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Server-Sent Events

Juneau REST Server now includes SSE response helpers for event-at-a-time streaming endpoints.

For the SSE serializer, parser and event types themselves, see SSE.

RestResponse.sse() helper​

Use RestResponse.sse() to configure an SSE response and emit events without manually juggling writer state:

@RestGet(path="/stream", serializers=SseSerializer.class)
public void stream(RestResponse res) throws Exception {
    try (var sse = res.sse().heartbeat(Duration.ofSeconds(15))) {
        sse.sendEvent("tick", "one");
        sse.comment("ping");
        sse.sendEvent(new SseEvent("tick", "two"));
        sse.flush();
    }
}

The helper sets:

  • Content-Type: text/event-stream
  • Cache-Control: no-cache
  • X-Content-Type-Options: nosniff
  • Content-Encoding: identity

Broadcaster pattern​

Use SseBroadcaster and SseSubscription for fan-out from one producer to many subscribers.

@RestGet(path="/broadcast", serializers=SseSerializer.class)
public void broadcast(RestRequest req, RestResponse res, SseBroadcaster broadcaster) throws Exception {
    var id = req.getHttpServletRequest().getRequestId();
    var subscription = broadcaster.subscribe(id);
    try (var sse = res.sse().heartbeat(Duration.ofSeconds(15))) {
        sse.sendFrom(subscription);
    } finally {
        subscription.close();
    }
}

When a subscriber queue is full, the in-memory broadcaster drops the oldest event so recent events continue to flow.

See also​