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Fabrikt /ˈfa-brikt/ - Kotlin code from OpenApi3 specifications

This library was built to take advantage of the complex modeling features available in OpenApi3. 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)

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 OpenApi3 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 particulary well suited to API's built according to Zalando's REST API guidelines. It 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:

  • Jackson annotated data classes
  • Spring MVC annotated controller interfaces
  • Ktor server routes and controller interfaces
  • OkHttp Client - with the option for a resilience4j fault-tolerance wrapper

Example Generation

Consult test directory for OAS code generation examples, it forms a living documentation full of code examples generated from different OpenApi3 permutations.

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
)

Usage Instructions

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)
--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
--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
--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
--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
--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

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 example

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  
     ...
}

Maven

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

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.