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

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);
                }
            }
    );
}

With recent stable Spring WebFlux (5.2.4.RELEASE as of writing):

final WebClient client = WebClient.create("https://example.com");
final Flux<DataBuffer> dataBufferFlux = client.get()
        .accept(MediaType.TEXT_HTML)
        .retrieve()
        .bodyToFlux(DataBuffer.class); // the magic happens here

final Path path = FileSystems.getDefault().getPath("target/example.html");
DataBufferUtils
        .write(dataBufferFlux, path, CREATE_NEW)
        .block(); // only block here if the rest of your code is synchronous

For me the non-obvious part was the bodyToFlux(DataBuffer.class), as it is currently mentioned within a generic section about streaming of Spring's documentation, there is no direct reference to it in the WebClient section.

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();
}