This is a pattern for Data Access Objects with JDK 5.0, from the CaveatEmptor example application. It is also explained in the book Java Persistence with Hibernate. Two links you might find useful: Sessions and transactions and Open Session in View.

This time I based the DAO example on interfaces. Tools like Hibernate already provide database portability, so persistence layer portability shouldn't be a driving motivation for interfaces. However, DAO interfaces make sense in more complex applications, when several persistence services are encapsulate in one persistence layer. I'd say that you should use Hibernate (or Java Persistence APIs) directly in most cases, the best reason to use an additional DAO layer is higher abstraction (e.g. methods like getMaximumBid() instead of session.createQuery(...) repeated a dozen times).

The DAO interfaces

I use one interface per persistent entity, with a super interface for common CRUD functionality:

public interface GenericDAO<T, ID extends Serializable> { T findById(ID id, boolean lock); List<T> findAll(); List<T> findByExample(T exampleInstance); T makePersistent(T entity); void makeTransient(T entity); }

You can already see that this is going to be a pattern for a state-oriented data access API, with methods such as makePersistent() and makeTransient() . Furthermore, to implement a DAO you have to provide a type and an identifier argument. As for most ORM solutions, identifier types have to be serializable.

The DAO interface for a particular entity extends the generic interface and provides the type arguments:

public interface ItemDAO extends GenericDAO<Item, Long> { public static final String QUERY_MAXBID = "ItemDAO.QUERY_MAXBID"; public static final String QUERY_MINBID = "ItemDAO.QUERY_MINBID"; Bid getMaxBid(Long itemId); Bid getMinBid(Long itemId); }

We basically separate generic CRUD operations and actual business-related data access operations from each other. (Ignore the named query constants for now, they are convenient if you use annotations.) However, even if only CRUD operations are needed for a particular entity, you should still write an interface for it, even it it is going to be empty. It is important to use a concrete DAO in your controller code, otherwise you will face some refactoring once you have to introduce specific data access operations for this entity.

An implementation with Hibernate

An implementation of the interfaces could be done with any state-management capable persistence service. First, the generic CRUD implementation with Hibernate:

public abstract class GenericHibernateDAO<T, ID extends Serializable> implements GenericDAO<T, ID> { private Class<T> persistentClass; private Session session; public GenericHibernateDAO() { this.persistentClass = (Class<T>) ((ParameterizedType) getClass() .getGenericSuperclass()).getActualTypeArguments()[0]; } @SuppressWarnings("unchecked") public void setSession(Session s) { this.session = s; } protected Session getSession() { if (session == null) throw new IllegalStateException("Session has not been set on DAO before usage"); return session; } public Class<T> getPersistentClass() { return persistentClass; } @SuppressWarnings("unchecked") public T findById(ID id, boolean lock) { T entity; if (lock) entity = (T) getSession().load(getPersistentClass(), id, LockMode.UPGRADE); else entity = (T) getSession().load(getPersistentClass(), id); return entity; } @SuppressWarnings("unchecked") public List<T> findAll() { return findByCriteria(); } @SuppressWarnings("unchecked") public List<T> findByExample(T exampleInstance, String[] excludeProperty) { Criteria crit = getSession().createCriteria(getPersistentClass()); Example example = Example.create(exampleInstance); for (String exclude : excludeProperty) { example.excludeProperty(exclude); } crit.add(example); return crit.list(); } @SuppressWarnings("unchecked") public T makePersistent(T entity) { getSession().saveOrUpdate(entity); return entity; } public void makeTransient(T entity) { getSession().delete(entity); } public void flush() { getSession().flush(); } public void clear() { getSession().clear(); } /** * Use this inside subclasses as a convenience method. */ @SuppressWarnings("unchecked") protected List<T> findByCriteria(Criterion... criterion) { Criteria crit = getSession().createCriteria(getPersistentClass()); for (Criterion c : criterion) { crit.add(c); } return crit.list(); } }

There are some interesting things in this implementation. First, it clearly needs a Session to work, provided with setter injection. You could also use constructor injection. How you set the Session and what scope this Session has is of no concern to the actual DAO implementation. A DAO should not control transactions or the Session scope.

