Fabrikt /ˈfa-brikt/ - Kotlin code from OpenAPI 3

---

• Introduction
• Features
• Examples
• Usage Instructions
  • Command Line
  • Gradle w/ custom task
  • Gradle w/ plugin
  • Maven
• Configuration Options
• Building Locally
• Publishing
• Specific Features

---

Introduction

This library was built to take advantage of the complex modeling features available in OpenAPI 3. It generates Kotlin data classes with advanced support for features such as:

• Null Safety
• Inlined schema definitions
• Enumerations
• Sealed Classes
• Polymorphism (@JsonSubTypes)
• Maps of Maps of Maps
• GraalVM Native Reflection Registration
• Json Merge Patch (via JsonNullable) (add x-json-merge-patch: true to schemas)
• Override Jackson Include NonNull (via JsonInclude) (add x-jackson-include-non-null: true to schemas)

as well as HTTP clients and controllers for a number of popular frameworks (see Features).

More than just bootstrapping, this library can be permanently integrated into a gradle or maven build and will ensure contract and code always match, even as APIs evolve in complexity.

The team that built this tool initially contributed to the Kotlin code generation ability in OpenApiTools, but reached the limits of what could be achieved with template-based generation. This library leverages the rich OpenAPI 3 model provided by KaiZen-OpenApi-Parser and uses Kotlin Poet to programmatically construct Kotlin classes for maximum flexibility.

It was built at Zalando Tech and is battle-tested in production there. It is particularly well-suited to API's built according to Zalando's REST API guidelines.

The library is available on Maven Central at the following coordinates:

<dependency>
  <groupId>com.cjbooms</groupId>
  <artifactId>fabrikt</artifactId>
</dependency>

Features

The library currently has support for generating:

• Models
  • Jackson annotated data classes
  • Kotlinx.serialization annotated data classes
• Clients
  • OkHttp Client (w/ Jackson Models) - with the option for a resilience4j fault-tolerance wrapper
  • OpenFeign annotated client interfaces
• Controllers
  • Spring MVC annotated controller interfaces
  • Micronaut HTTP annotated controller interfaces
  • Ktor server routes and controller interfaces

Examples

Consult test directory for OpenAPI code generation examples.

It forms a living documentation full of code examples generated from different OpenAPI 3 permutations.

Furthermore, the end-to-end tests demonstrate how to integrate the library using Gradle.

Usage Instructions

The library can be used in a variety of ways, including as a command line tool, a Gradle task, or a Maven plugin.

Please refer to Configuration Options section for a list of available parameters.

Command Line

Fabrikt is packaged as an executable jar, allowing it to be integrated into any build tool.

The CLI can be invoked as follows:

java -jar fabrikt.jar \
    --output-directory '/tmp' \
    --base-package 'com.example' \
    --api-file '/path-to-api/open-api.yaml' \
    --targets 'client' \
    --targets 'http_models' \
    --http-client-opts resilience4j

Gradle w/ custom task

Here is an example of a Gradle task with code generated to the build/generated directory, and execution linked to the compile task.

val fabrikt: Configuration by configurations.creating

val generationDir = "$buildDir/generated"
val apiFile = "$buildDir/path-to-api/open-api.yaml"

sourceSets {
    main { java.srcDirs("$generationDir/src/main/kotlin") }
    test { java.srcDirs("$generationDir/src/test/kotlin") }
    ...
}

tasks {   
    ...
    val generateCode by creating(JavaExec::class) {
        inputs.files(apiFile)
        outputs.dir(generationDir)
        outputs.cacheIf { true }
        classpath(fabrikt)
        mainClass.set("com.cjbooms.fabrikt.cli.CodeGen")
        args = listOf(
            "--output-directory", generationDir,
            "--base-package", "com.example",
            "--api-file", apiFile,
            "--targets", "http_models",
            "--targets", "client",
            "--http-client-opts", "resilience4j"
        )
    }
    withType<org.jetbrains.kotlin.gradle.tasks.KotlinCompile> {
        kotlinOptions.jvmTarget = "17"
        dependsOn(generateCode)
    }
}

dependencies {
     fabrikt("com.cjbooms:fabrikt:+") // This should be pinned  
     ...
}

Gradle w/ plugin

The Fabrikt Gradle plugin serves as a convenient wrapper for Fabrikt, allowing seamless integration of code generation into a Gradle build.

Note: Since the plugin is maintained separately from the Fabrikt library, please refer to the Configuration section of the plugin's README for the most up-to-date information on how to use it.

Latest version of the plugin:

plugins {
    // find latest version: https://github.com/acanda/fabrikt-gradle-plugin/releases
    id("ch.acanda.gradle.fabrikt") version "1.7.0"
}

fabrikt {
    generate("dog") {
        apiFile = file("src/main/openapi/dog.yaml")
        basePackage = "com.example.api"
    }
}

Maven

The exec-maven-plugin is capable of downloading the Fabrikt library from Maven Central and executing its main method with defined arguments.

