Unable to create call adapter for class example.Simple

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

Solution 1

Short answer: return Call<Simple> in your service interface.

It looks like Retrofit 2.0 is trying to find a way of creating the proxy object for your service interface. It expects you to write this:

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

However, it still wants to play nice and be flexible when you don't want to return a Call. To support this, it has the concept of a CallAdapter, which is supposed to know how to adapt a Call<Simple> into a Simple.

The use of RxJavaCallAdapterFactory is only useful if you are trying to return rx.Observable<Simple>.

The simplest solution is to return a Call as Retrofit expects. You could also write a CallAdapter.Factory if you really need it.

Solution 2

In case of Kotlin and coroutines this situation happened when I forgot to mark api service function as suspend when I call this function from CoroutineScope(Dispatchers.IO).launch{}:

Usage:

    val apiService = RetrofitFactory.makeRetrofitService()

    CoroutineScope(Dispatchers.IO).launch {

        val response = apiService.myGetRequest()

        // process response...

    }

ApiService.kt

interface ApiService {

       @GET("/my-get-request")
       suspend fun myGetRequest(): Response<String>
}

Solution 3

add dependencies:

compile 'com.squareup.retrofit:retrofit:2.0.0-beta1'
compile 'com.squareup.retrofit:adapter-rxjava:2.0.0-beta1'
compile 'com.squareup.retrofit:converter-gson:2.0.0-beta1'

create your adapter this way:

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

addCallAdapterFactory () and addConverterFactory () both need to be called.

Service:

public interface SimpleService {

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

}

Modify Simple to Call<Simple>.

Solution 4

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()

Solution 5

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);
}

View more solutions

Comments

[rmuller]

Great link! Will check later.

[Vlado Pandžić]

In the documentation there is still RestAdapter and there is no example how to make most simple request with Retrofit 2.0. I get error Could not resolve RxJavaCallAdapterFactory. Spent few hours to make most simple android http request. It seems it is no so simple as they advertise. Maybe they should document a bit more.

[Himanshu Virmani]

Use Retrofit instead of RestAdapter if you are using Retrofit 2.0.

[Himanshu Virmani]

@rmuller Please mark the answer accepted if you find it working as that will help other too..

[Himanshu Virmani]

@Vlado Are you using all the needed gradle dependencies? Make sure you add compile "com.squareup.retrofit:adapter-rxjava:2.0.0-beta1" if you want to use rx related adapters. See the link mentioned in answer above.

[Vlado Pandžić]

Oh, I didn't installed that dependency.I installed version 1.9 which is documented a bit.more.

[rmuller]

Checked it out. Your code does not work for my example code. My conclusion is that I need a Custom CallAdapter for this use case. Also note that RxJavaCallAdapterFactory needs a extra dependency. Correct?

[Damien Diehl]

RxJavaCallAdapterFactory requires the adapter-rxjava artifact from the same repo. compile 'com.squareup.retrofit:adapter-rxjava:2.0.0-beta1'

[rmuller]

I know that changing Simple to Call<Simple> does the trick. However, I do not want to introduce type Call to the service interface

[rmuller]

This is exactly the same configuration as in my question. So, does not resolve anything :)

[rmuller]

This is the same as my setup.

[rmuller]

This still does not resolve my issue, see updated question

[rmuller]

Yes, this is indeed the right answer :(. I know from the start that Call<Simple> is the way it works. However, as stated in my question my preference is to just return Simple (as was the case in the earlier version). My conclusion is: not possible without extra code.

[shyam]

The link you've provided for Call<Simple> is broken. What package does Simple belong to?

[Hosam Aly]

Thanks for the note @shyam. I've updated the links. Simple is the class mentioned in the OP's question. The link is to Call<T>.

[markov00]

Awesome thanks. Do you know if there is a way to create an async call adapter? so I can call a service like that: service.getSimple("id",new Adapter(){ onFail(){} onSuccess(){} } instead of using the enqueue() and the callback?

[Shayan_Aryan]

also be sure to add this line when creating adapter: .addCallAdapterFactory(RxJavaCallAdapterFactory.create())

[George Papas]

for retrofit2, ensure you use the correct dependencies com.squareup.retrofit2:adapter-rxjava:2.1.0 and com.squareup.retrofit2:converter-gson:2.1.0

[ozmank]

Failed to resolve: com.squareup.retrofit:adapter-rxjava:2.1.0

[ozmank]

"implements CallAdapter.Factory" This is a class not an interface?

[ozmank]

Failed to resolve: com.squareup.retrofit:adapter-rxjava:2.1.0

[rahul.ramanujam]

@ozmank its retrofit2 , I have updated my answer too. Please try it out

[Mateusz Korwel]

@ozmank In version 2.0.0-beta3 it was an interface, now in version 2.1.0 it is a class. I edited my code, to be more actual. Thanks.

[Dr.jacky]

@MateuszKorwel It always get throw new RuntimeException(e)! I wanna return String instead of Simple.

[Mateusz Korwel]

@Mr.Hyde It should work. Maybe you rest service dont work? What exception you get when you call call.execute().body()?

[Jared Burrows]

Thank you posting this. I have created a library around this handling Simple getSimple() and Response<Simple> getSimple() requests: github.com/jaredsburrows/retrofit2-synchronous-adapter.

[Richard Collette]

The code in this answer does not throw an HttpException on a response error. I switched to using the solution from @JaredBurrows.

[Johannes Barop]

You can also use CompletableFuture See stackoverflow.com/questions/32269064/…

[Johannes Barop]

You don't need any other adapter if you're using CompletableFuture See stackoverflow.com/questions/32269064/…

[Guilherme Ramos]

You saved my live bro s2

[Abubakar Rafi]

whilst using Coroutines us must use suspend with your Function . that helped thanks buddy

[mmdreza baqalpour]

it worked for me tnx. adding suspend is the solution to this error

[user10997009]

Hello @chatlanin,I am new with MVVM and corotuines,i got the same problem.Can you help me.Here is my code github.com/diptoroy/RetrofitKotlin

[AbhayBohra]

putting suspend works..but how? can you please explain how did it work

[Barrufet]

Saved my time, ty :)

[Bitwise DEVS]

I was annoyed that all I see is RxJava related answer here and there, but the OP's actual code sample does not even use RxJava. This is the answer I was looking for!

[Максим Сыроватка]

In my case this error was because i forgot in service mark func us suspend. Thanx for answer. :)