Handle Optional<?> @RequestParam() when generating URL using MvcUriComponentsBuilder

[fprochazka]

Example of an action:

@GetMapping("/{userId}")
public ModelAndView productGroups(
    @PathVariable @AssertUuid final String userId,
    @RequestParam("parentId") final Optional<@AssertUuid String> rawParentId,
    @PageableDefault(size = 100) final Pageable pageable
) throws ControllerException

this action validates, that the parentId is UUID, if it's present (which works great), but it also rejects the request completely, if it's not (with 400 BAD REQUEST). Which means any of the following ends with 400 - ?parentId, ?parentId=, ?parentId=garbage

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• Generating URL using MvcUriComponentsBuilder.fromMethodCall doesn't work without the OptionalUnwrappingConverter I've provided
• Without the fix in RequestParamMethodArgumentResolver, it generates empty query args - resulting in url with ?parentId (without = and without value) - which is not valid and ends with 400

I'm not sure if this is the correct way to fix this, so if you'll please give me some feedback on this, I'd be happy to add some tests.

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Related issue #16785 - it states that spring handles Optional<?> in handler arguments

[rstoyanchev]

@fprochazka, couldn't this all be handled in RequestParamMethodArgumentResolver, i.e. unpacking the value from the optional and then formatting it as usual? That would be a simpler solution, minus the converter it seems.

[fprochazka]

I'm not sure, you tell me :) I know Spring well enough to bend it, but not yet to solve this kind of problems in systemic way.

FYI, I've just copied already existing convertor and just reversed it with few tweaks. Imho the convertor should be available anyway.

[rstoyanchev]

Could you revise the PR to change only the argument resolver please? Also ideally a test to make the change complete.