AbstractMessageConverterMethodProcessor results in 406 even when ResponseEntity presets content type

[xak2000]

The fixes introduced in #20720 and #20798 added ability to return ResponseEntity with defined ContentType header. Like this:

    @GetMapping("/test")
    public ResponseEntity<MyModel> test() {
        return ResponseEntity.ok().contentType(MediaType.APPLICATION_XML).body(new MyModel());
    }

AbstractMessageConverterMethodProcessor now takes into account provided Content-Type header and uses it instead of Accept header from request. So it just ignores Accept header. It is ok and good.

But in this case, when no HttpMessageConverter found that can write the response with selected Content-Type, then AbstractMessageConverterMethodProcessor throws HttpMediaTypeNotAcceptableException.

		if (body != null) {
			throw new HttpMediaTypeNotAcceptableException(this.allSupportedMediaTypes);
		}

I think this is not right, because it leads to 406 response, like if no acceptable representation was found, while the real reason of this error is that no converter found for selected representation (and Accept header was ignored altogether). So, this clearly indicates a server misconfiguration (server tries to return a response using provided content type without registering proper HttpMessageConverter, that supports this content type with this response body type). So it should lead to error 500 and should throw HttpMessageNotWritableException.

It should do this only when selectedMediaType is selected based on outputMessage.getHeaders().getContentType() (the Accept header was not taken into account), of course. In other cases HttpMediaTypeNotAcceptableException is still good.

Affected version:

Spring 5.1.8.RELEASE

[rstoyanchev]

Fair point, in this case it's the server application that chose the content type of the response, and that choice could not be matched to a compatible converter.

[xak2000]

Thanks, @rstoyanchev !

Just want to ask one question: isn't HttpMessageNotWritableException is more appropriate in this case than IllegalStateException?

In similar case several lines upper the writeWithMessageConverters method throws it:

if (body != null && producibleTypes.isEmpty()) {
    throw new HttpMessageNotWritableException(
            "No converter found for return value of type: " + valueType);
}

This case is similar, except in this case no converters for return type found at all, while in new case no converters for [return type and required content-type] found. But, because the server is responsible for selecting this content-type, then I think HttpMessageNotWritableException is still applies here. In both cases it is illegal configuration of server. And both cases mean: the response can't be written by provided HttpMessageConverters - add more converers or change return type/conten-type.

Also, writeWithMessageConverters method declares HttpMessageNotWritableException as possible result. Unfortunately without mention it in javadoc (what causes this exception?), while two other declared exceptions IOException and HttpMediaTypeNotAcceptableException are present in javadoc, describing cases when they are thrown.

I think, it is worth to add HttpMessageNotWritableException to javadoc of this method with quick description of what causes it to throw. And if you really think that IllegalStateException is more appropriate than HttpMessageNotWritableException in case of preset Content-Type, then maybe it is worth to add it to javadoc (and method declaration) too?

---

P.S. Actually, while I writing this comment, I found another incorrect behavior. If you declare a controller like this:

@GetMapping(path = "/xml", produces = "application/xml")
public Map<String, Object> producesContentTypeThatIsNotSupportedByAnyConverter() {
	return ImmutableMap.of("foo", "bar");
}

And make a request like this (httpie):

http -v :9080/xml

Then the result will be:

GET /xml HTTP/1.1
Accept: */*
Accept-Encoding: gzip, deflate
Connection: keep-alive
Host: localhost:9080
User-Agent: HTTPie/0.9.9



HTTP/1.1 406
...

AbstractMessageConverterMethodProcessor#writeWithMessageConverters method will throw org.springframework.web.HttpMediaTypeNotAcceptableException: Could not find acceptable representation.

This is exactly the same case as with ResponseEntity - the server is responsible for selecting content-type application/xml and no converters found that can write the result using this content-type.

The Accept header was taken into account but it was Accept: */* so it is obviously not "Could not find acceptable representation" case, but more a "Cound not find http message converter for totally acceptable representation" (application/xml is acceptable for client, that requsted it with Accept: */*).

[rstoyanchev]

I've made a change to raise HttpMesageNotWritableException and updated the Javadoc. As for the issue with produces that one is potentially more nuanced. Can you open a separate ticket for consideration?