9.1Mybatis的体系结构
    sqlSessionFactory 单个数据库映射关系经过编译后内存镜像，

    1. // 初始化mybatis，创建SqlSessionFactory类的实例
    2.     SqlSessionFactory sqlSessionFactory = new SqlSessionFactoryBuilder()
    3.             .build(inputStream);

    每一个MyBatis应用都以一个sqlSessionFactory 对象的实例为核心，一旦被创建，应用执行期间都应该存在，建议使用单例模式
    sqlSession 执行持久化操作的对象 单线程对象，线程不安全
    9.2深入Mybatis配置文件
    mybatis-config.xml
    sqlsessionFactory就是根据该配置文件创建的
    mabatis-config.xml文件中可以配置多个environments属性，即多个数据库连接属性。
    但是每个sqlSessionFactory只能对应一个数据库。8
    selectOne
    selectList
    selectMap
    9.3深入Mapper XML映射文件
    ResultMaps 最最重要强大的元素
    告诉Mybatis将结果集转换成开发者所需要的对象
    第9章 Mybatis的基本用法.note
    conetextConfigLocation
    contextConfigLocation

    如果这里没有,url为/时，直接去到index.jsp了
    type Status report
    message /hrmweb/main
    description The requested resource is not available.
    有    时
    type Status report
    message /hrmweb/loginForm
    description The requested resource is not available.
    request info
    requestURL—>http://localhost:8080/hrmapp/main
    requestContextPath—>/hrmapp
    servletPath—>/main
    going to forward to loginForm.jsp
    request info
    requestURL—>http://localhost:8080/hrmapp/loginForm
    requestContextPath—>/hrmapp
    servletPath—>/loginForm
    visiting /loginForm,/login,/404.html….
    return-flag-1=true
    return-flag-3=true
    return-flag-2=false
    request info
    requestURL—>http://localhost:8080/hrmapp/favicon.ico
    requestContextPath—>/hrmapp
    servletPath—>/favicon.ico
    going to forward to loginForm.jsp
    request info
    requestURL—>http://localhost:8080/hrmapp/loginForm
    requestContextPath—>/hrmapp
    servletPath—>/loginForm
    visiting /loginForm,/login,/404.html….
    return-flag-1=true
    return-flag-3=true
    return-flag-2=false
    request info
    requestURL—>http://localhost:8080/hrmweb/main
    requestContextPath—>/hrmweb
    servletPath—>/main
    going to forward to loginForm.jsp
    request info
    requestURL—>http://localhost:8080/hrmweb/loginForm
    requestContextPath—>/hrmweb
    servletPath—>/loginForm
    visiting /loginForm,/login,/404.html….
    return-flag-1=true
    return-flag-3=true
    return-flag-2=false