Extending Jackson JsonTypeInfo support to consider defaultImpl

[eleftherias]

The Jackson @JsonTypeInfo annotation has a property called defaultImpl.
From the defaultImpl Javadoc:

Optional property that can be used to specify default implementation class to use for deserialization if type identifier is either not present, or can not be mapped to a registered type.

Example usage:

@JsonTypeInfo(use = Id.NAME, property = "type", defaultImpl = Dog.class)

I imagine this project could support defaultImpl, by marking the Jackson subtype discriminator as non-required for the class in defaultImpl.

In the example above, the generated schema would contain:

    "Dog-2" : {
      "allOf" : [ {
        "$ref" : "#/definitions/Dog-1"
      }, {
        "type" : "object",
        "properties" : {
          "type" : {
            "const" : "Dog"
          }
        }
      } ]
    },
    "Cat-2" : {
      "allOf" : [ {
        "$ref" : "#/definitions/Cat-1"
      }, {
        "type" : "object",
        "properties" : {
          "type" : {
            "const" : "Cat"
          }
        },
        "required" : [ "type" ]
      } ]
    },
    "Mouse-2" : {
      "allOf" : [ {
        "$ref" : "#/definitions/Mouse-1"
      }, {
        "type" : "object",
        "properties" : {
          "type" : {
            "const" : "Mouse"
          }
        },
        "required" : [ "type" ]
      } ]
    },

Notice that "type" is not required for Dog-2, but is still required for Cat-2 and Mouse-2.

This way if "type" is missing, it defaults to "Dog", which is what defaultImpl describes.

[CarstenWickner]

The suggestion sounds good, and I've created PR #279 for that now.
It does cover only the first part though:

if type identifier is either not present,

For the second part, one would have to list all known identifiers and negate them:

or can not be mapped to a registered type.

While possible, I'd refrain from supporting this second half for now, as it seems rather excessive. If included, that should probably be hidden behind a new JacksonOptionthen, to enable easily toggling it on/off.

Or what do you reckon @eleftherias?

[eleftherias]

Thanks @CarstenWickner!
I agree that supporting the case where the type identifier is not present is adequate.

[CarstenWickner]

The change has been released as v4.26.0.