@Test
public void registerScheduledJobThrowExceptionWhenIdentifierContainsOnlyWhitespace() {

    // given
    var jobIdOnlyWhitespace = "   ";
    given(jobMock.getJobIdentifier()).willReturn(jobIdOnlyWhitespace);
    // when / then
    assertThatThrownBy(() -> scheduledJobExecutor.registerScheduledJob(jobMock))
            .isInstanceOf(IllegalArgumentException.class)
            .hasMessage("A job's identifier can't contain only whitespace!");
}