Following #23205 I found a similar issue.

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: */*
...



HTTP/1.1 406
...

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

This is very similar case to a case with ResponseEntity - the server is responsible for selecting content-type application/xml (if it is acceptable by a client, and in this case - it is) 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 like a "Cound not find http message converter for totally acceptable representation" (application/xml is acceptable for a client, that requsted it with Accept: */*).

So it is strange to receive a 406 result for a request with Accept: */* header. The real problem in this case is a server misconfiguration: the server declares that it produces application/xml, but at the same time it didn't register any HttpMessageConverter that can return this content type. How can this be a 4xx then?