Spring Webclient: How to Stream Large Byte[] to File

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

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.

Stream upload 'POST' in Spring WebClient

Your first try was almost correct, however you need to use body(...) instead of bodyValue(...):

body(BodyInserters.fromResource(new InputStreamResource(inputStream)))

This is because bodyValue(...) wraps your resource inserter in a value inserter, which will then try to serialize the resource inserter itself and fail with the error you received.

Post file using webclient webflux to API accepting APPLICATION_OCTET_STREAM

This is how i was able to upload file

Step 1 : Read file as bytes array

bytes = new FileSystemResource((filePath)).getInputStream()
                    .readAllBytes();

Step 2 : pass the byte array to bodyValue method

webClient.post()
         .uri("Some-uri)
         .contentType(MediaType.APPLICATION_OCTET_STREAM)
         .bodyValue(bytes)
         .retrieve()
         .toBodilessEntity()) 

Spring WebClient BodyInserters.fromResource() changes content-type?

It happens because of the specific handling of the application/octet-stream in ResourceHttpMessageWriter. It tries to detect mime type by file extension.

You could use InputStreamResource instead to keep application/octet-stream

var resource = new InputStreamResource(new FileInputStream(localFilename));

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