Server Instance

Creating a Server

To start a Socket.IO server, create a Configuration, set the bind address and port, and then start the server.

Configuration config = new Configuration();
config.setHostname("localhost");
config.setPort(9092);

SocketIOServer server = new SocketIOServer(config);

When using frameworks such as Spring Boot, ensure you import com.socketio4j.socketio.Configuration and not framework classes with the same name (for example, org.springframework.context.annotation.Configuration), to avoid import conflicts.

Starting the Server

Sync

server.start()

Async

Stopping the Server

Always stop the server gracefully during shutdown.

server.stop();

socketio4j adds shutdown hook after the server started, it gracefully stops the server in certain scenarios, not all. It is always recommended to stop the server explicitly.

Complete Example

Server.java

import com.socketio4j.socketio.Configuration;
import com.socketio4j.socketio.SocketIOServer;
import org.slf4j.Logger;
import org.slf4j.LoggerFactory;

public class SocketIoServerMain {
    
    private static final Logger log = LoggerFactory.getLogger(SocketIoServerMain.class);
    
    public static void main(String[] args) throws Exception {
        // Create configuration
        Configuration config = new Configuration();
        config.setHostname("localhost");
        config.setPort(9092);

        // Create server
        SocketIOServer server = new SocketIOServer(config);
        
        server.addConnectListener(client -> {
            log.info("[/] connected -> " + client.getSessionId());
        });
        server.addDisconnectListener(client -> {
            log.info("[/] disconnected -> " + client.getSessionId());
        });
        
        server.addEventListener("hi", String.class, (client, data, ack) -> {
            //listen to "reply" in client
            log.info("received data : " + data);
            client.sendEvent("reply", "hello");
        });
        // Start server
        server.start();
        log.info("Socket.IO server started on port 9092");

        // Keep JVM alive
        Thread.currentThread().join();
    }
}

Check Events for event handling related documentation

Notes

• hostname is optional. If not set, the server binds to all interfaces (0.0.0.0 / ::0).

• port must be set before starting the server.

• Threading, transports, and other advanced options can be customized via Configuration before calling start().