GwtMocktoAsnycCallback.java

  Mockito.doAnswer(new Answer() {
    public Object answer(InvocationOnMock invocation) throws Throwable {
      String msg = (String)invocation.getArguments()[0];
      AsyncCallback callback = (AsyncCallback) invocation.getArguments()[1];
      callback.onSuccess("Client said: \"" + msg + "\"<br>Server answered: \"Hi!\"");
      return null;
    }
  }).when(sampleApplicationService).getMessage(Mockito.any(String.class), (AsyncCallback) Mockito.any());