Add SubType information at runtime using Jackson for polymorphism

Question

I am using Jackson to unmarshal polymorphic types from JSON. I am using the @JsonTypeInfo, @JsonSubTypes, and @JsonTypeName annotations similar to Example 4 in this post. My question is, say now I need someone else to extend my code and add a 3rd class: public class Duck extends Animal outside of the original code base. How can I (or the others) add the SubType information without modifying the source code (annotation) of the public abstract Animal class?

UPDATE:

I am forced to use @JsonTypeName to solve the POJO version changes. For example:

package my.zoo;
@JsonTypeInfo(  
    use = JsonTypeInfo.Id.NAME,  
    include = JsonTypeInfo.As.PROPERTY,  
    property = "type")  
@JsonSubTypes({  
    @Type(value = Cat.class, name = "[email protected]"),  
    @Type(value = Dog.class, name = "[email protected]"),
    @Type(value = Catv2.class, name = "[email protected]")})
public abstract class Animal {
    ...
}

@JsonTypeName("[email protected]")
public class Cat extends Animal {
    ...
}

@JsonTypeName("[email protected]")
public class Catv2 extends Animal {
    ...
}

@JsonTypeName("[email protected]")
public class Dog extends Animal {
    ...
}

// in another java package/project
package another.zoo;
import my.zoo.*;

@JsonTypeName("[email protected]")
public class Dogv2 extends Animal {
    ...
}

Now the problem I am facing is that I cannot unmarshal a JSON that has type name "[email protected]" without adding @Type(value = another.zoo.Dogv2.class, name = "[email protected]")}) to the Animal class. So doing this with annotation is obviously not possible. Is there a way to do this at runtime?

UPDATE 2:

I just found this SO question with the same/similar use case. My concern here is that using annotation will prevent people from extending/implementing your base class/interface. I would like a way to still keep extensibility of my base class/interface and make sure my (un)marshalling logic will work with future concrete types.

Answer 1

I ended up using Reflections library to find all subtype of Animal class, and register JsonSubTypes with mapper.registerSubtypes(Class<?>... classes) method.

Answer 2

Don't use @JsonSubTypes. Use @JsonTypeInfo#use with JsonTypeInfo.Id.CLASS to identify types for serialization and deserialization.

@JsonTypeInfo(use = JsonTypeInfo.Id.CLASS, include = JsonTypeInfo.As.PROPERTY, property = "type")
abstract class Animal { /* whatever */ }

Jackson will then store the fully qualified name to serialize the object and use it again to determine a target class for deserialization.

You'll have to make sure none of the subclasses have a property named type.

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It's possible to make this work with @JsonSubTypes but it involves mixins and is not a very maintainable solution.