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WebSockets & Real-Time Systems with Spring · Lesson

Basic Client-Server Messaging

Implement simple message sending from client to server and vice versa, demonstrating basic real-time interaction.

Basic Client-Server Messaging is a free WebSockets & Real-Time Systems with Spring lesson on CoddyKit — lesson 3 of 4. You can read the complete lesson below for free — then practise it hands-on in the browser with a built-in code editor and a 24/7 AI tutor. It is part of the WebSockets & Real-Time Systems with Spring learning path, one of 4 lessons in the course, and your progress syncs across the web and the CoddyKit app.

Messaging Fundamentals

In previous lessons, you set up your Spring WebSocket application and configured its endpoints. Now, let's make it talk!

Real-time communication is all about sending messages back and forth between clients (like web browsers) and the server. This lesson will show you the basics of how this interaction works.

Client Sends to Server

The most common interaction starts with a client sending a message to the server. This message might be a user action, a chat message, or a command.

  • The client establishes a WebSocket connection.
  • It then sends data to a specific server "destination".
  • Spring handles receiving this message on the server side.

Server-Side Receiver (Spring)

Spring uses annotations to make handling incoming messages easy. The @MessageMapping annotation maps a method to a specific destination path, similar to @GetMapping for HTTP.

When a client sends a message to, say, /app/hello, the method annotated with @MessageMapping("/hello") will be invoked.

Implementing Server Receiver

Let's see a simple server-side method that receives a message. Notice how the input message can be directly mapped to a Java object (e.g., a String).

import org.springframework.messaging.handler.annotation.MessageMapping;
import org.springframework.stereotype.Controller;

@Controller
public class GreetingController {

    @MessageMapping("/hello")
    public String handleHello(String message) {
        System.out.println("Received from client: " + message);
        return "Server received: " + message; 
    }
}

Server Sends to Client

After processing a client's message, the server often needs to send a response or broadcast updates to clients. This is how real-time interaction truly happens.

Spring provides convenient ways to send messages back to clients, often using a message broker.

Implementing Server Sender

To send an immediate response to the client who sent the message, you can use the @SendTo annotation. This routes the return value of your @MessageMapping method to a specific topic.

import org.springframework.messaging.handler.annotation.MessageMapping;
import org.springframework.messaging.handler.annotation.SendTo;
import org.springframework.stereotype.Controller;

@Controller
public class GreetingController {

    @MessageMapping("/hello")
    @SendTo("/topic/greetings")
    public String handleHello(String message) {
        System.out.println("Received: " + message);
        return "Hello from Server: " + message;
    }
}

Client-Side Sending (JavaScript)

On the client side, typically a web browser, you'll use JavaScript to interact with the WebSocket endpoint. Libraries like SockJS and STOMP.js simplify this process.

  • Connect to the WebSocket endpoint.
  • Send messages to the server's @MessageMapping destinations.
  • Subscribe to server "topic" destinations to receive messages.

Simple JavaScript Client

Here's how a basic JavaScript client sends a message. It uses a STOMP client to connect and send data to the server's /app/hello destination.

// This assumes SockJS and STOMP.js are loaded
var socket = new SockJS('/ws'); // Your WebSocket endpoint
var stompClient = Stomp.over(socket);

stompClient.connect({}, function (frame) {
    console.log('Connected: ' + frame);

    // Subscribe to a topic to receive messages
    stompClient.subscribe('/topic/greetings', function (greeting) {
        console.log("Received from server: " + greeting.body);
    });

    // Send a message to the server
    stompClient.send("/app/hello", {}, "Hello Spring!");
});

Full Echo Example (Server)

Let's put it together! This Spring Boot server will receive a message, print it, and then send a greeting back to a public topic. You can run this as a Spring Boot application.

import org.springframework.boot.SpringApplication;
import org.springframework.boot.autoconfigure.SpringBootApplication;
import org.springframework.context.annotation.Configuration;
import org.springframework.messaging.handler.annotation.MessageMapping;
import org.springframework.messaging.handler.annotation.SendTo;
import org.springframework.messaging.simp.config.MessageBrokerRegistry;
import org.springframework.stereotype.Controller;
import org.springframework.web.socket.config.annotation.EnableWebSocketMessageBroker;
import org.springframework.web.socket.config.annotation.StompEndpointRegistry;
import org.springframework.web.socket.config.annotation.WebSocketMessageBrokerConfigurer;

@SpringBootApplication
public class WebSocketServerApp {
    public static void main(String[] args) {
        SpringApplication.run(WebSocketServerApp.class, args);
    }
}

@Configuration
@EnableWebSocketMessageBroker
class WebSocketConfig implements WebSocketMessageBrokerConfigurer {
    @Override
    public void registerStompEndpoints(StompEndpointRegistry registry) {
        registry.addEndpoint("/ws").withSockJS(); // WebSocket endpoint
    }

    @Override
    public void configureMessageBroker(MessageBrokerRegistry config) {
        config.enableSimpleBroker("/topic"); // Enable a simple broker for /topic destinations
        config.setApplicationDestinationPrefixes("/app"); // Client messages for @MessageMapping
    }
}

@Controller
class GreetingController {
    @MessageMapping("/hello") // Client sends to /app/hello
    @SendTo("/topic/greetings") // Server sends response to /topic/greetings
    public String greet(String message) throws Exception {
        Thread.sleep(1000); // Simulate processing delay
        System.out.println("Server received: " + message);
        return "Server says: " + message + "!";
    }
}

Message Flow Check

Consider a Spring WebSocket application where a client sends a message to /app/chat and the server responds to /topic/updates. Which of the following statements are true about this interaction?

Recap: Basic Messaging

Great job! You've learned the fundamentals of client-server messaging in Spring WebSockets:

  • Clients send messages to server destinations using client-side STOMP libraries.
  • Servers receive messages using @MessageMapping.
  • Servers can respond immediately using @SendTo to broadcast to topics.

This forms the foundation for all real-time interactions. Next, we'll dive deeper into the STOMP protocol for more structured messaging!

Frequently asked questions

Is the “Basic Client-Server Messaging” lesson free?

Yes — the full text of “Basic Client-Server Messaging” is free to read here on the web, and the WebSockets & Real-Time Systems with Spring course includes 4 lessons in total. To practise it interactively (a built-in code editor and a 24/7 AI tutor) and unlock the rest of the WebSockets & Real-Time Systems with Spring course, upgrade to CoddyKit PRO.

What will I learn in “Basic Client-Server Messaging”?

Implement simple message sending from client to server and vice versa, demonstrating basic real-time interaction. You practise WebSockets & Real-Time Systems with Spring with hands-on code you run directly in the browser, and a 24/7 AI tutor answers your questions as you work through the lesson.

Do I need any experience to start WebSockets & Real-Time Systems with Spring?

No prior experience is required. WebSockets & Real-Time Systems with Spring on CoddyKit is structured for beginners through advanced learners; this is — lesson 3 of 4, so you can start here or from the beginning and move at your own pace.

How long does the “Basic Client-Server Messaging” lesson take?

Most CoddyKit lessons take about 5–10 minutes. Each one is bite-sized and interactive, so you make steady progress and pick up exactly where you left off across the web and the app.

Can I write and run code in this WebSockets & Real-Time Systems with Spring lesson?

Yes. Every WebSockets & Real-Time Systems with Spring lesson includes a built-in code editor, so you write and run real code right in your browser and get instant AI feedback — no local setup required.

All lessons in this course

  1. Spring Boot for WebSockets
  2. WebSocket Endpoint Configuration
  3. Basic Client-Server Messaging
  4. Handling WebSocket Lifecycle Events in Spring
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