Kotlin multi-platform API utilities. Write APIs in Kotlin; generate Ktor bindings and Open API Specs.

@Api
interface IdentityApi {

    @GET("/user/{id}")
    suspend fun getUser(@Path id: String): User
}

fun Application.module() {
    routing {
        // Function is auto-generated. Provide an `IdentityApi` instance.
        registerIdentityApi(api = IdentityApiImpl())
    }
}

This project is in an early stage and under active development. Breaking changes may occur between released versions. Until the version is a release version of 1.0.0 or greater, this project is considered to be in an experimental state.

First, set up the KSP plugin. The following can be added to the module's build.gradle.kts file to add the KSP plugin:

plugins {
    id("com.google.devtools.ksp") version "1.8.10-1.0.9"
}

Then, add the repositories for this project:

repositories {
    maven { url = uri("https://repo.repsy.io/mvn/chrynan/public") }
}

Finally, add the dependencies of this project:

dependencies {
    // The annotation processor - see ksp documentation for multiplatform builds
    ksp("com.chrynan.kapi:kapi-server-ksp:$kapiVersion")

    // The runtime library
    implementation("com.chrynan.kapi:kapi-server-core:$kapiVersion")
}

Note: It may be required to add the generated sources of the KSP processor for the IDE to recognize the generated code and resources. The following is an example of adding the generated sources for a Kotlin JVM project:

kotlin {
    sourceSets.main {
        kotlin.srcDir("build/generated/ksp/main/kotlin")
        resources.srcDir("build/generated/ksp/main/resources/")
    }
}

It possible to configure the kapi server KSP plugin by providing KSP options in the build.gradle.kts file:

ksp {
    arg("kapi.config.jsonProcessorConfig.enabled", "true")
}

• kapi.config.jsonProcessorConfig.enabled - Enable/disable the generation of the apis.json file containing all the information about the processed APIs. Defaults to true.
• kapi.config.ktorProcessorConfig.enabled - Enable/disable the generation of Ktor bindings. Defaults to true.
• kapi.config.openApiProcessorConfig.enabled - Enabled/disable the generation of Open API Specification files. Defaults to true.

An API component can be a Kotlin interface, class, or object. Just annotate the API component with the @Api annotation for the processor to register the component. The @Api annotation contains properties that can be provided that are useful to the processor when it generates the Ktor bindings and Open API Spec files.

@Api(
    name = "IdentityApi",
    basePath = "/identity",
    info = Info(
        title = "Identity Service API",
        summary = "Provides API functions for accessing and updating user identities."
    )
)
interface IdentityApiComponent {}

API functions are normal Kotlin functions of the API component that are annotated with an HTTP method annotation. These functions correspond to HTTP methods at the specified path. For example, the following function, of the IdentityApiComponent created above, will create an HTTP GET endpoint at /identity/{id}:

@GET("/{id}")
suspend fun getIdentity(@Path id: String): Identity

• @GET
• @POST
• @PUT
• @PATCH
• @DELETE
• @HEAD
• @OPTIONS

An API function can only be annotated with one of the above supported HTTP method annotations. Each of the HTTP method annotations has a required path property. A function in an API component that is not annotated with a supported HTTP method annotation is not considered an API function and no endpoint binding will be created for it.

Parameters of an API function must either be annotated with a supported parameter annotation, have a default value, or be a supported type that can be provided by the generated code.

• @Body - Represents the HTTP request body.
• @Field - Represents a field from the HTTP request body when the content type is application/x-www-form-urlencoded.
• @Header - Represents an HTTP header extracted from the request.
• @Part - Represents a part of the HTTP request body when the content type is multipart/form-data.
• @Path - Represents a path parameter extracted from the URL of the request.
• @Query - Represents a query parameter extracted from the URL of the request.
• @Principal - Represents an auth principal obtained from the request.

API function parameters that are not annotated with the above annotations must either have a default value or be one of the following types:

• kotlin.Unit
• io.ktor.server.application.ApplicationCall
• io.ktor.server.routing.Route

API functions may be extension functions of the following types:

• io.ktor.server.application.ApplicationCall
• io.ktor.server.routing.Route

This provides a way of accessing the underlying Ktor components for the endpoints for advanced use-cases.

The value returned from an API function is the HTTP response of the endpoint. The content type of the HTTP response is determined by the Ktor content negotiation plugin that must be set up separately from this library. Any type that works with the Ktor content negotiation plugin can be returned from an API function. However, special handling is performed for the com.chrynan.kapi.core.Response type, which will result in the Response.body() value being the HTTP response of the endpoint.

The kapi library provides a way to establish the accepted content types for a particular API function. This can be done by annotating an API function with @Consumes (or an annotation whose class itself is annotated with @Consumes). An API function can only support accepting a single specified content type, or any content type that is supported by the Ktor content negotiation plugin that is set up. Here are some examples:

• @Consumes("application/json") - Accepts an HTTP request body with a content type of application/json.
• @Consumes("*/*") - Accepts any content type for the HTTP request body.
• @Consumes("") - Accepts any content type supported by the established Ktor content negotiation plugin.
• @ContentNegotiation - Same as the @Consumes("") usage - Accepts any content type supported by the established Ktor content negotiation plugin.
• @ApplicationJson - Accepts an HTTP request body with a content type of application/json.
• @ApplicationFormUrlEncoded - Accepts an HTTP request body with a content type of application/x-www-form-urlencoded.
• @MultipartFormData - Accepts an HTTP request body with a content type of multipart/form-data.

