How do you broadcast messages to multiple clients in Spring WebSocket?

Table of Contents

• Introduction
• Steps to Broadcast Messages to Multiple Clients in Spring WebSocket
  • 1. Set Up Spring WebSocket Configuration
    • Example WebSocket Configuration:
  • Explanation:
  • 2. Create a WebSocket Controller to Handle Messages
    • Example WebSocket Controller for Broadcasting:
  • Explanation:
  • 3. Client-Side Setup with STOMP and SockJS
    • Example JavaScript Client:
  • Explanation:
  • 4. Broadcasting the Message to Multiple Clients
  • Example Output:
  • 5. Handling User-Specific Messages
    • Example of Sending a Private Message to a User:
• Conclusion

Introduction

In Spring WebSocket, broadcasting messages to multiple clients is a powerful feature, especially for building real-time communication applications such as chat systems, live notifications, and collaborative tools. Broadcasting allows the server to send a message to multiple clients simultaneously, ensuring all subscribers to a particular destination receive updates in real time.

In this guide, we will walk through the steps to broadcast messages to multiple clients using Spring WebSocket and the STOMP protocol. We will explain how to configure the messaging system and use annotations like @SendTo to route messages to all clients subscribed to a specific topic.

Steps to Broadcast Messages to Multiple Clients in Spring WebSocket

1. Set Up Spring WebSocket Configuration

To broadcast messages, we need to configure WebSocket with STOMP support. This involves creating a configuration class that enables WebSocket communication and configures the message broker.

Example WebSocket Configuration:

Explanation:

• **@EnableWebSocketMessageBroker**: This annotation enables WebSocket and message broker functionality in the application.
• **configureMessageBroker()**: Sets up a message broker for destinations that start with /topic (for broadcasting messages) and /queue (for user-specific messages).
• **setApplicationDestinationPrefixes("/app")**: Routes client messages that start with /app to server-side methods.
• **registerStompEndpoints()**: Registers the /ws WebSocket endpoint for clients to connect, with SockJS as a fallback option for browsers that don't support WebSocket.

2. Create a WebSocket Controller to Handle Messages

The server needs a controller to handle incoming messages from clients and broadcast them to all other clients subscribed to a particular topic. We will use the @MessageMapping annotation to handle incoming messages, and @SendTo to specify where the response should be sent.

Example WebSocket Controller for Broadcasting:

Explanation:

• **@MessageMapping("/sendMessage")**: This method handles messages sent by clients to the /app/sendMessage destination.
• **@SendTo("/topic/chat")**: The message returned by the method is broadcasted to all clients that are subscribed to the /topic/chat destination.
• ChatMessage: This class represents the message object that will be sent to subscribers. It can contain properties like sender, content, and timestamp.

3. Client-Side Setup with STOMP and SockJS

To enable clients to send and receive WebSocket messages, we need to set up a STOMP client. This client will connect to the /ws endpoint, send messages to the /app/sendMessage destination, and subscribe to the /topic/chat destination to receive broadcast messages.

Example JavaScript Client:

Explanation:

• **SockJS('/ws')**: Creates a WebSocket connection to the /ws endpoint, with SockJS fallback options for unsupported browsers.
• **stompClient.connect()**: Establishes a connection to the WebSocket server.
• **stompClient.subscribe('/topic/chat')**: Subscribes to the /topic/chat destination to receive messages broadcasted by the server.
• **stompClient.send("/app/sendMessage")**: Sends a message to the /app/sendMessage destination on the server. The server then processes the message and broadcasts it to all clients subscribed to /topic/chat.

4. Broadcasting the Message to Multiple Clients

When a client sends a message to the /app/sendMessage destination, the message is passed to the sendMessage method in the server's ChatController. The method returns the ChatMessage object, which is then broadcast to all clients subscribed to /topic/chat.

All subscribed clients will receive the message in real time.

Example Output:

When a user sends a message via the WebSocket connection, all clients subscribed to /topic/chat will receive the message:

5. Handling User-Specific Messages

If you want to send messages to specific users rather than broadcasting to all clients, you can use **SimpMessagingTemplate** to send messages to individual users.

Example of Sending a Private Message to a User:

In this example:

• The sendPrivateMessage() method receives a message from a client and sends it to a specific user’s queue (/queue/private), ensuring that only that user receives the message.
• **convertAndSendToUser()**: Sends the message to a specific user based on their username, directing it to the /queue/private destination.

Conclusion

Broadcasting messages to multiple clients in Spring WebSocket is a straightforward process using the STOMP protocol. By configuring WebSocket endpoints, setting up message routing with @MessageMapping and @SendTo, and using STOMP clients (with SockJS as a fallback), you can easily implement real-time communication features in your applications.

• **@SendTo** is an essential tool for broadcasting messages to all subscribed clients to a specific topic.
• Clients can connect, send messages, and subscribe to receive real-time updates.
• For more advanced scenarios, you can use **SimpMessagingTemplate** to send messages to specific users or queues.

With Spring WebSocket and STOMP, you can build efficient, real-time messaging applications that cater to multiple clients simultaneously.