This works for me. Just change from Flux to Mono. Seem there is no converter for Flux<byte[]>

@GetMapping(value = "/attachment/{fileId}", produces = MediaType.APPLICATION_OCTET_STREAM_VALUE)
public Mono<byte[]> get(@PathVariable String fileId) {

    return this.webClient.get()
            .uri(builder ->
                    builder.scheme("https")
                            .host("external-service.com")
                            .path("/{fileId}")
                            .build(fileId)
            ).attributes(clientRegistrationId("credentials"))
            .accept(MediaType.APPLICATION_OCTET_STREAM)
            .retrieve()
            .bodyToMono(byte[].class);
}

As @MichaelBerry mentioned, you are missing the block call.

It is clearly mentioned in the method documentation that:

Note that the writing process does not start until the returned Flux is subscribed to.

Below code worked for me:

public class MySpringBootApp {

  public static void main(String[] args) throws IOException {

    Path path = Paths.get("src/main/resources/sample.zip");
    WebClient client = WebClient.builder()
        .baseUrl("https://www.learningcontainer.com/wp-content/uploads/2020/05/sample-zip-file.zip")
        .build();
    Flux<DataBuffer> dataBufferFlux = client.get().retrieve().bodyToFlux(DataBuffer.class);
    DataBufferUtils.write(dataBufferFlux, path, StandardOpenOption.CREATE).block(); //Creates new file or overwrites exisiting file

  }


}

You're right, zero-copy is supported for now when posting data from a File-based Resource.

So the following looks right:

client.post()
      .body(BodyInserters.fromResource(new FileSystemResource(new File("file.txt"))));

Now for the reading part, zero-copy is not supported on the reading side right now in Spring Framework; you could create an enhancement issue on jira.spring.io for that.

Your code sample should look like:

Flux<DataBuffer> incoming = client.post()
      .retrieve().bodyToFlux(DataBuffer.class);
Mono<Void> writeOperation = DataBufferUtils.write(incoming, channel)
      .map(DataBufferUtils::release)
      .then();
// subscribe to the returned value, which will complete when writing is done

Unfortunately, reading data to DataBuffer won't do zero-copy as data will be copied in memory. I don't think zero-copy is supported right know on the reading side, so this could be an enhancement request on https://jira.spring.io.

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