We need to suppress a few compile-time warnings about unchecked casts, because Hibernate's interfaces are JDK 1.4 only. What follows are the implementations of the generic CRUD operations, quite straightforward. The last method is quite nice, using another JDK 5.0 feature, varargs. It helps us to build Criteria queries in concrete entity DAOs. This is an example of a concrete DAO that extends the generic DAO implementation for Hibernate:

public class ItemDAOHibernate extends GenericHibernateDAO<Item, Long> implements ItemDAO { public Bid getMaxBid(Long itemId) { Query q = getSession().getNamedQuery(ItemDAO.QUERY_MAXBID); q.setParameter("itemid", itemId); return (Bid) q.uniqueResult(); } public Bid getMinBid(Long itemId) { Query q = getSession().getNamedQuery(ItemDAO.QUERY_MINBID); q.setParameter("itemid", itemId); return (Bid) q.uniqueResult(); } }

Another example which uses the findByCriteria() method of the superclass with variable arguments:

public class CategoryDAOHibernate extends GenericHibernateDAO<Category, Long> implements CategoryDAO { public Collection<Category> findAll(boolean onlyRootCategories) { if (onlyRootCategories) return findByCriteria( Expression.isNull("parent") ); else return findAll(); } }

Preparing DAOs with factories

We could bring it all together in a DAO factory, which not only sets the Session when a DAO is constructed but also contains nested classes to implement CRUD-only DAOs with no business-related operations:

public class HibernateDAOFactory extends DAOFactory { public ItemDAO getItemDAO() { return (ItemDAO)instantiateDAO(ItemDAOHibernate.class); } public CategoryDAO getCategoryDAO() { return (CategoryDAO)instantiateDAO(CategoryDAOHibernate.class); } public CommentDAO getCommentDAO() { return (CommentDAO)instantiateDAO(CommentDAOHibernate.class); } public ShipmentDAO getShipmentDAO() { return (ShipmentDAO)instantiateDAO(ShipmentDAOHibernate.class); } private GenericHibernateDAO instantiateDAO(Class daoClass) { try { GenericHibernateDAO dao = (GenericHibernateDAO)daoClass.newInstance(); dao.setSession(getCurrentSession()); return dao; } catch (Exception ex) { throw new RuntimeException("Can not instantiate DAO: " + daoClass, ex); } } // You could override this if you don't want HibernateUtil for lookup protected Session getCurrentSession() { return HibernateUtil.getSessionFactory().getCurrentSession(); } // Inline concrete DAO implementations with no business-related data access methods. // If we use public static nested classes, we can centralize all of them in one source file. public static class CommentDAOHibernate extends GenericHibernateDAO<Comment, Long> implements CommentDAO {} public static class ShipmentDAOHibernate extends GenericHibernateDAO<Shipment, Long> implements ShipmentDAO {} }

This concrete factory for Hibernate DAOs extends the abstract factory, which is the interface we'll use in application code:

public abstract class DAOFactory { /** * Creates a standalone DAOFactory that returns unmanaged DAO * beans for use in any environment Hibernate has been configured * for. Uses HibernateUtil/SessionFactory and Hibernate context * propagation (CurrentSessionContext), thread-bound or transaction-bound, * and transaction scoped. */ public static final Class HIBERNATE = org.hibernate.ce.auction.dao.hibernate.HibernateDAOFactory.class; /** * Factory method for instantiation of concrete factories. */ public static DAOFactory instance(Class factory) { try { return (DAOFactory)factory.newInstance(); } catch (Exception ex) { throw new RuntimeException("Couldn't create DAOFactory: " + factory); } } // Add your DAO interfaces here public abstract ItemDAO getItemDAO(); public abstract CategoryDAO getCategoryDAO(); public abstract CommentDAO getCommentDAO(); public abstract ShipmentDAO getShipmentDAO(); }

Note that this factory example is suitable for persistence layers which are primarily implemented with a single persistence service, such as Hibernate or EJB 3.0 persistence. If you have to mix persistence APIs, for example, Hibernate and plain JDBC, the pattern changes slightly. Keep in mind that you can also call session.connection() inside a Hibernate-specific DAO, or use one of the many bulk operation/SQL support options in Hibernate 3.1 to avoid plain JDBC.

