Web socket servers

INFO

Although web sockets aren't specific to Javalin, this part of the docs focuses heavily on servers, so we felt these notes belonged here.

Creating a web socket server

To create a WebSocket server in Javalin, we need the Javalin WebSocket plugin, which is included in the core Javalin library.

Then, create the Javalin app and enable WebSocket functionality:

import io.javalin.Javalin;

public class WebSocketServer {
    public static void main(String[] args) {
        Javalin app = Javalin.create(config -> {
            config.wsFactoryConfig(ws -> {
                // WebSocket settings can be configured here
            });
        }).start(5001);
    }
}

Listening for connections

Listening for new WebSocket connections is straightforward. Use the ws method to define WebSocket endpoints.

import io.javalin.Javalin;
import io.javalin.websocket.WsConnectContext;

public class WebSocketServer {
    public static void main(String[] args) {
        Javalin app = Javalin.create().start(5001);

        app.ws("/websocket", ws -> {
            ws.onConnect(ctx -> {
                System.out.println("New connection: " + ctx.getSessionId());
                // do what we want with the connection
            });
        });
    }
}

When a client connects to the WebSocket endpoint (/websocket), the onConnect handler is triggered, passing a WsConnectContext object (ctx) representing the connection.

Listening for socket events

We can listen for messages from the client using the onMessage event and handle connection closures using the onClose event.

import io.javalin.Javalin;
import io.javalin.websocket.WsContext;

public class WebSocketServer {
    public static void main(String[] args) {
        Javalin app = Javalin.create().start(5001);

        app.ws("/websocket", ws -> {
            ws.onConnect(ctx -> System.out.println("Connected: " + ctx.getSessionId()));
            ws.onMessage((ctx, message) -> handleMessage(ctx, message));
            ws.onClose((ctx, status, reason) -> cleanUp(ctx));
        });
    }

    private static void handleMessage(WsContext ctx, String message) {
        System.out.println("Received message: " + message);
    }

    private static void cleanUp(WsContext ctx) {
        System.out.println("Connection closed: " + ctx.getSessionId());
    }
}

• onMessage: Triggered when a message is received from the client.

• onClose: Triggered when the client closes the connection, allowing us to perform cleanup.

Sending a message back

At any point, we can send a message to the client using the send method on the WsContext.

app.ws("/websocket", ws -> {
    ws.onConnect(ctx -> {
        ctx.send("Hello from the server!");
    });
});

The client will receive the message immediately, provided the connection is still open.