Object Serialization in Kotlin: Libraries and Code Examples

Object Serialization in Kotlin: A Complete Guide to Libraries and Tools

Serialization is the process of converting an object into a format that can be saved (e.g., to a file) or transmitted over a network (via REST API, Kafka messages, etc.). The reverse process — deserialization — restores the object from that format. In the Kotlin ecosystem, there are several powerful serialization libraries, each with its own features. In this article, we will cover four main approaches: kotlinx.serialization, Gson, Moshi, and Jackson.

1. kotlinx.serialization — The Native Choice for Kotlin

Developed by JetBrains, the kotlinx.serialization library is the "gold standard" for Kotlin projects. It supports multiplatform (JVM, JS, Native) and uses annotations for compile-time code generation.

Setup (Gradle)

plugins {    kotlin("jvm") version "1.9.0"    kotlin("plugin.serialization") version "1.9.0"}

dependencies {    implementation("org.jetbrains.kotlinx:kotlinx-serialization-json:1.6.0")}

Usage Example

import kotlinx.serialization.Serializableimport kotlinx.serialization.json.Json

@Serializabledata class User(val name: String, val age: Int, val email: String? = null)

fun main() {    val user = User("Ivan", 30, "ivan@example.com")

    // Serialization to JSON    val jsonString = Json.encodeToString(user)    println(jsonString) // {"name":"Ivan","age":30,"email":"ivan@example.com"}

    // Deserialization from JSON    val decoded = Json.decodeFromString<User>(jsonString)    println(decoded) // User(name=Ivan, age=30, email=ivan@example.com)}

Features

• Null safety: support for nullable fields via ?
• Polymorphism: serialization of sealed classes and hierarchies
• Custom formats: JSON, CBOR, Protobuf, Properties
• Performance: code generation without reflection

2. Gson — The Classic from Google

Gson is one of the oldest libraries for Java/Kotlin. It is simple to use but relies on reflection, which can reduce performance on large data volumes.

Setup

implementation("com.google.code.gson:gson:2.10.1")

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", 79999.99)

    // Serialization    val json = gson.toJson(product)    println(json) // {"id":1,"title":"Laptop","price":79999.99}

    // Deserialization    val restored = gson.fromJson(json, Product::class.java)    println(restored) // Product(id=1, title=Laptop, price=79999.99)}

Features

• Simplicity: minimal configuration
• Flexibility: custom TypeAdapter for complex types
• Drawbacks: reflection, no native support for Kotlin data classes (requires a plugin or @JvmField)

3. Moshi — A Modern Alternative to Gson

Moshi from Square is a lightweight library with built-in Kotlin support. It uses code generation via annotations, making it faster than Gson.

Setup

implementation("com.squareup.moshi:moshi-kotlin:1.15.0")kapt("com.squareup.moshi:moshi-kotlin-codegen:1.15.0") // for code generation

Example

import com.squareup.moshi.Moshiimport com.squareup.moshi.kotlin.reflect.KotlinJsonAdapterFactory

@JsonClass(generateAdapter = true)data class Book(val isbn: String, val title: String, val authors: