Serialize nested object json jackson

Question

I am using Jackson json library to convert my POJOs to json:

    public class A {
        public String name;
        public B b;
    }

    public class B {
        public Object representation;
        public String bar;
    }

I want to serialize an instance of A into JSON. I am going to use the ObjectMapper class from Jackson:

objectMapperPropertiesBuilder.setSerializationFeature(SerializationFeature.WRAP_ROOT_VALUE);
objectMapperPropertiesBuilder.setAnnotationIntrospector(new CustomAnnotionIntrospector());

Here annotation introspector picks root element as all these are JAXB classes with annotations like @XmlRootElement and @XmlType:

Ex: If I set in Object representation:

    public class C {
        public BigInteger ste;
        public String cr;
    }

Using this code, my JSON would look like this:

rootA: {
  "name": "MyExample",
  "b": {
    "rep": {
      "ste": 7,
      "cr": "C1"
    },
    "bar": "something"
  }
}

But I want the root element appended to my nested Object too. Object can be any custom POJO.

So in this case, I would want root element of class C appended in my JSON conversion. So:

rootA: {
  "name": "MyExample",
  "b": {
    "rep": {
      "rootC": {
        "ste": 7,
        "cr": "C1"
      }
    },
    "bar": "something"
  }
}

How can I add the root element of a nested object in JSON conversion ? All the objectMapper properties I specified will be applicable to class A. Do I have to write a custom serializer to apply some properties to nested object ?

Solution

You could use a custom serializer for that. However, the easiest way to achieve what you want is using a Map to wrap the C instance:

Map<String, Object> map = new HashMap<>();
map.put("rootC", c);

Your classes would be like:

@JsonRootName(value = "rootA")
public class A {
    public String name;
    public B b;
}

public class B {
    @JsonProperty("rep")
    public Object representation;
    public String bar;
}

public class C {
    public BigInteger ste;
    public String cr;
}

The code to create the JSON would be:

A a = new A();
a.name = "MyExample";

B b = new B();
b.bar = "something";

C c = new C();
c.cr = "C1";
c.ste = new BigInteger("7");

a.b = b;

Map<String, Object> map = new HashMap<>();
map.put("rootC", c);
b.representation = map;

ObjectMapper mapper = new ObjectMapper();
mapper.enable(SerializationFeature.WRAP_ROOT_VALUE);
mapper.enable(SerializationFeature.INDENT_OUTPUT);

String json = mapper.writeValueAsString(a);

And the generated JSON would be:

{
  "rootA" : {
    "name" : "MyExample",
    "b" : {
      "rep" : {
        "rootC" : {
          "ste" : 7,
          "cr" : "C1"
        }
      },
      "bar" : "something"
    }
  }
}