JSR-303 defines a metadata model and API for JavaBean validation using annotations. It allows for validation on both the server and client sides. Implementations include Hibernate Validator and Apache Bean Validation. Validation can be configured using annotations and custom constraints can be defined. Errors are handled by binding results to the model in Spring MVC. Both property and object level validation can be tested.

1. JSR-303
Bean Validation

2. Spring MVC Validator
public interface Validator {
boolean supports(Class<?> clazz);
void validate(Object target, Errors errors);
}

3. Spring MVC Validator
● Spring MVC specific
● Can not be used on client side
● All validation goes to single method
● Hard to make unit tests

4. Description
"This JSR will define a meta-data model and API
for JavaBeanTM validation based on annotations,
with overrides and extended meta-data through
the use of XML validation descriptors."

5. Key points
● Configured with annotations
● Not tied to a specific application tier
● Suits both server and client side

6. Implementations
● Hibernate Validator (reference implementation)
● Apache Bean Validation (incubation)
● GWT Validation

7. Annotation samples
@Min @Future @Null
@Past
@NotNull
@Digits
@MyCustomMegaConstraint
@Max
@Size
@Pattern

8. Custom constraint sample
@Target(TYPE)
@Retention(RUNTIME)
@Constraint(validatedBy = UniqueProductInOrderValidator.class)
public @interface UniqueProductInOrder {
String message() default "Only unique items";
Class<?>[] groups() default {};
Class<? extends Payload>[] payload() default {};
}

9. Bean sample
@UniqueProductInOrder
public class Item {
private String product;
@Valid private Order order;
@NotBlank
@Length(min = 5, max = 30)
public String getProduct() {
return product;
}
}

10. Validator sample
public class UniqueProductInOrderValidator implements
ConstraintValidator<UniqueProductInOrder, Item> {
@Override
public boolean isValid(Item v, ConstraintValidatorContext c) {
Collection<Item> items = value.getOrder().getItems();
Set<Item> set = new HashSet<Item>(items);
return set.size() == items.size();
}
}

11. Invoke validation sample
@RequestMapping(method = POST)
public String create(@Valid Item item) {
orderRepository.update(item.getOrder());
return "redirect:/orders/" + item.getOrder().getId();
}
@ExceptionHandler(BindException.class)
public ModelAndView itemHasErrors(BindException e) {
return new ModelAndView("addItem", e.getModel());
}

12. Another sample
@RequestMapping(value = "/orders", method = POST)
public String create(@Valid Order order, BindingResult errors) {
if (errors.hasErrors()) {
return null;
}
orderRepository.save(order);
return "redirect:/orders";
}

13. Groups sample
@GroupSequence({ Item.class, ComplexValidation.class })
@UniqueProductInOrder(groups = ComplexValidation.class)
public class Item {
@NotBlank
private String product;
}
interface ComplexValidation {}

14. Service layer validation
public class SomeService {
@Inject
private Validator validator;
public void doSomething(Item item) {
Set<ConstraintViolation<Order>> violations =
validator.validate(item);
}
}

15. Unit test sample
@Test
public void noValidationErrorsWhenItemIsValid() {
Item item = new Item(order, "x", 0);
Validator validator =
Validation.buildDefaultValidatorFactory().getValidator();
Set<ConstraintViolation<Item>> violations = validator.validate(item);
assertTrue(violations.isEmpty());
}

16. Single property unit test sample
@Test
public void nullProductIsNotAllowed() {
Item item = new Item(order, null, 0);
Validator validator =
Validation.buildDefaultValidatorFactory().getValidator();
Set<ConstraintViolation<Item>> violations =
validator.validateProperty(item, "product");
assertFalse(violations.isEmpty());
}

17. Hibernate Validator Annotation
Processor
Annotation processor based on JSR-269 which plugs
into the build process and raises compilation errors
whenever constraint annotations are incorrectly used.

18. https://github.com/aleksz/spring-
reference