Programming language: Kotlin
License: GNU General Public License v3.0 or later
Tags: Web    
Latest version: v1.2.19

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The atoms of your platform

Home Site | Quick Start

What is Hexagon

Hexagon is a microservices' toolkit (not a framework) written in Kotlin. Its purpose is to ease the building of server applications (Web applications, APIs or queue consumers) that run inside a cloud platform.

The Hexagon Toolkit provides several libraries to build server applications. These libraries provide single standalone features and are referred to as "Ports".

The main ports are:

  • The HTTP server: supports HTTPS, HTTP/2, mutual TLS, static files (serve and upload), forms processing, cookies, CORS and more.
  • The HTTP client: which supports mutual TLS, HTTP/2, cookies, form fields and files among other features.
  • Template Processing: allows template processing from URLs (local files, resources or HTTP content) binding name patterns to different engines.

Each of these features or ports may have different implementations called "Adapters".

Hexagon is designed to fit in applications that conform to the [Hexagonal Architecture] (also called Clean Architecture or Ports and Adapters Architecture). Also, its design principles also fits in this architecture.

The Hexagon's goals and design principles are:

  • Put you in Charge: There is no code generation, no runtime annotation processing, no classpath based logic, and no implicit behaviour. You control your tools, not the other way around.

  • Modular: Each feature (Port) or adapter is isolated in its own module. Use only the modules you need without carrying unneeded dependencies.

  • Pluggable Adapters: Every Port may have many implementations (Adapters) using different technologies. You can swap adapters without changing the application code.

  • Batteries Included: It contains all the required pieces to make production-grade applications: logging utilities, serialization, resource handling and build helpers.

  • Kotlin First: Take full advantage of Kotlin instead of just calling Java code from Kotlin. The library is coded in Kotlin for coding with Kotlin. No strings attached to Java (as a Language).

  • Properly Tested: The project's coverage is checked in every Pull Request. It is also stress-tested at TechEmpower Frameworks Benchmark.

For more information check the Quick Start Guide.

Simple HTTP service

You can clone a starter project (Gradle Starter or Maven Starter). Or you can create a project from scratch following these steps:

  1. Configure Kotlin in Gradle or Maven.
  2. Add the dependency:
  • In Gradle. Import it inside build.gradle:

    repositories {
  • In Maven. Declare the dependency in pom.xml:

  1. Write the code in the src/main/kotlin/Hello.kt file:
// hello_world
import com.hexagonkt.http.server.jetty.serve

lateinit var server: HttpServer

 * Start a Hello World server, serving at path "/hello".
fun main() {
    server = serve {
        get("/hello/{name}") {
            val name = pathParameters["name"]
            ok("Hello $name!", contentType = ContentType(PLAIN))
// hello_world
  1. Run the service and view the results at: http://localhost:2010/hello


Books Example

A simple CRUD example showing how to manage book resources. Here you can check the [full test](http_test/src/main/kotlin/com/hexagonkt/http/test/examples/BooksTest.kt).

// books
data class Book(val author: String, val title: String)

private val books: MutableMap<Int, Book> = linkedMapOf(
    100 to Book("Miguel de Cervantes", "Don Quixote"),
    101 to Book("William Shakespeare", "Hamlet"),
    102 to Book("Homer", "The Odyssey")

private val path: PathHandler = path {

    post("/books") {
        val author = queryParameters["author"] ?: return@post badRequest("Missing author")
        val title = queryParameters["title"] ?: return@post badRequest("Missing title")
        val id = (books.keys.maxOrNull() ?: 0) + 1
        books += id to Book(author, title)

    get("/books/{id}") {
        val bookId = pathParameters.require("id").toInt()
        val book = books[bookId]
        if (book != null)
            ok("Title: ${book.title}, Author: ${book.author}")
            notFound("Book not found")

    put("/books/{id}") {
        val bookId = pathParameters.require("id").toInt()
        val book = books[bookId]
        if (book != null) {
            books += bookId to book.copy(
                author = queryParameters["author"] ?: book.author,
                title = queryParameters["title"] ?: book.title

            ok("Book with id '$bookId' updated")
        else {
            notFound("Book not found")

    delete("/books/{id}") {
        val bookId = pathParameters.require("id").toInt()
        val book = books[bookId]
        books -= bookId
        if (book != null)
            ok("Book with id '$bookId' deleted")
            notFound("Book not found")

    // Matches path's requests with *any* HTTP method as a fallback (return 404 instead 405)
    after(ALL - DELETE - PUT - GET, "/books/{id}", status = NOT_FOUND) {

    get("/books") {
        ok(books.keys.joinToString(" ", transform = Int::toString))
// books

Error Handling Example

Code to show how to handle callback exceptions and HTTP error codes. Here you can check the [full test](http_test/src/main/kotlin/com/hexagonkt/http/test/examples/ErrorsTest.kt).

