I ran into a weird problem with Mockito and JUnit 5. A test passed in IntelliJ, but failed at the command line.

MockitoWeirdness.java

import org.junit.jupiter.api.Assertions;
import org.junit.jupiter.api.Test;
import org.junit.jupiter.api.extension.ExtendWith;
import org.mockito.Mockito;
import org.mockito.junit.jupiter.MockitoExtension;

import static org.mockito.ArgumentMatchers.anyString;
import static org.mockito.ArgumentMatchers.eq;
import static org.mockito.Mockito.doReturn;
import static org.mockito.Mockito.when;

/**
 * Last tested with Mockito 3.x and JUnit 5.4.x
 * Weird behavior: the first test fails when run from CLI: ant, java -jar, but passes when run by IntelliJ
 * The second call to Mockito.when throws NPE
 */
@ExtendWith(MockitoExtension.class)
public class MockitoJUnitIsBrokenTest {
    public interface StringProvider {
        String getString(String s);
    }

    /**
     * This test only fails when the annotation `@ExtendWith(MockitoExtension.class)` is used on the class.
     * The annotation is needed if using @Mock on fields, using Mockito.mock and removing the ExtendWith
     * annotation fixes the problem. The alternative mocking strategy of the other two tests also fixes it.
     */
    @Test
    public void testWorksOnlyInIde() {
        final StringProvider sp = Mockito.mock(StringProvider.class);
        when(sp.getString(eq("one"))).thenReturn("ONE");
        when(sp.getString(eq("two"))).thenReturn("TWO"); // throws NPE when run from CLI, but works in IntelliJ o_O

        final String f = sp.getString("one");
        final String s = sp.getString("two");
        Assertions.assertEquals("ONE", f);
        Assertions.assertEquals("TWO", s);
    }

    @Test
    public void testWorksInIdeAndCli() {
        final StringProvider sp = Mockito.mock(StringProvider.class);
        when(sp.getString(anyString())).thenAnswer(invocationOnMock -> {
            final String a = invocationOnMock.getArgument(0);
            if (a.equals("one")) {
                return "ONE";
            } else if (a.equals("two")) {
                return "TWO";
            }
            return null;
        });

        final String f = sp.getString("one");
        final String s = sp.getString("two");
        Assertions.assertEquals("ONE", f);
        Assertions.assertEquals("TWO", s);
    }

    @Test
    public void testAlsoWorksInBoth() {
        final StringProvider sp = Mockito.mock(StringProvider.class);
        doReturn("ONE").when(sp).getString(eq("one"));
        doReturn("TWO").when(sp).getString(eq("two"));

        final String f = sp.getString("one");
        final String s = sp.getString("two");
        Assertions.assertEquals("ONE", f);
        Assertions.assertEquals("TWO", s);
    }
}