Unable to create call adapter for class example.Si

Question

I am using retrofit 2.0.0-beta1 with SimpleXml. I want the retrieve a Simple (XML) resource from a REST service. Marshalling/Unmarshalling the Simple object with SimpleXML works fine.

When using this code (converted form pre 2.0.0 code):

final Retrofit rest = new Retrofit.Builder()
    .addConverterFactory(SimpleXmlConverterFactory.create())
    .baseUrl(endpoint)
    .build();
SimpleService service = rest.create(SimpleService.class);
LOG.info(service.getSimple("572642"));

Service:

public interface SimpleService {

    @GET("/simple/{id}")
    Simple getSimple(@Path("id") String id);

}

I get this exception:

Exception in thread "main" java.lang.IllegalArgumentException: Unable to create call adapter for class example.Simple
    for method SimpleService.getSimple
    at retrofit.Utils.methodError(Utils.java:201)
    at retrofit.MethodHandler.createCallAdapter(MethodHandler.java:51)
    at retrofit.MethodHandler.create(MethodHandler.java:30)
    at retrofit.Retrofit.loadMethodHandler(Retrofit.java:138)
    at retrofit.Retrofit$1.invoke(Retrofit.java:127)
    at com.sun.proxy.$Proxy0.getSimple(Unknown Source)

What am i missing? I know that wrapping the return type by a Call works. But I want the service to return business objects as type (and working in sync mode).

UPDATE

After added the extra dependancies and .addCallAdapterFactory(RxJavaCallAdapterFactory.create()) as suggested by different answers, I still get this error:

Caused by: java.lang.IllegalArgumentException: Could not locate call adapter for class simple.Simple. Tried:
 * retrofit.RxJavaCallAdapterFactory
 * retrofit.DefaultCallAdapter$1

Answer 1

If you want use retrofit2 and you don't want always return retrofit2.Call<T>, you have to create your own CallAdapter.Factory which return simple type as you expected. The simple code can look like this:

import retrofit2.Call;
import retrofit2.CallAdapter;
import retrofit2.Retrofit;

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

public class SynchronousCallAdapterFactory extends CallAdapter.Factory {
    public static CallAdapter.Factory create() {
        return new SynchronousCallAdapterFactory();
    }

    @Override
    public CallAdapter<Object, Object> get(final Type returnType, Annotation[] annotations, Retrofit retrofit) {
        // if returnType is retrofit2.Call, do nothing
        if (returnType.toString().contains("retrofit2.Call")) {
            return null;
        }

        return new CallAdapter<Object, Object>() {
            @Override
            public Type responseType() {
                return returnType;
            }

            @Override
            public Object adapt(Call<Object> call) {
                try {
                    return call.execute().body();
                } catch (Exception e) {
                    throw new RuntimeException(e); // do something better
                }
            }
        };
    }
}

Then simple register the SynchronousCallAdapterFactory in Retrofit should solved your problem.

Retrofit rest = new Retrofit.Builder()
        .baseUrl(endpoint)
        .addConverterFactory(SimpleXmlConverterFactory.create())
        .addCallAdapterFactory(SynchronousCallAdapterFactory.create())
        .build();

After that you can return simple type without retrofit2.Call.

public interface SimpleService {
    @GET("/simple/{id}")
    Simple getSimple(@Path("id") String id);
}

Answer 2

With the new Retrofit(2.+) you need to add addCallAdapterFactory which can be a normal one or a RxJavaCallAdapterFactory(for Observables). I think you can add more than both too. It automatically checks which one to use. See a working example below. You can also check this link for more details.

 Retrofit retrofit = new Retrofit.Builder().baseUrl(ApiConfig.BASE_URL)
        .addConverterFactory(GsonConverterFactory.create())
        .addCallAdapterFactory(RxJavaCallAdapterFactory.create())
        .build()