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kaml

Pipeline Coverage License Maven Central

What is this?

This library adds YAML support to kotlinx.serialization.

Currently, only Kotlin/JVM is fully supported.

Kotlin/JS and Kotlin/Wasm support are considered highly experimental. It is not yet fully functional, and may be removed or modified at any time.

(Follow this issue for a discussion of adding support for other targets.)

YAML version 1.2 is supported.

Usage samples

Parsing from YAML to a Kotlin object

@Serializable
data class Team(
    val leader: String,
    val members: List<String>
)

val input = """
        leader: Amy
        members:
          - Bob
          - Cindy
          - Dan
    """.trimIndent()

val result = Yaml.default.decodeFromString(Team.serializer(), input)

println(result)

Serializing from a Kotlin object to YAML

@Serializable
data class Team(
    val leader: String,
    val members: List<String>
)

val input = Team("Amy", listOf("Bob", "Cindy", "Dan"))

val result = Yaml.default.encodeToString(Team.serializer(), input)

println(result)

Parsing into YamlNode

It is possible to parse a string or an InputStream directly into a YamlNode, for example the following code prints Cindy.

val input = """
        leader: Amy
        members:
          - Bob
          - Cindy
          - Dan
    """.trimIndent()

val result = Yaml.default.parseToYamlNode(input)

println(
    result
        .yamlMap.get<YamlList>("members")!![1]
        .yamlScalar
        .content
)

Referencing kaml

Add the following to your Gradle build script:

Groovy DSL

plugins {
    id 'org.jetbrains.kotlin.jvm' version '1.4.20'
    id 'org.jetbrains.kotlin.plugin.serialization' version '1.4.20'
}

dependencies {
  implementation "com.charleskorn.kaml:kaml:<version number here>" // Get the latest version number from https://github.com/charleskorn/kaml/releases/latest
}

Kotlin DSL

plugins {
    kotlin("jvm") version "1.4.20"
    kotlin("plugin.serialization") version "1.4.20"
}

dependencies {
  implementation("com.charleskorn.kaml:kaml:<version number here>") // Get the latest version number from https://github.com/charleskorn/kaml/releases/latest
}

Check the releases page for the latest release information, and the Maven Central page for examples of how to reference the library in other build systems.

Features

  • Supports most major YAML features:

  • Supports parsing YAML to Kotlin objects (deserializing) and writing Kotlin objects as YAML (serializing)

  • Supports kotlinx.serialization's polymorphism for sealed and unsealed types

    Two styles are available (set YamlConfiguration.polymorphismStyle when creating an instance of Yaml):

    • using YAML tags to specify the type:

      servers:
  - !<frontend>
    hostname: a.mycompany.com
  - !<backend>
    database: db-1

    • using a type property to specify the type:

      servers:
  - type: frontend
    hostname: a.mycompany.com
  - type: backend
    database: db-1

    The fragments above could be generated with:

    @Serializable
sealed class Server {
  @SerialName("frontend")
  @Serializable
  data class Frontend(val hostname: String) : Server()

  @SerialName("backend")
  @Serializable
  data class Backend(val database: String) : Server()
}

@Serializable
data class Config(val servers: List<Server>)

val config = Config(listOf(
  Frontend("a.mycompany.com"),
  Backend("db-1")
))

val result = Yaml.default.encodeToString(Config.serializer(), config)

println(result)

  • Supports Docker Compose-style extension fields

    x-common-labels: &common-labels
  labels:
    owned-by: myteam@mycompany.com
    cost-centre: myteam

servers:
  server-a:
    <<: *common-labels
    kind: frontend

  server-b:
    <<: *common-labels
    kind: backend

  # server-b and server-c are equivalent
  server-c:
    labels:
      owned-by: myteam@mycompany.com
      cost-centre: myteam
    kind: backend

    Specify the extension prefix by setting YamlConfiguration.extensionDefinitionPrefix when creating an instance of Yaml (eg. "x-" for the example above).

    Extensions can only be defined at the top level of a document, and only if the top level element is a map or object. Any key starting with the extension prefix must have an anchor defined (&...) and will not be included in the deserialised value.

Contributing to kaml

Pull requests and bug reports are always welcome!

kaml uses Gradle for builds and testing:

  • To build the library: ./gradlew assemble
  • To run the tests and static analysis tools: ./gradlew check
  • To run the tests and static analysis tools continuously: ./gradlew --continuous check

Reference links