hibernate的Configuration类和SessionFactory接口

1,Configuration类的主要作用是解析Hibernate的配置文件和映射文件中的信息，即负责管理Hibernate的配置信息。Hibernate在运行时需要获取一些底层实现的基本信息，如数据库驱动类，数据路URL，数据库登录名，数据库登录密码等。

     通过Configuration对象的buildSessionFactory()方法可创建SessionFactory对象之后，由于配置信息已经由Hibernate维护并绑定在返回的SessionFactory中，此时Configuration已没有价值。

<pre name="code" class="html"><pre name="code" class="java">Configuration configuration = new Configuration(); 
configuration.configure("/hibernate.cfg.xml");
sessionFactory = configuration.buildSessionFactory();

我们看到：要创建一个Configuration,可以使用
Configuration config = new   Configuration().configure(); 这里，Configuration是入口，通过它来获得配置文件。
同时Configuration还可以通过指定参数来传递：
下面看：

File file = new File("c:\\Hibernate.xml"); 
Configuration config = new Configuration().config(file); 

2，SessionFactory接口

SessionFactory接口负责初始化Hibernate。它充当数据存储元的代理，并负责创建Session对象。通过Configuration实例构建SessionFactory对象。

sessionFactory = configuration.buildSessionFactory();

SessionFactory是线程安全的，可以被多个线程调用。在我们实际开发中，我们可以在初始化的部分构造一个SessionFactory即可。多数情况下，一个应用只初始化一个SessionFactory，为不同的线程提供Session。

下面是myeclipse添加Hibernate框架时，自动创建的HibernateSessionFactory.java文件。文件中有如下方法：

package SessionFactory;

import org.hibernate.HibernateException;
import org.hibernate.Session;
import org.hibernate.cfg.Configuration;

/**
 * Configures and provides access to Hibernate sessions, tied to the
 * current thread of execution.  Follows the Thread Local Session
 * pattern, see {@link http://hibernate.org/42.html }.
 */
public class HibernateSessionFactory {

    /** 
     * Location of hibernate.cfg.xml file.
     * Location should be on the classpath as Hibernate uses  
     * #resourceAsStream style lookup for its configuration file. 
     * The default classpath location of the hibernate config file is 
     * in the default package. Use #setConfigFile() to update 
     * the location of the configuration file for the current session.   
     */
    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;

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

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

	/**
     *  Close the single hibernate session instance.
     *
     *  @throws HibernateException
     */
    public static void closeSession() throws HibernateException {
        Session session = (Session) threadLocal.get();
        threadLocal.set(null);

        if (session != null) {
            session.close();
        }
    }

	/**
     *  return session factory
     *
     */
	public static org.hibernate.SessionFactory getSessionFactory() {
		return sessionFactory;
	}

	/**
     *  return session factory
     *
     *	session factory will be rebuilded in the next call
     */
	public static void setConfigFile(String configFile) {
		HibernateSessionFactory.configFile = configFile;
		sessionFactory = null;
	}

	/**
     *  return hibernate configuration
     *
     */
	public static Configuration getConfiguration() {
		return configuration;
	}

}