// errors
class CustomException : IllegalArgumentException()

private val path: PathHandler = path {

     * Catching `Exception` handles any unhandled exception, has to be the last executed (first
     * declared)
    exception<Exception>(NOT_FOUND) {
        internalServerError("Root handler")

    exception<IllegalArgumentException> {
        val error = exception?.message ?: exception?.javaClass?.name ?: fail
        val newHeaders = response.headers + Header("runtime-error", error)
        send(HttpStatus(598), "Runtime", headers = newHeaders)

    exception<UnsupportedOperationException> {
        val error = exception?.message ?: exception?.javaClass?.name ?: fail
        val newHeaders = response.headers + Header("error", error)
        send(HttpStatus(599), "Unsupported", headers = newHeaders)

    get("/exception") { throw UnsupportedOperationException("error message") }
    get("/baseException") { throw CustomException() }
    get("/unhandledException") { error("error message") }
    get("/invalidBody") { ok(LocalDateTime.now()) }

    get("/halt") { internalServerError("halted") }
    get("/588") { send(HttpStatus(588)) }

    // It is possible to execute a handler upon a given status code before returning
    on(pattern = "*", status = HttpStatus(588)) {
        send(HttpStatus(578), "588 -> 578")
// errors

Filters Example

This example shows how to add filters before and after route execution. Here you can check the [full test](http_test/src/main/kotlin/com/hexagonkt/http/test/examples/FiltersTest.kt).

// filters
private val users: Map<String, String> = mapOf(
    "Turing" to "London",
    "Dijkstra" to "Rotterdam"

private val path: PathHandler = path {
    filter("*") {
        val start = System.nanoTime()
        // Call next and store result to chain it
        val next = next()
        val time = (System.nanoTime() - start).toString()
        // Copies result from chain with the extra data
        next.send(headers = response.headers + Header("time", time))

    filter("/protected/*") {
        val authorization = request.authorization ?: return@filter unauthorized("Unauthorized")
        val credentials = authorization.value
        val userPassword = String(credentials.decodeBase64()).split(":")

        // Parameters set in call attributes are accessible in other filters and routes
        send(attributes = attributes
          + ("username" to userPassword[0])
          + ("password" to userPassword[1])

    // All matching filters are run in order unless call is halted
    filter("/protected/*") {
        if(users[attributes["username"]] != attributes["password"])
            send(FORBIDDEN, "Forbidden")

    get("/protected/hi") {
        ok("Hello ${attributes["username"]}!")

    path("/after") {
        after(PUT) {

        after(PUT, "/second") {

        after("/second") {

        after {
// filters

Files Example

The following code shows how to serve resources and receive files. Here you can check the [full test](http_test/src/main/kotlin/com/hexagonkt/http/test/examples/FilesTest.kt).

// files
private val path: PathHandler = path {

    // Serve `public` resources folder on `/*`
        methods = setOf(GET),
        pattern = "/*",
        status = NOT_FOUND,
        callback = UrlCallback(URL("classpath:public"))

    path("/static") {
        get("/files/*", UrlCallback(URL("classpath:assets")))
        get("/resources/*", FileCallback(File(directory)))

    get("/html/*", UrlCallback(URL("classpath:assets"))) // Serve `assets` files on `/html/*`
    get("/pub/*", FileCallback(File(directory))) // Serve `test` folder on `/pub/*`

    post("/multipart") {
        val headers: HttpFields<Header> = parts.first().let { p ->
            val name = p.name
            val bodyString = p.bodyString()
            val size = p.size.toString()
                Header("name", name),
                Header("body", bodyString),
                Header("size", size),

        ok(headers = headers)

    post("/file") {
        val part = parts.first()
        val content = part.bodyString()
        val submittedFile = part.submittedFileName ?: ""
        ok(content, headers = response.headers + Header("submitted-file", submittedFile))

    post("/form") {
        fun serializeMap(map: Map<String, List<String>>): List<String> = listOf(
            map.map { "${it.key}:${it.value.joinToString(",")}}" }.joinToString("\n")

        val queryParams = serializeMap(queryParameters.allValues)
        val formParams = serializeMap(formParameters.allValues)
        val headers =
            HttpFields(Header("query-params", queryParams), Header("form-params", formParams))

        ok(headers = response.headers + headers)
// files

You can check more sample projects and snippets at the examples page.


This project is supported by:


The toolkit is properly tested. This is the coverage report:


Performance is not the primary goal, but it is taken seriously. You can check performance numbers in the TechEmpower Web Framework Benchmarks.


If you like this project and want to support it, the easiest way is to give it a star :v:.

If you feel like you can do more. You can contribute to the project in different ways:

To know what issues are currently open and be aware of the next features you can check the Organization Board at GitHub.

You can ask any question, suggestion or complaint at the project's Slack channel. You can be up-to-date of project's news following @hexagon_kt on Twitter.

Thanks to all project's contributors!



The project is licensed under the [MIT License]. This license lets you use the source for free or commercial purposes as long as you provide attribution and don’t hold any project member liable.

*Note that all licence references and agreements mentioned in the hexagon README section above are relevant to that project's source code only.