Spring WebClient: How to stream large byte[] to file?

I cannot test whether or not the following code effectively does not buffer the contents of webClient payload in memory. Nevertheless, i think you should start from there:

public Mono<Void> testWebClientStreaming() throws IOException { Flux<DataBuffer> stream = webClient .get().accept(MediaType.APPLICATION_OCTET_STREAM) .retrieve() .bodyToFlux(DataBuffer.class); Path filePath = Paths.get("filename"); AsynchronousFileChannel asynchronousFileChannel = AsynchronousFileChannel.open(filePath, WRITE); return DataBufferUtils.write(stream, asynchronousFileChannel) .doOnNext(DataBufferUtils.releaseConsumer()) .doAfterTerminate(() -> { try { asynchronousFileChannel.close(); } catch (IOException ignored) { } }).then(); } 

Store the body to a temporary file and consume

static <R> Mono<R> writeBodyToTempFileAndApply( final WebClient.ResponseSpec spec, final Function<? super Path, ? extends R> function) { return using( () -> createTempFile(null, null), t -> write(spec.bodyToFlux(DataBuffer.class), t) .thenReturn(function.apply(t)), t -> { try { deleteIfExists(t); } catch (final IOException ioe) { throw new RuntimeException(ioe); } } ); } 

Pipe the body and consume

static <R> Mono<R> pipeBodyAndApply( final WebClient.ResponseSpec spec, final ExecutorService executor, final Function<? super ReadableByteChannel, ? extends R> function) { return using( Pipe::open, p -> { final Future<Disposable> future = executor.submit( () -> write(spec.bodyToFlux(DataBuffer.class), p.sink()) .log() .doFinally(s -> { try { p.sink().close(); log.debug("p.sink closed"); } catch (final IOException ioe) { throw new RuntimeException(ioe); } }) .subscribe(DataBufferUtils.releaseConsumer()) ); return just(function.apply(p.source())) .log() .doFinally(s -> { try { final Disposable disposable = future.get(); assert disposable.isDisposed(); } catch (InterruptedException | ExecutionException e) { e.printStackTrace(); } }); }, p -> { try { p.source().close(); log.debug("p.source closed"); } catch (final IOException ioe) { throw new RuntimeException(ioe); } } ); } 

I'm not sure if you have access to RestTemplate in your current usage of spring, but this one have worked for me.

 RestTemplate restTemplate // = ...; RequestCallback requestCallback = request -> request.getHeaders() .setAccept(Arrays.asList(MediaType.APPLICATION_OCTET_STREAM, MediaType.ALL)); // Streams the response ResponseExtractor<Void> responseExtractor = response -> { // Here I write the response to a file but do what you like Path path = Paths.get("http://some/path"); Files.copy(response.getBody(), path); return null; }; restTemplate.execute(URI.create("www.something.com"), HttpMethod.GET, requestCallback, responseExtractor);