Using PowerMockRunner with LocalDate class to mock today's date

Question

Consider a DateUtilTest class as follows that uses PowerMockRunner:

import com.reporting.utils.DateUtil;
import org.junit.Test;
import org.junit.runner.RunWith;
import org.powermock.core.classloader.annotations.PrepareForTest;
import org.powermock.modules.junit4.PowerMockRunner;
import java.time.LocalDate;
import java.util.Date;

import static org.junit.Assert.assertEquals;
import static org.mockito.Mockito.when;
import static org.powermock.api.mockito.PowerMockito.mockStatic;

@PrepareForTest(DateUtil.class)
@RunWith(PowerMockRunner.class)
public class DateUtilTest {


    @Test
    public void getPreviousWorkingDayAsDate_whenMonday() {
        //given
        LocalDate date = LocalDate.of(2017, 10, 16);
        LocalDate expected = LocalDate.of(2017, 10, 13);

        mockStatic(LocalDate.class);
        when(LocalDate.now()).thenReturn(date);

        //when
        Date previousWorkingDay = DateUtil.getPreviousWorkingDayAsDate();


        //then
        assertEquals(DateUtil.getDateFromLocalDate(expected), previousWorkingDay);
    }

    @Test
    public void getPreviousWorkingDayAsDate2_whenMonday() {
        //given
        LocalDate date = LocalDate.of(2017, 10, 16);


        mockStatic(LocalDate.class);
        when(LocalDate.now()).thenReturn(date);

        //when
        Date previousWorkingDay = DateUtil.getPreviousWorkingDayAsDate();
        LocalDate expected = LocalDate.of(2017, 10, 13);


        //then
        assertEquals(DateUtil.getDateFromLocalDate(expected), previousWorkingDay);
    }

}

I want to understand why @Test ==> getPreviousWorkingDayAsDate2_whenMonday fails when I move the expected LocalDate initialization to after the mocking of the LocalDate.class?

Further, could this tests be improved?

Answer 1

Further, could this tests be improved?

Yes - refactor DateUtil to be able to use a specific Clock. For example:

public class DateUtil {
    private static Clock clock = Clock.systemDefaultZone();

    public static setClock(Clock clock) {
        assertNotProduction();  // optionally check for an environment/system variable to throw exception if used in production
        DateUtil.clock = clock;  
    }

   public static Date getPreviousWorkingDayAsDate() {
      LocalDate today = LocalDate.now(clock);   // use clock
      ...
     return ...;
   }
}

Then the unit test does not need any mocking. For example:

@Test
public void getPreviousWorkingDayAsDate_whenMonday() {
    //given
    LocalDate monday = LocalDate.of(2017, 10, 16);
    Clock clock = Clock.fixed(monday.atStartOfDay(ZoneId.systemDefault()).toInstant(), ZoneId.systemDefault());
    DateUtil.setClock(clock);

    LocalDate lastFriday = LocalDate.of(2017, 10, 13);

    //when
    Date previousWorkingDay = DateUtil.getPreviousWorkingDayAsDate();

    //then
    assertEquals(DateUtil.getDateFromLocalDate(lastFriday), previousWorkingDay);
}

@After
public void resetClock() {
    DateUtil.setClock(Clock.systemDefaultZone());
}