Server Sent Events in Spring - why executor service is used there?

I wonder why SseEmitter events are sent inside the executor in most tutorials.

        SseEmitter emitter = new SseEmitter();
        ExecutorService sseMvcExecutor = Executors.newSingleThreadExecutor();

        sseMvcExecutor.execute(() -> { ...

What would happen if they would be run without it?

When you send events inside an executor, the event processing occurs in a separate thread.

Without an Executor, the events are still sent asynchronously because SseEmitter itself doesn’t block the main thread when you call send(). But, if your events require processing time or come from an external source (e.g., a message queue or a database fetch), handling everything directly in the main thread will make that thread wait, especially for slow or I/O-bound tasks. For example -

SseEmitter emitter = new SseEmitter();
try {
    emitter.send("Hello, World!");
    // Some processing logic --> This will block your main thread !
    emitter.send("More data!");
} catch (Exception e) {
    emitter.completeWithError(e);
}
return emitter;

using an Executor provides more flexibility and isolation for event generation.