Finally, this is how data access now looks like in controller/command handler code (pick whatever transaction demarcation strategy you like, the DAO code doesn't change):

// EJB3 CMT: @TransactionAttribute(TransactionAttributeType.REQUIRED) public void execute() { // JTA: UserTransaction utx = jndiContext.lookup("UserTransaction"); // JTA: utx.begin(); // Plain JDBC: HibernateUtil.getCurrentSession().beginTransaction(); DAOFactory factory = DAOFactory.instance(DAOFactory.HIBERNATE); ItemDAO itemDAO = factory.getItemDAO(); UserDAO userDAO = factory.getUserDAO(); Bid currentMaxBid = itemDAO.getMaxBid(itemId); Bid currentMinBid = itemDAO.getMinBid(itemId); Item item = itemDAO.findById(itemId, true); newBid = item.placeBid(userDAO.findById(userId, false), bidAmount, currentMaxBid, currentMinBid); // JTA: utx.commit(); // Don't forget exception handling // Plain JDBC: HibernateUtil.getCurrentSession().getTransaction().commit(); // Don't forget exception handling }

The database transaction, either JTA or direct JDBC, is started and committed in an interceptor that runs for every execute() , following the Open Session in View pattern. You can use AOP for this or any kind of interceptor that can be wrapped around a method call, see Session handling with AOP.

Preparing DAOs with manual dependency injection

You don't need to write the factories. You can as well just do this:

// EJB3 CMT: @TransactionAttribute(TransactionAttributeType.REQUIRED) public void execute() { // JTA: UserTransaction utx = jndiContext.lookup("UserTransaction"); // JTA: utx.begin(); // Plain JDBC: HibernateUtil.getCurrentSession().beginTransaction(); ItemDAOHibernate itemDAO = new ItemDAOHibernate(); itemDAO.setSession(HibernateUtil.getSessionFactory().getCurrentSession()); UserDAOHibernate userDAO = new UserDAOHibernate(); userDAO.setSession(HibernateUtil.getSessionFactory().getCurrentSession()); Bid currentMaxBid = itemDAO.getMaxBid(itemId); Bid currentMinBid = itemDAO.getMinBid(itemId); Item item = itemDAO.findById(itemId, true); newBid = item.placeBid(userDAO.findById(userId, false), bidAmount, currentMaxBid, currentMinBid); // JTA: utx.commit(); // Don't forget exception handling // Plain JDBC: HibernateUtil.getCurrentSession().getTransaction().commit(); // Don't forget exception handling }

The disadvantage here is that the implementation classes (i.e. ItemDAOHibernate and UserDAOHibernate ) of the persistence layer are exposed to the client, the controller. Also, constructor injection of the current Session might be more appropriate.

Preparing DAOs with lookup

Alternatively, call HibernateUtil.getSessionFactory().getCurrentSession() as a fallback, if the client didn't provide a Session when the DAO was constructed:

public abstract class GenericHibernateDAO<T, ID extends Serializable> implements GenericDAO<T, ID> { private Class<T> persistentClass; private Session session; public GenericHibernateDAO() { this.persistentClass = (Class<T>) ((ParameterizedType) getClass() .getGenericSuperclass()).getActualTypeArguments()[0]; } public void setSession(Session session) { this.session = session; } protected void getSession() { if (session == null) session = HibernateUtil.getSessionFactory().getCurrentSession(); return session; } ...

