Support multipart/related and multipart/mixed MediaTypes in RestTemplate

[sethcleveland]

Affects: <v5.2.0.M3>

At the moment, the RestTemplate posts multipart with Content-Type: multipart/form-data.

We have a requirement to post multipart data with a Content-Type of multipart/mixed or multipart/related.

C# provides flexibility for this with a MultipartContent with a subtype parameter.

https://docs.microsoft.com/en-us/dotnet/api/system.net.http.multipartcontent.-ctor?view=netframework-4.8#System_Net_Http_MultipartContent__ctor_System_String_

Any recommendations?

[sbrannen]

Any recommendations?

What happens if you configure explicit support for those content types as follows?

restTemplate.getMessageConverters().stream()
	.filter(FormHttpMessageConverter.class::isInstance)
	.map(FormHttpMessageConverter.class::cast)
	.findFirst()
	.ifPresent(converter -> {
		List<MediaType> supportedMediaTypes = new ArrayList<>(converter.getSupportedMediaTypes());
		supportedMediaTypes.add(new MediaType("multipart", "mixed"));
		supportedMediaTypes.add(new MediaType("multipart", "related"));
		converter.setSupportedMediaTypes(supportedMediaTypes);
	});

[sethcleveland]

That was our first inclination; however, it didn't work because of another check in the FormHttpMessageConverter, specifically this one:

spring-framework/spring-web/src/main/java/org/springframework/http/converter/FormHttpMessageConverter.java

Lines 279 to 291 in da582da

	private boolean isMultipart(MultiValueMap<String, ?> map, @Nullable MediaType contentType) {
	if (contentType != null) {
	return MediaType.MULTIPART_FORM_DATA.includes(contentType);
	}
	for (List<?> values : map.values()) {
	for (Object value : values) {
	if (value != null && !(value instanceof String)) {
	return true;
	}
	}
	}
	return false;
	}

The isMultipart method is coded to only check for multipart/form-data. For the proposed approach to work for us, it needs to be coded such that multipart/* is supported.

[sbrannen]

That was our first inclination; however, it didn't work because of another check in the FormHttpMessageConverter

Thanks for the feedback.

The isMultipart method is coded to only check for multipart/form-data. For the proposed approach to work for us, it needs to be coded such that multipart/* is supported.

@rstoyanchev, what do you think about making changes along those lines?

Also, what do you think about introducing MULTIPART_MIXED and MULTIPART_RELATED constants in MediaType?