@Test
public void testSingleStringGetMandatoryConfig() throws IOException {

  // Test null value.
  Configuration config = new Configuration();
  assertThrows(IOException.class, () -> getMandatoryConfig(config, PROPERTY_ONE));
  // Test empty string.
  config.set(PROPERTY_ONE.getKey(), "");
  assertThrows(IOException.class, () -> getMandatoryConfig(config, PROPERTY_ONE));
  // Test proper setting.
  config.set(PROPERTY_ONE.getKey(), VALUE_ONE);
  assertThat(getMandatoryConfig(config, PROPERTY_ONE)).isEqualTo(VALUE_ONE);
}