Generic DAO and JPA

Generic DAO is a design pattern to reduce and prevent code replacing on basic CRUD operations. Hibernate-Generic-DAO (http://code.google.com/p/hibernate-generic-dao/) provides exactly this pattern solution for Hibernate or JPA.

UML Overview of this example:

BaseDAO is an extension of GenericDAO and it must be extended by each DAO class of the project.

JPAGenericDAO is a generic, abstract class to be extended by each concrete DAO class.

Sample code:

1. /**
2.  * @author Hasan Oezdemir
3.  * @since 01.10.2011
4.  *
5.  */
6. public abstract class JPAGenericDAO<T, ID extends Serializable> extends GenericDAOImpl<T, ID> implements
7.         BaseDAO<T, ID> {
9.     private static final Logger logger = LoggerFactory.getLogger(JPAGenericDAO.class);
10.    
11.     @PersistenceContext(unitName="scheduling")
12.     private EntityManager entityManager;
13.    
14.     public JPAGenericDAO(){
15.         super();
16.     }
17.    
18.     @PostConstruct
19.     public void initialized(){
20.         logger.info( "initializing generic dao, setting entity manager: {}", entityManager);
21.         this.setEntityManager(entityManager);
22.         JPASearchProcessor searchProcessor = new JPASearchProcessor(new JPAAnnotationMetadataUtil());
23.         this.setSearchProcessor(searchProcessor);
24.     }
26.     @Override
27.     public T findByExampleUnique(T example) {
28.         Search s = this.createSearch(example);
29.         return searchUnique(s);
30.     }
32.     @Override
33.     public Collection<T> findByExample(T example) {
34.         Search s = this.createSearch(example);
35.         return search(s);
36.     }
37.    
38.     protected Search createSearch(T example) {
39.         Search s = new Search(example.getClass());
40.         s.addFilter(this._getFilterFromExample(example));
41.         return s;
42.     }
43. }

UserDAO extends the BaseDAO :

1. /**
2.  * @author Hasan Oezdemir
3.  * @since 01.10.2011
4.  *
5.  */
6. public interface UserDAO extends BaseDAO<User, Long> {
8. }

and a sample reference implementation of DAOs :

1. /**
2.  * @author Hasan Oezdemir
3.  * @since 01.10.2011
4.  *
5.  */
6. @Singleton
7. public class JPAUserDAO extends JPAGenericDAO<User, Long> implements UserDAO {
9.     public JPAUserDAO(){
10.         super();
11.     }
12. }

We keep basic CRUD operations in our abstract class but we have also seperate interfaces to provide 'Model' specific method and keep them seperate from generic DAO solutions.