Transform map into key value pairs with Genson

Question

I am using Genson to serialize a Java class into JSON. One of the class members is a Map, which I need to serialize directly into name/value pairs. For example:

public class Demo {

    String name;
    Map<String, String> mp = new HashMap<>();

    ...
    name = "MyName";
    mp.put("Book", "My book title");
    mp.put("Fruit", "Orange");
    ...

}

Serialized I need:

{
    "name":"Myname",
    "Book": "My book title",
    "Fruit": "Orange"
}

I tried to apply Genson, and I am getting close with its default operation, the output is:

{
    "name":"Myname",
    "mp":{
        "Book": "My book title",
        "Fruit": "Orange"
    }
}

The keys in mp are guaranteed not to name-clash with any members of Demo.

How can this use-case by implemented with Genson?

Answer 1

It is possible to achieve this output by implementing a custom Converter.

For example:

import com.owlike.genson.Context;
import com.owlike.genson.Converter;
import com.owlike.genson.stream.ObjectReader;
import com.owlike.genson.stream.ObjectWriter;

public class DemoConverter implements Converter<Demo> {

  @Override
  public void serialize(Demo demo, ObjectWriter objectWriter, Context context) {
    objectWriter.beginObject();
    objectWriter.writeString("name", demo.getName());
    demo.getMp().forEach((prop, value) ->
      objectWriter.writeString(prop, value)
    );
    objectWriter.endObject();
  }

  @Override
  public Demo deserialize(ObjectReader objectReader, Context context) throws Exception {
    // TODO
  }

}

Now, Genson should be instantiated and used with this converter:

Genson genson = new GensonBuilder().withConverters(new DemoConverter()).create();
String json = genson.serialize(demo);
// and the JSON is
{"name":"MyName","Fruit":"Orange","Book":"My book title"}

More doc, in "Custom Converter" section.