only one MultipartFile object populated when using an java.util.Optional MutipartFile array or list @RequestParam [SPR-15919]

[spring-projects-issues]

Brice Roncace opened SPR-15919 and commented

Having the multipart/form-data form:

<form action="<c:url value="/optionalPost"/>" method="post" enctype="multipart/form-data">
  <input type="hidden" name="${_csrf.parameterName}" value="${_csrf.token}"/>
  <input type="file" name="files" multiple/>
  <input type="submit"/>
</form>

When selecting multiple files for the file input, only one file is actually received by the controller when handling this request:

 // Does NOT work for multiple files (only one comes in)
@PostMapping("/optionalPost")
public String postTest2(@RequestParam Optional<List<MultipartFile>> files) {
  if (files != null && files.isPresent()) {
    System.out.println(files.get().size());
    files.get().forEach(file -> System.out.println(file.getOriginalFilename()));
  }
  return "index";
}

This fails in the same way:

// Does NOT work for multiple files (only one comes in)
@PostMapping("/optionalPost")
public String postTest3(@RequestParam Optional<MultipartFile[]> files) {
  if (files != null && files.isPresent()) {
    System.out.println(files.get().length);
    Stream.of(files.get()).forEach(file -> System.out.println(file.getOriginalFilename()));
  }
  return "index";
}

The workaround is to avoid the use of Optional:

// Works for multiple files
@PostMapping("/optionalPost")
public String postTest4(@RequestParam MultipartFile[] files) {
  System.out.println(files.length);
  Stream.of(files).forEach(file -> System.out.println(file.getOriginalFilename()));
  return "index";
}

// Works for multiple files
@PostMapping("/optionalPost")
public String postTest5(@RequestParam List<MultipartFile> files) {
  System.out.println(files.size());
  files.forEach(file -> System.out.println(file.getOriginalFilename()));
  return "index";
}

---

Affects: 4.3.10

Issue Links:

• Request params Optional<List<String> and List<String> are inconsistent [SPR-15676] #20235 Request params Optional<List and List are inconsistent
• java.util.Optional MultipartFile[] @RequestParam argument is null in multipart/form-data POST [SPR-15918] #20472 java.util.Optional MultipartFile[] @RequestParam argument is null in multipart/form-data POST

Referenced from: commits 53a9697, 15c82af

Backported to: 4.3.12

[spring-projects-issues]

Juergen Hoeller commented

Like #20472, this turns out to be a general limitation in the conversion infrastructure. The eventual fix for both issues was to make ObjectToOptionalConverter aware of array and collection sources, with special emptiness handling built in.