Kotlin cross-platform / multi-format serialization
Branch: master
Clone or download
Latest commit 55c564f Feb 15, 2019


Kotlin cross-platform / multi-format reflectionless serialization

JetBrains incubator project GitHub license TeamCity build Download

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

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 shipped with the rest of Kotlin compiler distribution, and the 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 1.3.20 or higher. Make sure that you have corresponding Kotlin plugin installed in the IDE. Since serialization is now bundled into Kotlin plugin, no additional plugins for IDE are required (but make sure you have deleted old additional plugin for 1.2, if you had one). 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 = '1.3.20'
    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' // or 'kotlin-multiplatform' for multiplatform projects
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:0.10.0"

Gradle (with plugins block)

You can setup serialization plugin with the kotlin plugin using Gradle plugins DSL instead of traditional apply plugin:

plugins {
    id 'kotlin-multiplatform' version '1.3.20'
    id 'kotlinx-serialization' version '1.3.20'

In this case, since serialization plugin is not published to Gradle plugin portal yet, you'll need to add plugin resolution rules to your settings.gradle:

pluginManagement {
    resolutionStrategy {
        eachPlugin {
            if (requested.id.id == "kotlin-multiplatform") {
            if (requested.id.id == "kotlinx-serialization") {

Don't forget to drop classpath dependency on the plugin from the buildscript dependencies, otherwise, you'll get an error about conflicting versions.

Runtime library should be added to dependencies the same way as before.


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 4.8 or higher, because older versions have unsupported format of metadata.

Sample project can be found in example-native folder.

Incompatible changes

All versions of library before 0.10.0 are using Gradle metadata v0.3 and therefore require Gradle 4.7 for build. Maven plugin coordinates before Kotlin 1.3.20 were kotlinx-maven-serialization-plugin.

Troubleshooting IntelliJ IDEA

Serialization support should work out of the box, if you have 1.3.x Kotlin plugin installed. If you have Kotlin 1.3.10 or lower, you have to delegate build to Gradle (Settings - Build, Execution, Deployment - Build Tools - Gradle - Runner - tick Delegate IDE build/run actions to gradle). Starting from 1.3.11, no delegation is required. In case of problems, force project re-import from Gradle.