Deserialzing using static method for types of a given super class

[sentinel0815]

I Already asked the same question on stackoverflow

I provided a sample project on GitHub to showcase my problem: I would like to configure to use a static method during deserialization of classes that extend AbstractCodestable (or implements the interface Codestable). How is this possible and if there are multiple solutions, what are the pros and cons?

In my project there are some base classes:

• Interface Codestable,
• an abstract implementation AbstractCodestable
• CodestableObjectMapper for configuring the ObjectMapper

There are also some codestable implementation classes

• like GenderCodestable and InterestsCodestable.
• All these codestable implementation classes have:
• no no-args constructor
• a private args constructor
• a public static method valueOf(String) to get an instance

Imagine that in real the base classes are defined in a maven module "base". The codestable implementation classes are generated in another maven module "codestable". The "base" module DOES NOT depend on "codestable" (and therefore does not know these classes). The "codestable" module" depends on the "base" module.

I would like to configure Jackson's ObjectMapper (or the CodestableObjectMapper in the sample project on GitHub) to use these valueOf methods to create concrete codestable objects during deserialization. But in real this object mapper class only knows the class AbstractCodestable (and the interface Codestable) but does not know the concrete implementation classes!

Currently the following exception occurs:

com.fasterxml.jackson.databind.exc.MismatchedInputException: Cannot construct instance of 
`jackson.example.deserialzer.codestable.type.GenderCodestable` (although at least one Creator exists): 
cannot deserialize from Object value (no delegate- or property-based Creator)

I can adjust all the classes as I want, but actually

• I do not want do make the construtor(s) public (because a developer should not be able to create a new *Codestable instances with e.g value -99 which has no functional meaning).
• I do not want to adjust all my concrete ~100 codestable classes
• I prefer not to add any Jackson annotations into the classes. I prefer to configure the deserializer's mapping separately. But if there is one simple solution with annotation in the abstract super class, I could agree with that.

Is there any way to let Jackson know how to deserialize the codestable classes?

[pjfanning]

Jackson let's you write your own custom serializers and deserializers.

• https://www.baeldung.com/jackson-custom-serialization
• https://www.baeldung.com/jackson-deserialization

[sentinel0815]

Hi. Thank you for the fast reply! But that does not help in my case.
Yes, I could write one Deserializer like I did (https://github.com/sentinel0815/jackson-deserializer/blob/main/src/main/java/jackson/example/deserialzer/base/mapper/CodestableDeserializer.java) which could use reflection. But I must declare the custom deserializer to use for each single concrete codestable implementation class and I do not know these classes (as mentioned above: 'base' does not depend on 'codestable' which contains the concrete codestable implementation classes)

[pjfanning]

Polymorphic types in jackson are usually handled using annotations - eg https://www.baeldung.com/java-jackson-polymorphic-deserialization

But if you don't want to add annotations, you can include hints in your serialized JSON. Your deserializer can use the hints.

• the JSON can be serialized to have something like "gender":"F" or "gender:F"
• you can include an extra markers in the serialized JSON if you don't want to serialize the polymorphic type in the main JSON fields. Something like:

 "value": "F",
 "valueType":"Gender"

There are probably other ways to do this too. I would suggest using a search engine and looking up 'polymorphism in Jackson', 'polymorphism in JSON' - things like that.

[sentinel0815]

I already did a lot of research and evaluated a lot of the solutions, but nothing helped yet.
E.g. https://www.baeldung.com/jackson-inheritance
I will try out your hints!

[cowtowncoder]

Some notes:

• No requirement for public constructors, Jackson can access any visibility
• One alternative to adding annotations in classes is to use "mix-in" annotations; associated (but may be lots of work here)
• Anything done via annotations can be triggered from other sources by custom AnnotationIntrospector -- default JacksonAnnotationIntrospector supports @JsonTypeInfo and all, but there's no requirement custom AI even uses annotations; may use any logic
• While factory method usage is supported, no auto-detection; need @JsonCreator (or equivalent detection via AnnotationIntrospector)

I don't think you can get what you are asking for, out of the box, but there are a few pieces that combined might allow it.

[sentinel0815]

Thanks for the JsonCreator hint. This works if I add the @JsonCreator Annotation (with @JsonProperty) at the valueOf method of all my concrete codestable classes - Sample on Git Hub

Can I abstain of the JsonCreator Annotation mabye by using the AnnotationIntrospector instead? I will dig into the JacksonAnnotationIntrospector the next days and try out...

[cowtowncoder]

@sentinel0815 Yes, you should be able to achieve equivalent of @JsonCreator with AnnotationIntrospector.

[sentinel0815]

I solved this by adding the @JsonCreator annotation to all concrete codestable classes e.g.

    @JsonCreator
    public static GenderCodestable valueOf(final @JsonProperty("code") String code) {
        if (BY_CODE.containsKey(code)) {
            return BY_CODE.get(code);
        } else {
            throw new NoSuchElementException("No GenderCodestable found with the code: " + code);
        }
    }

As the codestable classes are generated and I am able to adjust the generator this seems to be the easiest solution for me in this case.

[cowtowncoder]

Excellent! Thank you for sharing.