Serialization

Serialization is the process of converting data used by an application to a format that can be transferred over a network or stored in a database or a file. In turn, deserialization is the opposite process of reading data from an external source and converting it into a runtime object. Together, they are essential to most applications that exchange data with third parties.

Some data serialization formats, such as JSON and protocol buffers are particularly common. Being language-neutral and platform-neutral, they enable data exchange between systems written in any modern language.

In Kotlin, data serialization tools are available in a separate component, kotlinx.serialization. It consists of several parts: the org.jetbrains.kotlin.plugin.serialization Gradle plugin, runtime libraries, and compiler plugins.

Compiler plugins, kotlinx-serialization-compiler-plugin and kotlinx-serialization-compiler-plugin-embeddable, are published directly to Maven Central. The second plugin is designed for working with the kotlin-compiler-embeddable artifact, which is the default option for scripting artifacts. Gradle adds compiler plugins to your projects as compiler arguments.

Libraries

kotlinx.serialization provides sets of libraries for all supported platforms – JVM, JavaScript, Native – and for various serialization formats – JSON, CBOR, protocol buffers, and others. You can find the complete list of supported serialization formats below.

All Kotlin serialization libraries belong to the org.jetbrains.kotlinx: group. Their names start with kotlinx-serialization- and have suffixes that reflect the serialization format. Examples:

org.jetbrains.kotlinx:kotlinx-serialization-json provides JSON serialization for Kotlin projects.
org.jetbrains.kotlinx:kotlinx-serialization-cbor provides CBOR serialization.

Platform-specific artifacts are handled automatically; you don't need to add them manually. Use the same dependencies in JVM, JS, Native, and multiplatform projects.

Note that the kotlinx.serialization libraries use their own versioning structure, which doesn't match Kotlin's versioning. Check out the releases on GitHub to find the latest versions.

Formats

kotlinx.serialization includes libraries for various serialization formats:

Note that all libraries except JSON serialization (kotlinx-serialization-json) are Experimental, which means their API can be changed without notice.

There are also community-maintained libraries that support more serialization formats, such as YAML or Apache Avro. For detailed information about available serialization formats, see the kotlinx.serialization documentation.

Example: JSON serialization

Let's take a look at how to serialize Kotlin objects into JSON.

Add plugins and dependencies

Before starting, you must configure your build script so that you can use Kotlin serialization tools in your project:

Apply the Kotlin serialization Gradle plugin org.jetbrains.kotlin.plugin.serialization (or kotlin("plugin.serialization") in the Kotlin Gradle DSL).
plugins { kotlin("jvm") version "1.9.23" kotlin("plugin.serialization") version "1.9.23" }
plugins { id 'org.jetbrains.kotlin.jvm' version '1.9.23' id 'org.jetbrains.kotlin.plugin.serialization' version '1.9.23' }
Add the JSON serialization library dependency:org.jetbrains.kotlinx:kotlinx-serialization-json:1.6.0
dependencies { implementation("org.jetbrains.kotlinx:kotlinx-serialization-json:1.6.0") }
dependencies { implementation 'org.jetbrains.kotlinx:kotlinx-serialization-json:1.6.0' }

Now you're ready to use the serialization API in your code. The API is located in the kotlinx.serialization package and its format-specific subpackages, such as kotlinx.serialization.json.

Serialize and deserialize JSON

Make a class serializable by annotating it with @Serializable.

import kotlinx.serialization.Serializable

@Serializable
data class Data(val a: Int, val b: String)

Serialize an instance of this class by calling Json.encodeToString().

import kotlinx.serialization.Serializable
import kotlinx.serialization.json.Json
import kotlinx.serialization.encodeToString

@Serializable
data class Data(val a: Int, val b: String)

fun main() {
   val json = Json.encodeToString(Data(42, "str"))
}

As a result, you get a string containing the state of this object in the JSON format: {"a": 42, "b": "str"}

Use the decodeFromString() function to deserialize an object from JSON:

import kotlinx.serialization.Serializable
import kotlinx.serialization.json.Json
import kotlinx.serialization.decodeFromString

@Serializable
data class Data(val a: Int, val b: String)

fun main() {
   val obj = Json.decodeFromString<Data>("""{"a":42, "b": "str"}""")
}

That's it! You have successfully serialized objects into JSON strings and deserialized them back into objects.

What's next

For more information about serialization in Kotlin, see the Kotlin Serialization Guide.

You can explore different aspects of Kotlin serialization in the following resources:

Last modified: 28 November 2023