Myabtis 官网文档

官网网站：https://mybatis.org/mybatis-3/zh/index.html

搭建环境

项目结构

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引入依赖

创建Maven项目，pom依赖文件中加入mybatis和jdbc驱动依赖。

		<dependency>
            <groupId>org.mybatis</groupId>
            <artifactId>mybatis</artifactId>
            <version>3.5.11</version>
        </dependency>
        <!-- https://mvnrepository.com/artifact/com.mysql/mysql-connector-j -->
        <dependency>
            <groupId>com.mysql</groupId>
            <artifactId>mysql-connector-j</artifactId>
            <version>8.0.31</version>
        </dependency>

配置文件

在resources资源文件下创建xml配置文件，文件名随意。
mybatis-config.xml

<?xml version="1.0" encoding="UTF-8" ?>
<!DOCTYPE configuration
        PUBLIC "-//mybatis.org//DTD Config 3.0//EN"
        "https://mybatis.org/dtd/mybatis-3-config.dtd">
<configuration>
    <environments default="development">
        <environment id="development">
            <transactionManager type="JDBC"/>
            <dataSource type="POOLED">
                <property name="driver" value="com.mysql.cj.jdbc.Driver"/>
                <property name="url" value="jdbc:mysql://localhost:3306/mybatis"/>
                <property name="username" value="root"/>
                <property name="password" value="123456"/>
            </dataSource>
        </environment>
    </environments>
</configuration>

数据库

创建名为mybatis的数据库，添加用户表。主键id自增
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创建实体类

User.java

public class User implements Serializable {
    private static final long serialVersionUID = -47554981711793793L;

    private Integer id;

    private String username;

    private String sex;
 
    private String address;

    public User() {
    }

    public User(Integer id, String username, String sex, String address) {
        this.id = id;
        this.username = username;
        this.sex = sex;
        this.address = address;
    }
    ...toString Set、Get 方法

创建Mapper接口

java代码中创建UserMapper接口，封装user表的数据库操作。
UserMapper.java

public interface UserMapper {
    List<User> selectAll();

    User selectById(int id);

    int insert(User user);
    
    int update(User user);

    int delete(int[] ids);
}

创建Mapper的配置文件
UserMapper.xml

<?xml version="1.0" encoding="UTF-8" ?>
<!DOCTYPE mapper
        PUBLIC "-//mybatis.org//DTD Mapper 3.0//EN"
        "https://mybatis.org/dtd/mybatis-3-mapper.dtd">
<!--namespace为UserMapper接口的全类名-->
<mapper namespace="com.rzg.mapper.UserMapper">
<!--    id：对应的UserMapper接口中的方法名   resultType：返回值类型的全类名-->
    <select id="selectAll" resultType="com.rzg.entity.User">
        select id,username,sex,address from user
    </select>

    <select id="selectById" resultType="com.rzg.entity.User">
        select id,username,sex,address from user where id = #{id}
    </select>

    <insert id="insert">
        insert into user(id,username,sex,address) values(null,#{username},#{sex},#{address})
    </insert>

    <delete id="delete">
        delete from user where id in
        <foreach collection="array" item="id" separator="," open="(" close=")">
            #{id}
        </foreach>
    </delete>
    
    <update id="update">
        update user set username = #{username},sex=#{sex},address = #{address} where id = #{id}
    </update>

</mapper>

myabtis配置文件添加mapper配置文件

myabtis-config.xml

<?xml version="1.0" encoding="UTF-8" ?>
<!DOCTYPE configuration
        PUBLIC "-//mybatis.org//DTD Config 3.0//EN"
        "https://mybatis.org/dtd/mybatis-3-config.dtd">
<configuration>
    <environments default="development">
        <environment id="development">
            <transactionManager type="JDBC"/>
            <dataSource type="POOLED">
                <property name="driver" value="com.mysql.cj.jdbc.Driver"/>
                <property name="url" value="jdbc:mysql://localhost:3306/mybatis"/>
                <property name="username" value="root"/>
                <property name="password" value="123456"/>
            </dataSource>
        </environment>
    </environments>
    <mappers>
        <mapper resource="com/rzg/mapper/UserMapper.xml"/>
    </mappers>
</configuration>

运行

package com.rzg;

import com.rzg.entity.User;
import com.rzg.mapper.UserMapper;
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 javax.annotation.Resource;
import java.io.IOException;
import java.io.InputStream;
import java.util.List;

public class App {

    public static void main(String[] args) throws IOException {

        //使用myabtis自带的工具类Resources，加载xml配置文件
        InputStream is = Resources.getResourceAsStream("mybatis-config.xml");
        //使用配置文件输入流，构建出sqlSessionFactory
        SqlSessionFactory sqlSessionFactory = new SqlSessionFactoryBuilder().build(is);
        //sqlSessionFactory构建出sqlSession对象，用于一次会话，官方建议将sqlSession放在方法域中
        SqlSession sqlSession = sqlSessionFactory.openSession();
        //获得UserMapper的代理对象
        UserMapper mapper = sqlSession.getMapper(UserMapper.class);

        User user = new User();
        user.setUsername("张三");
        user.setSex("男");
        user.setAddress("上海");

        int i = mapper.delete(new int[]{32, 34});
        //提交事务
        sqlSession.commit();
        //关闭sqlSession释放资源
        sqlSession.close();
        System.out.println(i);
    }
}

typeAliases 别名

在Mapper的xml配置文件中，需要为每一个方法指定返回类型resultType，每一都要写一长串的全限定类名好麻烦，Mybatis提供了别名。
例子：在mybatis的配置文件myabtis-config中为com.rzg.entity.User设置别名User

