Validation services for complex domain objects.

Generally you will write `Validator`s for each of your domain objects, and add a {@link ValidationManager} to your application context. You will need to wire a suitable {@link IntrospectionManager} against the `ValidationManager` so that children of a domain object presented for validation can be identified and in turn also validated.

The {@link ValidationInterceptor} and {@link ValidationAdvisor} are provided, although their use is not recommended against DAOs given many `Validator`s require a DAO and this will cause a loop that results in the DAO not being advised. Instead your DAO implementations should have their mutator methods pass the object to the `ValidationManager` prior to persistence. This is a non-AOP approach, but represetns a practical solution.

If you domain objects themselves wish to ensure they are in a valid state prior to internal business methods being invoked, it is suggested they provide a `ValidationManager` collaborator, and fire its validate method. Such collaborator can be autowired during both instance retrieval and creation. It should generally also be marked as `transient`, to avoid possible serialisation issues if used inside a `HttpSession` or similar.

Sometimes domain objects need to internally update themselves before being validated. Any such domain objects should implement {@link BindBeforeValidation}. The `ValidationManager` will fire the related method just prior to validation, and you can do it manually using {@link BindBeforeValidationUtils}. Using the utility class is generally preferred over calling the method directly, as it ignores classes that do not implement `BindBeforeValidation`.

Finally, sometimes `Validator`s might need to perform queries against a persistence or services layer. For example, the `Validator` may be checking no other user has this username, or comparing the object's old state to detect modifications that violate business rules. If using an ORM tool such as Hibernate, it is recommended you use the `EvictionUtils` static methods to remove objects from the session.