Spring "No converter ..." for wildcard ResponseEntity with StreamingResponseBody as body

Affects: 2.2.4

That controller works perfectly:

@GetMapping("/test")
public ResponseEntity<StreamingResponseBody> test() {
    var response = ResponseEntity.ok().contentType(MediaType.APPLICATION_OCTET_STREAM)
            .header(HttpHeaders.CONTENT_DISPOSITION, "attachment;filename=\"test.txt\"")
            .body(os -> os.write("simple streaming test".getBytes(StandardCharsets.UTF_8)));
    return response;
}

But that controller

@GetMapping("/test")
public ResponseEntity<?> test() {
    var response = ResponseEntity.ok().contentType(MediaType.APPLICATION_OCTET_STREAM)
            .header(HttpHeaders.CONTENT_DISPOSITION, "attachment;filename=\"test.txt\"")
            .body((StreamingResponseBody)os -> os.write("simple streaming test".getBytes(StandardCharsets.UTF_8)));
    return response;
}

produces an exception in response:

{
    "timestamp": "2020-10-29T17:46:55.769+0000",
    "status": 500,
    "error": "Internal Server Error",
    "message": "No converter for [class local.pryadko.streamtest.TestControlle$$Lambda$1667/0x0000000840c3c040] with preset Content-Type 'application/octet-stream'",
    "path": "/api/test"
}

Looks like Spring can't correctly find out type :neutral_face:

Comment From: poutsma

Because in the second controller, the response entity returned by the method is not bound, so Spring cannot determine that it's a StreamingResponseBody you're returning.

Comment From: eroux

I'm getting the exact same error, @poutsma can you please elaborate on your answer? it doesn't seem quite straightforward to me...

Comment From: MikePryadko

@eroux, thru time I've changed my mind: this is not “real” problem because using raw generics (<?>) is not a good practice.

Comment From: eroux

well, I've changed my code too... but if using generics is not properly supported by Spring... why allow it at all?

Comment From: rstoyanchev

@eroux generally speaking, given ResponseEntity<?> we will take a look at the actual value to determine how to handle it. The problem here is that the response body is a lambda and it is not possible to determine its type. Either the ResponseEntity has to be declared with the target type or the response body has to be instantiated as a class.