Spring-hateoas: Curies relation can be added to `_links` without any curie given by the curie provider

[KenErikson]

If there is a : (colon) in a relation of a link, then curiedLinkPresent = true in Jackson2HalModule. This leads to that if a curie provider is configured, but no curie is given by curieProvider.getCurieInformation(Links.of(links)), the sortedLinks map suddenly has a "curies -> []".

Completed serialization then looks something like: "...,_links":{"colon:rel":{"href":"/test"},"curies"}...
This is wrong. "curies" should never be added in that case.

Introduced by:
https://github.com/spring-projects/spring-hateoas/commit/fc9d2a6777e0d3c8775d62c0d260727e7a450b91

Proposed solution, add in Jackson2HalModule:

if( !curies.isEmpty() ){
  sortedLinks.put(HalLinkRelation.CURIES, curies);
}

[odrotbohm]

Shouldn't we rather reject that case if a CurieProvider is registered? Would colon:rel make sense standalone without any curie defined for colon?

[KenErikson]

Depends on what you mean by reject. curies should not be set if no curies are present to avoid invalid serialization.
This could be done by not settings curiedLinkPresent = true when no curieProvider for current rel or by some other check.

I guess it would be an edge/invalid case where one would want to show colon:rel without accompanying curie. Don't think it should be removed or anything, but it shouldn't break the serializing.

[odrotbohm]

The spec defines link object keys to either be a registered link relation or a URI. colon:rel is indeed a valid URI, so we can and have to keep it as is even without a curie present. Definitely also a back-port candidate into 1.2.x.