Working with JSON in Kotlin: Data Parsing and Serialization

Introduction to Working with JSON in Kotlin

JSON (JavaScript Object Notation) is one of the most popular formats for data exchange between a server and a client. In modern Kotlin development, whether it's Android applications or server-side projects, the ability to work efficiently with JSON is an essential skill. In this article, we will thoroughly examine how to perform JSON parsing and serialization in Kotlin using the most popular libraries: kotlinx.serialization and Gson.

Whether you are creating a REST API, processing responses from external services, or saving local settings, understanding the principles of working with JSON will help you write cleaner and more efficient code.

Basics of JSON Parsing in Kotlin

What are Serialization and Deserialization?

Before moving to the code, it is important to understand two key concepts:

• Serialization — converting a Kotlin object (data class, list, etc.) into a JSON string.
• Deserialization (Parsing) — the reverse process: creating a Kotlin object from a JSON string.

Kotlin does not have built-in JSON support, so we use third-party libraries. Let's look at the two most popular ones.

The kotlinx.serialization Library

kotlinx.serialization is an official library from JetBrains that integrates perfectly with the Kotlin language. It works at compile time, ensuring high performance and type safety.

Adding the Library

First, add the plugin to build.gradle.kts:

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

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

Defining the Data Model

Let's create a class to represent a user:

import kotlinx.serialization.Serializable

@Serializabledata class User(    val name: String,    val age: Int,    val email: String? = null, // nullable field    val roles: List<String> = emptyList())

The @Serializable annotation tells the compiler that this class can be serialized.

Serialization (Object → JSON)

import kotlinx.serialization.encodeToStringimport kotlinx.serialization.json.Json

fun main() {    val user = User(        name = "Anna",        age = 28,        email = "anna@example.com",        roles = listOf("admin", "user")    )

    val jsonString = Json.encodeToString(user)    println(jsonString)    // {"name":"Anna","age":28,"email":"anna@example.com","roles":["admin","user"]}}

Deserialization (JSON → Object)

import kotlinx.serialization.decodeFromString

fun main() {    val json = """{"name":"Ivan","age":35,"roles":["user"]}"""        val user = Json.decodeFromString<User>(json)    println(user.name) // Ivan    println(user.email) // null (not specified in JSON)}

Configuring Json Settings

You can customize the parser's behavior:

val json = Json {    prettyPrint = true          // pretty formatting    ignoreUnknownKeys = true    // ignore extra fields in JSON    isLenient = true            // allow lenient parsing (e.g., quotes)}

val user = json.decodeFromString<User>(jsonString)

The Gson Library

Gson from Google is another popular library, especially common in Android development.

Adding Gson

dependencies {    implementation("com.google.code.gson:gson:2.10.1")}

Usage Example

import com.google.gson.Gson

data class Product(    val id: Int,    val title: String,    val price: Double)

fun main() {    val gson = Gson()    val product = Product(1, "Laptop", 999.99)        // Serialization    val json = gson.toJson(product)    println(json) // {"id":1,"title":"Laptop","price":999.99}        // Deserialization    val jsonString = """{"id":2,"title":"Mouse","price":25.50}"""    val parsedProduct = gson.fromJson(jsonString, Product::class.java)    println(parsedProduct.title) // Mouse}