hibernate官方实现Generic Data Access Objects

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.