Jackson JSON library: how to instantiate a class that contains abstract fields

Question

I want to convert a JSON string into java object, but the class of this object contains abstract fields, which Jackson can't instantiate, and doesn't produce the object. What is the easiest way to tell it about some default implementation of an abstract class, like

setDefault(AbstractAnimal.class, Cat.class);

or to decide about the implementation class based on JSON attribute name, eg. for JSON object:

{
    ...
    cat: {...}
    ...
}

i would just wite:

setImpl("cat", Cat.class);

I know it's possible in Jackson to embed class information inside JSON, but I don't want to complicate the JSON format I use. I want to decide what class to use just by setting default implementation class, or by the attribute name ('cat') - like in XStream library, where you write:

xStream.alias("cat", Cat.class);

Is there a way to do so, especially in one line, or does it require some more code?

Answer 1

There are multiple ways; before version 1.8, simplest way is probably to do:

@JsonDeserialize(as=Cat.class)
public abstract class AbstractAnimal { ... }

as to deciding based on attribute, that is best done using @JsonTypeInfo, which does automatic embeddeding (when writing) and use of type information.

There are multiple kinds of type info (class name, logical type name), as well as inclusion mechanisms (as-included-property, as-wrapper-array, as-wrapper-object). This page: https://github.com/FasterXML/jackson-docs/wiki/JacksonPolymorphicDeserialization explains some of the concepts.