Flyinsky's Codes
879 字
4 分钟
持久层框架Mybatis
2025-01-03
Java
Spring

一、概述#

MyBatis 是一个优秀的持久层框架，它基于 SQL 映射的思想，通过 XML 或注解方式将 SQL 语句与 Java 对象关联起来。与 Hibernate 等全自动 ORM 框架相比，MyBatis 提供了对 SQL 操作的更多控制，适合复杂 SQL 查询和性能优化场景。

特点：

  1. 支持直接编写 SQL。
  2. 动态 SQL 提供了灵活的 SQL 拼接方式。
  3. 简化了 JDBC 操作，避免了繁琐的 ResultSet 和 Statement 处理。
  4. 提供一级缓存和二级缓存机制。

二、环境配置#

1. 引入依赖#

在 Maven 项目中引入 MyBatis 相关依赖：

<dependencies>
    <!-- MyBatis 核心 -->
    <dependency>
        <groupId>org.mybatis</groupId>
        <artifactId>mybatis</artifactId>
        <version>3.5.13</version>
    </dependency>
    <!-- MyBatis-Log4j 日志支持 -->
    <dependency>
        <groupId>org.apache.logging.log4j</groupId>
        <artifactId>log4j-core</artifactId>
        <version>2.20.0</version>
    </dependency>
    <!-- MySQL 驱动 -->
    <dependency>
        <groupId>mysql</groupId>
        <artifactId>mysql-connector-java</artifactId>
        <version>8.0.33</version>
    </dependency>
</dependencies>
2. 配置数据库连接#

创建 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>
    <!-- 数据库环境配置 -->
    <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/testdb?useSSL=false&amp;serverTimezone=UTC"/>
                <property name="username" value="root"/>
                <property name="password" value="password"/>
            </dataSource>
        </environment>
    </environments>
    <!-- 全局设置 -->
    <settings>
        <setting name="mapUnderscoreToCamelCase" value="true"/> <!-- 驼峰命名映射 -->
        <setting name="logImpl" value="LOG4J"/> <!-- 日志实现 -->
    </settings>
</configuration>
3. Mapper XML 文件配置#

以 UserMapper.xml 为例：

<?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 namespace="com.example.mapper.UserMapper">
    <!-- 查询所有用户 -->
    <select id="findAll" resultType="com.example.entity.User">
        SELECT id, username, email FROM user
    </select>
</mapper>

三、开发步骤#

1. 数据库表结构#

假设有以下两张表：

  • user：用户表

    CREATE TABLE user (
    id INT AUTO_INCREMENT PRIMARY KEY,
    username VARCHAR(50) NOT NULL,
    email VARCHAR(100),
    role_id INT
);

  • role：角色表

    CREATE TABLE role (
    id INT AUTO_INCREMENT PRIMARY KEY,
    role_name VARCHAR(50) NOT NULL
);
2. 配置 mybatis-config.xml#
<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/testdb?useSSL=false&amp;serverTimezone=UTC"/>
                <property name="username" value="root"/>
                <property name="password" value="password"/>
            </dataSource>
        </environment>
    </environments>
    <mappers>
        <mapper resource="com/example/mapper/UserMapper.xml"/>
        <mapper resource="com/example/mapper/RoleMapper.xml"/>
    </mappers>
</configuration>

四、Mapper 配置#

1. 实体类定义#
User 实体类：#
package com.example.entity;

public class User {
    private Integer id;
    private String username;
    private String email;
    private Role role; // 对应的角色对象

    // Getters and Setters
}
Role 实体类：#
package com.example.entity;

public class Role {
    private Integer id;
    private String roleName;

    // Getters and Setters
}
2. UserMapper.xml 定义#
增删改查接口：#
<?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 namespace="com.example.mapper.UserMapper">

    <!-- resultMap 定义 -->
    <resultMap id="UserResultMap" type="com.example.entity.User">
        <id property="id" column="id"/>
        <result property="username" column="username"/>
        <result property="email" column="email"/>
        <association property="role" javaType="com.example.entity.Role">
            <id property="id" column="role_id"/>
            <result property="roleName" column="role_name"/>
        </association>
    </resultMap>

    <!-- 查询所有用户 -->
    <select id="findAll" resultMap="UserResultMap">
        SELECT u.id, u.username, u.email, r.id AS role_id, r.role_name
        FROM user u
        LEFT JOIN role r ON u.role_id = r.id
    </select>

    <!-- 动态查询 -->
    <select id="findByCondition" resultMap="UserResultMap">
        SELECT u.id, u.username, u.email, r.id AS role_id, r.role_name
        FROM user u
        LEFT JOIN role r ON u.role_id = r.id
        <where>
            <if test="username != null and username != ''">
                AND u.username = #{username}
            </if>
            <if test="email != null and email != ''">
                AND u.email = #{email}
            </if>
        </where>
    </select>

    <!-- 插入用户 -->
    <insert id="insertUser" parameterType="com.example.entity.User">
        INSERT INTO user (username, email, role_id)
        VALUES (#{username}, #{email}, #{role.id})
    </insert>

    <!-- 更新用户 -->
    <update id="updateUser" parameterType="com.example.entity.User">
        UPDATE user
        SET username = #{username},
            email = #{email},
            role_id = #{role.id}
        WHERE id = #{id}
    </update>

    <!-- 删除用户 -->
    <delete id="deleteUser" parameterType="int">
        DELETE FROM user WHERE id = #{id}
    </delete>

</mapper>
3. RoleMapper.xml 定义#
<?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 namespace="com.example.mapper.RoleMapper">

    <!-- 查询所有角色 -->
    <select id="findAll" resultType="com.example.entity.Role">
        SELECT id, role_name FROM role
    </select>

</mapper>

五、代码实现#

1. Mapper 接口#
package com.example.mapper;

import com.example.entity.User;
import org.apache.ibatis.annotations.Param;

import java.util.List;

public interface UserMapper {
    List<User> findAll();
    List<User> findByCondition(@Param("username") String username, @Param("email") String email);
    void insertUser(User user);
    void updateUser(User user);
    void deleteUser(int id);
}
2. 测试代码#
package com.example;

import com.example.entity.User;
import com.example.entity.Role;
import com.example.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 java.io.IOException;
import java.io.Reader;
import java.util.List;

public class MyBatisTest {
    public static void main(String[] args) {
        try (Reader reader = Resources.getResourceAsReader("mybatis-config.xml")) {
            SqlSessionFactory sqlSessionFactory = new SqlSessionFactoryBuilder().build(reader);
            try (SqlSession sqlSession = sqlSessionFactory.openSession(true)) {
                UserMapper userMapper = sqlSession.getMapper(UserMapper.class);

                // 查询所有用户
                List<User> users = userMapper.findAll();
                users.forEach(user -> System.out.println(user.getUsername() + " - " + user.getRole().getRoleName()));

                // 插入用户
                Role role = new Role();
                role.setId(1); // 假设角色 ID 为 1
                User newUser = new User();
                newUser.setUsername("John");
                newUser.setEmail("john@example.com");
                newUser.setRole(role);
                userMapper.insertUser(newUser);

                // 更新用户
                newUser.setEmail("updated@example.com");
                userMapper.updateUser(newUser);

                // 按条件查询
                List<User> filteredUsers = userMapper.findByCondition("John", null);
                filteredUsers.forEach(user -> System.out.println(user.getUsername()));

                // 删除用户
                userMapper.deleteUser(newUser.getId());
            }
        } catch (IOException e) {
            e.printStackTrace();
        }
    }
}
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