How to Deserialize Polymorphic JSON Using Jackson's ObjectMapper without Compilation Errors?

Jackson and Polymorphic JSON: Addressing Compilation Errors for ObjectNode Deserialization

This inquiry seeks to resolve a compilation error faced when deserializing polymorphic JSON using Jackson's ObjectMapper. Specifically, the error lies in the line:

return mapper.readValue(root, animalClass);

Issue Explanation:

The error stems from a mismatch between the method signature of readValue() and the arguments provided. readValue() expects two arguments: a JsonParser and a Class, but in this code, an ObjectNode (not a JsonParser) and a subtype of Animal (not Animal itself) are being passed.

Annotation-Based Polymorphism Solution:

As an alternative to the registry-based approach mentioned in the tutorial, consider using Jackson's annotation-based approach for polymorphic deserialization. This involves:

1. Annotating the Animal class: Use @JsonTypeInfo to define how type information is stored in the JSON, along with @JsonSubTypes to specify the possible subtypes.
2. Annotating the subclasses (e.g., Dog and Cat): Use @JsonTypeId to define the name of the property that holds the type information.

Example Implementation:

// An abstract base class for animals
@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 { ... }

// Subclass for dogs
public class Dog extends Animal { ... }

// Subclass for cats
public class Cat extends Animal { ... }

// Test class
public class Test {

  public static void main(String[] args) {
    ObjectMapper mapper = new ObjectMapper();
    ... // Serialize and deserialize animals as shown in the provided solution
  }
}

By utilizing annotations, Jackson can infer the type of the object based on the provided type information in the JSON, avoiding the need for a registry and resolving the compilation error.

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