Serialization with kotlinx.serialization

Apps constantly turn objects into JSON to send over the network or save to disk, and back again. kotlinx.serialization is Kotlin's official, compiler-driven library for exactly that.

Learn Serialization with kotlinx.serialization in our free Kotlin course — a beginner-friendly interactive lesson with worked examples, a practice exercise…

Part of the free Kotlin course at LearnCodingFast — hands-on lessons with examples you run in your browser, plus practice exercises and a quick quiz.

By the end you'll annotate classes with @Serializable , convert objects with Json.encodeToString and decodeFromString , and remap keys with @SerialName .

What You'll Learn in This Lesson

1️⃣ @Serializable, encode, and decode

Annotate a class with @Serializable , then use Json.encodeToString to turn an instance into JSON and Json.decodeFromString to parse it back.

No manual JSON building, no reflection at runtime — the compiler plugin generated a type-safe serializer for User .

2️⃣ Remapping Keys with @SerialName

When the JSON uses different key names than your Kotlin properties, @SerialName bridges the gap so your code stays idiomatic.

3️⃣ Serializing Collections

Collections of serializable types work directly — encode and decode a List<Task> with the same calls.

Your turn. Replace the TODO , then run and compare.

Encode a small object to JSON, decode it back, and prove the data survived the round trip.

📋 Quick Reference — Serialization

Practice quiz

What does serialization mean?

Compiling code to bytecode
Converting an object into a transferable format like JSON text
Encrypting a password
Running tests in order

Answer: Converting an object into a transferable format like JSON text. Serialization turns an in-memory object into a format (e.g. JSON) you can store or send.

Which annotation marks a class for kotlinx.serialization?

@Json
@SerializedName
@Serializable
@Serialize

Answer: @Serializable. @Serializable (from kotlinx.serialization) makes a class serializable.

Which call converts an object to a JSON string?

Json.encodeToString(value)
Json.toJson(value)
value.serialize()
Json.write(value)

Answer: Json.encodeToString(value). Json.encodeToString(value) produces the JSON text for a @Serializable object.

Which call parses a JSON string back into an object?

Json.parse(text)
Json.fromJson(text)
Json.decodeFromString<T>(text)
text.deserialize()

Answer: Json.decodeFromString<T>(text). Json.decodeFromString<T>(jsonString) reconstructs the object of type T.

What does @SerialName("user_id") do?

Renames the class
Maps a Kotlin property to a different JSON key name
Makes the property private
Marks it transient

Answer: Maps a Kotlin property to a different JSON key name. @SerialName customizes the JSON key for a property, e.g. userId <-> user_id.

Deserialization is the opposite of serialization. What does it produce?

A JSON string
An object reconstructed from the serialized data
A compiled class
A network request

Answer: An object reconstructed from the serialized data. Deserialization turns serialized text back into an in-memory object.

Which import provides the Json object in kotlinx.serialization?

import org.json.JSONObject
import com.google.gson.Gson
import java.util.Json
import kotlinx.serialization.json.Json

Answer: import kotlinx.serialization.json.Json. The Json entry point lives in kotlinx.serialization.json.

How do you serialize/deserialize a list of @Serializable items?

You cannot serialize collections
Convert each element to a string manually
Json.encodeToString and decodeFromString work with List<T> directly
Use a database

Answer: Json.encodeToString and decodeFromString work with List<T> directly. kotlinx.serialization supports List<T> (and other collections) of serializable types out of the box.

What is a common reason to configure Json { ignoreUnknownKeys = true }?

To speed up the app
To ignore JSON keys not present in your data class instead of throwing
To encrypt the output
To pretty-print only

Answer: To ignore JSON keys not present in your data class instead of throwing. ignoreUnknownKeys lets you tolerate extra fields in the JSON your model does not declare.

Why is kotlinx.serialization well suited to Kotlin compared to reflection-based libraries?

It runs on the GPU
It only works on Android
It uses a compiler plugin for type-safe, reflection-free serialization
It requires no annotations ever

Answer: It uses a compiler plugin for type-safe, reflection-free serialization. A compiler plugin generates serializers at build time, avoiding runtime reflection and supporting multiplatform.