The controller now uses these stateless data access objects through direct instantiation:

// EJB3 CMT: @TransactionAttribute(TransactionAttributeType.REQUIRED) public void execute() { // JTA: UserTransaction utx = jndiContext.lookup("UserTransaction"); // JTA: utx.begin(); // Plain JDBC: HibernateUtil.getCurrentSession().beginTransaction(); ItemDAO itemDAO = new ItemDAOHibernate(); UserDAO userDAO = new UserDAOHibernate(); Bid currentMaxBid = itemDAO.getMaxBid(itemId); Bid currentMinBid = itemDAO.getMinBid(itemId); Item item = itemDAO.findById(itemId, true); newBid = item.placeBid(userDAO.findById(userId, false), bidAmount, currentMaxBid, currentMinBid); // JTA: utx.commit(); // Don't forget exception handling // Plain JDBC: HibernateUtil.getCurrentSession().getTransaction().commit(); // Don't forget exception handling }

The only disadvantage of this very simple strategy is that the implementation classes (i.e. ItemDAOHibernate and UserDAOHibernate ) of the persistence layer are again exposed to the client, the controller. You can still supply a custom Session if needed (integration test, etc).

Each of these methods (factories, manual injection, lookup) for setting the current Session and creating a DAO instance has advantages and drawbacks, use whatever you feel most comfortable with.

Naturally, the cleanest way is managed components and EJB 3.0 session beans:

Writing DAOs as managed EJB 3.0 components

Turn your DAO superclass into a base class for stateless session beans (all your concrete DAOs are then stateless EJBs, they already have a business interface). This is basically a single annotation which you could even move into an XML deployment descriptor if you like. You can then use dependency injection and get the "current" persistence context provided by the container:

@Stateless public abstract class GenericHibernateDAO<T, ID extends Serializable> implements GenericDAO<T, ID> { private Class<T> persistentClass; @PersistenceContext private EntityManager em; public GenericHibernateDAO() { setSession( (Session)em.getDelegate() ); } ...

You can then cast the delegate of an EntityManager to a Hibernate Session .

This only works if you use Hibernate as a Java Persistence provider, because the delegate is the Session API. In JBoss AS you could even get a Session injected directly. If you use a different Java Persistence provider, rely on the EntityManager API instead of Session . Now wire your DAOs into the controller, which is also a managed component:

@Stateless public class ManageAuctionController implements ManageAuction { @EJB ItemDAO itemDAO; @EJB UserDAO userDAO; @TransactionAttribute(TransactionAttributeType.REQUIRED) // This is even the default public void execute() { Bid currentMaxBid = itemDAO.getMaxBid(itemId); Bid currentMinBid = itemDAO.getMinBid(itemId); Item item = itemDAO.findById(itemId, true); newBid = item.placeBid(userDAO.findById(userId, false), bidAmount, currentMaxBid, currentMinBid); } }

P.S. Credit has to be given to Eric Burke, who first posted the basics for this pattern on his blog. Unfortunately, not even the Google cache is available anymore.

A Better typed Generic DAO? You decide!

We are missing something on <T> end, since T allows you to "domain-ify" everything! and the Identifier type should ideally match the identifier type of T, but there's no way to do that on the code above. You decide which approach is better.

// Our common Model interface that an abstract Domain model will implement and all domain // models will extend. public interface IModel<ID extends Serializable> { public abstract ID getId(); public abstract void setId(final ID pId); } // Our generic DAO, NOTE: MODEL's ID type is the same as ID now, which makes sense. // Also model type is more restrictive, dis-allowing all kinds of funky stuff to go in. public abstract class GenericHibernateDAO<MODEL extends IModel<ID>, ID extends Serializable> implements GenericDAO<T, ID> { private Class<MODEL> persistentClass; private Session session; public GenericHibernateDAO() { // FIXME : I don't like magic number in the code, is there any way to fix 0 to something dynamic? this.persistentClass = (Class<MODEL>) ((ParameterizedType) getClass() .getGenericSuperclass()).getActualTypeArguments()[0]; } public final void setSession(final Session pSession) { this.session = session; } protected void getSession() { if (session == null) session = HibernateUtil.getSessionFactory().getCurrentSession(); return session; } ...

In addition, we could add things like:

public final String getRootAlias(){ this.getPersistentClass().getSimpleName() + String.valueOf('_'); }

Alhough this is not necessary or part of the enhanced version, but when criteria API is in use, this comes in handy.