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
@Serializableand standard collections. - Provides JSON, CBOR, and Protobuf formats.
- Complete multiplatform support: JVM, JS and Native.
- Quick example
- Runtime overview
- Current status
- Library installing
- Troubleshooting IntelliJ IDEA
- Usage
- More examples of supported Kotlin classes
- Writing JSON transformations
- Writing custom serializers
- Multiplatform polymorphic serialization
- Add-on formats
- Building library and compiler plugin from source
- Instructions for old versions under Kotlin 1.2 and migration guide
import kotlinx.serialization.*
import kotlinx.serialization.json.*
@Serializable
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")
}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 and JVM.
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.
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 1.3.30 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 and Maven.
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.
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:$kotlin_version"
}
}Then you can apply plugin (example in Groovy):
apply plugin: 'kotlin' // or 'kotlin-multiplatform' for multiplatform projects
apply plugin: 'kotlinx-serialization'After setting up the plugin one way or another, you have to add a dependency on the serialization runtime library. Note that while the plugin has version the same as the compiler one, runtime library has different coordinates, repository and versioning.
Kotlin DSL:
repositories {
// artifacts are published to JCenter
jcenter()
}
dependencies {
implementation(kotlin("stdlib", KotlinCompilerVersion.VERSION)) // or "stdlib-jdk8"
implementation("org.jetbrains.kotlinx:kotlinx-serialization-runtime:0.20.0") // JVM dependency
}Groovy DSL:
repositories {
jcenter()
}
dependencies {
implementation "org.jetbrains.kotlin:kotlin-stdlib:$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.
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 folder.
Multiplatform example is located at example-multiplatform folder.
Ensure the proper version of Kotlin and serialization version:
<properties>
<kotlin.version>1.3.70</kotlin.version>
<serialization.version>0.20.0</serialization.version>
</properties>You can also use JCenter or https://kotlin.bintray.com/kotlinx Bintray repository.
Add serialization plugin to Kotlin compiler plugin:
<build>
<plugins>
<plugin>
<groupId>org.jetbrains.kotlin</groupId>
<artifactId>kotlin-maven-plugin</artifactId>
<version>${kotlin.version}</version>
<executions>
<execution>
<id>compile</id>
<phase>compile</phase>
<goals>
<goal>compile</goal>
</goals>
</execution>
</executions>
<configuration>
<compilerPlugins>
<plugin>kotlinx-serialization</plugin>
</compilerPlugins>
</configuration>
<dependencies>
<dependency>
<groupId>org.jetbrains.kotlin</groupId>
<artifactId>kotlin-maven-serialization</artifactId>
<version>${kotlin.version}</version>
</dependency>
</dependencies>
</plugin>
</plugins>
</build>Add dependency on serialization runtime library:
<dependency>
<groupId>org.jetbrains.kotlinx</groupId>
<artifactId>kotlinx-serialization-runtime</artifactId>
<version>${serialization.version}</version>
</dependency>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.
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.