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MyBatis总结（2）- MyBatis实现原理（三）

news 来源：原创 2024/9/29 5:38:17

核心配置

JavaBeanMapper.xml（sql映射）

作用

JavaBeanMapper.xml实现：

用来干什么？
- 定义Sql语句映射。相对照JDBC的实现，是将原本的Sql代码提取出来，最终根据映射关系执行Sql操作。
好处？
- 解耦，mapper只关心定义Sql的映射关系，与java代码分离，更易维护。
如何使用？
- 先来展示一个基本的mapper xml，这里涉及到主要的几个标签元素：
  - Select
  - Insert
  - Update
  - Delete
  - ResultMap
  - Sql
  - Cache

<?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="org.example.daos.UserMapper"><resultMap id="userMap" type="Customer"><result column="pwd" property="password"/></resultMap><select id="getUserList" resultMap="userMap">SELECT * FROM mybatis.user</select><select id="getUserListByRowBounds" resultMap="userMap">SELECT * FROM mybatis.user</select><!-- 模糊查询1--><!--<select id="getUserListForFuzzyQuery" resultType="org.example.pojo.User">SELECT * FROM mybatis.user where name like #{name}</select>--><!-- 模糊查询2： "%"--><select id="getUserListForFuzzyQuery" resultType="org.example.pojo.User">SELECT * FROM mybatis.user where name like "%"#{name}</select><!-- 形参只有一个，且为基本类型时，parameterType可省略（parameterType="int" ）--><select id="getUserById" resultType="org.example.pojo.User">SELECT * FROM mybatis.user where id = #{id}</select><insert id="addUser" parameterType="org.example.pojo.User">INSERT INTO mybatis.user(id, name, pwd) values (#{id},#{name},#{pwd})</insert><update id="updateUserByUser" parameterType="org.example.pojo.User">UPDATE mybatis.user set name=#{name}, pwd=#{pwd} where id=#{id}</update><update id="updateUserByMap" parameterType="map">UPDATE mybatis.user set name=#{userName}, pwd=#{userPwd} where id=#{userId}</update><delete id="deleteUser" parameterType="int">DELETE FROM mybatis.user where id=#{id}</delete>
</mapper>

具体的标签元素：
- Select：
  - 这里的重点是，resultType，resultMap的使用，两者只能二选一
    - ResultType：语句中返回结果的类全限定名或别名。一般是该sql映射方法的返回值类型。特殊的，如果是集合类型，则只需定义集合的泛型类型即可。
    - ResultMap：对外部 resultMap 的命名引用。一般用于处理复杂的映射结果查询，比如：多表查询（一对多，多对一）：

多对一查询：

<?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="org.example.daos.StudentMapper"><!-- 多对一查询方式1：Teacher再查一次:子查询相当于:select id, name, tid from student where tid = (select id from teacher where id = tid)--><!--<resultMap id="StudentTeacher1" type="Student"><id property="id" column="id"/><result column="name" property="name"/><association property="teacher" column="tid" select="getTeacherById" javaType="Teacher"/></resultMap><select id="getStudentList" resultMap="StudentTeacher1">select * from student</select><select id="getTeacherById" resultType="Teacher">select * from teacher where id=#{tid}</select>--><!-- 多对一查询方式2：按照结果嵌套, 联表查询相当于：select s.id sid, s.name sname, t.id tid, t.name tname from student s, teacher t where s.tid = t.id--><select id="getStudentList" resultMap="StudentTeacher2">select s.id sid, s.name sname, t.id tid, t.name tname from student s, teacher t where s.tid = t.id</select><resultMap id="StudentTeacher2" type="Student"><result property="id" column="sid"/><result property="name" column="sname"/><association property="teacher" javaType="Teacher"><id column="id" property="tid"/><result property="name" column="tname"/></association></resultMap></mapper>

一对多查询：

<?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="org.example.daos.TeacherMapper"><!--一对多：方式1：联表查询 --><select id="getTeacherById" resultMap="TeacherStudent">select t.id tid, t.name tname, s.id sid, s.name sname from teacher t, student s where t.id = s.tid and t.id=#{tid}</select><resultMap id="TeacherStudent" type="Teacher"><result property="id" column="tid"/><result property="name" column="tname"/><collection property="studentList" ofType="Student"><result property="id" column="sid"/><result property="name" column="sname"/><result property="tid" column="tid"/></collection></resultMap><!--一对多：方式2：子查询 --><select id="getTeacherById2" resultMap="TeacherStudent2">select * from teacher where id=#{tid}</select><resultMap id="TeacherStudent2" type="Teacher"><result column="id" property="id"/><collection property="studentList" column="id" ofType="Student" select="getStudentByTid"/></resultMap><select id="getStudentByTid" resultType="Student">select * from student where tid=#{tid}</select>
</mapper>

