Question

I want to test effect of network connection drop exactly before the execution of specific line of a program. Such as consider this execution sequence:

1. Connect to database.
2. Get country list from database.
3. Get a random country from list.
4. Get city list of this country from database.

I want to test behavior of program if network connection drops just before statement 4. I’m able test it with debugger by putting a break point, closing database when it hits break point, and then continuing to run program.

I wonder how can I accomplish this in a more systematic and robust way.

Answer 1

Let’s make the assumption that the 4 lines of pseudo code are in MyService, and that MyService uses MyDAO to access the database, you would have the following:

public class MyService {

    private MyDAO myDAO;

    public MySErvice(MyDAO myDAO) {
        this.myDAO = myDAO;
    }

    public List<City> getRandomCityList() {
        List<Country> countries = myDAO.getCountries();
        Country c = pickRandom(countries);
        return myDAO.getCities(country);
    }
}

To test it, use a mocking framework like Mockito to mock MyDAO, and inject this mock into a MyService instance. Make the mock throw the same exception as the one thrown by a real MyDAO when the network goes down when its getCities() method is thrown, and see is MyService does the right thing:

MyDAO mockDAO = mock(MyDAO.class);
List<Country> countries = Arrays.asList(new Country(...));
when(mockDAO.getCountries()).thenReturn(countries);
when(mockDAO.getCities((Country) any(Country.class))).thenThrow(new NetworkIsDownException());

MyService underTest = new MyService(mockDAO);
// TODO call underTest.getRandomCityList() and check that it does what it should do.