การส่งและรับข้อความ STOMP

In this lesson, we'll learn how to send messages from a client to our Spring server and how the server can then send messages back, either to a general topic or a specific user.

STOMP provides a clear, structured way to route these real-time messages, making our applications more predictable and easier to manage.

Think of messaging in terms of producers and consumers:

• Producers send messages (e.g., a chat client sending a message).
• Consumers receive messages (e.g., another chat client displaying the message).

In a real-time app, clients often act as both!

A topic is a public destination. When a message is sent to a topic, all clients currently subscribed to that topic will receive the message.

It's like a broadcast channel or a public chat room where everyone hears what's said.

Common topic paths start with /topic/, e.g., /topic/public-chat or /topic/news-feed.

On the Spring server, we use @MessageMapping to handle incoming messages from clients to a specific destination. We can then use SimpMessagingTemplate to send messages back out to a topic.

This snippet shows a server method receiving a message and then broadcasting it to a topic.

On the client-side (e.g., JavaScript), you subscribe to a topic to receive messages. When the server sends a message to /topic/public-chat, all subscribed clients get it.

A client might subscribe like this:

Sometimes you need to send a message directly to a single, specific user, not a public topic. This is called user-specific messaging.

It's perfect for:

• Private chat messages
• Personal notifications
• Direct alerts

STOMP and Spring handle the routing automatically for you.

Spring's SimpMessagingTemplate has a special method, convertAndSendToUser, to target messages to a specific user. You provide the username and the specific destination within that user's queue.

The framework translates /user/{username}/queue/messages into a unique queue for that user.

A client subscribes to their own user-specific queue to receive private messages. The path usually looks like /user/queue/private-messages.

The key here is that the /user prefix is special. The STOMP broker automatically routes messages sent to a particular user to their unique session-specific queue.

Messages sent over STOMP usually carry data in their payload (the message body). The most common format for this data is JSON.

Spring's STOMP support automatically converts Java objects to JSON (and vice-versa) when you use @Payload and convertAndSend methods, making it super easy to work with structured data.

We've explored how to send and receive STOMP messages in Spring applications:

• Topics (e.g., /topic/public-chat) are for broadcasting messages to all subscribed clients.
• User-specific destinations (e.g., /user/queue/private-messages) are for sending private messages to a single user.
• Spring's SimpMessagingTemplate and @MessageMapping annotations simplify both sending and receiving.

You can now build interactive features like chat rooms and private notifications!