MyBatis持久层框架深度解析

MyBatis作为一款优秀的持久层框架，以其灵活的SQL映射和简单的配置方式深受开发者欢迎。本文将深入探讨MyBatis的核心特性和最佳实践。

1. 基础配置

1.1 XML配置文件

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<?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.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="UserMapper.xml"/>
  </mappers>
</configuration>

1.2 Mapper接口

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public interface UserMapper {
    @Select("SELECT * FROM users WHERE id = #{id}")
    User getUserById(int id);
    
    @Insert("INSERT INTO users(name, email) VALUES(#{name}, #{email})")
    int insertUser(User user);
}

2. 动态SQL

2.1 条件查询

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<select id="findUsers" resultType="User">
  SELECT * FROM users
  <where>
    <if test="name != null">
      AND name LIKE #{name}
    </if>
    <if test="email != null">
      AND email = #{email}
    </if>
  </where>
</select>

2.2 批量操作

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<insert id="batchInsert" parameterType="java.util.List">
  INSERT INTO users (name, email) VALUES
  <foreach collection="list" item="user" separator=",">
    (#{user.name}, #{user.email})
  </foreach>
</insert>

3. 结果映射

3.1 一对一关联

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<resultMap id="userWithProfile" type="User">
  <id property="id" column="user_id"/>
  <result property="name" column="user_name"/>
  <association property="profile" javaType="Profile">
    <id property="id" column="profile_id"/>
    <result property="address" column="address"/>
  </association>
</resultMap>

3.2 一对多关联

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<resultMap id="userWithOrders" type="User">
  <id property="id" column="user_id"/>
  <result property="name" column="user_name"/>
  <collection property="orders" ofType="Order">
    <id property="id" column="order_id"/>
    <result property="amount" column="amount"/>
  </collection>
</resultMap>

4. 缓存机制

4.1 一级缓存配置

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<select id="getUser" resultType="User" useCache="true">
  SELECT * FROM users WHERE id = #{id}
</select>

4.2 二级缓存配置

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<cache
  eviction="LRU"
  flushInterval="60000"
  size="512"
  readOnly="true"/>

5. 插件开发

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@Intercepts({
  @Signature(type = Executor.class, method = "update",
    args = {MappedStatement.class, Object.class})
})
public class ExamplePlugin implements Interceptor {
  @Override
  public Object intercept(Invocation invocation) throws Throwable {
    // 插件逻辑
    return invocation.proceed();
  }
}

6. 分页查询

6.1 物理分页

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<select id="getUsersByPage" resultType="User">
  SELECT * FROM users
  LIMIT #{offset}, #{pageSize}
</select>

6.2 使用PageHelper插件

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@Service
public class UserService {
    public List<User> getUsersByPage(int pageNum, int pageSize) {
        PageHelper.startPage(pageNum, pageSize);
        return userMapper.getAllUsers();
    }
}

7. 性能优化

7.1 批量操作优化

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public void batchInsert(List<User> users) {
    SqlSession sqlSession = sqlSessionFactory.openSession(ExecutorType.BATCH);
    UserMapper mapper = sqlSession.getMapper(UserMapper.class);
    try {
        for (User user : users) {
            mapper.insertUser(user);
        }
        sqlSession.commit();
    } finally {
        sqlSession.close();
    }
}

7.2 延迟加载

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<resultMap id="userMap" type="User">
  <id property="id" column="id"/>
  <result property="name" column="name"/>
  <association property="profile" 
               select="selectProfile"
               column="profile_id"
               fetchType="lazy"/>
</resultMap>

最佳实践建议

合理使用动态SQL，避免过度复杂的SQL拼接
适当配置缓存，提高查询性能
使用批量操作处理大量数据
配置合适的日志级别便于调试
注意SQL注入安全问题

常见问题解决

N+1查询问题

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<!-- 使用关联查询替代多次单独查询 -->
<select id="getUserWithOrders" resultMap="userWithOrders">
  SELECT u.*, o.*
  FROM users u
  LEFT JOIN orders o ON u.id = o.user_id
  WHERE u.id = #{id}
</select>

大数据量处理

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public void handleLargeData() {
    try (SqlSession session = sqlSessionFactory.openSession()) {
        session.select("selectLargeData", null, new ResultHandler<User>() {
            @Override
            public void handleResult(ResultContext<? extends User> context) {
                User user = context.getResultObject();
                // 处理单个对象
            }
        });
    }
}

总结

MyBatis通过灵活的SQL映射和简单的配置方式，为Java应用提供了强大的数据持久化能力。合理使用MyBatis的各项特性，可以显著提高开发效率和应用性能。建议开发团队制定统一的MyBatis使用规范，并在实践中不断优化和改进。