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http4k is a lightweight but fully-featured HTTP toolkit written in pure Kotlin that enables the serving and consuming of HTTP services in a functional and consistent way. http4k applications are just Kotlin functions. For example, here’s a simple echo server:

val app: HttpHandler = { request: Request -> Response(OK).body(request.body) }
val server = app.asServer(SunHttp(8000)).start()

http4k consists of a lightweight core library, http4k-core, providing a base HTTP implementation and Server/Client implementations based on the JDK classes. Further servers, clients, serverless, templating, websockets capabilities are then implemented in add-on modules. http4k apps can be simply mounted into a running Server, Serverless platform, or compiled to GraalVM and run as a super-lightweight binary.

You can read about the rationale and ethos of http4k here

Installation

dependencies {
    // install the platform...
    implementation(platform("org.http4k:http4k-bom:5.32.4.0"))

    // ...then choose any moduless but at least the core
    implementation("org.http4k:http4k-core")
}

Quickstart

Bored with reading already and just want to get coding? Read the quickstart or take a look at the examples repo, which showcases a variety of http4k use-cases and features.

Module feature overview

If you’re needed it to build an HTTP application, there’s an excellent chance that http4k has a module for it. Check out the reference guide sections for details on the over 65 different built in integrations. Here’s a selection:

• Seamless app running in all these runtimes:
  • Server: 8 server integrations including Jetty, Helidon, Undertow and Ktor
  • Servlets: Plug into any HTTP Servlet container
  • Serverless: Support for 6 platforms including AWS Lambda, GCP, Alibaba and Azure
  • Custom Lambda runtime: For running performance sensitive AWS Lambdas without the overhead of the AWS runtime
  • Native: http4k apps can compile to GraalVM with no changes or configuration
  • In-memory: Running apps in a JVM for lightning fast tests
  • Realtime: WebSockets and Server Sent Events (SSE)
• OpenAPI: Document your APIs with industry leading support
• Protocol formats: Support for lots of protocol formats including JSONRpc, Graphql
• Pluggable wire format integrations for: JSON, YAML, CSV, XML, DataFrame
• Clients are available for these technologies:
  • HTTP: 6 clients including Apache, Jetty and OkHttp
  • WebSocket & SSE: For realtime connectivity
• Observability: Measure http4l with integrations including OpenTelemetry and Micrometer
• Metrics: Protect your app with Resilience4k and Failsafe
• Security: Seamless OAuth and Digest integrations
• HTML Templating: 7 serverside rendering engines including Handlebars, Pug4J and Rocker
• Testing styles: Support for innovative testing techniques such as TracerBullet, Approval and Chaos testing
• Testing Integrations: Battle harden your apps with integrations for WebDriver, Playwright, KoTest and others

Acknowledgments

• Dan Bodart’s utterlyidle
• Ivan Moore for pairing on the original hackday project - Barely Magical.
• You can see the amazing people and companies who have helped us to make http4k here.