Serialize Query-Parameter in Retrofit

Question

Imagine the following request:

@POST(\"/recipes/create\")
void createRecipe(@Query(\"recipe\") Recipe recipe, Callback callback);

Answer 1

This is now possible when registering a custom Converter.Factory that overrides the stringConverter method, which is called when resolving parameters. The Github issue that @William referred to 2 years ago doesn't seem to be updated since support was added.

Method Javadoc:

Returns a Converter for converting type to a String, or null if type cannot be handled by this factory. This is used to create converters for types specified by @Field, @FieldMap values, @Header, @HeaderMap, @Path, @Query, and @QueryMap values.

The example below delegates to Gson, but in the same way any type of conversion can be applied to the parameters.

Example: GsonStringConverterFactory

class GsonStringConverterFactory
    extends Converter.Factory {

    private final transient Gson gson;

    GsonStringConverterFactory(final Gson gson) {

        this.gson = gson;
    }

    @Override
    public Converter stringConverter(final Type type, final Annotation[] annotations, final Retrofit retrofit) {

        final TypeAdapter typeAdapter;
        typeAdapter = gson.getAdapter(TypeToken.get(type));

        return new StringConverter<>(typeAdapter);
    }

    private static class StringConverter
            implements Converter {

        private final TypeAdapter typeAdapter;

        private StringConverter(final TypeAdapter typeAdapter) {

            this.typeAdapter = typeAdapter;
        }

        @Override
        public String convert(final T value)
                throws IOException {

            /* This works in our case because parameters in this REST api are always some kind of scalar 
             * and the toJson method converts these to simple json types. */
            final String jsonValue;
            jsonValue = typeAdapter.toJson(value));

            if (jsonValue.startsWith("\"") && jsonValue.endsWith("\"") {
                /* Strip enclosing quotes for json String types */
                return jsonValue.substring(1, jsonValue.length() - 1);
            } else {
                return jsonValue;
            }
        }
    }
}

Registering the converter:

To register the custom converter, your Retrofit builder could look something like this:

    new Retrofit.Builder().baseUrl(BASE_URL)
                          .addConverterFactory(GsonConverterFactory.create(gson))
                          .addConverterFactory(new GsonStringConverterFactory(gson))
                          .build();