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 (repo for webpage is at github.com/javalin/javalin.github.io).
- Documentation: javalin.io/documentation
- Chat on Gitter: https://gitter.im/javalin-io/general
- Chat on Discord: https://discord.gg/sgak4e5NKv
- Contributions are very welcome: CONTRIBUTING.md
- License summary: https://tldrlegal.com/license/apache-license-2.0-(apache-2.0)
- Interesting issues: /tipsy/javalin/issues?q=label:INFO
<dependency>
<groupId>io.javalin</groupId>
<artifactId>javalin</artifactId>
<version>3.13.5</version>
</dependency>compile "io.javalin:javalin:3.13.5"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"));
}
}import io.javalin.Javalin
fun main() {
val app = Javalin.create().start(7000)
app.get("/") { ctx -> ctx.result("Hello World") }
}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).
val app = Javalin.create { config ->
config.defaultContentType = "application/json"
config.autogenerateEtags = true
config.addStaticFiles("/public")
config.asyncRequestTimeout = 10_000L
config.dynamicGzip = true
config.enforceSsl = true
}.routes {
path("users") {
get(UserController::getAll)
post(UserController::create)
path(":user-id") {
get(UserController::getOne)
patch(UserController::update)
delete(UserController::delete)
}
ws("events", userController::webSocketEvents)
}
}.start(port)app.ws("/websocket/:path") { ws ->
ws.onConnect { ctx -> println("Connected") }
ws.onMessage { ctx ->
val user = ctx.message<User>(); // 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") }
}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)
app.wsBefore("/some-path/*") { ws -> ... } // runs before ws events on /some-path/*
app.wsBefore { ws -> ... } // runs before all ws events
app.wsAfter { ws -> ... } // runs after all ws events
app.wsException(Exception.class) { e, ctx -> ... } // runs if uncaught Exception in ws handlervar 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)
}app.post("/upload") { ctx ->
ctx.uploadedFiles("files").forEach { (contentType, content, name, extension) ->
FileUtil.streamToFile(content, "upload/$name")
}
}Javalin has an OpenAPI (Swagger) plugin. Documentation can be enabled both through a DSL and through annotations, and Javalin can render docs using both SwaggerUI and ReDoc. Read more at https://javalin.io/plugins/openapi.
- 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