# Kotlin multiplatform / multi-format reflectionless serialization

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

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

## Table of contents

<!--- TOC -->

* [Introduction and references](#introduction-and-references)
* [Setup](#setup)
  * [Gradle](#gradle)
    * [1) Setting up the serialization plugin](#1-setting-up-the-serialization-plugin)
    * [2) Dependency on the JSON library](#2-dependency-on-the-json-library)
  * [Android](#android)
  * [Multiplatform (Common, JS, Native)](#multiplatform-common-js-native)
  * [Maven](#maven)
  * [Bazel](#bazel)

<!--- END -->

* **Additional links**
  * [Kotlin Serialization Guide](docs/serialization-guide.md)
  * [Full API reference](https://kotlinlang.org/api/kotlinx.serialization/)
  * [Submitting issues and PRs](CONTRIBUTING.md)
  * [Building this library](docs/building.md)

## Introduction and references

Here is a small example.

```kotlin
import kotlinx.serialization.*
import kotlinx.serialization.json.*

@Serializable
data class Project(val name: String, val language: String)

fun main() {
    // Serializing objects
    val data = Project("kotlinx.serialization", "Kotlin")
    val string = Json.encodeToString(data)
    println(string) // {"name":"kotlinx.serialization","language":"Kotlin"}
    // Deserializing back into objects
    val obj = Json.decodeFromString<Project>(string)
    println(obj) // Project(name=kotlinx.serialization, language=Kotlin)
}
```

> You can get the full code [here](guide/example/example-readme-01.kt).

<!--- TEST_NAME ReadmeTest -->

<!--- TEST
{"name":"kotlinx.serialization","language":"Kotlin"}
Project(name=kotlinx.serialization, language=Kotlin)
-->

**Read the [Kotlin Serialization Guide](docs/serialization-guide.md) for all details.**

You can find auto-generated documentation website on [kotlinlang.org](https://kotlinlang.org/api/kotlinx.serialization/).

## Setup

[New versions](https://plugins.gradle.org/plugin/org.jetbrains.kotlin.plugin.serialization) of the serialization plugin are released in tandem with each new Kotlin compiler version.

Using Kotlin Serialization requires Kotlin compiler `1.4.0` or higher.
Make sure you have the corresponding Kotlin plugin installed in the IDE, no additional plugins for IDE are required.

### Gradle

To set up kotlinx.serialization, you have to do two things:
1) Add the **[serialization plugin](#1-setting-up-the-serialization-plugin)**.
2) Add the **[serialization library dependency](#2-dependency-on-the-json-library)**.

#### 1) Setting up the serialization plugin

You can set up the serialization plugin with the Kotlin plugin using the
[Gradle plugins DSL](https://docs.gradle.org/current/userguide/plugins.html#sec:plugins_block):

Kotlin DSL:

```kotlin
plugins {
    kotlin("jvm") version "1.9.22" // or kotlin("multiplatform") or any other kotlin plugin
    kotlin("plugin.serialization") version "1.9.22"
}
```

Groovy DSL:

```gradle
plugins {
    id 'org.jetbrains.kotlin.multiplatform' version '1.9.22'
    id 'org.jetbrains.kotlin.plugin.serialization' version '1.9.22'
}
```

> Kotlin versions before 1.4.0 are not supported by the stable release of Kotlin serialization.

<details>
  <summary>Using <code>apply plugin</code> (the old way)</summary>

First, you have to add the serialization plugin to your classpath as the other [compiler plugins](https://kotlinlang.org/docs/reference/compiler-plugins.html):

Kotlin DSL:

```kotlin
buildscript {
    repositories { mavenCentral() }

    dependencies {
        val kotlinVersion = "1.9.22"
        classpath(kotlin("gradle-plugin", version = kotlinVersion))
        classpath(kotlin("serialization", version = kotlinVersion))
    }
}
```

Groovy DSL:

```gradle
buildscript {
    ext.kotlin_version = '1.9.22'
    repositories { mavenCentral() }

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

Then you can `apply plugin` (example in Groovy):

```gradle
apply plugin: 'kotlin' // or 'kotlin-multiplatform' for multiplatform projects
apply plugin: 'kotlinx-serialization'
```
</details>

#### 2) Dependency on the JSON library

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

Kotlin DSL:

