Jaxb : Validation

JAXB also has the capability to validate an object graph in memory without actually writing it to XML. This allows client apps to check if a graph is okay and ready to process; if not, validation will identify objects that contain errors so that, for example, client apps can ask users to fix those.
The following code validates the object "o".

Validator v = context.createValidator();
    if(!v.validate(o))    System.err.println("error");

To receive detailed information about errors, you need to register ValidationEventHandler with the Validator, just like you did in Unmarshaller and Marshaller. This is analogous to registering an ErrorHandler for a SAX parser.
You can also first marshal an object graph and then validate XML (for example by Java API for validators). But doing so makes it much harder to associate errors with their sources, which makes debugging harder for humans. Validation after marshalling will give you errors like "missing <foo> element," but you can hardly know what is actually wrong in the object graph.
Validity is not enforced while you are modifying an object graph; you always have to explicitly validate it. To edit a valid object graph into another valid object graph, you may need to go through invalid intermediate states. If validity is enforced on every step of mutation, this becomes impossible.