JsonException during converting Java object to JSON

Somehow below code is failing even after creating a custom type adapter. Its working for other type like string. Any idea how can I resolve the issue. Below is the code:

package com;

import java.io.IOException;

import javax.xml.datatype.DatatypeConfigurationException;
import javax.xml.datatype.DatatypeFactory;
import javax.xml.datatype.XMLGregorianCalendar;

import com.google.gson.Gson;
import com.google.gson.GsonBuilder;
import com.google.gson.TypeAdapter;
import com.google.gson.stream.JsonReader;
import com.google.gson.stream.JsonWriter;

public class DateConversion {

    public static void main(String[] args) throws DatatypeConfigurationException {

        XMLGregorianCalendar date = DatatypeFactory.newInstance().newXMLGregorianCalendar();

        Request req = new Request();
        req.setDate(date);
        TypeAdapter<XMLGregorianCalendar> adapter = new TypeAdapter<XMLGregorianCalendar>() {

            @Override
            public void write(JsonWriter writer, XMLGregorianCalendar value) throws IOException {
                System.out.println("Inside write");
                if(value==null) {
                    writer.nullValue();
                    return;
                }
                writer.value("value");

            }

            @Override
            public XMLGregorianCalendar read(JsonReader reader) throws IOException {
                System.out.println("Inside read");
                return null;
            }
        };
        Gson gsonBuilder = (new GsonBuilder()).registerTypeAdapter(XMLGregorianCalendar.class, adapter).serializeNulls().create();
        System.out.println(gsonBuilder.toJson(req));
    }

}

class Request {
    private XMLGregorianCalendar date;

    public XMLGregorianCalendar getDate() {
        return date;
    }

    public void setDate(XMLGregorianCalendar date) {
        this.date = date;
    }
}

And below is the error stacktrace:

Exception in thread "main" com.google.gson.JsonIOException: Failed making field 'com.sun.org.apache.xerces.internal.jaxp.datatype.XMLGregorianCalendarImpl#eon' accessible; either change its visibility or write a custom TypeAdapter for its declaring type

Java object should be successfully parsed to JSON but it is not working