MyBatis 作为一款优秀的持久层框架，凭借其灵活的 SQL 映射、强大的动态 SQL 能力、轻量级的架构设计，在企业级 Java 应用中占据重要地位。它消除了几乎所有的 JDBC 代码和参数的手工设置以及结果集的检索，通过简单的 XML 或注解来配置和映射原生信息，将接口和 Java 的 POJOs 映射成数据库中的记录。本文将系统全面地梳理 MyBatis 的核心知识点，从基础概念到高级特性，帮助初学者建立完整的知识体系，也为有经验的开发者提供深入的技术参考。
一、MyBatis 概述
1.1 什么是 MyBatis
MyBatis 是一款优秀的持久层框架，支持自定义 SQL、存储过程以及高级映射。它通过 XML 或注解的方式，将 Java 对象与 SQL 语句进行映射，简化了 JDBC 编程的复杂性。
核心特性：
消除 JDBC 冗余代码，简化持久层开发
支持动态 SQL，灵活构建查询条件
提供强大的映射器，支持复杂对象关系映射
支持与 Spring 等框架无缝集成
支持缓存机制，提升查询性能
支持存储过程调用
轻量级，学习成本低
1.2 架构原理
MyBatis 架构层次：
┌─────────────────────────────────────────┐
│ Interface Layer │
│ (SqlSession, Mapper Interface) │
├─────────────────────────────────────────┤
│ Data Processing Layer │
│ (Parameter Mapping, SQL Execution) │
├─────────────────────────────────────────┤
│ SQL Mapping Layer │
│ (XML Mapping, Annotation Mapping) │
├─────────────────────────────────────────┤
│ Base Support Layer │
│ (Transaction, Cache, Connection) │
└─────────────────────────────────────────┘
工作流程：
读取配置文件（mybatis-config.xml）
创建 SqlSessionFactory
通过 SqlSessionFactory 创建 SqlSession
通过 SqlSession 获取 Mapper 代理对象
执行 Mapper 方法，执行 SQL
返回结果映射到 Java 对象
1.3 环境搭建
Maven 依赖：
<dependencies>
<!-- MyBatis 核心依赖 -->
<dependency>
<groupId>org.mybatis</groupId>
<artifactId>mybatis</artifactId>
<version>3.5.13</version>
</dependency>
<!-- 数据库驱动 -->
<dependency>
<groupId>mysql</groupId>
<artifactId>mysql-connector-java</artifactId>
<version>8.0.33</version>
</dependency>
<!-- 日志 -->
<dependency>
<groupId>ch.qos.logback</groupId>
<artifactId>logback-classic</artifactId>
<version>1.4.11</version>
</dependency>
</dependencies>
核心配置文件 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>
<!-- 配置属性 -->
<properties resource="jdbc.properties"/>
<!-- 配置全局设置 -->
<settings>
<!-- 开启驼峰命名转换 -->
<setting name="mapUnderscoreToCamelCase" value="true"/>
<!-- 开启延迟加载 -->
<setting name="lazyLoadingEnabled" value="true"/>
<!-- 开启二级缓存 -->
<setting name="cacheEnabled" value="true"/>
<!-- 日志实现 -->
<setting name="logImpl" value="SLF4J"/>
</settings>
<!-- 类型别名 -->
<typeAliases>
<package name="com.example.entity"/>
</typeAliases>
<!-- 环境配置 -->
<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>
<!-- 映射器配置 -->
<mappers>
<package name="com.example.mapper"/>
</mappers>
</configuration>
jdbc.properties：
driver=com.mysql.cj.jdbc.Driver
url=jdbc:mysql://localhost:3306/mybatis_demo?useSSL=false&serverTimezone=UTC
username=root
password=123456
初始化 SqlSessionFactory：
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.InputStream;
public class MyBatisUtil {
private static SqlSessionFactory sqlSessionFactory;
static {
try {
String resource = "mybatis-config.xml";
InputStream inputStream = Resources.getResourceAsStream(resource);
sqlSessionFactory = new SqlSessionFactoryBuilder().build(inputStream);
} catch (Exception e) {
e.printStackTrace();
}
}
public static SqlSession getSqlSession() {
return sqlSessionFactory.openSession();
}
public static SqlSession getSqlSession(boolean autoCommit) {
return sqlSessionFactory.openSession(autoCommit);
}
}
二、核心组件
2.1 SqlSessionFactory 与 SqlSession
public class SqlSessionDemo {
// SqlSessionFactory 是线程安全的，应该作为单例
