Android Retrofit 2 multiple converters (Gson & SimpleXML) error

Android Retrofit 2: Handling Multiple Converters (Gson & SimpleXML)

Retrofit 2, a popular REST client for Android, offers flexibility in handling different data formats. This article explores the challenges and solutions for using both Gson and SimpleXML converters within a single Retrofit instance.

The Challenge: Conflicting Converters

Retrofit’s default behavior assumes a single converter. When multiple converters are registered, conflicts can arise. The library might struggle to determine which converter to use for different API endpoints, leading to errors.

Solution: Custom Converter Factories

Understanding Converter Factories

Retrofit utilizes converter factories to define how data is converted between network responses and Java objects. Each converter factory corresponds to a specific format (e.g., JSON, XML). By leveraging custom factories, we can explicitly manage which converter handles different endpoints.

Creating Custom Factories

import com.google.gson.Gson;
import com.google.gson.GsonBuilder;
import com.google.gson.JsonDeserializer;
import com.google.gson.JsonSerializer;
import com.google.gson.reflect.TypeToken;
import org.simpleframework.xml.Serializer;
import org.simpleframework.xml.core.Persister;
import retrofit2.Converter;
import retrofit2.Retrofit;
import retrofit2.converter.gson.GsonConverterFactory;
import retrofit2.converter.simplexml.SimpleXmlConverterFactory;

import java.lang.annotation.Annotation;
import java.lang.reflect.Type;
import java.util.List;

public class CustomConverterFactories {

    // Gson Converter Factory
    public static class GsonConverterFactory extends Converter.Factory {

        private Gson gson;

        public GsonConverterFactory(Gson gson) {
            this.gson = gson;
        }

        @Override
        public Converter responseBodyConverter(Type type, Annotation[] annotations, Retrofit retrofit) {
            TypeToken typeToken = TypeToken.get(type);
            return new GsonResponseBodyConverter<>(gson, typeToken);
        }
    }

    // SimpleXML Converter Factory
    public static class SimpleXmlConverterFactory extends Converter.Factory {

        private Serializer serializer;

        public SimpleXmlConverterFactory(Serializer serializer) {
            this.serializer = serializer;
        }

        @Override
        public Converter responseBodyConverter(Type type, Annotation[] annotations, Retrofit retrofit) {
            return new SimpleXmlResponseBodyConverter<>(serializer, type);
        }
    }

    // Custom Annotation for Endpoint Specificity
    @interface GsonResponse { }
    @interface SimpleXmlResponse { }

    public static Retrofit createRetrofit() {
        Gson gson = new GsonBuilder()
                .registerTypeAdapter(MyGsonObject.class, new JsonSerializer() {
                    // Custom Gson serializer for MyGsonObject
                })
                .registerTypeAdapter(MyGsonObject.class, new JsonDeserializer() {
                    // Custom Gson deserializer for MyGsonObject
                })
                .create();
        Serializer serializer = new Persister(); // Initialize SimpleXML serializer

        return new Retrofit.Builder()
                .baseUrl("https://api.example.com/")
                .addConverterFactory(new GsonConverterFactory(gson)) // Register Gson factory
                .addConverterFactory(new SimpleXmlConverterFactory(serializer)) // Register SimpleXML factory
                .build();
    }
}

Using Custom Factories with Retrofit

// Using the custom annotation
@GsonResponse // Indicates this endpoint should use the Gson converter
@GET("gson_endpoint")
Call getGsonData();

@SimpleXmlResponse // Indicates this endpoint should use the SimpleXML converter
@GET("xml_endpoint")
Call getXmlData();

Comparison Table

Conclusion

Handling multiple data formats in Retrofit effectively requires leveraging custom converter factories. This approach ensures clarity and avoids conflicts when dealing with APIs that provide responses in various formats. By creating factories for Gson and SimpleXML, we establish a flexible framework for working with different data types within a single Retrofit instance.