[Java知识库]SSM——MyBatis概述

一. MyBatis简介

1. 什么是mybatis

• MyBatis 是一款优秀的持久层框架

• MyBatis 避免了几乎所有的 JDBC 代码和手动设置参数以及获取结果集的过程

• MyBatis 可以使用简单的 XML 或注解来配置和映射原生信息，将接口和 Java 的 实体类映射成数据库中的记录。

• MyBatis 本是apache的一个开源项目ibatis, 2010年这个项目由apache 迁移到了google code，并且改名为MyBatis 。

• Mybatis官方文档 : http://www.mybatis.org/mybatis-3/zh/index.html
  

2. 什么是持久化

持久化是将程序数据在持久状态和瞬时状态间转换的机制。

• 即把数据（如内存中的对象）保存到可永久保存的存储设备中（如磁盘）。持久化的主要应用是将内存中的对象存储在数据库中，或者存储在磁盘文件中、XML数据文件中等等。

• JDBC就是一种持久化机制。文件IO也是一种持久化机制。

持久层：

• 完成持久化工作的代码块 . ----> dao层 【DAO (Data Access Object) 数据访问对象】

• 说白了就是用来操作数据库存在的！

二. MyBatis开发步骤

1. 创建数据库

CREATE DATABASE `mybatis`;

USE `mybatis`;

DROP TABLE IF EXISTS `user`;

CREATE TABLE `user` (
`id` int(20) NOT NULL,
`name` varchar(30) DEFAULT NULL,
`pwd` varchar(30) DEFAULT NULL,
PRIMARY KEY (`id`)
) ENGINE=InnoDB DEFAULT CHARSET=utf8;

insert  into `user`(`id`,`name`,`pwd`) values (1,'狂神','123456'),(2,'张三','abcdef'),(3,'李四','987654');

2. IDEA中导入MyBatis所需jar包

<dependency>
   <groupId>org.mybatis</groupId>
   <artifactId>mybatis</artifactId>
   <version>3.5.2</version>
</dependency>
<dependency>
   <groupId>mysql</groupId>
   <artifactId>mysql-connector-java</artifactId>
   <version>5.1.47</version>
</dependency>

3. pojo层下编写实体类

public class User {
   
   private int id;  //id
   private String name;   //姓名
   private String pwd;   //密码
   
   //构造,有参,无参
   //set/get
   //toString()
   
}

4. resources下编写MyBatis核心配置文件

<?xml version="1.0" encoding="UTF8" ?>
<!DOCTYPE configuration
        PUBLIC "-//mybatis.org//DTD Config 3.0//EN"
        "http://mybatis.org/dtd/mybatis-3-config.dtd">

<configuration>
    <!--导入properties文件-->
    <properties resource="db.properties"/>

    <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>
<!--每一个Mapper.xml都需要在mybatis核心配置文件中注册-->
    <mappers>
        <mapper resource="com/pudding/dao/UserMapper.xml"/>
    </mappers>
</configuration>

MyBatis核心配置文件层级关系

driver=com.mysql.jdbc.Driver
url=jdbc:mysql://localhost:3306/mybatis?useSSL=true&useUnicode=true&characterEncoding=utf8
username=root
password=root

typeAliases给实体类起别名

5. utils包下编写MyBatis工具类

在工具类中获得resources包下的mybatis核心配置文件，进而获取SqlSession连接

package com.pudding.utils;

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;
import java.io.InputStream;

//SqlSessionFactory --> SqlSession
public class MybatisUtils {

    private static SqlSessionFactory sqlSessionFactory;
    static {
        try {
            //使用mybatis第一步：获取SqlSessionFactory对象
            String resource = "mybatis-config.xml";
            InputStream inputStream = Resources.getResourceAsStream(resource);
            sqlSessionFactory = new SqlSessionFactoryBuilder().build(inputStream);
        } catch (IOException e) {
            e.printStackTrace();
        }
    }

    //获取SqlSession连接
    //既然有了 SqlSessionFactory，顾名思义，我们可以从中获得 SqlSession 的实例
    //SqlSession 提供了在数据库执行 SQL 命令所需的所有方法
    public static SqlSession getSession(){
        return sqlSessionFactory.openSession();
    }
}

6. 在dao层下编写实体类的Mapper接口类

package com.pudding.dao;

import com.pudding.pojo.User;

import java.util.List;
import java.util.Map;

public interface UserMapper {

    //查询所有用户
    List<User> getUserList();

    //根据ID查询用户
    User getUserById(int id);

    User getUserById(Map<String,Object> map);

    //insert一个用户
    int addUser(User user);

    //万能的Map
    int addUser2(Map<String,Object> map);

    //修改用户
    int updateUser(User user);

