title: Where is implementation of @Future defined?
body: <p>Java persistence &amp; hibernate make it easy to add property-level constraints to entities.</p>
  
  <pre><code>@Entity
  @Table(name = "party")
  public class PartyEntity {
      @Future
      public DateTime start;
  }
  </code></pre>
  
  <p>The <code>javax.validation.constraint.Future</code> annotation is defined without a specific implementation.</p>
  
  <pre><code>@Target({ METHOD, FIELD, ANNOTATION_TYPE, CONSTRUCTOR, PARAMETER })
  @Retention(RUNTIME)
  @Documented
  @Constraint(validatedBy = {})
  public @interface Future {
      // ...
  }
  </code></pre>
  
  <p>Where is actual validation implemented? How can this be extended to allow an administrator override?</p>
  
  <p>Though not shown here, I am already using groups for another purpose on this constraint.</p>