Jackson does not include type info for fields while serialization

Question

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"} 

UPDATE: Here is the explanation and solution from jackson dev for the problem https://github.com/FasterXML/jackson-databind/issues/1015

Answer 1

It seems you were pretty close, all you need to add is a way to tell jackson how to handle your interface type.

@JsonTypeInfo(
        use = JsonTypeInfo.Id.NAME,
        include = JsonTypeInfo.As.PROPERTY,
        property = "@class")
@JsonSubTypes({
        @JsonSubTypes.Type(value = ConcreteA.class)})
interface A {
}

So adding the @JsonTypeInfo to your interface will add the type information in a property you can configure to be the one you expect, in this case I chose @class.

The Json output I got from my example:

{"@class":"com.company.jackson.Test$ConcreteB","a":{"@type":"Test$ConcreteA"}}

Hope it helps,

José Luis