管理Session

傳統方法

Hibernate每次操作db時，需建立session。

// session factory
Configuration config = new Configuration().configure();
SessionFactory sessionFactory = config.buildSessionFactory();
// 操作db
Session session = sessionFactory.openSession();
session.beginTransaction();
session.saveOrUpdate(entity); 
session.getTransaction().commit();
session.close();

但每次的操作流程都是[建session]->操作DB->[關session]除了重複的code太多，也不優雅。若能整合Spring，則有HibernateDaoSupport或HibernateTemplate可以使用。但如果有些專案單純想使用Hibernate卻不想使用Srping呢？這時就要自行管理了。

Spring的HibernateTemplate

先看看使用Srping的HibernateTemplate如何實作。

public class MyDao extends HibernateDaoSupport {
    public void save(User e) {
        getHibernateTemplate().saveOrUpdate(e);
    }
    public List<User> queryXXX() {
        return getHibernateTemplate().execute(new HibernateCallback<List<User>>() {
            @SuppressWarnings("unchecked")
            @Override
            public List<User> doInHibernate(Session session) throws HibernateException, SQLException {
                // do your work
            }
        });
    }
}

說明:

• 使用Spring的HibernateDaoSupport，只要呼叫getHibernateTemplate()就可以直接操作db，而且也不必手動關閉session。
• 比較複雜的操作也是使用HibernateCallback.doInHibernate(Session)介面完成。並不直接操作session。

自己實作

使用HibernateSessionFactory提供session，並用HibernateSessionManager管理session。

public class HibernateSessionFactory {  
    private static String CONFIG_FILE_LOCATION = "/hibernate.cfg.xml";  
    private static final ThreadLocal<Session> threadLocal = new ThreadLocal<Session>();  
    private static Configuration configuration = new Configuration();      
    private static org.hibernate.SessionFactory sessionFactory;  
    private static String configFile = CONFIG_FILE_LOCATION;  

    private HibernateSessionFactory() {  
    }  

    /** 
     * Returns the ThreadLocal Session instance.  Lazy initialize 
     * the <code>SessionFactory</code> if needed. 
     * 
     *  @return Session 
     *  @throws HibernateException 
     */  
    public static Session getSession() throws HibernateException {  
        Session session = (Session) threadLocal.get();  
        if (session == null || !session.isOpen()) {  
            if (sessionFactory == null) {  
                rebuildSessionFactory();  
            }  
            session = (sessionFactory != null) ? sessionFactory.openSession() : null;  
            threadLocal.set(session);  
        }  
        return session;  
    }  

    public static void rebuildSessionFactory() {  
        try {  
            configuration.configure(configFile);  
            sessionFactory = configuration.buildSessionFactory();  
        } catch (Exception e) {  
            System.err.println("%%%% Error Creating SessionFactory %%%%");  
            e.printStackTrace();  
        }  
    }  

    public static void closeSession() throws HibernateException {  
        Session session = (Session) threadLocal.get();  
        threadLocal.set(null);  
        if (session != null && session.isOpen()) {  
            session.close();  
        }  
    }  

    public static org.hibernate.SessionFactory getSessionFactory() {  
        return sessionFactory;  
    }  

    public static void setConfigFile(String configFile) {  
        HibernateSessionFactory.configFile = configFile;  
        sessionFactory = null;  
    }  

    public static Configuration getConfiguration() {  
        return configuration;  
    }  

    public static Statistics getStatistics() {
        return sessionFactory.getStatistics();
    }

    public static long getSessionOpenCount() {
        return sessionFactory.getStatistics().getSessionOpenCount();
    }
}

public class HibernateSessionManager {
    public interface HibernateCallback<T> {
        public T doInHibernate(Session session);
    }

    public <E> E execTemplate(HibernateCallback<E> hc) {
        E result = null;
        Session s = getSession();
        Transaction tx = null;

        try {
            tx = s.beginTransaction();
            result = hc.doInHibernate(s); // equals to: s.saveOrUpdate(entity);
            tx.commit();
        } catch(Exception e) {
            e.printStackTrace();
            if(tx != null) tx.rollback();
        } finally {
            if(s != null) {
                s.close();
                s = null;
            }
        }
        return result;
    }

    protected Session getSession() {
        return HibernateSessionFactory.getSession();
    }
}