MyBatis基础

1.概述

• MyBatis是支持定制化SQL、存储过程以及高级映射的优秀持久层框架

  1. 避免传统JDBC硬编码

  2. XML配置或者注解

  3. POJO对象和数据库记录直接映射

  4. 完善的文档支持

• MyBatis使用场景

  1. 更加关注SQL优化的项目

  2. 需求频繁更新改动的项目

2.开发环境搭建（以User实体类为例）

• 安装（maven中添加依赖到pom.xml）

  <dependency>
        <groupId>org.mybatis</groupId>
        <artifactId>mybatis</artifactId>
        <version>版本号</version>
  </dependency>

  若要使用数据库

  <dependency>
        <groupId>mysql</groupId>
        <artifactId>mysql-connector-java</artifactId>
        <version>5.1.41</version>
  </dependency>

• 在resources文件中添加mybatis-config.xml配置文件

  <xml version="1.0" encoding="UTF-8" >
  <!DOCTYPE configuration
          PUBLIC "-//mybatis.org//DTD Config 3.0//EN"
          "http://mybatis.org/dtd/mybatis-3-config.dtd">
  <configuration>
      <!--properties配置 用于加载外部的properties配置文件-->
      <properties resource="db.properties"></properties>
      <!--environments 主要用于数据源的配置，可以配置多个数据源-->
      <environments default="development">
          <environment id="development">
              <transactionManager type="JDBC"/>
              <dataSource type="POOLED">
                  <property name="driver" value="${driver}"/>
                  <property name="url" value="${url}"/>
                  <property name="username" value="${username}"/>
                  <property name="password" value="${password}"/>
              </dataSource>
          </environment>
      </environments>
      <mappers>
          <!--引入某一个具体的映射文件，resource进行路径方式引入 -->
          <mapper resource="mapper/usersMapper.xml"/>
      </mappers>
  </configuration>

  db.properties文件中：

  driver=com.mysql.jdbc.Driver
  url=jdbc:mysql://localhost:3306/mydb
  username=root
  password=

  usersMapper.xml配置文件中：

  <xml version="1.0" encoding="UTF-8" >
  <!DOCTYPE mapper
          PUBLIC "-//mybatis.org//DTD Mapper 3.0//EN"
          "http://mybatis.org/dtd/mybatis-3-mapper.dtd">
  <!--Mapper 用于定义一个映射文件的根节点
      namespace属性是用来配置命名空间，主要进行session级别的缓存管理
      命名空间默认情况下，使用我们当前操作的实体类的全路径
   -->
  <mapper namespace="com.imooc.entity.Users">
  
     <!-- <select id = "findById" resultType="com.imooc.entity.Users">
          SELECT * FROM users where id = #{ id }
      </select> -->
  
      <select id = "findAll" resultType="com.imooc.entity.Users">
          SELECT * FROM users
          <if test="id != null">
              where id = #{id}
          </if>
      </select>
  </mapper>

3.utils类 - SqlSessionFactoryUtils

public class SqlSessionFactoryUtils {
    private static String RESOURCE = "mybatis-config.xml";

    private static SqlSessionFactory sqlSessionFactory;

    //将SqlSessionFactory对象存放在ThreadLocal进行管理
    private static ThreadLocal<SqlSessionFactory>  threadLocal = new ThreadLocal<SqlSessionFactory>();

    /*
    初始化SqlsessionFactory方法
     */
    public static void initSqlSessionFactory(){
        try{
            InputStream is = Resources.getResourceAsStream(RESOURCE);

            sqlSessionFactory = new SqlSessionFactoryBuilder().build(is);
        }catch(IOException e){
            e.printStackTrace();
        }
    }
    /*
    获取工厂对象方法
     */
    public static SqlSessionFactory getSqlSessionFactory(){
        return sqlSessionFactory;
    }
    /*
    关闭Sqlsession方法
     */
    public static void close(){
        SqlSession session = (SqlSession) threadLocal.get();
        if(session != null){
            session.close();
            threadLocal.set(null);
        }
    }
}

4.dao类 - UsersDAO

public class UsersDAO {
    //执行查询操作 获取数据
    private SqlSession sqlSession ;
    private List<Users> list;
    private Users user;

    private SqlSession getSqlSession(){
        sqlSession = SqlSessionFactoryUtils.getSqlSessionFactory().openSession();
        return sqlSession;
    }
    /*
    全部数据查询
    */

    public List<Users> findAll(){
        try{
            list = getSqlSession().selectList("findAll");
        }catch(Exception e){
            e.printStackTrace();
        }finally{
            sqlSession.close();
        }
        return list;
    }
    /*
    根据编号查询
     */
    public Users findById(@Param("id")Integer id){
        try{
            user = getSqlSession().selectOne("findById",id);
        }catch(Exception e){
            e.printStackTrace();
        }finally{
            sqlSession.close();
        }
        return user;
    }
}

5.servlet类 - 多条数据查询

@WebServlet("/index")
public class UsersFindServlet extends HttpServlet {
    private UsersDAO usersDAO = new UsersDAO();

    @Override
    protected void doGet(HttpServletRequest req, HttpServletResponse resp) throws ServletException, IOException {
        this.doPost(req,resp);
    }

    @Override
    protected void doPost(HttpServletRequest req, HttpServletResponse resp) throws ServletException, IOException {
        List<Users> list = usersDAO.findAll();
        req.setAttribute("usersList",list);
        req.getRequestDispatcher("index.jsp").forward(req,resp);
    }
}

6.servlet类 - 单条数据查询

@WebServlet("/detail")
public class UsersFindByIdServlet extends HttpServlet {
    private UsersDAO usersDAO = new UsersDAO();

    @Override
    protected void doGet(HttpServletRequest req, HttpServletResponse resp) throws ServletException, IOException {
        this.doPost(req,resp);
    }

    @Override
    protected void doPost(HttpServletRequest req, HttpServletResponse resp) throws ServletException, IOException {
        String id = req.getParameter("id");
        Users user = usersDAO.findById(Integer.parseInt(id));
        req.setAttribute("user",user);
        req.getRequestDispatcher("detail.jsp").forward(req,resp);
    }
}

7.listener类 - 初始化Listener

//监听器 监听容器启动和销毁
@WebListener //注解配置 监听器注册到容器中
public class InitSqlSessionListener implements ServletContextListener {

    public void contextInitialized(ServletContextEvent servletContextEvent) {
        System.out.println("容器加载中...");
        //初始化SqlSessionFactory对象
        SqlSessionFactoryUtils.initSqlSessionFactory();
    }

    public void contextDestroyed(ServletContextEvent servletContextEvent) {
        System.out.println("容器销毁中...");
        //关闭SqlSession对象
        SqlSessionFactoryUtils.close();
    }
}