Allow `valueToTree()` via `SerializationContext`, add to `ObjectWriter`

[XakepSDK]

Currently i have this serializer code:

if(value.hasStyling()) {
    JsonNode jsonNode = JacksonComponentSerializer.MAPPER.valueToTree(value.style());
    if (jsonNode.isObject()) {
        Iterator<Map.Entry<String, JsonNode>> fields = jsonNode.fields();
        while (fields.hasNext()) {
            Map.Entry<String, JsonNode> next = fields.next();
            gen.writeFieldName(next.getKey());
            gen.writeTree(next.getValue());
        }
    }
}

Can we have some way to convert object to TreeNode without static access to mapper?

[XakepSDK]

Still required, as found approach doesn't fit for all serializers, only for some.

[cowtowncoder]

This is difficult thing to support, since valueToTree() (or conversions in general) are sequences of serialization-then-deserialization, and serialization side does not have support for deserialization (nor is there linkage back to ObjectMapper from serializer).
But I agree that ability to perform such conversions for serialization (and deserialization) is useful.

In short term, a work-around is to use getCodec() method of JsonGenerator that is passed to serialize(): that can be used to call conversion methods.

[cowtowncoder]

Then again, maybe worth thinking through... since serialization into TokenBuffer is easy and exists, so perhaps it'd be possible to have some sort of short-cut and then applicable without fully configured JsonNode deserializer.

[cowtowncoder]

Ugggggh. The missing piece for otherwise simple implementation is lack of JsonNodeFactory from SerializerProvider (and SerializationConfig).
So to implement this, would need to solve that problem first.

And, as usual, 3.0 (master) has moved JsonNodeFactory to be accessible from serialization side, so I think I'll just implement this there at first.

[cowtowncoder]

Hmmh. Ok, while 3.0 does give access to JsonNodeFactory, there is the secondary problem: readTree() used within ObjectMapper does everything by the book, creating DeserializationContext and this does not exist on serialization-only side of things.
And this access to deserialization things is not just a theoretical problem but there is one problem wrt BigDecimal handling, reads; as well as possibility of custom JsonNode (or subtype) deserializers.

One possible way forward would be to have alternative shortcut between JsonNode and TokenBuffer (constructing former from latter, passing in JsonNodeFactory. I'll see if that is doable as that seems reasonable, for now.