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

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 and 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()
            .flatMap(response -> response.bodyToMono(ByteArrayResource.class))

Oleg Estekhin's answer gives the OP what he asked for, but it's loading the entire response content in the memory, which is an issue for large responses. To get a chunk of bytes at a time instead, we can do the following:

  .flatMapMany { it.body(BodyExtractors.toDataBuffers()) }

The size of these buffers would be 8192 kb by default; see this answer for changing that if needed.

Note that attempting to do dataBuffer.asByteBuffer().array() thrwos an exception if the ByteBuffer is not backed by an array.