JForum“Support for several popular databases,including MysqL,Postgresql,Oracle and HsqlDB”。

JForum系统中用于持久化的实体类被放在net.jforum.entities包下，例如：User、UserSession、Post等。关于实体类的DAO操作被定义在多个接口中，例如：关于User的DAO操作被定义在net.jforum.dao.UserDAO接口中。针对不同的数据库，有不同的UserDAO实现类。

如上图，GenericUserDAO是JForum的默认实现，对应的数据库是MysqL。GenericUserDAO、HsqldbUserDAO、OracleUserDAO和sqlServerUserDAO类都有public的无参数构造函数。JForum不是根据“配置文件 + if else”的方式决定初始化哪个UserDAO，而是新建了一个抽象类DataAccessDriver。

public abstract class DataAccessDriver 
{
	private static DataAccessDriver driver;
	
	protected DataAccessDriver() {}
	
	/**
	 * Starts the engine.
	 * This method should be called when the system
	 * is starting. 
	 * 
	 * @param implementation The dao.driver implementation
	 */
	public static void init(DataAccessDriver implementation)
	{
		driver = implementation;
	}
	
	/**
	 * Gets a driver implementation instance. 
	 * You MUST use this method when you want a instance
	 * of a valid <code>DataAccessDriver</code>. Never access
	 * the driver implementation directly.  
	 * 
	 * @return <code>DataAccessDriver</code> instance
	 */
	public static DataAccessDriver getInstance()
	{
		return driver;
	}
	
	public abstract ForumDAO newForumDAO();
	public abstract GroupDAO newGroupDAO();
	public abstract PostDAO newPostDAO();
	public abstract PollDAO newPollDAO();
	public abstract RankingDAO newRankingDAO();
	public abstract TopicDAO newTopicDAO();
	public abstract UserDAO newUserDAO();
	public abstract CategoryDAO newCategoryDAO();
	public abstract TreeGroupDAO newTreeGroupDAO();
	public abstract SmilieDAO newSmilieDAO();
	public abstract GroupSecurityDAO newGroupSecurityDAO();
	public abstract PrivateMessageDAO newPrivateMessageDAO();
	public abstract UserSessionDAO newUserSessionDAO();
	public abstract ConfigDAO newConfigDAO();
	public abstract KarmaDAO newKarmaDAO();
	public abstract BookmarkDAO newBookmarkDAO();
	public abstract AttachmentDAO newAttachmentDAO();
	public abstract ModerationDAO newModerationDAO();
	public abstract BannerDAO newBannerDAO();
        public abstract SummaryDAO newSummaryDAO();
        public abstract MailIntegrationDAO newMailIntegrationDAO();
	public abstract ApiDAO newApiDAO();
	public abstract BanlistDAO newBanlistDAO();
	public abstract ModerationLogDAO newModerationLogDAO();
	public abstract LuceneDAO newLuceneDAO();
}

init()方法仅在系统启动时被调用一次。JForum系统中有多处使用此类init()，有一点反向注入的味道。

系统启动时，初始化DataAccessDriver：

	public static void loadDaoImplementation()
	{
		// Start the dao.driver implementation
		String driver = SystemGlobals.getValue(ConfigKeys.DAO_DRIVER);

		logger.info("Loading JDBC driver " + driver);

		try {
			Class c = Class.forName(driver);
			DataAccessDriver d = (DataAccessDriver)c.newInstance();
			DataAccessDriver.init(d);
		}
		catch (Exception e) {
			throw new ForumException(e);
		}
    }

如果是MysqL，String driver值为：net.jforum.dao.MysqL.MysqLDataAccessDriver