How to parse polymorphic JSON?

[CypherpunkSamurai]

I have an API that returns 2 different JSON. I'm using retrofit in Java, How can I parse this?

How do I create a Java class to represent it?

{
    "status": "running",
    "result": "result info"
}

in case of Error:

{
   "error": "Error Code"
}

[pjfanning]

Have a look at JsonTypeInfo annotation - plenty of blogs and stackoverflow entries about it - eg https://www.baeldung.com/jackson-annotations

The JSON will need a type field to specify which class the message relates to.

An alternative non-polymorphic approach is to load the JSON as an AST and check for the existence of the 'error' field and then to deserialize the AST as an Error instance, otherwise deserialize as a Status/Result instance. See the 'readTree' section in https://www.baeldung.com/jackson-object-mapper-tutorial

[cowtowncoder]

+1 for @pjfanning's answer. In addition, mode of use = Id.DEDUCTION would probably work well for this particular case -- it does not use explicit type id but bases it on "unique properties" found. An article that explains this is:

https://www.baeldung.com/jackson-deduction-based-polymorphism

(see paragraph 3, "Deduction-based Polymorphism")

[CypherpunkSamurai]

Thank you very much :) It worked, I just had to sit and focus on reading :)

For future reference, I've made a repo https://github.com/CypherpunkSamurai/LearnJackson/tree/jackson-polymorphic-json

[CypherpunkSamurai]

Can you guys please provide a link to example app / codebase i can follow along? I find it a lot complicated and it would help if I can follow a working example.

Like the Github API Auth is a good example for polymorphic json. It returns different json for auth cases.

[CypherpunkSamurai]

I made a replit project here, but as you can see its returning exception when parsing the json.
I refered to the guide https://www.baeldung.com/jackson-deduction-based-polymorphism.

What could be the issue please help?

https://replit.com/@CypherSamurai/JacksonTest#Main.java

[CypherpunkSamurai]

Oh... it worked :D I just had to implement public getters and setters so the classes are different. Also does having private variables in the implemented classes affect the deduction ?

[cowtowncoder]

Note that private fields can be automatically detected but this is not the default. Alternatively you can annotate them with @JsonProperty to include. Such fields may be "pulled in" by public getters and/or setters, but otherwise will not be used.

So deduction and operation are all based on observed logical properties, which in turn are determined based on auto-detection with configurable levels (by default "setters" of all visibility, including private, but getters and fields only if public) or explicit annotations. Once accessors (fields, getters/setters, parameters of annotated constructors) are determined, they are unified (so a property can have a "mutator" (field, setter, constructor parameter) and/or "accessors" (field, getter).
For inclusion in serialization, an accessor is needed; for deserialization "mutator".

To change visibility level for auto-detection you can either use @JsonAutoDetect annotation on class, or configure ObjectMapper (setDefaultVisibility() or something like that).

[CypherpunkSamurai]

Ahh, Nice. I'll copy this text into the example repo i made. Thanks for help :)

Jackson seems cool, gonna be using it for all my projects :D

[cowtowncoder]

@CypherpunkSamurai cool!