    //根据id删除用户
    int deleteUser(int id);
}

7. 编写实体类对应的Mapper.xml配置文件

<?xml version="1.0" encoding="UTF8" ?>
<!DOCTYPE mapper
        PUBLIC "-//mybatis.org//DTD Mapper 3.0//EN"
        "http://mybatis.org/dtd/mybatis-3-mapper.dtd">
        
<!--namespace=绑定一个对应的Dao/Mapper接口-->
<mapper namespace="com.pudding.dao.UserMapper">
    <select id="getUserList" resultType="com.pudding.pojo.User">
        select * from user
    </select>

    <select id="getUserById" parameterType="int" resultType="com.pudding.pojo.User">
        select * from user where id = #{id}
    </select>

    <select id="getUserById2" parameterType="map" resultType="com.pudding.pojo.User">
        select * from user where id = #{id} and name = #{name}
    </select>

    <insert id="addUser" parameterType="com.pudding.pojo.User">
        insert into user (id, name ,pwd) value (#{id},#{name},#{pwd});
    </insert>

    <update id="updateUser" parameterType="com.pudding.pojo.User">
        update user set name=#{name},pwd=#{pwd} where id = #{id}
    </update>

    <delete id="deleteUser" parameterType="com.pudding.pojo.User">
        delete from user where id = #{id}
    </delete>
</mapper>

mybatis映射文件概述

7.1 Mybatis的Dao（Mapper）层实现

8. 通过测试类进行测试

package com.pudding.dao;

import com.pudding.pojo.User;
import com.pudding.utils.MybatisUtils;
import org.apache.ibatis.session.SqlSession;
import org.junit.Test;

import java.util.List;

public class UserMapperTest {
    @Test
    public void testfindAll(){
        //第一步：获得SQLSession对象
        SqlSession sqlSession = MybatisUtils.getSession();

        //方式一：getMapper
        UserMapper userMapper = sqlSession.getMapper(UserMapper.class);
        List<User> userList = userMapper.getUserList();

        for(User user : userList){
            System.out.println(user);
        }

        sqlSession.close();


    }

    @Test
    public void tsetSelectUserById() {
        SqlSession session = MybatisUtils.getSession();  //获取SqlSession连接
        UserMapper mapper = session.getMapper(UserMapper.class);
        User user = mapper.getUserById(1);
        System.out.println(user);
        session.close();
    }

    //增删改需要提交事务
    @Test
    public void testAddUser() {
        SqlSession session = MybatisUtils.getSession();
        UserMapper mapper = session.getMapper(UserMapper.class);
        User user = new User(4,"王五","zxcvbn");
        int i = mapper.addUser(user);
        System.out.println(i);
        session.commit(); //提交事务,重点!不写的话不会提交到数据库
        session.close();
    }

    @Test
    public void testUpdateUser() {
        SqlSession session = MybatisUtils.getSession();
        UserMapper mapper = session.getMapper(UserMapper.class);

        mapper.updateUser(new User(4, "哈哈", "123123"));

        session.commit(); //提交事务,重点!不写的话不会提交到数据库
        session.close();
    }

    @Test
    public void testDeleteUser() {
        SqlSession session = MybatisUtils.getSession();
        UserMapper mapper = session.getMapper(UserMapper.class);
        int i = mapper.deleteUser(4);
        System.out.println(i);
        session.commit(); //提交事务,重点!不写的话不会提交到数据库
        session.close();
    }
}

9. 注意事项

Maven中的build下要配置静态资源过滤问题

<?xml version="1.0" encoding="UTF8"?>
<project xmlns="http://maven.apache.org/POM/4.0.0"
         xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance"
         xsi:schemaLocation="http://maven.apache.org/POM/4.0.0 http://maven.apache.org/xsd/maven-4.0.0.xsd">
    <parent>
        <artifactId>Mybatis-Study</artifactId>
        <groupId>com.pudding</groupId>
        <version>1.0-SNAPSHOT</version>
    </parent>
    <modelVersion>4.0.0</modelVersion>

    <artifactId>mybatis-01</artifactId>

    <build>
        <resources>
            <resource>
                <directory>src/main/java</directory>
                <includes>
                    <include>**/*.properties</include>
                    <include>**/*.xml</include>
                </includes>
                <filtering>false</filtering>
            </resource>
            <resource>
                <directory>src/main/resources</directory>
                <includes>
                    <include>**/*.properties</include>
                    <include>**/*.xml</include>
                </includes>
                <filtering>false</filtering>
            </resource>
        </resources>
    </build>

</project>

三. MyBatis相应的API

1. SqlSessionFactoryBuilder（SqlSession工厂构建器）

2. SqlSessionFactory（SqlSession工厂对象）

3. SqlSession（会话对象）