ssesnippet.java

    // The two things I see in every SSE example: 
@GET @Produces(MediaType.SERVER_SENT_EVENTS)
public void eventStream(@Context SseEventSink eventSink, @Context Sse sse) {

    // starting a separate thread to handle the complete request within
    // the context of the method
    executor.execute(() -> {                     
    
        // try-with-resources: closing the resource also in the method
        try(SseEventSink sink = eventSink) {      
            eventSink.onNext(sse.newEvent("event1")); 
            // don't think the close (which is in the blog) is needed? 
        } catch (IOException e) { /* handle exception */ }
    });
}

  // my usual case
private static final Map<String, SseEventSink> openListeners = new HashMap<>(); // hmmm... cluster anybody?  
  
@GET @Path("monitor") @Produces(MediaType.SERVER_SENT_EVENTS)
public void startListening(@Context SseEventSink eventSink, @Context Sse sse) {
    String key = RandomUtils.randomString();
    
    // store the eventSink, no closing means the method emits a warning
    openListeners.put(key, eventSink);    
    
    // have some other process know where to find the eventSink
    otherProcess.registerCallback("callback/"+key);
    
    // no action here
}

// some other process has done something and wants the user to know, 
// it calls this method. Other than the callback that was registered
// the two calls are unrelated
@GET @Path("callback/{key}")
public void callback(@PathParam("key") String key, @Context Sse sse) {
    try {      
        SseEventSink eventSink = openListeners.get(key);
        eventSink.onNext(sse.newEvent("event1"));  
        // optionally 
        eventSink.close();
    } catch (IOException e) { /* handle exception */ }
}