Why does Gson.toJson serialize a generic field to an empty JSON object?

Question

I have a generic class containing a field of type T, Gson serializes this field as an empty object. I have included code below to demonstrate the issue. Reading the JSON back seems fine (as long as you supply the correct type token).

import java.lang.reflect.Type;
import com.google.gson.Gson;
import com.google.gson.reflect.TypeToken;

public class GsonIssue {
    static class AbstractThing {
        private String fieldA = "valueA";

        public String getFieldA() {
            return fieldA;
        }

        public void setFieldA(String fieldA) {
            this.fieldA = fieldA;
        }

        @Override
        public String toString() {
            return "AbstractThing [fieldA=" + fieldA + "]";
        }
    }

    static class Thing extends AbstractThing {
        private String fieldB = "valueB";

        @Override
        public String toString() {
            return "Thing [fieldB=" + fieldB + ", fieldA=" + getFieldA() + "]";
        }
    }

    static class Wrapper<T extends AbstractThing> {
        private T abstractThing;
        private String standardField = "standard value";

        public Wrapper(T abstractThing) {
            this.abstractThing = abstractThing;
        }

        @Override
        public String toString() {
            return "Wrapper [abstractThing=" + abstractThing + ", standardField=" + standardField + "]";
        }
    }

    public static void main(String[] args) {
        Wrapper<Thing> wrapper = new Wrapper<Thing>(new Thing());

        Gson gson = new Gson();
        String json = gson.toJson(wrapper);
        System.out.println(json);
        // prints : {"abstractThing":{},"standardField":"standard value"}
        // so standardField is correctly serialized but not abstractThing.

        // but if we manually construct the expected json string, and parse it back, we get the expected object structure
        json = "{\"standardField\": \"some text\", " +
                "\"abstractThing\":{\"fieldB\" : \"arb value\", \"fieldA\" : \"another arb value\"}}";

        Type type = new TypeToken<Wrapper<Thing>>() {}.getType();
        Object fromJson = gson.fromJson(json, type);
        System.out.println(fromJson);
        // prints : Wrapper [abstractThing=Thing [fieldB=arb value, fieldA=another arb value], standardField=some text]
        // which is as expected
    }
}

Answer 1

From their docs:

When you call toJson(obj), Gson calls obj.getClass() to get information on the fields to serialize. Similarly, you can typically pass MyClass.class object in the fromJson(json, MyClass.class) method. This works fine if the object is a non-generic type. However, if the object is of a generic type, then the Generic type information is lost because of Java Type Erasure

You can solve this problem by specifying the correct parameterized type for your generic type. You can do this by using the TypeToken class.

They give the following example for a List<T>:

Type listType = new TypeToken<List<String>>() {}.getType();
gson.toJson(myStrings, listType);

So for your code, you'd need ...

Type myType = new TypeToken<Wrapper<Thing>>() {}.getType();
String json = gson.toJson(wrapper, myType);

https://sites.google.com/site/gson/gson-user-guide#TOC-Serializing-and-Deserializing-Gener