What are @JsonTypeInfo and @JsonSubTypes used for in jackson

Question

What are @JsonTypeInfo and @JsonSubTypes annotations using for in jackson ?

public class Lion extends Animal {

private String name;

@JsonCreator
public Lion(@JsonProperty("name") String name) {
    this.name = name;
}

public String getName() {
    return name;
}

public String getSound() {
    return "Roar";
}

public String getType() {
    return "carnivorous";
}

public boolean isEndangered() {
    return true;
}

@Override
public String toString() {
    return "Lion [name=" + name + ", getName()=" + getName() + ", getSound()=" + getSound() + ", getType()=" + getType() + ", isEndangered()="
            + isEndangered() + "]";
}

}

========================================

public class Elephant extends Animal {

@JsonProperty
private String name;

@JsonCreator
public Elephant(@JsonProperty("name") String name) {
    this.name = name;
}

public String getName() {
    return name;
}

public String getSound() {
    return "trumpet";
}

public String getType() {
    return "herbivorous";
}

public boolean isEndangered() {
    return false;
}

@Override
public String toString() {
    return "Elephant [name=" + name + ", getName()=" + getName() + ", getSound()=" + getSound() + ", getType()=" + getType()
            + ", isEndangered()=" + isEndangered() + "]";
}
}

==============================================

@JsonTypeInfo (use = JsonTypeInfo.Id.CLASS, include = As.PROPERTY, property = "classNameExtenral")
@JsonSubTypes ({@Type (value = Lion.class, name = "lion"), @Type (value = Elephant.class, name = "elephant")})

public abstract class Animal {

@JsonProperty ("name")
String name;
@JsonProperty ("sound")
String sound;
@JsonProperty ("type")
String type;
@JsonProperty ("endangered")
boolean endangered;
}

public static void main(String[] args){
    Lion lion = new Lion("Simba");
    Elephant elephant = new Elephant("Manny");
    List<Animal> animals = new ArrayList<>();
    animals.add(lion);
    animals.add(elephant);
}

What I understand is that it additionally preserves the concrete type of object being serialised along with the actual data.

What is not clear to me is what is the actual advantage/gain during deserialization.

Not getting any significant documentation apart from the java docs. Can any1 please help out here or provide some docs around the same.

Answer 1

The purpose of these annotations is to support polymorphism on deserialization. When deserializing the actual code being executed will know the class of what it expects. E.g., the type of some field being deserialized into. But if that class has subclasses (i.e., subtypes) how does the generic Jackson deserializer know which actual class the string being deserialized is? It's got to create an instance of some concrete type (the class or one of its subclasses) and fill it up. The only way it can know which one to create is if that information is written into the serialization in the first place.

As this answer says there are three ways to do it - you pick the one that's appropriate for your use case. @JsonTypeInfo + @JsonSubtypes is one of those ways - it works great when you know, at compile time, all of the possible subtypes that could exist for the class in question.