<?xml version="1.0" encoding="UTF-8" ?>
<!DOCTYPE configuration
        PUBLIC "-//mybatis.org//DTD Config 3.0//EN"
        "https://mybatis.org/dtd/mybatis-3-config.dtd">
<configuration>
    <typeAliases>
        <typeAlias  alias="User" type="com.rzg.entity.User"/>
    </typeAliases>
    <environments default="development">
        <environment id="development">
            <transactionManager type="JDBC"/>
            <dataSource type="POOLED">
                <property name="driver" value="com.mysql.cj.jdbc.Driver"/>
                <property name="url" value="jdbc:mysql://localhost:3306/mybatis"/>
                <property name="username" value="root"/>
                <property name="password" value="123456"/>
            </dataSource>
        </environment>
    </environments>
    <mappers>
        <mapper resource="com/rzg/mapper/UserMapper.xml"/>
    </mappers>
</configuration>

UserMapper.xml的配置文件中的com.rzg.entity.User可以改成User

 <select id="selectAll" resultType="User">
        select id,username,sex,address from user
 </select>

如果很多实体需要设置别名会很麻烦，可以指定包，将该包下面所有实体设置为首字母小写的别名。

<typeAliases>
     <package name="com.rzg.entity"/>
</typeAliases>

设置别名后，resultType中可以不区分大小写，甚至可以全部大写。

<select id="selectAll" resultType="USER">
        select id,username,sex,address from user
</select>

属性（properties）

properties标签

在mybatis-config配置文件中，我们将dataSource 属性driver、url、username、password写死了，不方便以后的修改。可以将其提取出来，作为properties，像下面这样。

<?xml version="1.0" encoding="UTF-8" ?>
<!DOCTYPE configuration
        PUBLIC "-//mybatis.org//DTD Config 3.0//EN"
        "https://mybatis.org/dtd/mybatis-3-config.dtd">
<configuration>

    <properties>
        <property name="driver" value="com.mysql.cj.jdbc.Driver"/>
        <property name="url" value="jdbc:mysql://localhost:3306/mybatis"/>
        <property name="username" value="root"/>
        <property name="password" value="123456"/>
    </properties>

    <typeAliases>
        <package name="com.rzg.entity"/>
    </typeAliases>
    <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>
        <mapper resource="com/rzg/mapper/UserMapper.xml"/>
    </mappers>
</configuration>

外部properties文件

甚至可以将dataSource 属性提取出来，作为一个properties文件。
dataSource.properties

driver=com.mysql.cj.jdbc.Driver
url=jdbc:mysql://localhost:3306/mybatis
username=root
password=123456

<?xml version="1.0" encoding="UTF-8" ?>
<!DOCTYPE configuration
        PUBLIC "-//mybatis.org//DTD Config 3.0//EN"
        "https://mybatis.org/dtd/mybatis-3-config.dtd">
<configuration>

    <properties resource="dataSource.properties">

    </properties>

    <typeAliases>
        <package name="com.rzg.entity"/>
    </typeAliases>
    <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>
        <mapper resource="com/rzg/mapper/UserMapper.xml"/>
    </mappers>
</configuration>

java动态修改dataSource 属性

不只是配置文件中可以设置dataSource 属性，java创建sqlSessionFactory时，可以传入Properties对象，覆盖配置文件中的属性

public class App1 {

    public static void main(String[] args) throws IOException {
        //使用myabtis自带的工具类Resources，加载xml配置文件
        InputStream is = Resources.getResourceAsStream("mybatis-config.xml");

        Properties properties = new Properties();
        properties.setProperty("driver","com.mysql.cj.jdbc.Driver");
        properties.setProperty("url","jdbc:mysql://localhost:3306/mybatis");
        properties.setProperty("username","root");
        properties.setProperty("password","123456");
        //使用配置文件输入流，构建出sqlSessionFactory
        SqlSessionFactory sqlSessionFactory = new SqlSessionFactoryBuilder().build(is,properties);
        //sqlSessionFactory构建出sqlSession对象，用于一次会话，官方建议将sqlSession放在方法域中
        SqlSession sqlSession = sqlSessionFactory.openSession();
        //获得UserMapper的代理对象
        UserMapper mapper = sqlSession.getMapper(UserMapper.class);

        User user = new User();
        user.setUsername("张三");
        user.setSex("男");
        user.setAddress("上海");

        List<User> users = mapper.selectAll();
        //提交事务
        sqlSession.commit();
        //关闭sqlSession释放资源
        sqlSession.close();
        System.out.println(users);

    }
}

那么问题来了，通过这三种方式设置的dataSource，谁的优先级最高呢？

如果在多个地方设置了properties，那么，MyBatis 将按照下面的顺序来加载：

首先读取在 properties 元素体内指定的属性。
然后根据 properties 元素中的 resource 属性读取类路径下属性文件，或根据 url 属性指定的路径读取属性文件，并覆盖之前读取过的同名属性。
最后读取作为方法参数传递的属性，并覆盖之前读取过的同名属性。因此，通过方法参数传递的属性具有最高优先级，resource/url 属性中指定的配置文件次之，最低优先级的则是 properties 元素中指定的属性。