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Javalin is a very lightweight web framework for Kotlin and Java which supports WebSockets, HTTP2 and async requests. Javalin’s main goals are simplicity, a great developer experience, and first class interoperability between Kotlin and Java.

Javalin is more of a library than a framework. Some key points:

You don't need to extend anything
There are no @Annotations
There is no reflection
There is no other magic; just code.

General information

The project webpage is javalin.io (the source is at javalin/javalin.github.io).
Read the documentation on: javalin.io/documentation
A summary of the license can be found at TLDR Legal
Interesting issues

Community

We have a very active Discord server where you can get help quickly. We also have a (much less active) Slack if you prefer that.

Contributions are very welcome, you can read more about contributing in our guide: CONTRIBUTING

Please consider Sponsoring or starring Javalin if you want to support the project.

Quickstart

Add dependency

Maven


    io.javalin
    javalin
    6.3.0

Gradle

implementation("io.javalin:javalin:6.3.0")

Start programming (Java)

import io.javalin.Javalin;public class HelloWorld {    public static void main(String[] args) {        var app = Javalin.create(/*config*/)
            .get("/", ctx -> ctx.result("Hello World"))
            .start(7070);
    }
}

Start programming (Kotlin)

import io.javalin.Javalinfun main() {    val app = Javalin.create(/*config*/)
        .get("/") { it.result("Hello World") }
        .start(7070)
}

Examples

This section contains a few examples, mostly just extracted from the docs. All examples are in Kotlin, but you can find them in Java in the documentation (it's just syntax changes).

You can find more examples in the javalin-samples repository.

Api structure and server config

import io.javalin.Javalinimport io.javalin.apibuilder.ApiBuilder.*fun main() {    val app = Javalin.create { config ->
        config.useVirtualThreads = true
        config.http.asyncTimeout = 10_000L
        config.staticFiles.add("/public")
        config.staticFiles.enableWebjars()
        config.router.apiBuilder {
            path("/users") {
                get(UserController::getAll)
                post(UserController::create)
                path("/{userId}") {
                    get(UserController::getOne)
                    patch(UserController::update)
                    delete(UserController::delete)
                }
                ws("/events", userController::webSocketEvents)
            }
        }
    }.start(7070)
}

WebSockets

app.ws("/websocket/{path}") { ws ->
    ws.onConnect { ctx -> println("Connected") }
    ws.onMessage { ctx ->        val user = ctx.message() // convert from json string to object
        ctx.send(user) //  convert to json string and send back
    }
    ws.onClose { ctx -> println("Closed") }
    ws.onError { ctx -> println("Errored") }
}

Filters and Mappers

app.before("/some-path/*") { ctx -> ... } // runs before requests to /some-path/*app.before { ctx -> ... } // runs before all requestsapp.after { ctx -> ... } // runs after all requestsapp.exception(Exception.class) { e, ctx -> ... } // runs if uncaught Exceptionapp.error(404) { ctx -> ... } // runs if status is 404 (after all other handlers)app.wsBefore("/some-path/*") { ws -> ... } // runs before ws events on /some-path/*app.wsBefore { ws -> ... } // runs before all ws eventsapp.wsAfter { ws -> ... } // runs after all ws eventsapp.wsException(Exception.class) { e, ctx -> ... } // runs if uncaught Exception in ws handler

JSON-mapping

var todos = arrayOf(...)
app.get("/todos") { ctx -> // map array of Todos to json-string
    ctx.json(todos)
}
app.put("/todos") { ctx -> // map request-body (json) to array of Todos
    todos = ctx.body>()
    ctx.status(204)
}

File uploads

app.post("/upload") { ctx ->
    ctx.uploadedFiles("files").forEach { uploadedFile ->        FileUtil.streamToFile(uploadedFile.content(), "upload/${uploadedFile.filename()}")
    }
}

Plugins

Javalin has a plugin system that allows you to add functionality to the core library. You can find a list of plugins here.

Installing a plugin is as easy as adding a dependency to your project and registering it with Javalin:

Javalin.create { config ->
    config.registerPlugin(MyPlugin())
}

Some of the most popular plugins are:

OpenAPI Plugin

The Javalin OpenAPI plugin allows you to generate an OpenAPI 3.0 specification for your API at compile time.

Annotate your routes with @OpenApi to generate the specification:

@OpenApi(
    summary = "Get all users",
    operationId = "getAllUsers",
    tags = ["User"],
    responses = [OpenApiResponse("200", [OpenApiContent(Array::class)])],
    path = "/users",
    methods = [HttpMethod.GET]
)fun getAll(ctx: Context) {
    ctx.json(UserService.getAll())
}

Swagger UI and ReDoc UI implementations for viewing the generated specification in your browser are also available.

For more information, see the Javalin OpenAPI Wiki.

SSL Plugin

The Javalin SSL plugin allows you to easily configure SSL for your Javalin server, supporting a variety of formats such as PEM, PKCS12, DER, P7B, and JKS.

Enabling SSL on the 443 port is as easy as:

val plugin = SSLPlugin { conf ->
    conf.pemFromPath("/path/to/cert.pem", "/path/to/key.pem")
}Javalin.create { javalinConfig ->
    javalinConfig.plugins.register(plugin)
}.start()