Spring WebFlux detect client disconnect

Suppose the following @RestController:

@GetMapping("listen")
public Flux<Object> listen() {
    return Flux.create(sink -> process(sink));
}

And somewhere

sink.next(new Object());

This code has no information about sink state or completion

Tried using isCanceled, it returns false every time.

Is it possible to detect is FluxSink is still being used by the client?

In spring-webflux if the client close the connection the subscription will be canceled and disposed. If in the process method you add a callback onCancel and onDispose you will see that.

private <T> void process(final FluxSink<T> sink) {
    sink.onCancel(new Disposable() {
        @Override
        public void dispose() {
            System.out.println("Flux Canceled");
        }
    });

    sink.onDispose(new Disposable() {

        @Override
        public void dispose() {
            System.out.println("Flux dispose");
        }
    });
}

Then send an http request to your endpoint and cancel it before your flux complete. You will see that both callbacks are triggered.