How to convert a Flux<DataBuffer> into a StreamingResponseBody in a Spring Controller

I have a Flux of DataBuffer in my Spring Controller and I want to attach it to the StreamingResponseBody as a stream. I have read many answers for similar approaches, but nothing quite matches this. I do not want to load the entire file in memory. I want it streamed.

@GetMapping(value="/attachment")
public ResponseEntity<StreamingResponseBody> get(@PathVariable long attachmentId) {

    Flux<DataBuffer> dataBuffer = this.myService.getFile(attachmentId);

    StreamingResponseBody stream = out -> {
       // what do do here?
    }

    return ResponseEntity.ok()
        .contentType(MediaType.APPLICATION_OCTET_STREAM)
        .body(stream);
    }
}

EDIT: myService.getFile()

public Flux<DataBuffer> gteFile(long attachmentId) {

    return this.webClient.get()
        .uri("https://{host}/attachments/{attachmentId}", host, attachmentId)
        .attributes(clientRegistrationId("attachmentClient"))
        .accept(MediaType.ALL)
        .exchangeToFlux(clientResponse -> clientResponse.bodyToFlux(DataBuffer.class));
}

If you're using WebFlux there's no need to return StreamingResponseBody, you can just return Flux<DataBuffer> directly, so it will be streaming and also non-blocking.

If you want to add some headers/customize your response, the you can return Mono<ResponseEntity<Flux<DataBuffer>>> :

<!-- language: java -->
@GetMapping("/attachment/{attachmentId}")
public Mono<ResponseEntity<Flux<DataBuffer>>> get(@PathVariable long attachmentId) {

    Flux<DataBuffer> dataBuffers = this.myService.getFile(attachmentId);
    
    ContentDisposition contentDisposition = ContentDisposition.inline()
            .filename("example.txt")
            .build();

    HttpHeaders headers = new HttpHeaders();
    headers.setContentDisposition(contentDisposition);

    return Mono.just(ResponseEntity.ok()
            .headers(headers)
            .contentType(MediaType.APPLICATION_OCTET_STREAM)
            .body(dataBuffers));
}

If you want to use StreamingResponseBody (which means you're using blocking Spring MVC on web-layer), then you can do the following:

<!-- language: java -->

StreamingResponseBody stream = outputStream ->
        Mono.create(sink -> DataBufferUtils.write(dataBuffers, outputStream)
                .subscribe(DataBufferUtils::release, sink::error, sink::success)
        ).block();

and then return it from your controller