private SqlSessionFactory sqlSessionFactory;
// 获取 SqlSession
public void testSqlSession() {
// 默认不自动提交
try (SqlSession session = sqlSessionFactory.openSession()) {
// 执行操作
UserMapper mapper = session.getMapper(UserMapper.class);
User user = mapper.selectById(1L);
System.out.println(user);
// 手动提交
session.commit();
} catch (Exception e) {
// 回滚
session.rollback();
}
// 自动提交事务
try (SqlSession session = sqlSessionFactory.openSession(true)) {
UserMapper mapper = session.getMapper(UserMapper.class);
mapper.insert(new User("张三", 25));
}
// 设置执行器类型
try (SqlSession session = sqlSessionFactory.openSession(
ExecutorType.BATCH)) { // SIMPLE, REUSE, BATCH
UserMapper mapper = session.getMapper(UserMapper.class);
// 批量操作
for (int i = 0; i < 1000; i++) {
mapper.insert(new User("user" + i, i));
}
session.commit();
}
}
}
2.2 Mapper 接口与 XML 映射
实体类：
package com.example.entity;
import java.io.Serializable;
import java.util.Date;
public class User implements Serializable {
private Long id;
private String username;
private String password;
private Integer age;
private String email;
private Date createTime;
private Date updateTime;
// 构造函数、getter、setter
public User() {}
public User(String username, Integer age) {
this.username = username;
this.age = age;
}
// getter/setter 省略
}
Mapper 接口：
package com.example.mapper;
import com.example.entity.User;
import org.apache.ibatis.annotations.*;
import java.util.List;
import java.util.Map;
public interface UserMapper {
// 基本 CRUD 方法
User selectById(@Param("id") Long id);
List<User> selectAll();
int insert(User user);
int update(User user);
int deleteById(Long id);
// 多条件查询
List<User> selectByCondition(@Param("username") String username,
@Param("age") Integer age);
// 使用 Map 参数
List<User> selectByMap(Map<String, Object> params);
// 返回 Map
Map<String, Object> selectMapById(Long id);
// 分页查询
List<User> selectByPage(@Param("offset") int offset,
@Param("limit") int limit);
// 批量插入
int batchInsert(@Param("list") List<User> users);
}
XML 映射文件 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">
<!-- 结果映射 -->
<resultMap id="userResultMap" type="com.example.entity.User">
<id property="id" column="id" jdbcType="BIGINT"/>
<result property="username" column="username" jdbcType="VARCHAR"/>
<result property="password" column="password" jdbcType="VARCHAR"/>
<result property="age" column="age" jdbcType="INTEGER"/>
<result property="email" column="email" jdbcType="VARCHAR"/>
<result property="createTime" column="create_time" jdbcType="TIMESTAMP"/>
<result property="updateTime" column="update_time" jdbcType="TIMESTAMP"/>
</resultMap>
<!-- 基础字段列表 -->
<sql id="baseColumns">
id, username, password, age, email, create_time, update_time
</sql>
<!-- 根据ID查询 -->
<select id="selectById" resultMap="userResultMap" parameterType="long">
SELECT <include refid="baseColumns"/>
FROM user
WHERE id = #{id}
</select>
<!-- 查询所有 -->
<select id="selectAll" resultType="com.example.entity.User">
SELECT <include refid="baseColumns"/>
FROM user
ORDER BY id DESC
</select>
<!-- 插入用户 -->
<insert id="insert" parameterType="com.example.entity.User"
useGeneratedKeys="true" keyProperty="id">
