Make Spring HATEOS links serializion/deserialization JSON:API 1.1 compliant

[toedter]

Spring HATEOAS provides a generic link concept. In JSON:API, for relationships between rest resources, the relationsships attribute is in most cases the proper solution. Furthermore, JSON:API provides a links attribute, both for the top-level document, as well as for resource links. The only allowed member of resource-level links is self. So, if a Spring HATEOAS representation model contains only a self-link, the current serialization would be JSON:API compliant. An example is:

{
  "data": {
    "id": "4",
    "type": "movies",
    "attributes": {
      "title": "The Matrix"
    },
    "links": {
      "self": "/api/movies/4"
    }
  }
}

But if you would add a link with the relation imdb, the current serialization would NOT be JSON:API compliant:

{
  "data": {
    "id": "4",
    "type": "movies",
    "attributes": {
      "title": "The Matrix"
    },
    "links": {
      "imdb": "https://www.imdb.com/title/tt0133093/"
    }
  }
}

Furthermore, Spring HATEOAS allows adding several links with the same relation name. Currently this library serializes this as an array, which is NOT JSON:API complient. The JSON:API spec demats that a link with a unique name is always a JSON object, never an array.

I am currently thinking about solutions, that the always library creates JSON:API compliant results. Furthermore, I am thinking of validation capabilities and more specific link building support.

[toedter]

serialization and configuration done.