Question: ### Introduction

I need to serialize a java object (`Fahrweg`) that contains nested objects (all implement the interface `FahrwegEreignis`) to json.
My code to do this is as follows:

```
Fahrweg fahrweg = new Fahrweg();

ObjectMapper mapper = new ObjectMapper();

mapper.addMixIn(FahrwegEreignis.class, FahrwegEreignisMixIn.class);

ObjectWriter ow = mapper.writer().withDefaultPrettyPrinter();

try {
  String json = ow.writeValueAsString(fahrweg);
  System.out.print(json);
} catch (JsonProcessingException e) {
  e.printStackTrace();
}

```

By default, the type of the object is not contained in the resulting json (I assume because JS is dynamically typed).

I would like to add a field to every object about like so: `"type" : "classname"`.

I've read about the Jackson annotations, and I'm using mixin because I cannot modify the classes in question. My mixin interface (`FahrwegEreignisMixIn`) looks like this:

```
import com.fasterxml.jackson.annotation.JsonProperty;

public interface FahrwegEreignisMixIn {
    @JsonProperty("type")
    //TODO
}

```

### Question

What do I need to put at `//TODO`? 

My first instinct was `Class<?> getClass();` but that doesn't work.

### Answer

The answer in the duplicate question worked for me. I had to change
my `FahrwegEreignisMixIn` to this:

```
import com.fasterxml.jackson.annotation.JsonTypeInfo;

@JsonTypeInfo(use = JsonTypeInfo.Id.CLASS, property = "type")
public interface FahrwegEreignisMixIn {}

```

Answer: 
I managed to solve the problem.
By mistake my AndroidManifest.xml file did not include the Service.
In my case the problem was that I declared the Service in the androidTest Manifest file, which isn't working for any reason.