Programming language: Kotlin
Tags: Web    

wasabi alternatives and similar libraries

Based on the "Web" category.
Alternatively, view wasabi alternatives based on common mentions on social networks and blogs.

Do you think we are missing an alternative of wasabi or a related project?

Add another 'Web' Library


Download TeamCity

Wasabi - An HTTP Framework


tl;dr We are merging with Ktor.

Ktor and Wasabi both have the same goals - define a simple, extensible HTTP framework for the JVM, built with Kotlin. Wasabi was created before Ktor and Ktor is influenced significantly by Wasabi. The recent commitment by the core team (@swishy and @hhariri) has unfortunately been impacted due to other commitments and we've decided that the best way to move forward is to join efforts with the #Ktor team to provide the best possible experience for our users. Given Wasabi and Ktor similarity, migration shouldn't be an issue and both the Wasabi team as well as the Ktor team are here to help (reach out to us on Kotlin Slack).

You can of course fork this project and continue it, or contribute your efforts to #Ktor!


An HTTP Framework built with Kotlin for the JVM.

Wasabi combines the conciseness and expressiveness of Kotlin, the power of Netty and the simplicity of Express.js (and other Sinatra-inspired web frameworks) to provide an easy to use HTTP framework

What it is

An HTTP framework that allows you to easily create back-end services for a web application or any other type of application that might require an HTTP API.

What it is not

It is not an MVC framework. There is no View Engine or templating language. You can combine it with client-side frameworks such as AngularJS or Ember. If you want a fully-fledged MVC framework in Kotlin, take a look at Kara

It is not a REST framework. Primarily because there's no such thing, and while calling it REST might sell better, it would be false. However it does provide (and will provide) features that allow you to create resource-orientated systems that help comply with ReSTful constraints.

Current Status

In development. A lot of the stuff here is not ready for production. There are many experiments, quite a few hacks and things that are just wrong. So be warned.


A lot of the API and design of this framework is based on experience developing applications with other HTTP frameworks and the needs that have arised. However, like any framework, until you don't have several usages of it, it's hard to know if you got it right. So while I'm hoping that there won't be major API changes, some things might change over time.

Getting Started

The Hello World of Wasabi

var server = AppServer()

server.get("/", { response.send("Hello World!") })


Starting a new application

The easiest way to use Wasabi is with Gradle.

  1. Download Gradle.

  2. Create the build.gradle file and import it into your IDE.

To create this file you can either:

  • Have Gradle generate the file for you (easiest) running the build.gradle file under tools (Recommended). To use this option download the file in [tools/build.gradle](tools/build.gradle) and type


    for more info

  • Create it manually. See the [sample for the structure](tools/sample.gradle). Make sure you fill in the TODOs

Including as a dependency (Gradle)

Follow the instructions on BinTray for adding a dependency in your build system.

You will need to add the repository to the repositories section also as per below.

repositories {
        url 'https://dl.bintray.com/wasabifx/wasabifx/'

Important: Versioning

Kotlin is still in development and so is Wasabi. The current master trunk of Wasabi uses the latest released version of Kotlin.

The AppServer

Each Wasabi application is composed of a single AppServer on which you define route handlers. A route handler can respond to any of the HTTP verbs: GET, POST, PUT, DELETE, OPTIONS, HEAD. A normal application consists of a section where you define a series of parameters for the application, followed by your handlers (i.e. your routing table).

var server = AppServer()

server.get("/customers", { .... } )
server.post("/customer", { .... } )


Route Handlers

In Wasabi, every request is processed by one or more route handlers. In the previous example, we are responding to a GET to "/" with the text "Hello World!". You can chain route handlers. For instance, if you want to log information about a request (this is actually built-in so no need to do it manually), you could do

      val log = Log()

      log.info("URI requested is ${request.uri}")
      response.send("Hello World!")

or as follows enforces JSON serialisation regardless of the request accept header.

      val log = Log()

      log.info("URI requested is ${request.uri}")
      response.send("Hello World!", "application/json")

By calling next() on each handler, the processing will continue.

All verbs on AppServer have the following signature

fun get(path: String, vararg handlers: RouteHandler.() -> Unit) {
  addRoute(HttpMethod.GET, path, *handlers)

where you can pass one or multiple route handlers. Each one of these is an extension method to the class RouteHandler. This class has various properties, amongst which are request and response. That is how you can access these properties from inside each of the functions without an explicit declaration.

