@Test
 public void testNonNullValue() {
   try {
     onlyNull.validate("Not a null value");
     fail("expected onlyNull validator to fail with an exception");
   } catch (Validator.InvalidValueException ex) {
     assertEquals("Only null accepted", ex.getMessage());
   }
   try {
     notNull.validate("Not a null value");
   } catch (Validator.InvalidValueException ex) {
     fail("notNull should not throw exception for \"Not a null value\"");
   }
 }
 @Test
 public void testNullValue() {
   try {
     notNull.validate(null);
     fail("expected null to fail with an exception");
   } catch (Validator.InvalidValueException ex) {
     assertEquals("Null not accepted", ex.getMessage());
   }
   try {
     onlyNull.validate(null);
   } catch (Validator.InvalidValueException ex) {
     fail("onlyNull should not throw exception for null");
   }
 }