Allow custom JsonNode implementations

[marschall]

Is your feature request related to a problem? Please describe.
com.fasterxml.jackson.databind.ObjectReader#readValue(JsonNode) currently only works with JsonNode implementations from the jackson-databind module. It does not work with custom JsonNode implementations. We have a use case where we would like to use custom JsonNode implementations.

Describe the solution you'd like
com.fasterxml.jackson.databind.ObjectReader#readValue(JsonNode) should work with any JsonNode implementation. The reason this currently does not work is because ObjectCursor currently casts to ObjectNode

jackson-databind/src/main/java/com/fasterxml/jackson/databind/node/NodeCursor.java

Line 198 in 9e3a311

_contents = ((ObjectNode) n).fields();

There is no need for this as #fields() is defined on JsonNode. ArrayCursor for example does not cast to ArrayNode and just calls JsonNode#elements().

Usage example

JsonNode jsonNode = new CustomObjectNode();

this.objectMapper.readerFor(Custom.class).readValue(jsonNode);

Additional context
On our project we settled on Jackson and jackson-databind for our JSON parsing and object mapping needs. So far this has worked well for us. We also store JSON in the database as LOBs. Our database vendor has introduced a native JSON datatype. Part of this is a custom binary format to send JSON preparsed over the wire to the driver. The driver can use this format directly without the need to serialize to text first. The driver exposes this as javax.json.JsonObject objects to our code.

We are experimenting with adapting javax.json.JsonObject to com.fasterxml.jackson.databind.JsonNode. This would give us the efficiency of being able to use the driver to parse the database internal format while still being able to use jackson-databind for the mapping.

Simply removing the cast seems to do the trick. An additional check could be introduced, on the other hand ArrayCursor has no such check.

marschall@1209c84

[cowtowncoder]

@marschall Sounds like a good improvement if as you say fields() exists in JsonNode.
If you have time to provide a PR against 2.14 branch (seems like safe enough to add in a patch) I'd be happy to get it merged. A simple test to verify ability to use custom Object implementation would be awesome.

[marschall]

There you to #3701. I have previously contributed to jackson-core and signed the CLA. See FasterXML/jackson-core#798

[cowtowncoder]

Thank you @marschall! Will be in 2.14.2 patch when released.