SqlSessionFactory是mybatis的基础中的基础，必须实例！
逻辑思路：
它的实例化需要（依赖）"mybatis-config.xml"文件，
其中有三大抽象：1、数据源；2、别名；3、注册mapper
可以把依赖（作为属性）注入（DI）到SqlSessionFactoryBean中，
来完成SqlSessionFactory的实例化。
pom：junit、webmvc、mysql-connector、spring-jdbc、mybatis、mybatis-spring、lombok
1、spring-dao.xml：bean约束
<beans xmlns="http://www.springframework.org/schema/beans"
xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance"
xmlns:context="http://www.springframework.org/schema/context"
xsi:schemaLocation="http://www.springframework.org/schema/beans
http://www.springframework.org/schema/beans/spring-beans.xsd
http://www.springframework.org/schema/context
http://www.springframework.org/schema/context/spring-context.xsd">
</beans>
2、db.properties
jdbc.driver=com.mysql.cj.jdbc.Driver
jdbc.url=jdbc:mysql://localhost:3306/数据库?serverTimezone=GMT%2B8
jdbc.username=root
jdbc.password=123
3、引入数据库配置文件
<context:property-placeholder location="classpath:db.properties"/>
4、从spring自带jdbc配置数据源
<bean id="dataSource" class="org.springframework.jdbc.datasource.DriverManagerDataSource">
<property name="driverClassName" value="${jdbc.driver}"/>
<property name="url" value="${jdbc.url}"/>
<property name="username" value="${jdbc.username}"/>
<property name="password" value="${jdbc.password}"/>
</bean>
5、利用SqlSessionFactoryBean获取配置SqlSessionFactory实例
<bean id="sqlSessionFactory" class="org.mybatis.spring.SqlSessionFactoryBean">
<property name="dataSource" ref="dataSource"/>
<property name="mapperLocations" value="classpath*:mapper/*.xml"/>
<property name="typeAliasesPackage" value="pojo"/>
</bean>
6、扫描dao包，同时生成sqlsessionTemplate和注入mapper接口的实现类
<bean class="org.mybatis.spring.mapper.MapperScannerConfigurer">
<property name="basePackage" value="dao" />
<property name="sqlSessionFactoryBeanName" value="sqlSessionFactory"/>
</bean>
7、加载spring-dao.xml获取上下文，从而为dao接口自动装配
ClassPathXmlApplicationContext context = new ClassPathXmlApplicationContext("spring/spring-dao.xml");
StudentDao studentDao = (StudentDao) context.getBean("studentDao");
List<Student> students = studentDao.selectAll();
