Deep merge for `JsonNode` using `ObjectReader.readTree()`

[sirianni]

Is your feature request related to a problem? Please describe.

Describe the solution you'd like
Ability to deep merge two JsonNode objects, and/or to deserialize a stream by updating an existing JsonNode rather than creating a new one.

Usage example

    var mapper = new ObjectMapper();
    var node1 = mapper.readTree("{\"root\":{\"a\":\"aaa\",\"foo\":\"hello\"}}");
    var node2 = mapper.readerForUpdating(node1)
        .readTree("{\"root\":{\"b\":\"bbb\",\"foo\":\"goodbye\"}}");

Additional context
#584 implies that this capability exists, but it does not appear to be functioning propery for JsonNode. There also doesn't seem to be a way to specify shallow or deep merge using the tree API readerForUpdating()

[cowtowncoder]

It should just work, with Jackson 2.12 at least. There is no (and there is unlikely ever to be a way) to select deep/shallow merge as this depends on POJOs in question (or more accurately, deserializers).

If JsonNode merge does not work, it'd be good to explain what exactly you see, and how that differs from the expectation. Ideally a unit test.

[sirianni]

Thanks @cowtowncoder for the quick reply! I've created a repo https://github.com/sirianni/jsonMerge with a failing unit test.

[sirianni]

select deep/shallow merge as this depends on POJOs in question (or more accurately, deserializers).

What are the semantics for JsonNode?

[cowtowncoder]

Since JsonNode is mutable, it should allow deep merge; JsonDeserializer associated with a type should indicate this via Boolean supportsUpdate() (true, false or null (not known)).
Technically only ObjectNode and ArrayNode are updateable since leaf scalar types are immutable.

But actual implementation relies on deserialize() override which takes existing instance; if that is implemented as mutable operation, merges are possible.

[cowtowncoder]

Ah. Quick fix is to use readValue(), not readTree() -- there is no benefit from trying to use readTree() here.

But I am not sure why the difference should matter and will see if that can be fixed. Thanks!

[cowtowncoder]

Fixed by simply delegating calls to readValue() in case "valueToUpdate" is defined; seems that this should work (and does handle tests I added). Added in 2.13 just to minimize any possibility of breakage for 2.12.x; I would recommend just using readValue() directly here in general.