How to guarantee a class is never exposed client side with Vaadin

I understand that Vaadin uses some logic to only expose the UI code and then calls the server to access the code through Javascript. However let's say I have a Listener for example with some code that references other classes. So let's say I have something like:

PojoClass data = DatabaseClass.getDataFromDatabase();
doSomeProcessingOnData(data);
..
myTextField.setValue(data.getSomeValue());
..

Now how I can be sure that the code behind DatabaseClass is not exposed as Javascript. I assume this just happens, but more specifically how can say the doSomeProcessingOnData(data) method which may have calls to say 4-5 different classes that has some internal IP property not be exposed? I understand PojoClass will be exposed but I don't want the internals of doSomeProcessingOnData(data) to be exposed.