fabrikt

KO | EN

gitlite — search

#typescript #ai-agents #deepseek-harness #dsh-plugin #open-source #ai #claude-code #cli #windows #codex #developer-tools #dsh

★ 283 Open GitHub ↗

Generates Kotlin Code from OpenAPI 3 Specifications

// repository documentation

Was this content helpful?

★ 0 (0 ratings)

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

* [Introduction](#introduction)
* [Features](#features)
* [Examples](#examples)
* [Usage Instructions](#usage-instructions)
  * [Command Line](#command-line)
  * [Gradle w/ custom task](#gradle-w-custom-task)
  * [Gradle w/ plugin](#gradle-w-plugin)
  * [Maven](#maven)
  * [Docker](#docker)
* [Getting the Most from Fabrikt](#getting-the-most-from-fabrikt)
* [Configuration Options](#configuration-options)
* [Original Motivation](#original-motivation)
* [Building Locally](#building-locally)
* [Publishing](#publishing)
* [Specific Features](#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)
 - Jackson's polymorphic subtype deduction (add `x-jackson-subtype-deduction: true` to the `oneOf` schema) to gain Sealed Interface polymorphism, without needing to declare a discriminator property. Requires subtypes to have distinguishing required fields.
 
as well as HTTP clients and controllers for a number of popular frameworks (see [Features](#features)).

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

### Try Fabrikt Online

Try Fabrikt with your own API spec in the [Fabrikt Playground](https://try.fabrikt.io) and see how it can help you generate code for your API clients and servers.

[![Screenshot of Fabrikt Playground](https://github.com/user-attachments/assets/34ac6aff-f27e-4f48-bc53-cbb2a00e3e78)](https://try.fabrikt.io)

### Coordinates

```xml
<dependency>
  <groupId>io.fabrikt</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
  * **Ktor Client (w/ Jackson & Kotlin Serialization models)**
  * **Spring HTTP Interface** 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](src/test/resources/examples) generated from different OpenAPI 3 permutations.

Furthermore, the [end-to-end tests](/end2end-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](#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
```

__Tip__: You can also run the latest version without a manual download via [JBang](https://www.jbang.dev/):

```
jbang run io.fabrikt:fabrikt:RELEASE [args...]
```

### 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.

```kotlin
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("io.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("io.fabrikt:fabrikt:+") // This should be pinned  
     ...
}
```

### Gradle w/ plugin

The [Fabrikt Gradle plugin](https://github.com/acanda/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](https://github.com/acanda/fabrikt-gradle-plugin?tab=readme-ov-file#configuration) section of the 
plugin's README for the most up-to-date information on how to use it.

Latest version of the plugin: [![Gradle Plugin Portal Version](https://img.shields.io/gradle-plugin-portal/v/ch.acanda.gradle.fabrikt?style=flat)](https://plugins.gradle.org/plugin/ch.acanda.gradle.fabrikt)

```kotlin
plugins {
    // find latest version: https://github.com/acanda/fabrikt-gradle-plugin/releases
    id("ch.acanda.gradle.fabrikt") version "1.39.1"
}

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

### Maven

The [exec-maven-plugin](http://www.mojohaus.org/exec-maven-plugin/examples/example-exec-using-plugin-dependencies.html) is capable of downloading the Fabrikt library from Maven Central and executing its main method with defined arguments.

### Docker

Fabrikt is also available as a Docker image, which can be convenient for CI/CD pipelines or environments where you prefer not to install Java directly.

The Docker image can be invoked as follows:

