java - How to best get a byte array from a ClientResponse from Spring WebClient?

Question

I'm trying out the new WebClient from Spring 5 (5.0.0.RC2) in a codebase that uses reactive programming and I've had success mapping the JSON response from an endpoint to a DTO in my app, which works very nice:

WebClient client = WebClient.create(baseURI);
Mono<DTO> dto = client.get()
        .uri(uri)
        .accept(MediaType.APPLICATION_JSON)
        .exchange()
        .flatMap(response -> response.bodyToMono(DTO.class));

However, now I'm trying to the response body from an endpoint which uses Protocol Buffers (binary data served as application/octet-stream), so I'd like to get the raw bytes from the response, which I'll then map to an object myself.

I got it to work like this using Bytes from Google Guava:

Mono<byte[]> bytes = client.get()
        .uri(uri)
        .accept(MediaType.APPLICATION_OCTET_STREAM)
        .exchange()
        .flatMapMany(response -> response.body(BodyExtractors.toDataBuffers()))
        .map(dataBuffer -> {
            ByteBuffer byteBuffer = dataBuffer.asByteBuffer();
            byte[] byteArray = new byte[byteBuffer.remaining()];
            byteBuffer.get(byteArray, 0, bytes.length);
            return byteArray;
        })
        .reduce(Bytes::concat)

This works, but is there an easier, more elegant way to get these bytes?

Answer 1

ClientResponse.bodyToMono() in the end uses some org.springframework.core.codec.Decoder which claims to support the specified class.

So we should check the class hierarchy of the Decoder, in particular where and how the decodeToMono() method is implemented.

There is a StringDecoder which supports decoding to String, a bunch of Jackson-related decoders (used in your DTO example under the hood), and there is also a ResourceDecoder which is of particular interest.

ResourceDecoder supports org.springframework.core.io.InputStreamResource and org.springframework.core.io.ByteArrayResource. ByteArrayResource is essentially a wrapper around byte[], so the following code will provide an access to the response body as a byte array:

Mono<byte[]> mono = client.get()
            ...
            .exchange()
            .flatMap(response -> response.bodyToMono(ByteArrayResource.class))
            .map(ByteArrayResource::getByteArray);