Using gwtmockito's AsyncAnswers (http://klimer.eu/2014/08/28/testing-asynchronous-gwt-rpc-services/)

AnyAsyncCallback.java

public class AnyAsyncCallback<T> extends ArgumentMatcher<AsyncCallback<T>> {
    public AnyAsyncCallback() {
    }
    
    @Override
    public boolean matches(Object argument) {
        return argument instanceof AsyncCallback;
    }
}

anyAsyncCallbackUsage.java

protected static <T> AsyncCallback<T> anyAsyncCallback(Class<T> clazz) {
    return Matchers.argThat(new AnyAsyncCallback<T>());
}

Result.java

doAnswer(returnSuccess(somePojoInstance)).when(service).login(eq(login), eq(password), anyAsyncCallback(SomPojo.class));

Service.java

public interface LoginServiceAsync {
    void login(String login, String password, AsyncCallback<SomePojo> callback);
}

UsageFailure.java

doAnswer(AsyncAnswers.returnFailure(new LoginException("Unknown user"))).when(service).login(eq(login), eq(password), Matchers.<AsyncCallback<SomePojo>>.any());

UsageSuccess.java

doAnswer(AsyncAnswers.returnSuccess(somePojoInstance)).when(service).login(eq(login), eq(password), Matchers.<AsyncCallback<SomePojo>>.any());