Programming language: Kotlin
Tags: JSON    
Latest version: v0.20.0

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Kotlin multiplatform / multi-format reflectionless serialization

JetBrains incubator project GitHub license TeamCity build Download

Kotlin serialization consists of a compiler plugin, that generates visitor code for serializable classes, runtime library with core serialization API and JSON format, and support libraries with ProtoBuf, CBOR and properties formats.

  • Supports Kotlin classes marked as @Serializable and standard collections.
  • Provides JSON, [CBOR](formats/README.md#CBOR), and [Protobuf](formats/README.md#ProtoBuf) formats.
  • Complete multiplatform support: JVM, JS and Native.

Table of contents

Quick example

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

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

fun main() {
    // Json also has .Default configuration which provides more reasonable settings,
    // but is subject to change in future versions
    val json = Json(JsonConfiguration.Stable)
    // 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}""") // b is optional since it has default value
    println(obj) // Data(a=42, b="42")

Runtime overview

This project contains the runtime library. Runtime library provides:

  • Ready-to-use JSON serialization format.
  • Core primitives for plugin-generated code (Encoder, Decoder, SerialDescriptor).
  • Basic skeleton implementations of core primitives for custom serialization formats.
  • Built-ins and collections serializers.

You can open example projects for [JS](examples/example-js) and [JVM](examples/example-jvm).

To learn more about JSON usage and other formats, see [usage](docs/runtime_usage.md). More examples of various kinds of Kotlin classes that can be serialized can be found [here](docs/examples.md).

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](docs/old12.md).


Using Kotlin Serialization requires Kotlin compiler 1.3.30 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](examples/example-jvm/build.gradle) and [Maven](examples/example-jvm/pom.xml).


Using the plugins block

You can setup the serialization plugin with the Kotlin plugin using Gradle plugins DSL:

Kotlin DSL:

plugins {
    kotlin("multiplatform") // or kotlin("jvm") or any other kotlin plugin
    kotlin("plugin.serialization") version "1.3.70"

Groovy DSL:

plugins {
    id 'org.jetbrains.kotlin.multiplatform' version '1.3.70' // or any other kotlin plugin
    id 'org.jetbrains.kotlin.plugin.serialization' version '1.3.70'

Note: plugin marker for serialization has been published in Kotlin 1.3.50. If you need to use the earlier Kotlin version, see KT-27612 for workaround with plugin resolution rules.

Using apply plugin (the old way)

First, you have to add the serialization plugin to your classpath as the other compiler plugins:

Kotlin DSL:

buildscript {
    repositories { jcenter() }

    dependencies {
        val kotlinVersion = "1.3.70"
        classpath(kotlin("gradle-plugin", version = kotlinVersion))
        classpath(kotlin("serialization", version = kotlinVersion))

Groovy DSL:

buildscript {
    ext.kotlin_version = '1.3.70'
    repositories { jcenter() }

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

Then you can apply plugin (example in Groovy):

apply plugin: 'kotlin' // or 'kotlin-multiplatform' for multiplatform projects
apply plugin: 'kotlinx-serialization'

Dependency on the runtime library

After setting up the plugin one way or another, you have to add a dependency on the serialization runtime library. Note that while the plugin has version the same as the compiler one, runtime library has different coordinates, repository and versioning.

Kotlin DSL:

repositories {
    // artifacts are published to JCenter

dependencies {
    implementation(kotlin("stdlib", KotlinCompilerVersion.VERSION)) // or "stdlib-jdk8"
    implementation("org.jetbrains.kotlinx:kotlinx-serialization-runtime:0.20.0") // JVM dependency

Groovy DSL:

repositories {

dependencies {
    implementation "org.jetbrains.kotlin:kotlin-stdlib:$kotlin_version" // or "kotlin-stdlib-jdk8"
    implementation "org.jetbrains.kotlinx:kotlinx-serialization-runtime:0.20.0" // JVM dependency


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.

Multiplatform (common, JS, Native)

Platform artifacts have the same names as JVM one, but with additional suffix (e.g. org.jetbrains.kotlinx:kotlinx-serialization-runtime-native). For Native artifact, Gradle metadata is required (put the line enableFeaturePreview('GRADLE_METADATA') in your gradle.properties) and minimal supported version of Gradle is 5.3.

Typically, you need the following dependencies in your multiplatform project (don't forget to rename source sets according to your setup):

sourceSets {
    commonMain {
        dependencies {
            implementation kotlin('stdlib-common')
            implementation "org.jetbrains.kotlinx:kotlinx-serialization-runtime-common:$serialization_version"
    commonTest {
        dependencies {
            implementation kotlin('test-common')
            implementation kotlin('test-annotations-common')
    jvmMain {
        dependencies {
            implementation kotlin('stdlib-jdk8')
            implementation "org.jetbrains.kotlinx:kotlinx-serialization-runtime:$serialization_version"
    jvmTest {
        dependencies {
            implementation kotlin('test')
            implementation kotlin('test-junit')
    jsMain {
        dependencies {
            implementation kotlin('stdlib-js')
            implementation "org.jetbrains.kotlinx:kotlinx-serialization-runtime-js:$serialization_version"
    jsTest {
        dependencies {
            implementation kotlin('test-js')
    nativeMain {
        dependencies {
            implementation "org.jetbrains.kotlinx:kotlinx-serialization-runtime-native:$serialization_version"
    nativeTest {}

JavaScript example is located at [example-js](examples/example-js) folder. Multiplatform example is located at [example-multiplatform](examples/example-multiplatform) folder.


Ensure the proper version of Kotlin and serialization version:


You can also use JCenter or https://kotlin.bintray.com/kotlinx Bintray repository.

Add serialization plugin to Kotlin compiler plugin:


Add dependency on serialization runtime library:


Incompatible changes

Library versions 0.20.0 and higher require Kotlin 1.3.70 and higher and incompatible with previous versions.

Library version 0.14.0 require Kotlin 1.3.60/61 and incompatible with other versions.

All versions of library before 0.13.0 are using Gradle metadata v0.4 and therefore it is recommended to use Gradle 4.8-5.1 to build.

Library versions 0.11.0 and higher require Kotlin 1.3.30 and higher and incompatible with previous versions.

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 and have imported the project from Maven or Gradle with serialization enabled in their build scripts. 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.

*Note that all licence references and agreements mentioned in the kotlinx.serialization README section above are relevant to that project's source code only.