```kotlin
repositories {
    mavenCentral()
}

dependencies {
    implementation("org.jetbrains.kotlinx:kotlinx-serialization-json:1.6.3")
}
```

Groovy DSL:

```gradle
repositories {
    mavenCentral()
}

dependencies {
    implementation "org.jetbrains.kotlinx:kotlinx-serialization-json:1.6.3"
}
```

>We also provide `kotlinx-serialization-core` artifact that contains all serialization API but does not have a bundled serialization format with it

### Android

By default, proguard rules are supplied with the library.
[These rules](rules/common.pro) keep serializers for _all_ serializable classes that are retained after shrinking,
so you don't need additional setup.

**However, these rules do not affect serializable classes if they have named companion objects.**

If you want to serialize classes with named companion objects, you need to add and edit rules below to your `proguard-rules.pro` configuration.

Note that the rules for R8 differ depending on the [compatibility mode](https://r8.googlesource.com/r8/+/refs/heads/master/compatibility-faq.md) used.

<details>
<summary>Example of named companion rules for ProGuard and R8 compatibility mode</summary>

```proguard
# Serializer for classes with named companion objects are retrieved using `getDeclaredClasses`.
# If you have any, replace classes with those containing named companion objects.
-keepattributes InnerClasses # Needed for `getDeclaredClasses`.

-if @kotlinx.serialization.Serializable class
com.example.myapplication.HasNamedCompanion, # <-- List serializable classes with named companions.
com.example.myapplication.HasNamedCompanion2
{
    static **$* *;
}
-keepnames class <1>$$serializer { # -keepnames suffices; class is kept when serializer() is kept.
    static <1>$$serializer INSTANCE;
}
```
</details>


<details>
<summary>Example of named companion rules for R8 full mode</summary>

```proguard
# Serializer for classes with named companion objects are retrieved using `getDeclaredClasses`.
# If you have any, replace classes with those containing named companion objects.
-keepattributes InnerClasses # Needed for `getDeclaredClasses`.

-if @kotlinx.serialization.Serializable class
com.example.myapplication.HasNamedCompanion, # <-- List serializable classes with named companions.
com.example.myapplication.HasNamedCompanion2
{
    static **$* *;
}
-keepnames class <1>$$serializer { # -keepnames suffices; class is kept when serializer() is kept.
    static <1>$$serializer INSTANCE;
}

# Keep both serializer and serializable classes to save the attribute InnerClasses
-keepclasseswithmembers, allowshrinking, allowobfuscation, allowaccessmodification class
com.example.myapplication.HasNamedCompanion, # <-- List serializable classes with named companions.
com.example.myapplication.HasNamedCompanion2
{
    *;
}
```
</details>

In case you want to exclude serializable classes that are used, but never serialized at runtime,
you will need to write custom rules with narrower [class specifications](https://www.guardsquare.com/manual/configuration/usage).

### Multiplatform (Common, JS, Native)

Most of the modules are also available for Kotlin/JS and Kotlin/Native.
You can add dependency to the required module right to the common source set:
```gradle
commonMain {
    dependencies {
        // Works as common dependency as well as the platform one
        implementation "org.jetbrains.kotlinx:kotlinx-serialization-json:$serialization_version"
    }
}
```
The same artifact coordinates can be used to depend on platform-specific artifact in platform-specific source-set.

### Maven

Ensure the proper version of Kotlin and serialization version:

```xml
<properties>
    <kotlin.version>1.9.22</kotlin.version>
    <serialization.version>1.6.3</serialization.version>
</properties>
```

Add serialization plugin to Kotlin compiler plugin:

```xml
<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:

```xml
<dependency>
    <groupId>org.jetbrains.kotlinx</groupId>
    <artifactId>kotlinx-serialization-json</artifactId>
    <version>${serialization.version}</version>
</dependency>
```

### Bazel

To setup the Kotlin compiler plugin for Bazel, follow [the
example](https://github.com/bazelbuild/rules_kotlin/tree/master/examples/plugin/src/serialization)
from the `rules_kotlin` repository.