Important: This news article covers an old version of Javalin (v0.5.1). The current version is v6.3.0.
See the documentation page for up-to-date information.

Functional WebSockets

There is only one change in this release, but it’s a big one. Version 0.5.1 introduces a WebSocket API, built on top of Jetty.

WebSockets can be enabled in three different ways:

Lambda approach

  • Java
  • Kotlin
app.ws("/websocket", ws -> {
    ws.onConnect(session -> System.out.println("Connected"));
    ws.onMessage((session, message) -> {
        System.out.println("Received: " + message);
        session.getRemote().sendString("Echo: " + message);
    });
    ws.onClose((session, statusCode, reason) -> System.out.println("Closed"));
    ws.onError((session, throwable) -> System.out.println("Errored"));
});

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") }
}

Annotated class

You can pass an annotated class to the ws() function:

app.ws("/websocket", WebSocketClass.class);

Annotation API can be found on Jetty’s docs page

WebSocket object

You can pass any object that fulfills Jetty’s requirements (annotated/implementing WebSocketListener, etc):

app.ws("/websocket", new WebSocketObject());