Configuration Options

This section documents the available CLI parameters for controlling what gets generated. This documentation is generated using: ./gradlew printCodeGenUsage

Parameter	Description
--api-file	This must be a valid Open API v3 spec. All code generation will be based off this input.
--api-fragment	A partial Open API v3 fragment, to be combined with the primary API for code generation purposes.
* --base-package	The base package which all code will be generated under.
--external-ref-resolution	Specify to which degree referenced schemas from external files are included in model generation. Default: TARGETED
	CHOOSE ONE OF:
	TARGETED - Generate models only for directly referenced schemas in external API files.
	AGGRESSIVE - Referencing any schema in an external API file triggers generation of every external schema in that file.
--http-client-opts	Select the options for the http client code that you want to be generated.
	CHOOSE ANY OF:
	RESILIENCE4J - Generates a fault tolerance service for the client using the following library "io.github.resilience4j:resilience4j-all:+" (only for OkHttp clients)
	SUSPEND_MODIFIER - This option adds the suspend modifier to the generated client functions (only for OpenFeign clients)
	SPRING_RESPONSE_ENTITY_WRAPPER - This option adds the Spring-ResponseEntity generic around the response to be able to get response headers and status (only for OpenFeign clients).
--http-client-target	Optionally select the target client that you want to be generated. Defaults to OK_HTTP
	CHOOSE ONE OF:
	OK_HTTP - Generate OkHttp client.
	OPEN_FEIGN - Generate OpenFeign client.
--http-controller-opts	Select the options for the controllers that you want to be generated.
	CHOOSE ANY OF:
	SUSPEND_MODIFIER - This option adds the suspend modifier to the generated controller functions
	AUTHENTICATION - This option adds the authentication parameter to the generated controller functions
	COMPLETION_STAGE - This option makes generated controller functions have Type CompletionStage (works only with Spring Controller generator)
--http-controller-target	Optionally select the target framework for the controllers that you want to be generated. Defaults to Spring Controllers
	CHOOSE ONE OF:
	SPRING - Generate for Spring framework.
	MICRONAUT - Generate for Micronaut framework.
	KTOR - Generate for Ktor server.
--http-model-opts	Select the options for the http models that you want to be generated.
	CHOOSE ANY OF:
	X_EXTENSIBLE_ENUMS - This option treats x-extensible-enums as enums
	JAVA_SERIALIZATION - This option adds Java Serializable interface to the generated models
	QUARKUS_REFLECTION - This option adds @RegisterForReflection to the generated models. Requires dependency "'io.quarkus:quarkus-core:+"
	MICRONAUT_INTROSPECTION - This option adds @Introspected to the generated models. Requires dependency "'io.micronaut:micronaut-core:+"
	MICRONAUT_REFLECTION - This option adds @ReflectiveAccess to the generated models. Requires dependency "'io.micronaut:micronaut-core:+"
	INCLUDE_COMPANION_OBJECT - This option adds a companion object to the generated models.
	SEALED_INTERFACES_FOR_ONE_OF - This option enables the generation of interfaces for discriminated oneOf types
	NON_NULL_MAP_VALUES - This option makes map values non-null. The default (since v15) and most spec compliant is make map values nullable
--http-model-suffix	Specify custom suffix for all generated model classes. Defaults to no suffix.
--output-directory	Allows the generation dir to be overridden. Defaults to current dir
--resources-path	Allows the path for generated resources to be overridden. Defaults to src/main/resources
--serialization-library	Specify which serialization library to use for annotations in generated model classes. Default: JACKSON
	CHOOSE ONE OF:
	JACKSON - Use Jackson for serialization and deserialization
	KOTLINX_SERIALIZATION - Use kotlinx.serialization for serialization and deserialization
--src-path	Allows the path for generated source files to be overridden. Defaults to src/main/kotlin
--targets	Targets are the parts of the application that you want to be generated.
	CHOOSE ANY OF:
	HTTP_MODELS - Jackson annotated data classes to represent the schema objects defined in the input.
	CONTROLLERS - Spring / Micronaut / Ktor HTTP controllers for each of the endpoints defined in the input.
	CLIENT - Simple http rest client.
	QUARKUS_REFLECTION_CONFIG - This options generates the reflection-config.json file for quarkus integration projects
--type-overrides	Specify non-default kotlin types for certain OAS types. For example, generate Instant instead of OffsetDateTime
	CHOOSE ANY OF:
	DATETIME_AS_INSTANT - Use Instant as the datetime type. Defaults to OffsetDateTime
	DATETIME_AS_LOCALDATETIME - Use LocalDateTime as the datetime type. Defaults to OffsetDateTime
	BYTEARRAY_AS_INPUTSTREAM - Use InputStream as ByteArray type. Defaults to ByteArray
--validation-library	Specify which validation library to use for annotations in generated model classes. Default: JAVAX_VALIDATION
	CHOOSE ONE OF:
	JAVAX_VALIDATION - Use javax.validation annotations in generated model classes (default)
	JAKARTA_VALIDATION - Use jakarta.validation annotations in generated model classes
	NO_VALIDATION - Use no validation annotations in generated model classes

Building Locally

Fabrikt is built with Gradle and requires an initialised git repository. The easiest way to build it is to clone the repo locally before executing the build command:

git clone git@github.com:cjbooms/fabrikt.git
cd fabrikt/
./gradlew clean build

Publishing

This library is published to Sonatype's OSS staging repository using Github actions when a release is drafted. It can be manually promoted from there to the release repository which is indexed by Maven Central.