Route Parameters

Wasabi supports route parameters. Define as many parameters as you like using : followed by the name of the parameter. Access it via request.routeParams["name"]

server.get("/customer/:id", { val customerId = request.routeParams["id"] } )

Query Parameters

Access query parameters using queryParams property of the request.


server.get("/customer", { val customerName = request.queryParams["name"] } )

Form Parameters

Access form parameters using bodyParams property of the request.

server.post("/customer", { val customerNameFromForm = request.bodyParams["name"] } )

Organization of Route Handlers and Application layout

How you layout the code for your application or group your routes depends largely on your own choice. One thing I've always been against is forcing people to group routes per class for instance. Having said that, there are some bounds you need to stay in.

Option 1

Defining logic for each route handler inline:

val appServer = AppServer()

appServer.get("/customer", { response.send(customers) })

For very simple operations this might be ok however, it will soon become unmaintainable.

Option 2

Define route handlers as functions and reference them:

val appServer = AppServer()

appServer.get("/customer", getCustomers)

This means that your definition of route handlers pretty much becomes a routing table, which is what it should be.

This is the preferred option. You can then group functions however way you want:

Grouping by file

Group similar routes in its own file. As Kotlin allows top level functions, you do not need to have a class to group functions. As such, you could have a file name CustomerRouteHandlers.kt for instance with:

val getCustomers = routeHandler {



val getCustomerById = routeHandler {

routeHandler is a syntatic sugar to define the type of the route handler. You could also have writte that as:

val getCustomers : RouteHandler.() -> Unit = {



val getCustomerById  : RouteHandler.() -> Unit = {
Grouping by class

If for some reason you want to group by class, you can do so. Best way is to use a companion object

class CustomerRoutes {

    companion object {

        val createCustomer = routeHandler {




Note Kotlin allows you to refer to a function by name using :: syntax. In principle you could also do:

appServer.get("/customer", ::getCustomer)

and not require an explicit variable declaration for the function. However, this currently does not work with extension functions but hopefully will in the future.


In addition to handlers, Wasabi also has interceptors. These allow you to intercept a request and decide whether you want it to continue or not (returning false would stop processing). Since you have access to both the request and response, you can do whatever you need. Think of interceptors as a way to add functionality to every request, or a those matching a certain route pattern. Some frameworks have popularized the term middleware to refer to something that intercepts a request/response. I do not agree with such a broad and somewhat ambiguous term. I like to name things as close to what they actually do. An interceptor implements the following interface

interface Interceptor {
  fun intercept(request: Request, response: Response): Boolean

You return true if you want the process to continue or false if you want to interrupt the request.

To add an interceptor to the application, you use

server.interceptor(MyInterceptor(), path, position)

where path can be a specific route or *** to match all routes. Position indicates when the intercept occurs. Possible positions are

 enum class InterceptOn {

Out of the box, the following interceptors are available

  • BasicAuthenticationInterceptor: Basic authentication
  • ContentNegotiationInterceptor: Automatic Content negotation
  • FavIconInterceptor: Support for favicon
  • StaticFileInterceptor: Support for serving static files
  • LoggingInterceptor: Logging
  • FileBasedErrorInterceptor: Customize error messages based on convention (e.g. 404.html)
  • SessionManagementInterceptor: Session Management support

Most interceptor add extension methods to AppServer to make them easier (and more descriptive) to use. For instance the ContentNegotiationInterceptor and StaticFileInterceptor would be used as so

val appServer = AppServer()


Content Negotiation

Wasabi ships with content negotiation out of the box, via a couple of interceptors. In particular:

  • ContentNegotiationParserInterceptor (still looking for a better name) Allows you to indicate to Wasabi to not only take into account Accept Headers but also Query Fields and Extensions on documents

  • ContentNegotiationInterceptor Does the actual content negotiation, finding the appropriate serializer.

How it works

Content Negotiation Parsing

Sometimes you want to not only do Content Negotiation using the Accept headers, but also using a query field (for instance format=json) or extensions to documents (e.g. /customer.json).

