Spring Cannot test @RequestPart multipart controllers with Servlet MockPart

Tested with spring boot 2.3.4. Example project

We have controller that receives some arguments that are parts of multipart request.

@RestController
class HelloWorldController {
    @RequestMapping(path = "helloworld", method = RequestMethod.POST)
    @ResponseStatus(code = HttpStatus.OK)
    public String helloWorld(@RequestPart(name = "part0") String str0,
                             @RequestPart(name = "part1") String str1) {
        return str0+str1;
    }
}

For running application request works as intended:

POST http://localhost:8080/helloworld
Content-Type: multipart/form-data; boundary=6512c604-1edc-4c15-8916-dc2e91413c82

--6512c604-1edc-4c15-8916-dc2e91413c82
Content-Disposition: form-data;name="part0"
Content-Type: text/plain

Hello
--6512c604-1edc-4c15-8916-dc2e91413c82
Content-Disposition: form-data;name="part1"
Content-Type: text/plain

World
--6512c604-1edc-4c15-8916-dc2e91413c82--

## response HelloWorld

Now we want to test it:

@Test
void helloWorldTest() throws Exception {
        MockPart part0 = new MockPart("part0", "Hello".getBytes());
        part0.getHeaders().setContentType(MediaType.TEXT_PLAIN);
        MockPart part1 = new MockPart("part1", "World".getBytes());
        part1.getHeaders().setContentType(MediaType.TEXT_PLAIN);

        web.perform(multipart("/helloworld")
                .part(part0, part1))
                .andExpect(status().isOk())
                .andExpect(content().string("HelloWorld"));
}

But test is failing with NPE exception:

Request processing failed; nested exception is java.lang.NullPointerException
org.springframework.web.util.NestedServletException: Request processing failed; nested exception is java.lang.NullPointerException
    at javax.servlet.http.HttpServlet.service(HttpServlet.java:652)
Caused by: java.lang.NullPointerException
    at org.springframework.web.multipart.support.RequestPartServletServerHttpRequest.getBody(RequestPartServletServerHttpRequest.java:99)

The reason behind that is org.springframework.web.multipart.support.RequestPartServletServerHttpRequest:84; it's expecting StandardMultipartHttpServletRequest in multipartRequest field but got MockMultipartHttpServletRequest:

public InputStream getBody() throws IOException {
    if (this.multipartRequest instanceof StandardMultipartHttpServletRequest) {
        try {
            return this.multipartRequest.getPart(this.partName).getInputStream();
        }
        catch (Exception ex) {
            throw new MultipartException("Could not parse multipart servlet request", ex);
        }
    }
    <...>
}

Comment From: philwebb

RequestPartServletServerHttpRequest and MockMultipartHttpServletRequest are part of Spring Framework. I'll transfer your issue for that team to consider. Perhaps that instanceof check should be for a org.springframework.web.multipart.MultipartHttpServletRequest?

Comment From: JWThewes

Wouldn't this be ideal for a first contribution? I would be absolutely willing to fix this.

Comment From: JWThewes

I created a PR for this. This is one idea to solve the problem. Existing tests work and the mentioned problem is fixed (validated via test). I would love to get feedback for further PRs.

Comment From: rstoyanchev

I wonder if this isn't a duplicate of #25602?

Comment From: rstoyanchev

I confirm the example project works against master but fails with 5.2.x.

Comment From: dimone-kun

I wonder if this isn't a duplicate of #25602?

It seems that it is. Sorry didn't find that issue.

Comment From: rstoyanchev

No worries, it's not very obvious and at the time I fixed it for the upcoming 5.3 only but this does show it would be beneficial to have a fix in 5.2.x as well.