sessionFactory 为null

package com.choiwan;

import org.hibernate.SessionFactory;
import org.hibernate.cfg.Configuration;

public class HibernateDemo
{
    public static void main(String args[]) {
        //Configuration负责管理Hibernate配置消息
        Configuration config=new Configuration().configure();
        //根据config建立SessionFactory
        //SessionFactory将用于建立 Session
        //@SuppressWarnings("deprecation")
        SessionFactory sessionFactory=config.buildSessionFactory();
        
        IUserDAO userDAO=new UserDAO(sessionFactory);
        
        User user=new User();
        user.setName("caterpillar");
        user.setAge(new Integer(22));
        
        userDAO.insert(user);
        user=userDAO.find(new Integer(1));
        
        System.out.println("name:"+user.getName());
    }

}

<?xml version="1.0" encoding="utf-8"?> 
<!DOCTYPE hibernate-configuration PUBLIC
    "-//Hibernate/Hibernate Configuration DTD 3.0//EN"
    "http://hibernate.sourceforge.net/hibernate-configuration-3.0.dtd">
    
<hibernate-configuration> 
    <session-factory> 
        <property name="show_sql">
            true
        </property> 

        <property name="dialect">
            org.hibernate.dialect.MySQLDialect
        </property> 

        <property name="connection.driver_class">
            com.mysql.jdbc.Driver
        </property> 

        <property name="connection.url">
            jdbc:mysql://localhost/demo
        </property> 

        <property name="connection.username">
            root
        </property> 

        <property name="connection.password">
            4131025
        </property> 

        <!-- 以下设置对象与数据库表格的映射文件 --> 
        <mapping resource="com/choiwan/User.hbm.xml"/> 
    </session-factory> 
</hibernate-configuration>