Convert writes to OutputStream into a Flux<DataBuffer> usable by ServerResponse

问题

I have a legacy library that I have to use to retrieve a file. This legacy library doesn't return in InputStream, as you usually expect for reading stuff, but it expects that it is passed an open OutputStream, that it can write to.

I have to write a Webflux REST service, that writes this OutputStream to the org.springframework.web.reactive.function.server.ServerResponse body.

legacyLib.BlobRead(outputStream); // writes the stream to an outputstream, that has to be provided by me, and somehow has to end up in the ServerResponse

Since I want to pass along the Stream directly to the ServerResponse, I probably have to do something like this, right?

ServerResponse.ok().body(magicOutpuStreamToFluxConverter(), DataBuffer.class);

回答1:

Here a part of the RequestHandler that's important; I left out some errorhandling/catching of exceptions, that might generally not be needed. Note that i publishedOn a different Scheduler for the read (or at least, that's what I wanted to do), so that this blocking read doesn't interfere with my main event thread:

private Mono<ServerResponse> writeToServerResponse(@NotNull FPTag tag) {
    final long blobSize = tag.getBlobSize();
    return ServerResponse.ok()
        .contentType(MediaType.APPLICATION_OCTET_STREAM)
        .body(Flux.<DataBuffer>create((FluxSink<DataBuffer> emitter) -> {
      // for a really big blob I want to read it in chunks, so that my server doesn't use too much memory
      for(int i = 0; i < blobSize; i+= tagChunkSize) {
        // new DataBuffer that is written to, then emitted later
        DefaultDataBuffer dataBuffer = new DefaultDataBufferFactory().allocateBuffer();
        try (OutputStream outputStream = dataBuffer.asOutputStream()) {
          // write to the outputstream of DataBuffer
          tag.BlobReadPartial(outputStream, i, tagChunkSize, FPLibraryConstants.FP_OPTION_DEFAULT_OPTIONS);
          // don't know if flushing is strictly neccessary
          outputStream.flush();
        } catch (IOException | FPLibraryException e) {
          log.error("Error reading + writing from tag to http outputstream", e);
          emitter.error(e);
        }
        emitter.next(dataBuffer);
      }
      // if blob is finished, send "complete" to my flux of DataBuffers
      emitter.complete();
    }, FluxSink.OverflowStrategy.BUFFER).publishOn(Schedulers.newElastic("centera")).doOnComplete(() -> closeQuietly(tag)), DataBuffer.class);

}

来源：https://stackoverflow.com/questions/49426304/convert-writes-to-outputstream-into-a-fluxdatabuffer-usable-by-serverresponse