java.util.Optional MultipartFile[] @RequestParam argument is null in multipart/form-data POST [SPR-15918]

[spring-projects-issues]

Brice Roncace opened SPR-15918 and commented

If an Optional @RequestParam is not present in the request parameters, that value should be Optional.EMPTY as opposed to null. However, an Optional<MultipartFile[]> or Optional<List<MultipartFile>> parameter on a controller method handling a POSTed multipart/form-data request is NOT converting that parameter to Optional.EMPTY as expected but leaving it null.

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Affects: 4.3.10

Reference URL: https://stackoverflow.com/q/45949393/225217

Issue Links:

• Request params Optional<List<String> and List<String> are inconsistent [SPR-15676] #20235 Request params Optional<List and List are inconsistent
• Empty List instead of null in List<MultipartFile> files [SPR-16329] #20876 Empty List instead of null in List files
• only one MultipartFile object populated when using an java.util.Optional MutipartFile array or list @RequestParam [SPR-15919] #20473 only one MultipartFile object populated when using an java.util.Optional MutipartFile array or list @RequestParam

Referenced from: commits 15c82af, 53a9697

Backported to: 4.3.12

[spring-projects-issues]

Juergen Hoeller commented

This turned out to be a general bug in the DefaultConversionService's CollectionToObjectConverter: It unconditionally turned an empty collection into null, not sending it through the ConversionService for potential Optional adaptation.

[spring-projects-issues]

Maciej Zerkowski commented

Does it mean there is no way to get null instead of empty list for simple @RequestPart(required = false) List<MultipartFile> files rest controller method param?