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

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.

Answer 2

IllegalArgumentException: Unable to create call adapter for class java.lang.Object

Short answer: I have solved it by the following changes

ext.retrofit2Version = '2.4.0' -> '2.6.0'
implementation"com.squareup.retrofit2:retrofit:$retrofit2Version"
implementation "com.squareup.retrofit2:adapter-rxjava2:$retrofit2Version"
implementation "com.squareup.retrofit2:converter-gson:$retrofit2Version"

Good luck

Answer 3

in my case using this

compile 'com.jakewharton.retrofit:retrofit2-rxjava2-adapter:1.0.0'

with this

new Retrofit.Builder().addCallAdapterFactory(RxJava2CallAdapterFactory.create())...

solved the problem when nothing else worked

Answer 4

In my case I used

com.squareup.retrofit2:adapter-rxjava:2.5.0 //notice rxjava

instead of

com.squareup.retrofit2:adapter-rxjava2:2.5.0 //notice rxjava2

you should be using com.squareup.retrofit2:adapter-rxjava2:2.5.0 when using io.reactivex.rxjava2

Answer 5

The way I fixed this issue was adding this to the application gradle file, if the configuration is not set there will be conflicts, maybe this will be fixed in the stable release of the library:

configurations {
    compile.exclude group: 'stax'
    compile.exclude group: 'xpp3'
}

dependencies {
    compile 'com.squareup.retrofit:retrofit:2.0.0-beta1'
    compile 'com.squareup.retrofit:converter-simplexml:2.0.0-beta1'
}

and create your adapter this way:

  Retrofit retrofit = new Retrofit.Builder()
            .baseUrl(endPoint)
            .addConverterFactory(SimpleXmlConverterFactory.create())
            .build();

Hope it helps!

Answer 6

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>
}