INSERT INTO user (username, password, age, email, create_time, update_time)
VALUES (#{username}, #{password}, #{age}, #{email},
NOW(), NOW())
</insert>
<!-- 更新用户 -->
<update id="update" parameterType="com.example.entity.User">
UPDATE user
SET username = #{username},
password = #{password},
age = #{age},
email = #{email},
update_time = NOW()
WHERE id = #{id}
</update>
<!-- 删除用户 -->
<delete id="deleteById" parameterType="long">
DELETE FROM user WHERE id = #{id}
</delete>
<!-- 条件查询 -->
<select id="selectByCondition" resultMap="userResultMap">
SELECT <include refid="baseColumns"/>
FROM user
WHERE 1 = 1
<if test="username != null and username != ''">
AND username LIKE CONCAT('%', #{username}, '%')
</if>
<if test="age != null">
AND age = #{age}
</if>
</select>
<!-- Map参数查询 -->
<select id="selectByMap" resultMap="userResultMap" parameterType="map">
SELECT <include refid="baseColumns"/>
FROM user
WHERE 1 = 1
<if test="username != null">
AND username = #{username}
</if>
<if test="minAge != null">
AND age >= #{minAge}
</if>
<if test="maxAge != null">
AND age <= #{maxAge}
</if>
</select>
<!-- 返回Map -->
<select id="selectMapById" resultType="map" parameterType="long">
SELECT <include refid="baseColumns"/>
FROM user
WHERE id = #{id}
</select>
<!-- 分页查询 -->
<select id="selectByPage" resultMap="userResultMap">
SELECT <include refid="baseColumns"/>
FROM user
ORDER BY id DESC
LIMIT #{offset}, #{limit}
</select>
<!-- 批量插入 -->
<insert id="batchInsert" parameterType="list">
INSERT INTO user (username, age, create_time, update_time)
VALUES
<foreach collection="list" item="user" separator=",">
(#{user.username}, #{user.age}, NOW(), NOW())
</foreach>
</insert>
</mapper>
三、动态 SQL
3.1 if 标签
<!-- if 条件判断 -->
<select id="findUsers" resultType="User">
SELECT * FROM user
WHERE 1 = 1
<if test="username != null and username != ''">
AND username LIKE CONCAT('%', #{username}, '%')
</if>
<if test="age != null">
AND age = #{age}
</if>
<if test="minAge != null">
AND age >= #{minAge}
</if>
<if test="maxAge != null">
AND age <= #{maxAge}
</if>
</select>
<!-- 更优雅的写法，避免 WHERE 1=1 -->
<select id="findUsers" resultType="User">
SELECT * FROM user
<where>
<if test="username != null and username != ''">
username LIKE CONCAT('%', #{username}, '%')
</if>
<if test="age != null">
AND age = #{age}
</if>
<if test="minAge != null">
AND age >= #{minAge}
</if>
<if test="maxAge != null">
AND age <= #{maxAge}
</if>
</where>
</select>
3.2 choose/when/otherwise 标签
<!-- choose 相当于 switch-case -->
<select id="findUsersByChoice" resultType="User">
SELECT * FROM user
<where>
<choose>
<when test="username != null and username != ''">
username LIKE CONCAT('%', #{username}, '%')
</when>
<when test="age != null">
age = #{age}
</when>
<otherwise>
status = 1
</otherwise>
</choose>
</where>
</select>
3.3 where 标签
<!-- where 标签会自动处理 AND/OR 前缀 -->
<select id="findUsers" resultType="User">
SELECT * FROM user
<where>
<if test="username != null">
AND username LIKE CONCAT('%', #{username}, '%')
</if>
<if test="age != null">
AND age = #{age}
</if>
<if test="email != null">
AND email = #{email}
</if>
