Introduction: Kotlin cross-platform / multi-format serialization
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Kotlin serialization consists of a compiler plugin, which automatically produces visitor code for classes, and runtime library, which uses generated code to serialize objects without reflection.

  • Supports Kotlin classes marked as @Serializable and standard collections.
  • Supports JSON, CBOR, and Protobuf formats out-of-the-box.
  • The same code works on Kotlin/JVM, Kotlin/JS and Kotlin/Native (Native support is limited for now, see below in the corresponding section).

Runtime overview

This project contains the runtime library. Runtime library provides:

  • Interfaces which are called by compiler-generated code (Encoder, Decoder).
  • Basic skeleton implementations of these interfaces in which you should override some methods if you want to implement custom data format.
  • Some internal classes like built-ins and collections serializers.
  • Ready-to-use serialization formats.
  • Other useful classes that benefit from serialization framework (e.g. object-to-Map transformer)

You can open example projects for JVM or JS to get started playing with it.

Table of contents

Quick example

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

data class Data(val a: Int, @Optional val b: String = "42")

fun main(args: Array<String>) {
    // serializing objects
    val jsonData = JSON.stringify(Data.serializer(), Data(42))
    // serializing lists
    val jsonList = JSON.stringify(Data.serializer().list, listOf(Data(42)))
    println(jsonData) // {"a": 42, "b": "42"}
    println(jsonList) // [{"a": 42, "b": "42"}]

    // parsing data back
    val obj = JSON.parse(Data.serializer(), """{"a":42}""")
    println(obj) // Data(a=42, b="42")

To learn more about JSON usage and other formats, see usage. More examples of various kinds of Kotlin classes that can be serialized can be found here.

Current project status

Starting from Kotlin 1.3-RC2, serialization plugin is bundled into the Kotlin compiler, and IDEA plugin is bundled into the Kotlin plugin.

Runtime library is under reconstruction to match the corresponding KEEP, so some features described there can be not implemented yet. While library is stable and has successfully been used in various scenarios, there is no API compatibility guarantees between versions, that's why it is called experimental. This document describes setup for Kotlin 1.3 and higher. To watch instructions regarding 1.2, follow this document.


Using Kotlin Serialization requires Kotlin compiler 1.3.0 or higher. Make sure that you have corresponding Kotlin plugin installed in the IDE. Since serialization is now bundled into Kotlin plugin, no additional actions in IDE are required (but make sure you have deleted old additional plugin for 1.2, if you had one). Delegate build to Gradle: (Settings - Build, Execution, Deployment - Build Tools - Gradle - Runner - tick Delegate IDE build/run actions to gradle). Example projects on JVM are available for Gradle and Maven.


You have to add the serialization plugin as the other compiler plugins:

buildscript {
    ext.kotlin_version = '1.3.0'
    repositories { jcenter() }

    dependencies {
        classpath "org.jetbrains.kotlin:kotlin-gradle-plugin:$kotlin_version"
        classpath "org.jetbrains.kotlin:kotlin-serialization:$kotlin_version"

Don't forget to apply the plugin:

apply plugin: 'kotlin'
apply plugin: 'kotlinx-serialization'

Next, you have to add dependency on the serialization runtime library. Note that while plugin have version the same as compiler one, runtime library has different coordinates, repository and versioning.

repositories {
    // artifacts are published to this repository
    maven { url "https://kotlin.bintray.com/kotlinx" }

dependencies {
    compile "org.jetbrains.kotlin:kotlin-stdlib:$kotlin_version"
    compile "org.jetbrains.kotlinx:kotlinx-serialization-runtime:0.9.0"


Library should work on Android "as is". If you're using proguard, you need to add this to your proguard-rules.pro:

-keepattributes *Annotation*, InnerClasses
-dontnote kotlinx.serialization.SerializationKt
-keep,includedescriptorclasses class com.yourcompany.yourpackage.**$$serializer { *; } # <-- change package name to your app's
-keepclassmembers class com.yourcompany.yourpackage.** { # <-- change package name to your app's
    *** Companion;
-keepclasseswithmembers class com.yourcompany.yourpackage.** { # <-- change package name to your app's
    kotlinx.serialization.KSerializer serializer(...);

You may also want to keep all custom serializers you've defined.


Ensure the proper version of Kotlin and serialization version:


Include bintray repository for library:


Add serialization plugin to Kotlin compiler plugin:


Add dependency on serialization runtime library:


Multiplatform (JS and common)

Replace dependency on kotlinx-serialization-runtime with kotlinx-serialization-runtime-js or kotlinx-serialization-runtime-common to use it in JavaScript and common projects, respectively. Both kotlin-platform-*** and kotlin-multiplatform are supported. You have to apply kotlinx-serialization plugin to every module, including common and platform ones.

JavaScript example is located at example-js folder.


You can apply the plugin to kotlin-platform-native or kotlin-multiplatform projects. konan plugin is not supported and deprecated.

Important note: for kotlin-multiplatform project, apply usual kotlinx-serialization plugin. For kotlin-platform-native module, apply kotlinx-serialization-native plugin, since platform-native from K/N 0.9.3 uses infrastructure in which compiler plugins are shaded.

Use kotlinx-serialization-runtime-native artifact. Don't forget to enableFeaturePreview('GRADLE_METADATA') in yours settings.gradle. You must have Gradle 4.7, because higher versions have unsupported format of metadata.

Serialization compiler plugin for Native is still in active development, and is not as feature-full as JVM/JS plugins.

What works:

  • @Serializable classes with primitive or @Serializable properties
  • Standard collections
  • @Optional and @SerialName annotations

What does not work:

  • @Serializable classes with generics (except standard collections)
  • Enums and arrays (Array<T>, ByteArray, etc)
  • @Transient initializers and init blocks
  • @SerialInfo-based annotations

Sample project can be found in example-native folder.

Troubleshooting IntelliJ IDEA

Serialization support should work out of the box, if you have 1.3 Kotlin plugin installed. You still have to delegate build to Gradle (Settings - Build, Execution, Deployment - Build Tools - Gradle - Runner - tick Delegate IDE build/run actions to gradle). In case of problems, force project re-import from Gradle.

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