@Test
 public void testNotNullConstraint_not_there() throws NoSuchFieldException, SecurityException {
   Field f1 = TestClass.class.getDeclaredField("sizeString");
   ObjectContext<Object> s1 = uut.forField(f1);
   assertFalse(s1.isRequired());
 }
 @Test
 public void testCustomNotNullConstraint() throws NoSuchFieldException, SecurityException {
   Field f1 = TestClass.class.getDeclaredField("notEmptyString");
   ObjectContext<Object> s1 = uut.forField(f1);
   assertTrue(s1.isRequired());
 }