How to Deserialize JSON into Polymorphic Types with Jackson: Resolving the 'Cannot Map Directly' Compilation Error?

Deserialize JSON into Polymorphic Types with Jackson: Resolving the Compilation Error

In an attempt to solve the compilation error while deserializing polymorphic JSON using Jackson's ObjectMapper, the culprit lies in the type mismatch of the readValue() method's parameters.

The problematic line is:

return mapper.readValue(root, animalClass);

The generic readValue() method expects a TypeReference or a Class as the second parameter, but animalClass is an instance of Class, which the compiler cannot map directly.

Solution Using Annotations:

To resolve this error and allow polymorphic deserialization, we can leverage Jackson's JSON annotations. In this approach, the @JsonTypeInfo, @JsonSubTypes, and @JsonIgnoreProperties annotations are employed.

Modify the Animal class and its subclasses as follows:

Animal Class:

@JsonIgnoreProperties(ignoreUnknown = true)
@JsonTypeInfo(use = JsonTypeInfo.Id.NAME, include = JsonTypeInfo.As.PROPERTY)
@JsonSubTypes({
    @JsonSubTypes.Type(value = Dog.class, name = "Dog"),
    @JsonSubTypes.Type(value = Cat.class, name = "Cat")
}
public abstract class Animal {

    private String name;

    public String getName() {
        return name;
    }

    public void setName(String name) {
        this.name = name;
    }

}

Dog and Cat Subclasses:

public class Dog extends Animal {

    private String breed;

    public Dog() {

    }

    // Constructors and getters/setters omitted for brevity

}

public class Cat extends Animal {

    private String favoriteToy;

    // Constructors and getters/setters omitted for brevity


}

With these annotations, Jackson can now properly deserialize the polymorphic JSON data into the appropriate subclasses. The corrected readValue() line would be:

return mapper.readValue(root, Animal.class);

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