Spring: Returning empty HTTP Responses with ResponseEntity<Void> doesn't work

NOTE: This is true for the version mentioned in the question, 4.1.1.RELEASE.

Spring MVC handles a ResponseEntity return value through HttpEntityMethodProcessor.

When the ResponseEntity value doesn't have a body set, as is the case in your snippet, HttpEntityMethodProcessor tries to determine a content type for the response body from the parameterization of the ResponseEntity return type in the signature of the @RequestMapping handler method.

So for

public ResponseEntity<Void> taxonomyPackageExists( @PathVariable final String key ) {

that type will be Void. HttpEntityMethodProcessor will then loop through all its registered HttpMessageConverter instances and find one that can write a body for a Void type. Depending on your configuration, it may or may not find any.

If it does find any, it still needs to make sure that the corresponding body will be written with a Content-Type that matches the type(s) provided in the request's Accept header, application/xml in your case.

If after all these checks, no such HttpMessageConverter exists, Spring MVC will decide that it cannot produce an acceptable response and therefore return a 406 Not Acceptable HTTP response.

With ResponseEntity<String>, Spring will use String as the response body and find StringHttpMessageConverter as a handler. And since StringHttpMessageHandler can produce content for any media type (provided in the Accept header), it will be able to handle the application/xml that your client is requesting.

Spring MVC has since been changed to only return 406 if the body in the ResponseEntity is NOT null. You won't see the behavior in the original question if you're using a more recent version of Spring MVC.

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In iddy85's solution, which seems to suggest ResponseEntity<?>, the type for the body will be inferred as Object. If you have the correct libraries in your classpath, ie. Jackson (version > 2.5.0) and its XML extension, Spring MVC will have access to MappingJackson2XmlHttpMessageConverter which it can use to produce application/xml for the type Object. Their solution only works under these conditions. Otherwise, it will fail for the same reason I've described above.

According Spring 4 MVC ResponseEntity.BodyBuilder and ResponseEntity Enhancements Example it could be written as:

....
   return ResponseEntity.ok().build();
....
   return ResponseEntity.noContent().build();

UPDATE:

If returned value is Optional there are convinient method, returned ok() or notFound():

return ResponseEntity.of(optional)

You can also not specify the type parameter which seems a bit cleaner and what Spring intended when looking at the docs:

@RequestMapping(method = RequestMethod.HEAD, value = Constants.KEY )
public ResponseEntity taxonomyPackageExists( @PathVariable final String key ){
    // ...
    return new ResponseEntity(HttpStatus.NO_CONTENT);
}