MyBatis：灵活的ORM框架

概述

MyBatis是一款优秀的持久层框架，它支持自定义SQL、存储过程以及高级映射。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。

1. 核心配置

<?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="123456"/>
            </dataSource>
        </environment>
    </environments>
    <mappers>
        <mapper resource="UserMapper.xml"/>
    </mappers>
</configuration>

2. Mapper映射

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

<mapper namespace="com.example.mapper.UserMapper">
    <resultMap id="UserResultMap" type="User">
        <id property="id" column="id"/>
        <result property="username" column="username"/>
        <result property="email" column="email"/>
    </resultMap>

    <select id="findById" resultMap="UserResultMap">
        SELECT * FROM users WHERE id = #{id}
    </select>

    <insert id="insert" useGeneratedKeys="true" keyProperty="id">
        INSERT INTO users (username, email) VALUES (#{username}, #{email})
    </insert>

    <update id="update">
        UPDATE users SET username = #{username}, email = #{email} WHERE id = #{id}
    </update>

    <delete id="deleteById">
        DELETE FROM users WHERE id = #{id}
    </delete>
</mapper>

3. 动态SQL

<select id="findUsers" resultMap="UserResultMap">
    SELECT * FROM users
    <where>
        <if test="username != null">
            AND username = #{username}
        </if>
        <if test="email != null">
            AND email = #{email}
        </if>
    </where>
</select>

<insert id="batchInsert">
    INSERT INTO users (username, email) VALUES
    <foreach collection="users" item="user" separator=",">
        (#{user.username}, #{user.email})
    </foreach>
</insert>

4. 注解方式

import org.apache.ibatis.annotations.*;

@Mapper
public interface UserMapper {
    @Select("SELECT * FROM users WHERE id = #{id}")
    User findById(@Param("id") Long id);

    @Insert("INSERT INTO users (username, email) VALUES (#{username}, #{email})")
    @Options(useGeneratedKeys = true, keyProperty = "id")
    int insert(@Param("user") User user);

    @Update("UPDATE users SET username = #{username}, email = #{email} WHERE id = #{id}")
    int update(@Param("user") User user);

    @Delete("DELETE FROM users WHERE id = #{id}")
    int deleteById(@Param("id") Long id);
}

最佳实践

1. 使用Mapper接口：类型安全的SQL映射
2. 合理使用动态SQL：灵活构建查询条件
3. 使用ResultMap：处理复杂映射关系
4. 一级缓存：默认开启，提高查询性能
5. 二级缓存：跨Session共享缓存

总结

MyBatis是一款灵活的ORM框架，它提供了SQL映射、动态SQL等强大功能。掌握MyBatis的使用，可以高效地进行数据库操作，提高开发效率。