RequiredKeyAdapterFactory.java

RequiredKeyAdapterFactory.java

public class RequiredKeyAdapterFactory implements TypeAdapterFactory {

    public <T> TypeAdapter<T> create(Gson gson, TypeToken<T> type){

        final TypeAdapter<T> delegate = gson.getDelegateAdapter(this, type);

        return new TypeAdapter<T>() {
            @Override
            public void write(JsonWriter out, T value) throws IOException {
                    if (value != null) {

                    Field[] fields = value.getClass().getDeclaredFields();

                    for (int i = 0; i < fields.length; i++) {
                        if (fields[i].isAnnotationPresent(Required.class)) {
                            validateNullValue(value, fields[i]);
                        }
                    }
                }
                delegate.write(out, value);
            }

            private <T> void validateNullValue(T value, Field field) {
                field.setAccessible(true);
                Class t = field.getType();
                Object v = null;
                try {
                    v = field.get(value);
                } catch (IllegalArgumentException e) {
                    throw new IllegalArgumentException(e);
                } catch (IllegalAccessException e) {
                    throw new IllegalArgumentException(e);
                }
                // Put your exhastive if checks here
                if (!t.isPrimitive() && v == null) {
                    throw new JsonParseException(field + " is null");
                }
            }

            @Override
            public T read(JsonReader in) throws IOException {

                T value = delegate.read(in);
                if (value != null) {
                    Field[] fields = value.getClass().getDeclaredFields();
                    for (int i = 0; i < fields.length; i++) {
                        if (fields[i].isAnnotationPresent(Required.class)) {
                            validateNullValue(value, fields[i]);
                        }
                    }
                }
                return value;
            }

        };
    }
}

Usage

  Gson gson = new GsonBuilder()
                    .registerTypeAdapterFactory(new RequiredKeyAdapterFactory())
                    .create();