data.

Sample Data

{  "id": 123,  "name": "Anna",  "email": "anna@example.com",  "active": true}

Creating a Data Class

The same class is used for both libraries. Important: for kotlinx.serialization, you need to add the @Serializable annotation.

import kotlinx.serialization.Serializable

@Serializabledata class User(    val id: Int,    val name: String,    val email: String,    val active: Boolean)

Parsing with kotlinx.serialization

import kotlinx.serialization.json.Json

fun parseWithKotlinx(jsonString: String): User {    val json = Json { ignoreUnknownKeys = true } // ignore unknown fields    return json.decodeFromString<User>(jsonString)}

// Usage:fun main() {    val json = """{"id":123,"name":"Anna","email":"anna@example.com","active":true}"""    val user = parseWithKotlinx(json)    println(user.name) // Anna}

Parsing with Gson

import com.google.gson.Gson

fun parseWithGson(jsonString: String): User {    val gson = Gson()    return gson.fromJson(jsonString, User::class.java)}

// Usage:fun main() {    val json = """{"id":123,"name":"Anna","email":"anna@example.com","active":true}"""    val user = parseWithGson(json)    println(user.email) // anna@example.com}

Serialization: Converting an Object to JSON

The reverse process — creating a JSON string from an object — is equally important for sending data to the server.

Serialization with kotlinx.serialization

import kotlinx.serialization.encodeToString

fun serializeWithKotlinx(user: User): String {    val json = Json { prettyPrint = true }    return json.encodeToString(user)}

// Result:// {//   "id": 123,//   "name": "Anna",//   "email": "anna@example.com",//   "active": true// }

Serialization with Gson

fun serializeWithGson(user: User): String {    val gson = GsonBuilder().setPrettyPrinting().create()    return gson.toJson(user)}<