Spring Error 415 when handle a multipart form request with RequestEntity<MultiValueMap<String, Object>>

Affects: 5.2.0 I'm try to handle a multipart request with follow code:

// A RestController
@RequestMapping(path = "form", consumes = {MediaType.MULTIPART_FORM_DATA_VALUE, MediaType.APPLICATION_FORM_URLENCODED_VALUE})
public void testFrom(RequestEntity<MultiValueMap<String, Object>> requestEntity) {
    log.info(requestEntity.toString());
}

then tomcat return a 415 code with console output:

2020-11-25 14:42:16,244 WARN  (AbstractHandlerExceptionResolver.java:199) - Resolved [org.springframework.web.HttpMediaTypeNotSupportedException: Content type 'application/x-www-form-urlencoded;charset=UTF-8' not supported]

if i use code like this, everything works well:

@RequestMapping(path = "form", consumes = {MediaType.MULTIPART_FORM_DATA_VALUE, MediaType.APPLICATION_FORM_URLENCODED_VALUE})
public void testFrom(@RequestBody MultiValueMap<String, Object> requestEntity) {
    log.info(requestEntity.toString());
}

After some debug of spring's code, I found in org.springframework.web.servlet.mvc.method.annotation.HttpEntityMethodProcessor#resolveArgument, calling getHttpEntityType returns a ParameterizedType. But when calling org.springframework.web.servlet.mvc.method.annotation.AbstractMessageConverterMethodArgumentResolver#readWithMessageConverters(org.springframework.http.HttpInputMessage, org.springframework.core.MethodParameter, java.lang.reflect.Type) to convert request body, following code make the targetClass become RequestEntity instead of MultiValueMap, cause the fail of message convertion.

Class<T> targetClass = (targetType instanceof Class ? (Class<T>) targetType : null);
if (targetClass == null) {
    ResolvableType resolvableType = ResolvableType.forMethodParameter(parameter);
    targetClass = (Class<T>) resolvableType.resolve();
}

Is it a bug?

Comment From: poutsma

The cause of this behavior is the use of RestController, which means that all parameters use @ResponseBody semantics. The method signature would effectively be:

public void testFrom(@RequestBody RequestEntity<MultiValueMap<String, Object>> requestEntity)

Because the form converter can only convert to multi-value maps—and not request entities—the parameter resolution fails.

To use HttpEntity types in controller method signatures—either as parameters or returns types, do not annotate the class with @RestController, but with @Controller instead.