Search code examples
javascriptjavaserver-sent-eventsjavalin

How to make the client receive server sent event using Javalin?

I am trying to implement a server client project which needs the server to send data to the client every 5 minutes with the client only asking in the beginning of connection. Server-sent events seem to be the go-to solution.

I have tried to use the functions given in the Javalin Documents. I am able to receive a response with a simple get from the server. But I couldn't establish a sse connection. The code enters the lambda function in the server but the client does not receive anything. I am not sure if the client or the server, or even both have a problem. The only output we get from the codes below is "connected" on the server side. Thank you in advance.

Code for the server

import io.javalin.Javalin;

public class SimpleTwitter {

  public static void main(String[] args) {

    Javalin app = Javalin.create().start(7000);

    app.sse("/sse", client ->{
        System.out.println("connected");
        client.sendEvent("message","Hello, SSE");
        client.onClose(() -> System.out.println("Client disconnected"));
    });

    app.get("/", ctx -> ctx.result("Hello World"));

   }
}

Code for the client

<!DOCTYPE html>
<html>
<body>

<h1>Getting server updates</h1>
<div id="result"></div>

<script>
if(typeof(EventSource) !== "undefined") {
  var source = new EventSource("http://localhost:7000/sse");
  source.onmessage = function(event) {
    document.getElementById("result").innerHTML += event.data + "<br>";
  };
} else {
  document.getElementById("result").innerHTML = "Sorry, your browser does not support server-sent events...";
}
</script>

</body>
</html>
Solution

  • Turns out the problem was not the code. By looking at the developer tools on chrome, we saw the following:

    "Access to resource at 'http://localhost:7000/sse' from origin 'null' has been blocked by CORS policy: No 'Access-Control-Allow-Origin' header is present on the requested resource."

    When we installed a chrome extention called "Allow-Control-Allow-Origin: *", we were able to see the output.

    Also, here are the updated better working codes:

    > <<!DOCTYPE html> <html> <body>
> 
> <h1>Tweets</h1>
> 
> <script> new
> EventSource('http://localhost:7000/sse').addEventListener( "hi", msg
> =>{       document.write(msg.data); }); </script>
> 
> </body> </html>

    ...

    public static void main(String[] args) throws InterruptedException {

        Queue<SseClient> clients = new ConcurrentLinkedQueue<>();

        Javalin app = Javalin.create().start(7000);
                app.sse("/sse", client -> {
            clients.add(client);
            client.onClose(() -> clients.remove(client));
        });

        while (true) {
            for (SseClient client : clients) {
                client.sendEvent("hi", "hello world");
            }
            TimeUnit.SECONDS.sleep(1);
        }

    }