```bash
docker run --rm -v $(pwd):/workspace ghcr.io/fabrikt-io/fabrikt:latest \
  --output-directory '.' \
  --base-package 'com.example' \
  --api-file 'openapi.yaml' \
  --targets 'http_models'
```

The command mounts your current directory to `/workspace` in the container, where Fabrikt will read the OpenAPI specification and write the generated code.

## Getting the Most from Fabrikt

### 1. Prefer components to inline schemas
While inline schemas are perfectly valid they are not supported by Fabrikt in all circumstances.
This is especially true for request bodies and non-trivial parameters. Instead, define your schemas in the
components section of the OpenAPI spec (`components.parameters` & `components.requestBodies`). [#20](https://github.com/fabrikt-io/fabrikt/issues/20), [#187](https://github.com/fabrikt-io/fabrikt/issues/187)

### 2. Use `oneOf` with discriminator for polymorphism
`oneOf` along with the flag `SEALED_INTERFACES_FOR_ONE_OF` will generate polymorphic models with sealed interfaces.
The `discriminator` property is used by Fabrikt to determine the subtypes to be generated.

## 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). |
|                                |   `SPRING_CLOUD_OPENFEIGN_STARTER_ANNOTATION` - This option adds the @FeignClient annotation to generated client interface |
|                                |   `GROUP_BY_TAG` - This option groups clients based on the first tag rather than paths |
|   `--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. |
|                                |   `SPRING_HTTP_INTERFACE` - Generate Spring HTTP Interface. |
|                                |   `KTOR` - Generate Ktor 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 |
|                                |   `GROUP_BY_TAG` - This option groups controllers based on the first tag rather than paths |
|                                |   `COMPLETION_STAGE` - This option makes generated controller functions have Type CompletionStage<T> (works only with Spring Controller generator). Can be overridden per operation using the OpenAPI extension `x-async-support: true|false` |
|                                |   `SSE_EMITTER` - This option makes generated controller functions have Type SseEmitter (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:+" |
|                                |   `MICRONAUT_SERDEABLE` - This option adds @Serdeable to the generated models. Requires dependency "'io.micronaut.serde:micronaut-serde-jackson:+" |
|                                |   `INCLUDE_COMPANION_OBJECT` - This option adds a companion object to the generated models. |
|                                |   `SEALED_INTERFACES_FOR_ONE_OF` - This option is deprecated. Sealed interfaces are enabled by default in v26+. Use DISABLE_SEALED_INTERFACES_FOR_ONE_OF to disable. |
|                                |   `DISABLE_SEALED_INTERFACES_FOR_ONE_OF` - This option disables the default sealed interfaces for oneOf behavior in v26+ |
|                                |   `NON_NULL_MAP_VALUES` - This option makes map values non-null. The default (since v15) and most spec compliant is make map values nullable |
|                                |   `FAULT_TOLERANT_ENUMS` - This option adds an UNRECOGNIZED enum entry as a fallback for unmapped values, preventing deserialization exceptions. If jackson is used, the deserialization option **READ_UNKNOWN_ENUM_VALUES_USING_DEFAULT_VALUE** will need to be enabled as well |
|                                |   `FAULT_TOLERANT_OPEN_ENUMS` - This option converts the "open enum" pattern (an `anyOf` combining a string enum with an open `type: string`) into a fault-tolerant enum, i.e. an enum carrying the declared values plus an UNRECOGNIZED fallback, instead of collapsing the type to a plain `String`. Behaves like FAULT_TOLERANT_ENUMS for the affected enums |
|   `--http-model-suffix`        | Specify custom suffix for all generated model classes. Defaults to no suffix. |
|   `--instant-library`          | Specify which Instant library to use in generated model classes for kotlinx.serialization. Default: KOTLINX_INSTANT |
|                                | CHOOSE ONE OF: |
|                                |   `KOTLINX_INSTANT` - Use `kotlinx.datetime` Instant in generated classes (default) |
|                                |   `KOTLIN_TIME_INSTANT` - Use `kotlin.time` Instant in generated classes |
|   `--jackson-nullability-mode` | Configure advanced handling when serializing null values with Jackson. Default: NONE |
|                                | CHOOSE ONE OF: |
|                                |   `NONE` - Default Jackson behaviour |
|                                |   `ENFORCE_OPTIONAL_NON_NULL` - Omit null values for optional non-null fields |
