Spring Websocket Reply To User Message Flow

Question

I am trying to send a private message between users using spring-websocket.

I found the following:

https://github.com/rstoyanchev/springx2013-websocket/blob/master/spring-messaging/slides.md

which includes:

Send Reply To User

@Controller
public class GreetingController {

  // Message sent to "/user/{username}/queue/greetings"

  @MessageMapping("/greetings")
  @SendToUser
  public String greet(String greeting) {
      return "[" + getTimestamp() + "]: " + greeting;
  }

}

The above seems to imply that a message sent to "/user/{username}/queue/greetings" will invoke the greet method before sending the return value on to the specified user.

When I send a message to this destination, it gets sent to the directly to the user without being processed by the greet controller method.

Do I understand the expected flow correctly? If I don't, what do I need to do to be able to process the message using a controller method before it's sent to a user.

Answer 1

The @SendToUser annotation defines that the return value of the method should be send to a user destination prefixed with /user/{username} where the user name is extracted from the headers of the input message being handled (current user).

In Spring 4.2 you can use placeholders in @SendTo (only destination variable placeholders, see SPR-12170), if you pass the username as a destination variable you could do something like this:

@MessageMapping("/greetings/{u}")
@SendTo("/user/{u}/queue/greetings")
public String greet(String greeting) {
    return "[" + getTimestamp() + "]: " + greeting;
}

This approach uses the SimpMessagingTemplate internally, so if you are using a version previous to 4.2, there's nothing wrong in using the SimpMessagingTemplate for dynamic destinations:

@MessageMapping("/greetings/{username}")
public void greet(@Payload String greeting, @DestinationVariable("username") String username) {
  String message =  "[" + getTimestamp() + "]: " + greeting;
  simpMessagingTemplate.convertAndSend("/user/" + username + "/queue/greetings", message);
}