Specific Features

Polymorphism via allOf

The following example shows how allOf can be used to generate polymorphic Kotlin data classes. It does the following:

• A ChildDefinition schema defines both the discriminator mapping details and the schema for the discriminator property. In this case the discriminator property is an enumeration
• In each child schema, allOf is used to merge the ChildDefinition with the child's custom schema. This guarantees that each child schema inherits the correct discriminator property.
• In the Responses schema a oneOf lists only child schemas. This will be detected by Fabrikt and it will generate the list of parent types: List<ChildDefinition>

NOTE: A new feature has been added that allows Polymorphism to be achieved using only a discriminated oneOf. This feature makes use of Kotlin sealed interface and must be explicitly enabled via --http-model-opts, SEALED_INTERFACES_FOR_ONE_OF

openapi: 3.0.0
components:
  schemas:
    ChildDefinition:
      type: object
      discriminator:
        propertyName: some_enum
        mapping:
          obj_one_only: '#/components/schemas/DiscriminatedChild1'
          obj_two_first: '#/components/schemas/DiscriminatedChild2'
          obj_two_second: '#/components/schemas/DiscriminatedChild2'
          obj_three: '#/components/schemas/discriminated_child_3'
      properties:
        discriminating_property:
          $ref: '#/components/schemas/ChildDiscriminator'

    ChildDiscriminator:
      type: string
      enum:
        - obj_one_only
        - obj_two_first
        - obj_two_second
        - obj_three

    DiscriminatedChild1:
      allOf:
        - $ref: '#/components/schemas/ChildDefinition'
        - type: object
          properties:
            some_prop:
              type: string

    DiscriminatedChild2:
      allOf:
        - $ref: '#/components/schemas/ChildDefinition'
        - type: object
          properties:
            some_prop:
              type: string

    discriminated_child_3:
      allOf:
        - $ref: '#/components/schemas/ChildDefinition'

    Responses:
      type: "object"
      properties:
        entries:
          type: "array"
          items:
            oneOf:
              - $ref: "#/components/schemas/DiscriminatedChild2"
              - $ref: "#/components/schemas/DiscriminatedChild2"
              - $ref: "#/components/schemas/discriminated_child_3"

@JsonTypeInfo(
    use = JsonTypeInfo.Id.NAME,
    include = JsonTypeInfo.As.EXISTING_PROPERTY,
    property = "some_enum",
    visible = true
)
@JsonSubTypes(
    JsonSubTypes.Type(
        value = DiscriminatedChild1::class,
        name =
        "obj_one_only"
    ),
    JsonSubTypes.Type(
        value = DiscriminatedChild2::class,
        name =
        "obj_two_first"
    ),
    JsonSubTypes.Type(
        value = DiscriminatedChild2::class,
        name =
        "obj_two_second"
    ),
    JsonSubTypes.Type(value = DiscriminatedChild3::class, name = "obj_three")
)
sealed class ChildDefinition() {
    abstract val someEnum: ChildDiscriminator
}

enum class ChildDiscriminator(
    @JsonValue
    val value: String
) {
    OBJ_ONE_ONLY("obj_one_only"),

    OBJ_TWO_FIRST("obj_two_first"),

    OBJ_TWO_SECOND("obj_two_second"),

    OBJ_THREE("obj_three");

    companion object {
        private val mapping: Map<String, ChildDiscriminator> =
            values().associateBy(ChildDiscriminator::value)

        fun fromValue(value: String): ChildDiscriminator? = mapping[value]
    }
}

data class DiscriminatedChild1(
    @param:JsonProperty("some_prop")
    @get:JsonProperty("some_prop")
    val someProp: String? = null,
    @get:JsonProperty("some_enum")
    @get:NotNull
    override val someEnum: ChildDiscriminator = ChildDiscriminator.OBJ_ONE_ONLY
) : ChildDefinition()

data class DiscriminatedChild2(
    @get:JsonProperty("some_enum")
    @get:NotNull
    override val someEnum: ChildDiscriminator,
    @param:JsonProperty("some_prop")
    @get:JsonProperty("some_prop")
    val someProp: String? = null
) : ChildDefinition()

data class DiscriminatedChild3(
    @get:JsonProperty("some_enum")
    @get:NotNull
    override val someEnum: ChildDiscriminator = ChildDiscriminator.OBJ_THREE
) : ChildDefinition()

data class Responses(
    @param:JsonProperty("entries")
    @get:JsonProperty("entries")
    @get:Valid
    val entries: List<ChildDefinition>? = null
)