|                                |   `ENFORCE_REQUIRED_NULLABLE` - Include null values for required nullable fields |
|                                |   `STRICT` - Combines `ENFORCE_OPTIONAL_NON_NULL` and `ENFORCE_REQUIRED_NULLABLE` for strictest contract enforcement |
|   `--openfeign-client-name`    | Specify openfeign client name for spring-cloud-starter-openfeign. Defaults to 'fabrikt-client'. |
|   `--output-directory`         | Allows the generation dir to be overridden. Defaults to current dir |
|   `--output-opts`              | Select options for the output. |
|                                | CHOOSE ANY OF: |
|                                |   `ADD_FILE_DISCLAIMER` - This option adds a disclaimer to the generated files. |
|   `--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` |
|                                |   `BYTE_AS_STRING` - Ignore string format `byte` and use `String` as the type |
|                                |   `BINARY_AS_STRING` - Ignore string format `binary` and use `String` as the type |
|                                |   `URI_AS_STRING` - Ignore string format `uri` and use `String` as the type |
|                                |   `UUID_AS_STRING` - Ignore string format `uuid` and use `String` as the type |
|                                |   `DATE_AS_STRING` - Ignore string format `date` and use `String` as the type |
|                                |   `DATETIME_AS_STRING` - Ignore string format `date-time` and use `String` as the type |
|                                |   `BYTEARRAY_AS_INPUTSTREAM` - Use `InputStream` as ByteArray type. Defaults to `ByteArray` |
|                                |   `ANY_AS_JSONELEMENT` - Use `kotlinx.serialization.json.JsonElement` for untyped (any) schemas and `JsonObject` for untyped objects. Requires the KOTLINX_SERIALIZATION serialization library. Defaults to `Any` |
|   `--validation-library`       | Specify which validation library to use for annotations in generated model classes. Default: JAKARTA_VALIDATION |
|                                | CHOOSE ONE OF: |
|                                |   `JAVAX_VALIDATION` - Use `javax.validation` annotations in generated model classes |
|                                |   `JAKARTA_VALIDATION` - Use `jakarta.validation` annotations in generated model classes (default) |
|                                |   `NO_VALIDATION` - Use no validation annotations in generated model classes |
## Original Motivation

The team that built the first version of this tool initially contributed to the Kotlin code generation ability in
[OpenApiTools](https://github.com/OpenAPITools/openapi-generator), 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](https://github.com/RepreZen/KaiZen-OpenApi-Parser) and uses [Kotlin Poet](https://square.github.io/kotlinpoet/) to
programmatically construct Kotlin classes for maximum flexibility.

This project was started by engineers from [Zalando Tech](https://opensource.zalando.com/) and is battle-tested heavily in production there.

## 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](src/test/resources/examples/discriminatedOneOf). This feature makes use of Kotlin `sealed interface` and must be explicitly enabled via `--http-model-opts, SEALED_INTERFACES_FOR_ONE_OF`_

```yml
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"
```
```kotlin
@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
)
```

## Contributing

### 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:fabrikt-io/fabrikt.git
cd fabrikt/
./gradlew clean build
```

### Adjusting Test Examples

A utility function is available in [GeneratedCodeAsserter.kt](src/test/kotlin/com/cjbooms/fabrikt/util/GeneratedCodeAsserter.kt) to mass change all of the code generation examples in the test resources folder. This is useful when a global change is made to the code generation logic and all of the examples need to be updated.

### Publishing

1. Go to [Release Tab](https://github.com/fabrikt-io/fabrikt/releases)
2. Select `Draft a new release`.
3. Set tag to a version greater than current using symantic versioning, anticipating whether the changes made could break builds.
4. Click `Generate release notes`. Ensure that the tag and release version match.
5. Click `Publish release` buttom at the bottom.

Github Actions will publish the deployment to [Sonatype Central](https://central.sonatype.com/publishing/deployments). You must then log in to Sonatype and decide to either release or drop that deployment. After 30 minutes or so, Maven Central will have indexed the promoted release.

fabrikt

Explore Similar Repositories

Was this content helpful?

Recent Feedback

Download README