Javalin - A simple web framework for Java and Kotlin
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.
- Documentation: javalin.io/documentation
- Chat: https://gitter.im/javalin-io/general
- Contributions are very welcome: CONTRIBUTING.md
Quickstart
Add dependency
<dependency>
<groupId>io.javalin</groupId>
<artifactId>javalin</artifactId>
<version>2.8.0</version>
</dependency>
// or gradle, if you must
compile 'io.javalin:javalin:2.8.0'
Start programming (Java)
import io.javalin.Javalin;
public class HelloWorld {
public static void main(String[] args) {
Javalin app = Javalin.create().start(7000);
app.get("/", ctx -> ctx.result("Hello World"));
}
}
Start programming (Kotlin)
import io.javalin.Javalin
fun main(args: Array<String>) {
val app = Javalin.create().start(7000)
app.get("/") { ctx -> ctx.result("Hello World") }
}
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).
Api structure and server config
val app = Javalin.create().apply {
enableCorsForAllOrigins()
enableStaticFiles("/public")
enableStaticFiles("uploads", Location.EXTERNAL)
}.start(port)
app.routes {
path("users") {
get(UserController::getAll)
post(UserController::create)
path(":user-id") {
get(UserController::getOne)
patch(UserController::update)
delete(UserController::delete)
}
}
}
WebSockets
app.ws("/websocket") { ws ->
ws.onConnect { session -> println("Connected") }
ws.onMessage { session, message ->
println("Received: " + message)
session.remote.sendString("Echo: " + message)
}
ws.onClose { session, statusCode, reason -> println("Closed") }
ws.onError { session, throwable -> println("Errored") }
}
Filters and Mappers
app.before("/some-path/*") { ctx -> ... } // runs before requests to /some-path/*
app.before { ctx -> ... } // runs before all requests
app.after { ctx -> ... } // runs after all requests
app.exception(Exception.class) { e, ctx -> ... } // runs if uncaught Exception
app.error(404) { ctx -> ... } // runs if status is 404 (after all other handlers)
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<Array<Todo>>()
ctx.status(204)
}
File uploads
app.post("/upload") { ctx ->
ctx.uploadedFiles("files").forEach { (contentType, content, name, extension) ->
FileUtil.streamToFile(content, "upload/$name")
}
}
Special thanks
- Blake Mizerany, for creating Sinatra
- Per Wendel, for creating Spark
- Christian Rasmussen, for being a great guy
- Per Kristian Kummermo, also for being a great guy