MultipartBodyBuilder requires parts to be named

[jakub-bochenski]

Affects: 5.13.15

   public PartBuilder part(String name, Object part, @Nullable MediaType contentType) {
   	Assert.hasLength(name, "'name' must not be empty");

This is wrong and makes no sense for contents like multipart/mixed
Field names are only relevant for multipart/form-data

c.f. https://www.w3.org/Protocols/rfc1341/7_2_Multipart.html

[poutsma]

This is wrong and makes no sense for contents like multipart/mixed

Opening with a snarky comment like that does not help in getting attention drawn to your issue. If anything, the reverse is true.

MultipartBodyBuilder was not designed to be used for multipart/mixed, but for multipart/form-data where each part gets a Content-Disposition header with the part name. Given this context, I would argue that the method signature is correct and makes a lot of sense.

I see that the Javadoc of MultipartBodyBuilder can be improved to make this limitation clearer, and I will do so.

[jakub-bochenski]

Oh, nice, so WebClient now officially doesn't support multipart/mixed

[rstoyanchev]

There is some additional context on this.

Our multipart support has evolved historically from multipart form data requests, so it is rooted in that. We added "multipart/mixed" and "multipart/related" as supported media types to the codecs and message converters with #23159 and #23209, but in retrospect that was inadequate.

In any case, this would be a deeper change, most likely not involving MultipartBodyBuilder at all, since that prepares a MultiValueMap, and hence assumes parts have names.

It would help if you provide a bit more context around your case. Do you need both client and server support, or do you make requests to a 3rd party API? Considering the Servlet API also assumes parts have names, there can be more challenges.

[jakub-bochenski]

I'm using a 3rd party API that is receiving and responding with multipart/mixed. So I'm interested just in client support.

I'm able to send multipart/mixed using MultiValueMap just by setting all the names to empty string.
But MultiValueMap doesn't support "reactive" elements the way MultipartBodyBuilder does.

As for reading the response it seems RestTemplate can only write multipart/mixed, not read it.
For WebClient the best I could come up with is

.retrieve()
                .toEntityFlux((inputMessage, context) ->
                        partReader
                                .read(ResolvableType.forType(byte[].class), inputMessage, context))

but it's inconvenient to the have to deal with raw bytes from DataBuffer for each part.

Also I don't see how would it handle reading multipart nested in multipart --it's a real-world scenario

[poutsma]

I have created #30230 to see how much interest there is in the community for Spring to have proper multipart/mixed support.