JsonDeserializer getNullValue does not match NullValueProvider

[pjfanning]

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Describe the bug

NullValueProvider return Object
JsonDeserializer return T

https://github.com/FasterXML/jackson-databind/blob/2.19/src/main/java/com/fasterxml/jackson/databind/deser/NullValueProvider.java#L27

https://github.com/FasterXML/jackson-databind/blob/2.19/src/main/java/com/fasterxml/jackson/databind/JsonDeserializer.java#L313

Interestingly, getAbsentValue returns Object in both.

I'm hitting a real world issue in Scala. Scala 2.11 and 2.12 cannot handle the mixed typing and won't let me override getNullValue in jackson-module-scala. Scala 2.13 and Scala 3 are ok. Unfortunately, we need to support older Scala releases.

One option is to say that it might break things to chnage the signatures in Jackson 2 but could we consider changing Jackson 3 to make NullValueProvider<T> and to have getNullValue and getAbsentValue to return T?

I have a possible solution for Jackson 2 that I would suggest is a temp solution on Jackson 2 that does not get forward fit to Jackson 3. Jackson 3 only gets the NullValueProvider<T> change. In Jackson 2, we could add a JsonDeserializer method called with withNullValueProvider which returns a new JsonDeserializer instance that uses the provider from withNullValueProvider. This change is one that I'm entirely sure about.

Version Information

All recent versions

[cowtowncoder]

I do not think it is practical to go through the hoops to parameterize NullValueProvider (even just for 3.0), and dropping parameter from ValueDeserializer may be equally impractical.
In hindsight making ValueDeserializer typed was probably wrong choice since at most it helps implementors avoid some mistakes but provides absolutely no benefit to Jackson core components.
This is why newer abstractions tend to be non-parametric: Json-/ValueDeserializer, Json-/ValueSerializer started with type parameter and are stuck, newer providers not.

While it unfortunate that older Scala versions cannot be made to work, my initial thinking here is that no changes should be made on databind side.
Could support for 2.11 and 2.12 be dropped from Jackson 3.x at least? (I understood this is not an option for 2.x).

[pjfanning]

Could #5057 be a contender? It needs tests and I would still need to actually test that the jackson-module-scala class that I want to fix would be able to uptake (it's likely but not yet tested).

[pjfanning]

@cowtowncoder would it be possible to consider changing master/jackson3 to have Object getNullValue(ctxt)?

[cowtowncoder]

@pjfanning That's how it is already: T only included for "deserialize()" methods, but none of "getEmpty/Null/AbsentValue()" (return plain Object).

[cowtowncoder]

Could #5057 be a contender? It needs tests and I would still need to actually test that the jackson-module-scala class that I want to fix would be able to uptake (it's likely but not yet tested).

It feels way too much of an one-off by-passing other constructs (null/empty/etc value providers) so not really liking that approach.

[pjfanning]

@pjfanning That's how it is already: T only included for "deserialize()" methods, but none of "getEmpty/Null/AbsentValue()" (return plain Object).

My point is to change this (except only on master) to return Object again.
https://github.com/FasterXML/jackson-databind/blob/2.19/src/main/java/com/fasterxml/jackson/databind/JsonDeserializer.java#L313

#5057 is quite small. It doesn't yet work for me. But with some debugging I think I might get it work for me. 5057 basically just adds an extra constructor and only the Scala deserializers would use them.

[cowtowncoder]

It already returns Object:

https://github.com/FasterXML/jackson-databind/blob/master/src/main/java/tools/jackson/databind/ValueDeserializer.java#L342

    @Override
    public Object getNullValue(DeserializationContext ctxt) {
        return null;
    }

[pjfanning]

I found a workaround in Scala module. FasterXML/jackson-module-scala@31efc10

[cowtowncoder]

@pjfanning Ok great!