Can jackson determine root object type to deserialize to when json includes type property?

Question

Assume serialization to json includes the class name of the actual object, using this annotation on the Class:

@JsonTypeInfo(use = JsonTypeInfo.Id.CLASS, include = JsonTypeInfo.As.PROPERTY, property = "@type")
class MyClass {
    String foo;
}

So json is for example:

{"@type": "com.example.MyClass", "foo": "bar"}

Can this be deserialized without specifying the type? And I mean not even the super type. Just something like:

objectMapper.readValue(value, Object.class);

which doesn't actually work, it brings back a Map.

Answer 1

Well, it is certainly possible to do that although I have personally never used Jackson that way. You can deserialize it to a JsonNode object and then convert it to the proper type.

final ObjectMapper objectMapper = new ObjectMapper();
final MyClass myClass = new MyClass();
myClass.foo = "bar";

// Serialize
final String json = objectMapper.writeValueAsString(myClass);

// Deserialize
final JsonNode jsonNode = objectMapper.readTree(json);

// Get the @type
final String type = jsonNode.get("@type").asText();

// Create a Class-object
final Class<?> cls = Class.forName(type);

// And convert it
final Object o = objectMapper.convertValue(jsonNode, cls);

System.out.println(o.getClass());

The output is:

MyClass