insert calculated field during serialization

[Marcin Biegan]

Hi,

I'm rewriting a library which decouples model from serialized view definition and I want to do that with Jackson (as it's already used for other stuff). It all looks great - filters and mixins allow to easily include just a subset of fields. What I have trouble with is adding a new field, computed before/during serialization (just like additional getter).

I've seen that it was mentioned multiple times all over (e.g. http://stackoverflow.com/questions/20907488/injecting-additional-fields-in-serialization or http://stackoverflow.com/questions/19434888/inject-a-field-in-json-serialization), but all these solutions look like workarounds or require defining custom serializers.

Has anything changed recently in Jackson which would allow to achieve that in a prettier way? I actually think that when serializing object of A class, a decent approach would be to define a wrapper inheriting from A with additional attributes and serialize it, but sometimes it won't be feasible (e.g. final class).

What's currently the best approach to achieve this?

Thanks,
Marcin Biegan

[Tatu Saloranta]

Unfortunately this belongs to a list of (really-)nice-to-have features that have not yet been implemented. There is a github issue this, although I don't remember off-hand the id.

My idea is to allow decorator of sorts, possibly added via `@JsonSerialize`; but one question is as to where to allow addition of entries. Given streaming nature of data-binding, choices would really be before and/or after; perhaps callback would be called twice, once after START_OBJECT (and possibly type id), once right before END_OBJECT.

In the meantime, use of custom serializers is probably the way to go: use of delegating-serializer might help. You could, for example, use 'ObjectMapper.valueToTree()' to get a JsonNode that represents input value, then append fields, and serialize that. In case of delegating serializer, this would mean using `JsonNode` as the delegate value.

It should be possible to make this work in somewhat generic fashion by adding `BeanSerializerModifier`, taking original serializer, and maybe using that to serialize into `TokenBuffer`, then reading as tree... but it does get
unnecessarily complicated.

Come to think of this; another way to use `BeanSerializerModifier` would be to create and custom `BeanPropertyWriter` instances. This would probably be the cleanest way, as the property/-ies added would behave much like any other property. Quite a bit of work, but could work really well.

-+ Tatu +-

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[Marcin Biegan]

I think I'll go with BeanSerializerModifier, looks like the best approach.

Thanks,
Marcin