Jackson does not include type info in fields

[akshay-sharma]

I am trying to serialize an object to json using jackson 2.6.3 I want to include type info in the serialized json. This does not work for members nested within this class.

Here is the test code.

public class Test {

@JsonSubTypes({ @JsonSubTypes.Type(value = ConcreteA.class)})
interface A {
}

@JsonTypeInfo( use=JsonTypeInfo.Id.CLASS)
class ConcreteA implements A {
}

@JsonSubTypes({ @JsonSubTypes.Type(value = ConcreteB.class)})
interface B {
}

@JsonTypeInfo( use=JsonTypeInfo.Id.CLASS)
class ConcreteB implements B {
    A a;
    public A getA() {
        return a=new ConcreteA();
    }
}

@org.junit.Test
public void testSerialisation() throws JsonProcessingException {
    ObjectMapper objectMapper = new ObjectMapper();
    objectMapper.disable(SerializationFeature.FAIL_ON_EMPTY_BEANS);
    System.out.println(objectMapper.writeValueAsString(new ConcreteB()));
}
}

The json being converted by jackson is

{"@class":"Test$ConcreteB","a":{}}

Note that it does not include type info for field 'a'. The type info does get included while serializing just A.

{"@class":"Test$ConcreteA"} 

[cowtowncoder]

The problem here is that @JsonTypeInfo really has to be put in a common supertype, and should not be used in subtypes. It is not enough that values are of implementation types that have that annotation: it must be placed so it is visible from the declared (static) type. In this case it means that field a is only known to be of type A, and that type does not have necessary annotations to indicate type information is needed. Handling this way is done primarily because during deserialization only static type information is available (so deserialization would fail).

So: to solve the problem, you would need to move @JsonTypeInfo either on interface A, or on property a.

[akshay-sharma]

Putting jsonTypeInfo to just interfaces serializes it as :

{"@class":"Test$ConcreteB","a":{"@class":"Test$A"}}

Notice that is marks the @Class value as interface type A.

However, strangely, A alone gets serialized properly

A a  = new ConcreteA();
System.out.println(objectMapper.writeValueAsString(a));

as

{"@class":"Test$ConcreteA"}

I want to serialize and then deserialize objects which will hold different implementations of an interface. Thus concrete type info would be required for deserialization.(And hopefully can be used for deserialization)

[david-bakin]

Have you tried using @JsonTypeInfo (instead of @JsonSubTypes) as described here? (And also here, which is where I struggled with understanding what Jackson calls "polymorphic deserialization").

[cowtowncoder]

@akshay-sharma This is not enough to help: please include relevant class/interface definitions.

[akshay-sharma]

It looks like I was facing problems due to innner classes I wrote for testing. I put public and static qualifier on these and are able to get the desired behavior now.

Here is the modified test I began with case http://pastebin.com/4g84rsAd
Here is a better verbose test case http://pastebin.com/7kJcWfsH