Hibernate Validator has two built-in Hibernate event listeners. Whenever a PreInsertEvent or PreUpdateEvent occurs, the listeners will verify all constraints of the entity instance and throw an exception if any constraint is violated. Basically, objects will be checked before any inserts and before any updates made by Hibernate. This is the most convenient and the easiest way to activate the validation process. On constraint violation, the event will raise a runtime InvalidStateException which contains an array of InvalidValues describing each failure.

<hibernate-configuration>
    ...
    <event type="pre-update">
        <listener 
          class="org.hibernate.validator.event.ValidatePreUpdateEventListener"/>
    </event>
    <event type="pre-insert">
        <listener 
          class="org.hibernate.validator.event.ValidatePreInsertEventListener"/>
    </event>
</hibernate-configuration>