OpenAPI Generator Plus Kotlin Client Generator

This generator uses Kotlin Serialization, Ktor and OkHttp.

Configuration options

These options can be supplied in your config.yml (or equivalent) under the generator configuration.

Option	Type	Default	Description
package	string	com.example	Base package for generated code.
apiPackage	string	package	Package for the generated *Api classes.
modelPackage	string	{package}.model	Package for the generated model classes.
supportPackage	string	{package}.support	Package for the generated support classes (serializers, configuration, etc.).
securityPackage	string	{package}.security	Package for the generated security classes.
customTemplates	string	(none)	Path (relative to the config file) to a directory of custom Handlebars templates that override the built-in ones.
hideGenerationTimestamp	boolean	false	Omit the generation timestamp from the generated code.
dateImplementation	string	{supportPackage}.LocalDate	Kotlin type used for the date string format.
timeImplementation	string	{supportPackage}.LocalTime	Kotlin type used for the time string format.
dateTimeImplementation	string	{supportPackage}.Instant	Kotlin type used for the date-time string format.
binaryRepresentation	string	kotlin.ByteArray	Kotlin type used for binary / file data.
urlImplementation	string | object	java.net.URL	Kotlin type used for the url string format. See URL implementation below.
isJavaSerializable	boolean	true	Make generated models implement java.io.Serializable.
gradle	object	(none)	When present, generate build.gradle.kts, settings.gradle.kts and gradle/libs.versions.toml. See Gradle below.
android	object	(none)	When present, generate build.gradle.kts as an Android library module instead of a kotlin("jvm") library. See Android library module below.

URL implementation

By default, properties with format: url are mapped to java.net.URL. You can override this with urlImplementation.

For types that are constructed directly from a String (i.e. via a Type(string) constructor) you can pass the type name as a string:

urlImplementation: java.net.URI

For types that aren't constructed via a String constructor — such as android.net.Uri, which is created via Uri.parse(string) — use the object form and specify fromString, the expression used to construct the type from a String:

urlImplementation:
  type: android.net.Uri
  fromString: android.net.Uri.parse

fromString defaults to type, and the value is always serialized to a String via toString().

Note: android.net.Uri is part of the Android SDK and is not on the classpath of the default kotlin("jvm") module. To compile against it, generate the module as an Android library module (recommended), or — if you want to keep it a plain JVM module — add the Android SDK as a compileOnly dependency via gradle.dependencies (its implementation is provided by the Android runtime on-device):

gradle:
  dependencies:
    - compileOnly("com.google.android:android:4.1.1.4")

Gradle

When the gradle option is present, the generator emits build.gradle.kts, settings.gradle.kts and gradle/libs.versions.toml. The gradle object supports:

Key	Type	Default	Description
groupId	string	com.example	The Gradle group.
version	string	0.0.1	The Gradle version.
versions	object	{}	A map of version overrides.
plugins	string[]	[]	Extra lines appended verbatim to the generated plugins { } block.
dependencies	string[]	[]	Extra lines appended verbatim to the generated dependencies { } block.

plugins and dependencies entries are inserted exactly as written, so you can use any Gradle declaration — a version-catalog alias (implementation(libs.google.android)) or a literal coordinate (compileOnly("com.google.android:android:4.1.1.4")).

Note: build.gradle.kts is regenerated on every run, so anything you need in it must come from your config (gradle.plugins / gradle.dependencies) rather than hand-edits. settings.gradle.kts and gradle/libs.versions.toml are only generated if they don't already exist.

Android library module

If your generated types require the Android SDK — for example using android.net.Uri for urlImplementation — provide an android option. The generated build.gradle.kts is then produced as an Android library module: the com.android.library plugin replaces alias(libs.plugins.kotlin.jvm), and a configure<LibraryExtension> { … } block is added. This puts the Android SDK on the classpath, so Android types resolve without a compileOnly stub.

