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 CRUDfunctionality:
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.
出自：http://www.hibernate.org