注意到DaoFactory中的SqlSession不是全局唯一的并且没有关闭，这里SqlSession为什么不像SqlSessionFactory一样设置成static的熟悉然后全局唯一呢？麻烦老师详细讲解一下。

​package com.imooc.icake.global;

import org.apache.ibatis.io.Resources;
import org.apache.ibatis.session.SqlSession;
import org.apache.ibatis.session.SqlSessionFactory;
import org.apache.ibatis.session.SqlSessionFactoryBuilder;

import java.io.IOException;

public class DaoFactory {
    private static DaoFactory daoFactory;
    private static SqlSessionFactory sqlSessionFactory;

    private DaoFactory(){
        SqlSessionFactoryBuilder sqlSessionFactoryBuilder=new SqlSessionFactoryBuilder();
        try {
            sqlSessionFactory=sqlSessionFactoryBuilder.build(Resources.getResourceAsReader("/mybatis.xml"));
        } catch (IOException e) {
            e.printStackTrace();
        }
    }

    public static DaoFactory newInstance(){
        if(daoFactory==null){
            daoFactory=new DaoFactory();
        }
        return daoFactory;
    }

    public <T> T getDao(Class<T> tClass){
        SqlSession sqlSession=sqlSessionFactory.openSession(true);
        return sqlSession.getMapper(tClass);
    }
}