Background
The problem came up on my day job while implementing a POST multipart/form-data endpoint for a file upload with some meta information. I am not an expert in the Spring Boot ecosystem; it is likely that the problem is solved by a simple fix and that I am just missing the right term to search for.
Problem statement
To implement an endpoint for a file-upload with additional meta information, I wrote the following @RestController:
@RestController
@RequestMapping(Resource.ROOT)
@AllArgsConstructor(onConstructor = @__({@Inject}))
public class Resource {
public static final String ROOT = "/test";
private final Logger logger;
@PostMapping(consumes = MediaType.MULTIPART_FORM_DATA, produces = MediaType.APPLICATION_JSON)
public ResponseEntity<Void> test(@Valid final Request request) {
logger.info("request = {}", request);
return ResponseEntity.ok().build();
}
}
With Request being specified as:
@Value
@AllArgsConstructor
public class Request {
@NotNull
String name;
@NotNull
MultipartFile file;
}
And a small happy path test:
@SpringBootTest
@AutoConfigureMockMvc
class TestCase {
@Autowired
private MockMvc mockMvc;
@Test
void shouldReturnOk() throws Exception {
// GIVEN
final byte[] content = Files.readAllBytes(Path.of(".", "src/test/resources/PenPen.png"));
final String name = "name";
// WHEN
// @formatter:off
mockMvc.perform(MockMvcRequestBuilders
.multipart(Resource.ROOT)
.file("file", content)
.param("name", name))
// THEN
.andExpect(status().isOk());
// @formatter:on
}
}
A complete MRE can be found on Bitbucket, branch problem-with-immutable-request.
When running the test (./mvnw test), it fails with the endpoint returning a 400 BAD REQUEST instead of 200 OK. Reading the logs reveals that request parameter file is null:
...
Content type = text/plain;charset=UTF-8
Body = file: must not be null.
...
I partially understand why it is null. With this partial knowledge, I was able to circumvent the problem by making the field file in Request mutable:
@ToString
@Getter
@AllArgsConstructor
public class Request {
@NotNull
private final String name;
@Setter
@NotNull
private MultipartFile file;
}
The code "fixing" the problem can be found on Bitbucket, branch problem-solved-by-making-field-mutable.
This, however, makes the Request mutable, which I would like to prevent. To further investigate, I unrolled the lombok annotations on Request and added some logging:
public class Request {
private static final Logger LOGGER = LoggerFactory.getLogger(Request.class);
@NotNull
private final String name;
@NotNull
private MultipartFile file;
public Request(final String name, final MultipartFile file) {
this.name = name;
this.setFile(file);
}
public @NotNull String getName() {
return this.name;
}
public @NotNull MultipartFile getFile() {
return this.file;
}
public String toString() {
return "Request(name=" + this.getName() + ", file=" + this.getFile() + ")";
}
public void setFile(final MultipartFile file) {
LOGGER.info("file = {}", file);
this.file = file;
}
}
Code of unrolled version can be found on Bitbucket, branch lombok-unrolled-for-debugging.
When looking at the log statements of the now successful test, we can see that Request::setFile is called twice:
2020-09-05 09:42:31.049 INFO 11012 --- [ main] d.turing85.springboot.multipart.Request : file = null
2020-09-05 09:42:31.056 INFO 11012 --- [ main] d.turing85.springboot.multipart.Request : file = org.springframework.mock.web
The first call comes from the constructor invocation. The second call, I imagine, comes from somewhere within Spring's mapping mechanism for the form parameters.
I know that there is the possibility to define the form parameters individually on the endpoint and constructing the Request instance within the method:
public class Resource {
...
@PostMapping(consumes = MediaType.MULTIPART_FORM_DATA, produces = MediaType.APPLICATION_JSON)
public ResponseEntity<Void> test(
@RequestPart(name = "name") final String name,
@RequestPart(name = "file") final MultipartFile file) {
final Request request = new Request(name, file);
logger.info("request = {}", request);
return ResponseEntity.ok().build();
}
}
This will, however, result in other problems. For example, we would have to add an additional exception mapper for MissingServletRequestPartException and align the returned HTTP response with the existing response for BindException. I would like to avoid this if possible.
A search on the topic turned up Spring Boot controller - Upload Multipart and JSON to DTO. The solution, however, did not work for me since I do not use MVC (I think).
Question
Is there a possibility to keep Request immutable such that Spring is able to pass the MultipartFile to the all args constructor instead of setting it through the setter afterwards? Writing a custom mapper/converter is acceptable, but I did not find a possibility to write a mapper for either a specific endpoint or a specific type.