Key	Type	Default	Description
plugin	string	alias(libs.plugins.android.library)	The plugin declaration that replaces alias(libs.plugins.kotlin.jvm) in the plugins { } block.
compileSdk	number | string	35	The compileSdk value, inserted verbatim — a literal API level (36) or a Kotlin expression (libs.versions.android.compileSdk.get().toInt()).
namespace	string	gradle.groupId	The Android namespace.

android:
  plugin: "alias(libs.plugins.android.library)"
  compileSdk: "libs.versions.android.compileSdk.get().toInt()"

gradle:
  groupId: "com.letterboxd.api"

produces:

import com.android.build.api.dsl.LibraryExtension

plugins {
    alias(libs.plugins.android.library)
    alias(libs.plugins.kotlin.serialization)
}

group = "com.letterboxd.api"
version = "0.0.1"

configure<LibraryExtension> {
    namespace = "com.letterboxd.api"
    compileSdk = libs.versions.android.compileSdk.get().toInt()
}

dependencies {
    // …
}

Note: the android option requires gradle to also be configured (the Android configuration is written into the generated build.gradle.kts); the generator throws if android is set without gradle.

The android.library and kotlin.jvm plugins are mutually exclusive, which is why kotlin.jvm is removed when android is present. An Android library module also needs the Kotlin Android plugin (org.jetbrains.kotlin.android) applied for its Kotlin sources to compile — add it via gradle.plugins, e.g. alias(libs.plugins.kotlin.android).

The generated build.gradle.kts references version-catalog aliases such as libs.plugins.android.library and libs.versions.android.compileSdk. These are not declared in the generator's default gradle/libs.versions.toml, so you must define them yourself. We recommend managing all versions, libraries, and plugins through a Gradle version catalog (gradle/libs.versions.toml) in your project — this keeps versions consistent across the generated module and the rest of your build, and lets the generated references (libs.plugins.* / libs.versions.*) resolve.

Development

To use this library in development we use the link command.

Get started by ensuring that your target project is using the same node version so that the global link will be discoverable. We use a .nvmrc file in this project to set the version.

First check that this project builds

pnpm install
pnpm run build

Then to register this directory with the global registry

pnpm run link

Then in your target project run

pnpm link --global @openapi-generator-plus/kotlin-client-generator

Limitations

• This library uses Ktor with OkHttp engine for making network calls. Thus, it follows the current compatibility of OkHttp engines in Ktor, which are JVM and Android.
• This library assumes JSON is used for model representation during network calls.
• This library has a limitation in only supporting one-of discriminators of type string or an enum that is represented in string.
• The generated model objects are not java.io.Serializable, because Kotlin date and time classes are not java.io.Serializable. If you want to put model objects into an android.os.Bundle or android.os.Intent, you need to serialize the model object first (such as to json).

Requirements

• The library was developed with the latest version of Kotlin at the time of development (version 1.9.22), and so it utilises Kotlin language features that were introduced after version 1.9.0, such as data object. Therefore, it’s strongly recommended to use the latest version of Kotlin, if not, at least 1.9.22.
• Gradle version catalog is necessary for the generated build.gradle.kts to reference necessary versions. Ensure you have this in your project before including the API module to the project.

Implementation

There are two approaches you can take when implementing the generated API into an existing project.

1. File browser approach

1. Copy the folder containing the generated API files into the project root directory.
   • The folder generated by the API should contain src/ folder and build.gradle.kts.
   • The folder may be appropriately renamed, e.g. api/, but in this guide, we’ll reference it in the code blocks as {api_folder}.
2. At the root directory of the project, open settings.gradle.kts and add the api directory in the include(...) line along with all the other modules of the project.
   • If this is a plain, standard Android app, it should look something like include(":app", ":{api_folder}")
3. Inside gradle/ directory, create a version catalog if the project does not have it already. Conventionally, it should be named libs.versions.toml.
4. Add the required versions, libraries, and plugins inside the version catalog. Following code block contains the essential set of versions, libraries, and plugins, as specified in the auto-generated build.gradle.kts file.

