Support for implicit Json-Ld types

[jimmarino]

Feature Request

Often a containing object schema will define a Json-Ld @type for a child object, making the latter optional. For example:

{
  "@context":  "...",
  "@type": "dspace:ContractOfferMessage",
 
  "dspace:offer": {
    "@type": "odrl:Offer",
    "@id": "urn:uuid:d526561f-528e-4d5a-ae12-9a9dd9b7a518",
    "target": "urn:uuid:3dd1add8-4d2d-569e-d634-8394a8836a88"
  },
  
}

ContractOfferMessage defines a dspace:offer property that is always of type odrl:Offer. Specifying the @type on the policy is therefore not required.

We need a way to provide the type context information to child transformers. In the above case, JsonObjectToPolicyTransformer.

Solution Proposal

A general facility should be built into TransformerContext to provide this information generically. This can be done by adding the following methods:

    @Nullable
    default <T> T getData(String key, Class<T> expectedType) {
        return null;
    }

    @Nullable
    default <T> T setData(String key, Object data) {
        return null;
    }

    @Nullable
    default void clearData(String key) {
    }

JsonLdTransformer will then define a DEFAULT_TYPE constant transformers can use to set, access, and clear type data.