How to stream text data with JAX-RS continuously - jax-rs

Question

I read multiple questions that are similar to mine and found this: https://stackoverflow.com/a/34358215/12550134
But I am not able to do this. I use plain JAX-RS API and Open Liberty as my server. Unfortunately the ResourceConfig cannot be found, so I cannot disable the buffer, as described in the answer above.
This is my code:
#GET
#Produces(MediaType.TEXT_PLAIN)
public Response sayHelloStream() {
LOGGER.debug("calling sayHelloStream");
StreamingOutput out = outputStream -> {
Writer writer = new BufferedWriter(new OutputStreamWriter(outputStream));
for (int i = 0; i < 999999999; i++) {
writer.write("Hello\n");
writer.flush();
try {
LOGGER.debug("before sleep");
TimeUnit.SECONDS.sleep(3);
LOGGER.debug("after sleep");
} catch (InterruptedException e) {
LOGGER.error("error with the timer", e);
}
}
};
return Response.ok(out).build();
}
When calling it in the browser nothing happens. To my understanding due to the buffer. How am I able to stream text data like this using plain JAX-RS?

Answer 1

I would use the SSE extension. AFAIK it's part of the JAX-RS API, allthough you might need an extra module to enable it server-side:
https://eclipse-ee4j.github.io/jersey.github.io/documentation/latest/sse.html
...
import javax.ws.rs.sse.Sse;
import javax.ws.rs.sse.SseEventSink;
import javax.ws.rs.sse.OutboundSseEvent;
...
#Path("events")
public static class SseResource {
#GET
#Produces(MediaType.SERVER_SENT_EVENTS)
public void getServerSentEvents(#Context SseEventSink eventSink, #Context Sse sse) {
new Thread(() -> {
for (int i = 0; i < 10; i++) {
// ... code that waits 1 second
final OutboundSseEvent event = sse.newEventBuilder()
.name("message-to-client")
.data(String.class, "Hello world " + i + "!")
.build();
eventSink.send(event);
}
}).start();
}
}
It streams text data to the client in chunks in the SSE format, so it can easily be handled in the browser using e.g. the HTML5 <eventsource> element or the EventSource JavaScript API.
var source = new EventSource('.../events');
source.addEventListener('message-to-client', function(e) {
console.log(e.data);
}, false);