Singleton and unit testing

Question

The Effective Java has the following statement on unit testing singletons

Making a class a singleton can make it difficult to test its clients, as it’s im

Answer 1

It is possible, see the example

import static org.junit.Assert.assertEquals;
import static org.mockito.Mockito.atLeastOnce;
import static org.mockito.Mockito.mock;
import static org.mockito.Mockito.verify;
import static org.mockito.Mockito.when;

import java.lang.reflect.Field;

import org.junit.After;
import org.junit.Before;
import org.junit.Test;

public class DriverSnapshotHandlerTest {

private static final String MOCKED_URL = "MockedURL";
private FormatterService formatter;

@SuppressWarnings("javadoc")
@Before
public void setUp() {
    formatter = mock(FormatterService.class);
    setMock(formatter);
    when(formatter.formatTachoIcon()).thenReturn(MOCKED_URL);
}

/**
 * Remove the mocked instance from the class. It is important, because other tests will be confused with the mocked instance.
 * @throws Exception if the instance could not be accessible
 */
@After
public void resetSingleton() throws Exception {
   Field instance = FormatterService.class.getDeclaredField("instance");
   instance.setAccessible(true);
   instance.set(null, null);
}

/**
 * Set a mock to the {@link FormatterService} instance
 * Throws {@link RuntimeException} in case if reflection failed, see a {@link Field#set(Object, Object)} method description.
 * @param mock the mock to be inserted to a class
 */
private void setMock(FormatterService mock) {
    Field instance;
    try {
        instance = FormatterService.class.getDeclaredField("instance");
        instance.setAccessible(true);
        instance.set(instance, mock);
    } catch (Exception e) {
        throw new RuntimeException(e);
    }
}

/**
 * Test method for {@link com.example.DriverSnapshotHandler#getImageURL()}.
 */
@Test
public void testFormatterServiceIsCalled() {
    DriverSnapshotHandler handler = new DriverSnapshotHandler();
    String url = handler.getImageURL();

    verify(formatter, atLeastOnce()).formatTachoIcon();
    assertEquals(MOCKED_URL, url);
}

}