public class AkteIdGenerator implements IdentifierGenerator {
   public Serializable generate(SessionImplementor session, Object object)
         throws HibernateException {
      // if custom id is set -> use this id
      if (object instanceof SomeBean) {
         SomeBean someBean = (SomeBean) object;
         Long customId = someBean.getCustomId();
         if (customId != 0) {
            return customId;
         }
      }
      // otherwise --> call the SequenceGenerator manually
      SequenceStyleGenerator sequenceGenerator ...
   }
}

    @GeneratedValue(
            strategy = GenerationType.SEQUENCE,
            generator = "MY_SEQUENCE")
    @SequenceGenerator(
            allocationSize = 1,
            name = "MY_SEQUENCE",
            sequenceName = "MY_SEQUENCE_NAME")