Working with YAML in Kotlin: Libraries, Code Examples |…

Working with YAML in Kotlin: Library Overview and Usage Examples

YAML (YAML Ain't Markup Language) is a convenient data serialization format, often used for configuration files, describing API specifications (OpenAPI), and data exchange. In the Kotlin ecosystem, there are several mature libraries for working with YAML, each with its own features. In this article, we will look at three main approaches: using SnakeYAML, kaml, and kotlinx.serialization with the YAML format.

1. SnakeYAML — Classic Parser for JVM

SnakeYAML is the most popular Java library for working with YAML, which is fully compatible with Kotlin. It provides a low-level API for parsing and creating YAML documents.

Adding Dependency

Add the dependency to build.gradle.kts:

implementation("org.yaml:snakeyaml:2.2")

Example: Reading YAML

Consider the file config.yaml:

server:  host: localhost  port: 8080database:  url: jdbc:postgresql://localhost/mydb  user: admin  password: secret

Code for parsing:

import org.yaml.snakeyaml.Yamlimport java.io.File

fun main() {    val yaml = Yaml()    val inputStream = File("config.yaml").inputStream()    val data: Map<String, Any> = yaml.load(inputStream)

    val server = data["server"] as Map<String, Any>    println("Host: ${server["host"]}")    println("Port: ${server["port"]}")

    val db = data["database"] as Map<String, Any>    println("DB URL: ${db["url"]}")}

Example: Creating YAML

fun createYaml() {    val data = mapOf(        "appName" to "MyApp",        "version" to "1.0.0",        "features" to listOf("auth", "logging", "metrics")    )

    val yaml = Yaml()    val writer = FileWriter("output.yaml")    yaml.dump(data, writer)    writer.close()}

Pros of SnakeYAML:

Maximum compatibility with Java projects
Support for all YAML 1.1 features (anchors, tags, custom types)
Flexible configuration via DumperOptions

Cons:

Works with raw types Map<String, Any> — requires type casting
Does not support Kotlin data classes out of the box

2. kaml — Library for Kotlin with Data Class Support

kaml is a wrapper over SnakeYAML that adds support for deserialization into Kotlin data classes and serialization from them. The library uses Kotlin reflection for automatic mapping.

Adding Dependency

implementation("net.mamoe.yamlkt:yamlkt:0.13.0")

Example: Deserialization into a Data Class

import net.mamoe.yamlkt.Yamlimport net.mamoe.yamlkt.YamlElement

data class ServerConfig(    val host: String,    val port: Int)

data class AppConfig(    val server: ServerConfig,    val debug: Boolean = false)

fun main() {    val yaml = Yaml()    val input = """        server:          host: "0.0.0.0"          port: 8080        debug: true    """.trimIndent()

    val config = yaml.decodeFromString(AppConfig.serializer(), input)    println("Server: ${config.server.host}:${config.server.port}")    println("Debug mode: ${config.debug}")}

Example: Serialization

fun serializeConfig() {    val config = AppConfig(        server = ServerConfig("localhost", 9090),        debug = false    )

    val yaml = Yaml()    val output = yaml.encodeToString(AppConfig.serializer(), config)    println(output)}

Pros of kaml: