Exception on HAL serialization with object mapper configured to sort map entries by keys

[frank256]

This concerns JSON HAL serialization using Spring Boot 2.4.4 / Spring HATEOAS 1.2.5.

When using an object mapper which has the feature SerializationFeature.ORDER_MAP_ENTRIES_BY_KEYS enabled (which is, unfortunately, mandatory for me in a legacy project), I will get an HttpMessageNotWritableException due to a JsonMappingException with the cause being that org.springframework.hateoas.mediatype.hal.HalLinkRelation cannot be cast to java.lang.Comparable.
This is due to the Jackson MapSerializer being used to serialize the links, which expects the map keys to be comparable if the feature is enabled. However, HalLinkRelation does not implement Comparable nor is the map (I understand the links are already sorted?) that is passed into it implementing SortedMap (this is checked).

I tried to customize the object mapper with a custom map serializer that adds the links to a temporary TreeMap so that they are not sorted again by the MapSerializer, however, as the map serializer is constructed directly within the Jackson2HalModule (lines 202-5), I currently do not see how I can intervene here.

I'd consider this more or less a bug (?), however, if you have a hint for a workaround, I'd also appreciate that very much!

Disabling the feature is, unfortunately, not an option for me. It worked with Spring HATEOAS before 1.0.0.

[odrotbohm]

Can you provide an example that's reproducing the error? I think we should be able to work around this Jackson limitation by mapping the LinkRelation keys to their String representation.

[frank256]

Thanks for the fast reply! I cannot attach code from the original project, so I tried to prepare a very minimal demo using Spring Data Rest. Of course, there would not be any meaningful sorting taking place here. But you can see the exception.

The original project also makes heavy use of custom RepresentationModels, RepresentationModelProcessors and other HATEOAS functionality not related to Spring Data Rest.

spring-hateoas-hal-map-sort-demo.zip

edit: See SelfLinkTest for exceptions.

[odrotbohm]

Actually, it should be reproducible using an ObjectMapper with the HAL customizations registered and the serialization settings tweaked to what you described. I'll have a look at your sample and see where I can take this.

[odrotbohm]

This is in place for 1.4, 1.3 and 1.2 snapshots. Feel free to give it a spin.