[versions]
kotlin = "{latest_version that is >= 1.9.22}"
kotlinx-datetime = "{latest_version that is >= 0.5.0}"
kotlinx-serialization = "{latest_version that is >= 1.6.2}"
ktor = "{latest_version that is >= 2.3.7}"

[libraries]
kotlinx-datetime = { module = "org.jetbrains.kotlinx:kotlinx-datetime", version.ref = "kotlinx-datetime" }
kotlinx-serialization-json = { module = "org.jetbrains.kotlinx:kotlinx-serialization-json", version.ref = "kotlinx-serialization" }
ktor-client-content-negotiation = { module = "io.ktor:ktor-client-content-negotiation", version.ref = "ktor" }
ktor-client-core = { module = "io.ktor:ktor-client-core", version.ref = "ktor" }
ktor-client-okhttp = { module = "io.ktor:ktor-client-okhttp", version.ref = "ktor" }
ktor-serialization-kotlinx-json = { module = "io.ktor:ktor-serialization-kotlinx-json", version.ref = "ktor" }

[plugins]
kotlin-jvm = { id = "org.jetbrains.kotlin.jvm", version.ref = "kotlin" }
kotlin-serialization = { id = "org.jetbrains.kotlin.plugin.serialization", version.ref = "kotlin" }

5. Go to build.gradle.kts in the root directory, and append aliases to the two plugins required by the API module, as specified in the version catalog.

plugins {
    … existing list of plugins …

    /** Needed for :api library. */
    alias(libs.plugins.kotlin.jvm) apply false
    alias(libs.plugins.kotlin.serialization) apply false
}

6. [Optional] At this point, you may also go ahead and refactor version managements for all libraries and plugins into libs.versions.toml so that it’s consistent throughout the codebase.

7. Go to build.gradle.kts for the module that uses the generated API, and add the generated API module as a project dependency by appending the following line in the dependencies {…} closure.

implementation(project(":{api_folder}"))

8. Open command prompt in the project root directory and run the following command to build the project to see if all the configurations were done correctly. The build should succeed if every configuration was made correctly.

./gradlew build

2. Android Studio approach

1. Set up version catalog in your project if you haven't done so.

2. Add libraries and plugins required by the generated API module.

[versions]
kotlin = "{latest_version that is >= 1.9.22}"
kotlinx-datetime = "{latest_version that is >= 0.5.0}"
kotlinx-serialization = "{latest_version that is >= 1.6.2}"
ktor = "{latest_version that is >= 2.3.7}"

[libraries]
kotlinx-datetime = { module = "org.jetbrains.kotlinx:kotlinx-datetime", version.ref = "kotlinx-datetime" }
kotlinx-serialization-json = { module = "org.jetbrains.kotlinx:kotlinx-serialization-json", version.ref = "kotlinx-serialization" }
ktor-client-content-negotiation = { module = "io.ktor:ktor-client-content-negotiation", version.ref = "ktor" }
ktor-client-core = { module = "io.ktor:ktor-client-core", version.ref = "ktor" }
ktor-client-okhttp = { module = "io.ktor:ktor-client-okhttp", version.ref = "ktor" }
ktor-serialization-kotlinx-json = { module = "io.ktor:ktor-serialization-kotlinx-json", version.ref = "ktor" }

[plugins]
kotlin-jvm = { id = "org.jetbrains.kotlin.jvm", version.ref = "kotlin" }
kotlin-serialization = { id = "org.jetbrains.kotlin.plugin.serialization", version.ref = "kotlin" }

3. Go to build.gradle.kts in the root directory, and append aliases to the two plugins required by the API module, as specified in the version catalog.

plugins {
  …remaining plugins…

  /** Needed for :api library. */
  alias(libs.plugins.kotlin.jvm) apply false
  alias(libs.plugins.kotlin.serialization) apply false
}

4. Click [File -> New -> Import Module…] from the menu bar and specify the source directory to the generated API.

5. [Optional] Android Studio may have created settings.gradle to include the new module instead of utilising the existing settings.gradle.kts. In that case, remove settings.gradle, and manually append the newly-included module in settings.gradle.kts. e.g...

include(":app", ":api")

6. Go to build.gradle.kts for the module that uses the generated API, and add the generated API module as a project dependency by appending the following line in the dependencies {…} closure.

implementation(project(":{api_folder}"))