### Question:
ExactlyOneNonNullValidator implements ConstraintValidator<ValidIfExactlyOneNonNull, Object> { @Override public boolean isValid(Object value, ConstraintValidatorContext context) { context.disableDefaultConstraintViolation(); if (value == null) { return true; } Class<?> clazz = value.getClass(); int numNonNull = 0; for (String fieldName : fields) { try { Field f = clazz.getDeclaredField(fieldName); f.setAccessible(true); if (f.get(value) != null) { numNonNull++; } } catch (NoSuchFieldException e) { throw new IllegalStateException("Field " + fieldName + " listed in annotation is not present in class " + clazz); } catch (IllegalAccessException e) { throw new IllegalStateException("Failed to access field " + fieldName + " after setting to accessible"); } } switch (numNonNull) { case 0: ValidatorUtil.addConstraintViolation(context, "All fields " + Arrays.toString(fields) + " are null, but one must be non-null"); return false; case 1: return true; default: ValidatorUtil.addConstraintViolation(context, "Two or more fields among " + Arrays.toString(fields) + " are non-null, but only one may be"); return false; } }  @Override void initialize(ValidIfExactlyOneNonNull constraint); @Override boolean isValid(Object value, ConstraintValidatorContext context);  }### Answer:
@Test public void testNullIsValid() { assertTrue(underTest.isValid(null, context)); verify(context, never()).buildConstraintViolationWithTemplate(any(String.class)); }