You can also create your own content type annotation like this:

@Consumes(contentType = "text/*")
@Target(AnnotationTarget.FUNCTION)
annotation class TextAny

If an API function is set up to accept a particular content type that is not a blank String or any value ("/"), then this will result in the generated Ktor endpoint being wrapped in an accept(contentType) { ... } function call.

An API function can be annotated with the @Produces annotation to indicate the success and error responses that the function can return. For instance, the following function will respond with an Identity model and a status code of 200 for a successful response when the function returns without an exception being thrown:

@GET("/{id}")
@Produces(
    success = Success(
        statusCode = 200
    )
)
suspend fun getIdentity(@Path id: String): Identity

When an API function throws an exception, that exception can be caught and result in an ApiError model (which is a rough implementation of the RFC-7807 specification) being the HTTP response of the API endpoint, for example:

@GET("/{id}")
@Produces(
    success = Success(
        statusCode = 200
    ),
    errors = [
        Error(
            statusCode = 500,
            exception = IllegalStateException::class,
            title = "Internal Server Error"
        )
    ]
)
suspend fun getIdentity(@Path id: String): Identity

In the above example, if the getIdentity API function throws an IllegalStateException, that will result in an HTTP response of type ApiError with a status code of 500.

Authentication and authorization for APIs is handled through the concept of "SecuritySchemes", "SecurityRequirements", and the @Auth annotation. First, you must establish the Security Schemes for an API by adding them to the @Api annotation:

@Api(
    securitySchemes = [
        SecurityScheme(
            SecurityScheme(
                name = "OAuthToken",
                type = SecurityScheme.Type.OAUTH2,
                flows = [
                    OAuthFlow(
                        type = OAuthFlow.Type.AUTHORIZATION_CODE,
                        authorizationUrl = "https://auth.example.com",
                        scopes = [
                            OAuthScope("read"),
                            OAuthScope("write")
                        ]
                    )
                ]
            )
        )
    ]
)
interface IdentityApiComponent { ... }

Then, create a convenience annotation for applying a Security Requirement for an API component that points to the above created Security Scheme:

@Auth(
    SecurityRequirement(name = "OAuthToken", scopes = ["read"]),
    type = Auth.RequirementType.ALL
)
@Target(AnnotationTarget.CLASS, AnnotationTarget.FUNCTION)
annotation class RequiresReadAccess

Finally, apply the annotation to any API components that require that scope to access:

@GET("/{id}")
@RequiresReadAccess
suspend fun getIdentity(@Path id: String): Identity

Note: To obtain the Principal from the result of successful authentication, use the @Principal annotation on a parameter of an API function.

While OAuth scope-based Authorization is handled automatically within the generated code, the Authentication providers still have to be established manually. However, there is a convenience extension property automatically generated on the KapiSecurity model that can be used to gain access to all of an APIs SecurityDefinitions:

install(Authentication) {
    Kapi.security.exampleApi.definitions.forEach { definition ->
        definition.provider // String name value equivalent to the `@SecurityScheme.name` property value.
        definition.scheme // Open API `SecurityScheme` model instance.
    }
}

To register an API with Ktor, just invoke the generated Route extension function providing the API implementation:

routing {
    registerIdentityApi(api = IdentityApiImpl())
}

More detailed documentation is available in the docs folder. The entry point to the documentation can be found here.

For security vulnerabilities, concerns, or issues, please responsibly disclose the information either by opening a public GitHub Issue or reaching out to the project owner.

This project was inspired by the works of the following open source projects and libraries:

• retrofit
• ktor
• Ktorfit
• ktor-retrofit
• OpenAPI
• JAX-RS

This project arose from my desire to create server-side APIs in a reusable, compile-time safe, and Kotlin multiplatform friendly manner while still utilizing modern frameworks, such as Ktor. I didn't want to fiddle around with creating large JSON or YAML Open API files after already creating the Kotlin API endpoints just so client-side code can utilize my API. Also, I wanted to be able to write my APIs in Kotlin where each endpoint is a Kotlin function whose parameters are extracted from the HTTP request and whose return value is the HTTP response body. So, I created this library with the hope of providing a tool for anyone with similar desires for their APIs. The following can be considered benefits of utilizing this library over directly writing your server APIs in Ktor:

• The APIs can be written in Kotlin, just as you would write any other Kotlin function. This should expedite the API development as it no longer requires getting familiar with complex framework components.
• Reusable interfaces with HTTP endpoint metadata. This allows separation from the HTTP concepts from the implementation of the code itself. The interface function can contain all the annotation metadata about the HTTP and API concepts, while the implementations can simply be focused on performing the business logic.
• Auto-generation and hosting of Open API JSON files, so you don't have to fiddle with creating those files manually.

Support this project by becoming a sponsor of my work! And make sure to give the repository a ⭐

Outside contributions are welcome for this project. Please follow the code of conduct and coding conventions when contributing. If contributing code, please add thorough documents and tests. Thank you!

Copyright 2023 chRyNaN

Licensed under the Apache License, Version 2.0 (the "License");
you may not use this file except in compliance with the License.
You may obtain a copy of the License at

   http://www.apache.org/licenses/LICENSE-2.0

Unless required by applicable law or agreed to in writing, software
distributed under the License is distributed on an "AS IS" BASIS,
WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
See the License for the specific language governing permissions and
limitations under the License.