How should a Serializer discover DefaultTyping

[nsoft]

If a Serializer is producing output and Default typing is turned on, that serializer should be accounting for the default typing and ensuring it adds an appropriate attribute. However com.fasterxml.jackson.databind.JsonSerializer#serialize only gets a JsonGenerator and SerializerProvider in it's arguments and if there is a route to the mapper's configuration through those objects I wasn't able to find it.

Many other things that might be turned on or off could be discovered via isEnabled() but default typing isn't a feature so it's not exposed there. Furthermore details like the property being used (usually @class but IIUC it can vary), and the DefaultTyping mode (i.e. NON_FINAL, etc) would be important.

com.fasterxml.jackson.databind.JsonSerializer#serializeWithType looks promising but at least when serializing an ID for an ObjectIdGenerator , this is never called. (although the deserialization process does call the deserializeWithType method)

[pjfanning]

SerializerProvider has this method SerializationConfig getConfig().
And from SerializationConfig you can call SubtypeResolver getSubtypeResolver().

Activating DefaultTyping affects the SubtypeResolver but I haven't checked what gets returned by SubtypeResolver getSubtypeResolver() after you setup DefaultTyping and if this helps but it might be worth checking.

[nsoft]

Yeah I looked at that but the methods on a SubtypeResolver don't provide any information about configuration, just lookups for reachable subtypes. It appears to only have one subclass (StdSubtypeResolver) so which class is returned doesn't tell us much either.

[cowtowncoder]

Quick note: serializers should not handle type id addition; separate TypeSerializer and TypeDeserializer are designed or that purpose. Even in case of delegating (like array and Collection serialization) parent type (se)serializer needs to locate appropriate Type[De]Serializer to use.
This because of variety of Type Id inclusion mechanisms.

However, custom JsonSerializer will need to override second method -- serializeWithType() -- for polymorphic use case. Implementation needs to call TypeSerializer before and after serializing actual value: MapSerializer shows an implementation for cases where value is serialized as JSON Object; there are other implementations too.

Wrt mention of ObjectIdGenerator -- Object Ids are special types and cannot be polymorphic. So they should not contain Type Ids, at least not ones core databind deals with (i.e. if some type id is needed it must be fully custom, not enabled via @JsonTypeInfo or Default Typing).