</where>
</select>
3.4 set 标签
<!-- set 标签用于动态更新语句 -->
<update id="updateUser" parameterType="User">
UPDATE user
<set>
<if test="username != null">username = #{username},</if>
<if test="password != null">password = #{password},</if>
<if test="age != null">age = #{age},</if>
<if test="email != null">email = #{email},</if>
update_time = NOW()
</set>
WHERE id = #{id}
</update>
3.5 foreach 标签
<!-- 遍历集合 -->
<select id="selectByIds" resultType="User">
SELECT * FROM user
WHERE id IN
<foreach collection="ids" item="id" open="(" separator="," close=")">
#{id}
</foreach>
</select>
<!-- 批量插入 -->
<insert id="batchInsert">
INSERT INTO user (username, age, create_time)
VALUES
<foreach collection="list" item="user" separator=",">
(#{user.username}, #{user.age}, NOW())
</foreach>
</insert>
<!-- 批量更新 -->
<update id="batchUpdate">
<foreach collection="list" item="user" separator=";">
UPDATE user
SET username = #{user.username},
age = #{user.age}
WHERE id = #{user.id}
</foreach>
</update>
<!-- 遍历 Map 参数 -->
<select id="selectByMap" resultType="User">
SELECT * FROM user
WHERE 1 = 1
<foreach collection="params" index="key" item="value">
<if test="key == 'username'">
AND username = #{value}
</if>
<if test="key == 'age'">
AND age = #{value}
</if>
</foreach>
</select>
3.6 trim 标签
<!-- trim 标签可以自定义前缀、后缀和去除的字符 -->
<select id="findUsers" resultType="User">
SELECT * FROM user
<trim prefix="WHERE" prefixOverrides="AND |OR ">
<if test="username != null">
AND username LIKE CONCAT('%', #{username}, '%')
</if>
<if test="age != null">
AND age = #{age}
</if>
<if test="email != null">
AND email = #{email}
</if>
</trim>
</select>
<!-- trim 用于更新 -->
<update id="updateUser">
UPDATE user
<trim prefix="SET" suffixOverrides=",">
<if test="username != null">username = #{username},</if>
<if test="password != null">password = #{password},</if>
<if test="age != null">age = #{age},</if>
<if test="email != null">email = #{email},</if>
update_time = NOW(),
</trim>
WHERE id = #{id}
</update>
3.7 bind 标签
<!-- bind 用于创建绑定变量 -->
<select id="findUsers" resultType="User">
<bind name="pattern" value="'%' + username + '%'"/>
SELECT * FROM user
WHERE username LIKE #{pattern}
</select>
<!-- 复杂绑定 -->
<select id="findUsers" resultType="User">
<bind name="start" value="(page - 1) * size"/>
<bind name="end" value="page * size"/>
SELECT * FROM user
ORDER BY id DESC
LIMIT #{start}, #{size}
</select>
3.8 sql 与 include
<!-- 定义可重用的 SQL 片段 -->
<sql id="userColumns">
id, username, password, age, email, create_time, update_time
</sql>
<sql id="userWhere">
<where>
<if test="username != null">
AND username LIKE CONCAT('%', #{username}, '%')
</if>
<if test="age != null">
AND age = #{age}
</if>
</where>
</sql>
<!-- 引用 SQL 片段 -->
<select id="selectUsers" resultType="User">
SELECT <include refid="userColumns"/>
FROM user
<include refid="userWhere"/>
</select>
<!-- 带参数的 SQL 片段 -->
<sql id="userColumnsWithAlias">
${alias}.id, ${alias}.username, ${alias}.age
</sql>
<select id="selectUsersWithAlias" resultType="User">
SELECT <include refid="userColumnsWithAlias">
<property name="alias" value="u"/>
</include>
FROM user u
</select>