The ContentNegotiationParser allows you to do this. Easiest way to use it is via the extension function:

server.parseContentNegotiationHeaders() {

Based on the order in which you pass in onAcceptHeader, onExtension and onQueryParameter defines the priority. Above for instance first the Query Parameter is taking into account, then extension and lastly the Accept Header

QueryParameter defaults to the query parameter format but you can pass in a different one. Both onExtension and onQueryParameter also take a list of mappings, which an array of extension to media type. By default json maps to application/json and xml to application/xml

Automatic Content Negotiation

By default Content Negotiation is enabled. You can disable it via the AppConfiguration. You can then just send and object you want and Wasabi will automatically serialize it and send it back to the client.

server.get("/customer/:id", {
  val customer = getCustomerById(request.params["id"])

Wasabi will automatically serialize that into Json, Xml or any other media type supported (see Serializers below)


If you need to manually override Content Negotiation, you can do so using the negotiate function on response

   server.get("/customer/:id", {

        val customer = getCustomerById(request.params["id"])

        response.negotiate (
            "text/html" with { send("Joe Smith") },
            "application/json" with { send(customer) }


negotiate signature is:

fun negotiate(vararg negotiations: Pair<String, Response.() -> Unit>)

You can pass in an unlimited number of media type, functions. Also, as the function is an extension function to Response, you have access to the response functions directly. That is why we can just write send as opposed to response.send.

Note that even if you use manual content negotiation, Wasabi will still try and serialize the object for you based on the media type.


Both manual as well as automatic Content Negotiation use serializers. Wasabi ships with Json and XML (TODO) serializers. These are defined as a property of the AppServer.

Each Serializer can take as parameters a variable number of strings which define regular expressions for the media types it can handle. JsonSerializer for instance takes:

class JsonSerializer(): Serializer("application/json", "application/vnd\\.\\w*\\+json")

CORS Support

Wasabi provides CORS support via an Interceptor. You can enable CORS support in multiple ways:

  • Via AppConfiguration.enableCORSGlobally - which enables CORS for all routes, all verbs, all origins
  • Programatically using server.enableCORSGlobally - same as above
  • Programatically using server.enablesCORS([...COREntry...]) - providing an array of CORSEntry's

Each CORSEntry consists of:

class CORSEntry(val path: String = "*",
                       val origins: String = "*",
                       val methods: String = "GET, POST, PUT, DELETE",
                       val headers: String = "Origin, X-Requested-With, Content-Type, Accept",
                       val credentials: String = "",
                       val preflightMaxAge: String = "") {


with the corresponding defaults. Obviously you should only have one default.

Auto Options Support

Wasabi can automatically respond to OPTIONS for a specific path. You can enable this in multiple ways:

  • Via AppConfiguration.enableAutoOptions
  • Programatically using server.enableAutoOptions - same as above

Exception Handlers

Wasabi allows registration of custom handlers by exception type also. You can register them as follows:

val appServer = AppServer()
appServer.exception(MyKewlException::class, {
    reponse.setStatus(418, "My brew is not as strong as yours!")
    response.send("Out of beans: ${exception.message}")

like other Wasabi handlers the registered exception handler gets passed the request and response as well as the exception thrown.

WebSocket Support

Wasabi has initial WebSocket support which automatically handles websocket handshake and socket upgrade. Wasabi allows the registration of multiple 'channels' per appserver and you can declare a handler per channel. As our websocket support matures we intend to allow the registration of interceptors and serializers in the manner our HTTP / HTTP/2 pipeline does currently.

Example showing direct reply to the current client.

    appServer.channel("/foo", {
        if (frame is TextWebSocketFrame) {
            val textFrame = frame as TextWebSocketFrame
            respond(ctx!!.channel(), TextWebSocketFrame(textFrame.text().toUpperCase()))

Example showing message broadcast from channel.

    appServer.channel("/foo", {
        if (frame is TextWebSocketFrame) {
            val textFrame = frame as TextWebSocketFrame

Example showing triggering broadcast from webservice.

    appServer.post("/message", {
        val message = request.bodyParams["message"]
        broadcast("/foo", TextWebSocketFrame(message))


We're mostly hanging out on the #wasabi Channel on Kotlin's Slack. Join us there for questions and discussions.


There's a lot of work still pending and any help would be appreciated. Pull Requests welcome!

We have the project building on TeamCity. Click on the Icon to go to the build


Licensed under Apache 2 OSS License

Thank you's

Big thank you to JProfiler for the OSS license to help improve Wasabi performance

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