MyBatis入门：SQL映射框架

概述

MyBatis是一个优秀的持久层框架，它支持自定义SQL、存储过程以及高级映射。本教程介绍MyBatis的基础知识和使用方法。

1. 配置文件

<!-- mybatis-config.xml -->
<?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>
  <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/mydb"/>
        <property name="username" value="root"/>
        <property name="password" value="password"/>
      </dataSource>
    </environment>
  </environments>
  <mappers>
    <mapper resource="com/example/mapper/UserMapper.xml"/>
  </mappers>
</configuration>

2. Mapper接口和XML

// UserMapper.java
import java.util.List;

public interface UserMapper {
    User findById(Long id);
    List<User> findAll();
    List<User> findByName(String name);
    int insert(User user);
    int update(User user);
    int deleteById(Long id);
}

<!-- UserMapper.xml -->
<?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 id="UserResultMap" type="User">
    <id property="id" column="id"/>
    <result property="name" column="name"/>
    <result property="age" column="age"/>
  </resultMap>
  
  <select id="findById" parameterType="long" resultMap="UserResultMap">
    SELECT * FROM users WHERE id = #{id}
  </select>
  
  <select id="findAll" resultMap="UserResultMap">
    SELECT * FROM users
  </select>
  
  <select id="findByName" parameterType="string" resultMap="UserResultMap">
    SELECT * FROM users WHERE name LIKE CONCAT('%', #{name}, '%')
  </select>
  
  <insert id="insert" parameterType="User" useGeneratedKeys="true" keyProperty="id">
    INSERT INTO users (name, age) VALUES (#{name}, #{age})
  </insert>
  
  <update id="update" parameterType="User">
    UPDATE users SET name = #{name}, age = #{age} WHERE id = #{id}
  </update>
  
  <delete id="deleteById" parameterType="long">
    DELETE FROM users WHERE id = #{id}
  </delete>
  
</mapper>

3. 动态SQL

<!-- 动态SQL示例 -->
<select id="findUsers" parameterType="User" resultMap="UserResultMap">
  SELECT * FROM users
  <where>
    <if test="name != null and name != ''">
      AND name LIKE CONCAT('%', #{name}, '%')
    </if>
    <if test="age != null">
      AND age = #{age}
    </if>
    <if test="minAge != null">
      AND age &gt;= #{minAge}
    </if>
    <if test="maxAge != null">
      AND age &lt;= #{maxAge}
    </if>
  </where>
</select>

<!-- choose-when-otherwise -->
<select id="findUsersByCondition" parameterType="map" resultMap="UserResultMap">
  SELECT * FROM users
  <where>
    <choose>
      <when test="searchBy == 'name'">
        AND name LIKE CONCAT('%', #{keyword}, '%')
      </when>
      <when test="searchBy == 'age'">
        AND age = #{keyword}
      </when>
      <otherwise>
        AND (name LIKE CONCAT('%', #{keyword}, '%') OR age = #{keyword})
      </otherwise>
    </choose>
  </where>
</select>

<!-- foreach -->
<select id="findByIds" parameterType="list" resultMap="UserResultMap">
  SELECT * FROM users
  WHERE id IN
  <foreach collection="list" item="id" open="(" separator="," close=")">
    #{id}
  </foreach>
</select>

4. 使用示例

import org.apache.ibatis.io.Resources;
import org.apache.ibatis.session.*;
import java.io.IOException;
import java.util.List;

public class MyBatisDemo {
    public static void main(String[] args) throws IOException {
        String resource = "mybatis-config.xml";
        SqlSessionFactory sqlSessionFactory = new SqlSessionFactoryBuilder()
            .build(Resources.getResourceAsStream(resource));
        
        try (SqlSession session = sqlSessionFactory.openSession()) {
            UserMapper mapper = session.getMapper(UserMapper.class);
            
            // 查询
            User user = mapper.findById(1L);
            System.out.println("用户: " + user);
            
            // 插入
            User newUser = new User(null, "Bob", 30);
            mapper.insert(newUser);
            session.commit();
            
            // 更新
            newUser.setName("Bobby");
            mapper.update(newUser);
            session.commit();
            
            // 删除
            mapper.deleteById(newUser.getId());
            session.commit();
        }
    }
}

5. 最佳实践

1. 使用Mapper接口：类型安全，易于维护
2. 使用动态SQL：灵活构建查询条件
3. 使用ResultMap：处理复杂映射关系
4. 使用缓存：提高查询性能
5. 使用分页插件：简化分页查询

总结

MyBatis是一个优秀的SQL映射框架。掌握MyBatis的配置和使用，可以高效地进行数据库操作。