Syncing clients

Setting up the server

As a quick reminder, let's see again the basic server set up:

import io.javalin.Javalin;

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

        app.ws("/websocket", ws -> {
            ws.onConnect(ctx -> {
                // Handle new connections
                System.out.println("New connection: " + ctx.getSessionId());
            });
        });
    }
}

Tracking sockets

To update all clients, we need to keep track of the WebSocket sessions. Fortunately, Javalin provides a WsContext object for each connection, which we can store in a List or Set for easy tracking.

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

import java.util.Collections;
import java.util.Set;
import java.util.concurrent.ConcurrentHashMap;

public class WebSocketSync {
    private static final Set<WsContext> connectedClients =
        Collections.newSetFromMap(new ConcurrentHashMap<>());

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

        app.ws("/websocket", ws -> {
            ws.onConnect(ctx -> {
                connectedClients.add(ctx);
                System.out.println("Connected clients: " + connectedClients.size());
            });

            ws.onClose(ctx -> {
                connectedClients.remove(ctx);
                System.out.println("Connection closed. Remaining clients: " + connectedClients.size());
            });
        });
    }
}

• The connectedClients set holds references to all currently active WebSocket sessions.

• When a client connects, it is added to the connectedClients set.

• When a client disconnects, it is removed from the set.

Forwarding messages

To broadcast messages received from one client to all other connected clients, we iterate over the connectedClients set and call the send method for each session.

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

import java.util.Collections;
import java.util.Set;
import java.util.concurrent.ConcurrentHashMap;

public class WebSocketSync {
    private static final Set<WsContext> connectedClients =
        Collections.newSetFromMap(new ConcurrentHashMap<>());

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

        app.ws("/websocket", ws -> {
            ws.onConnect(ctx -> connectedClients.add(ctx));

            ws.onClose(ctx -> connectedClients.remove(ctx));

            ws.onMessage((ctx, message) -> {
                for (WsContext client : connectedClients) {
                    if (client != ctx) { // Avoid echoing the message back to the sender
                        client.send(message);
                    }
                }
            });
        });
    }
}

• The onMessage handler receives messages from a client.

• The message is forwarded to all other connected clients using the send method.

And that's it! Now all connected users will receive any message sent by any other user.