Insert：涉及到自动生成主键id的设置(keyProperty, useGeneratedKeys)，多行插入(foreach)
Update：
Delete：

<!-- 自动生成主键id -->
<insert id="insertAuthor" useGeneratedKeys="true"keyProperty="id">insert into Author (username,password,email,bio)values (#{username},#{password},#{email},#{bio})
</insert><!-- 多行插入 -->
<insert id="insertAuthor" useGeneratedKeys="true"keyProperty="id">insert into Author (username, password, email, bio) values<foreach item="item" collection="list" separator=",">(#{item.username}, #{item.password}, #{item.email}, #{item.bio})</foreach>
</insert><update id="updateAuthor">update Author setusername = #{username},password = #{password},email = #{email},bio = #{bio}where id = #{id}
</update><delete id="deleteAuthor">delete from Author where id = #{id}
</delete>

Sql：sql语句重用片段。也可动态赋值

<sql id="if_title_author"><if test="title!= null">AND title = #{title}</if><if test="author != null">AND author = #{author}</if>
</sql><select id="queryBlog1" parameterType="map">select * from blog where 1=1<include refid="if_title_author"/>
</select><!-- 动态赋值: ${include_target}, property -->
<sql id="someinclude">from<include refid="${include_target}"/>
</sql>
<select id="select" resultType="map">selectfield1, field2, field3<include refid="someinclude"><property name="prefix" value="Some"/></include>
</select>

参数的定义：
- 如果一个列允许使用 null 值，并且会使用值为 null 的参数，就必须要指定 JDBC 类型（jdbcType）

#{average,javaType=double,jdbcType=NUMERIC,typeHandler=MyTypeHandler,numericScale=2}

字符串替换: ${}方式不会被预编译转义，可以通过这种方式指定某个字符串column，而非对应的数值。但存在sql注入风险。

@Select("select * from user where ${column} = #{value}")
User findByColumn(@Param("column") String column, @Param("value") String value);

association & collection ：collection 用于一对多，association用于多对一。

association 联表查询
<association property="author" column="blog_author_id" javaType="Author"><id property="id" column="author_id"/><result property="username" column="author_username"/>
</association>association 子表查询
<resultMap id="blogResult" type="Blog"><association property="author" column="author_id" javaType="Author" select="selectAuthor"/>
</resultMap>collection子表查询
<collection property="posts" column="id" ofType="Post" select="selectPostsForBlog"/>collection联表查询
<resultMap id="blogResult" type="Blog"><id property="id" column="blog_id" /><result property="title" column="blog_title"/><collection property="posts" ofType="Post"><id property="id" column="post_id"/><result property="subject" column="post_subject"/><result property="body" column="post_body"/></collection>
</resultMap>

OfType：

<collection property="posts" javaType="ArrayList" column="id" ofType="Post" select="selectPostsForBlog"/>

可以读作： “posts 是一个存储 Post 的 ArrayList 集合” 。且在一般情况下，MyBatis 可以推断 javaType 属性，因此并不需要填写。

缓存Cache：
- 一级缓存：默认开启。SqlSession级别的缓存，也叫本地缓存
- 二级缓存：基于namespace级别的缓存，针对mapper <cache>, LRU, FIFO，开启二级缓存，需要在对于mapper上，加入标签元素<Cache>即可
- 当会话sqlSession提交commit或关闭close时，一级缓存的数据才会提交到二级缓存中！！！
- 缓存顺序：当查询业务来到DAO层时：
  - 先查看二级缓存；
  - 再查看一级缓存；
  - 最后再查数据库
动态Sql：解决在定义Sql映射时，拼接sql语句：where子句条件，SET子句，多条语句foreach的编写。参考链接
- If, choose, foreach, trim
分页：limit
- Select * from user limit startIndex, pageSize
- RowBounds（selectList (String statement, Object parameter, RowBounds rowBounds)）
- Mybatis PageHelper
注解开发：参考链接