3.Mapper.xml 详解

文章目录

Mapper.xml 详解
- 1. parameterType
- 2. resultType
- 3. 级联查询
- - 3.1 一对多
  - 3.2 多对多

Mapper.xml 详解

MyBatis 主要有两个的配置文件：config.xml 和 Mapper.xml，这两个配置文件可以自定义文件名。

config.xml 是全局配置文件，主要配置 MyBatis 的数据源(DataSource)，事务管理(TransactionManager)以及打印 SQL 语句，开启二级缓存，注册 Mapper.xml 等。
Mapper.xml 的作用是什么？因为 MyBatis 是“半自动”的ORM框架，即SQL语句需要开发者自定义，MyBatis 的关注点在 POJO 与 SQL 之间的映射关系，所以 Mapper.xml 主要配置接口方法对应的 SQL 语句，即接口方法的具体实现。

准备测试环境如下：

实体类 User：

package com.training.entity;import lombok.AllArgsConstructor;
import lombok.Data;
import lombok.NoArgsConstructor;//使用这个插件后，会默认生成属性的Getter与Setter方法@AllArgsConstructor
@NoArgsConstructor
@Data
public class User {private Integer id;private String name;private Double score;
}

自定义 UserMapper 接口

package com.training.mapper;public interface UserMapper {//定义相关的业务方法//...
}

创建接口对应的 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.training.mapper.UserMapper"><!--定义接口方法对应的 SQL 语句--></mapper>

测试类 Test

package com.training.test;import com.training.entity.User;
import com.training.mapper.UserMapper;
import org.apache.ibatis.session.SqlSession;
import org.apache.ibatis.session.SqlSessionFactory;
import org.apache.ibatis.session.SqlSessionFactoryBuilder;import java.io.InputStream;
import java.util.List;
public class Test {public static void main(String[] args) {//加载MyBatis配置信息InputStream inputStream = Test.class.getClassLoader().getResourceAsStream("config.xml");//构建SQLSessionFactoryBuilderSqlSessionFactoryBuilder builder = newSqlSessionFactoryBuilder();SqlSessionFactory factory = builder.build(inputStream);//获取SqlSessionSqlSession sqlSession = factory.openSession();//获取实现接口的代理对象UserMapper mapper = sqlSession.getMapper(UserMapper.class);//...}
}

1. parameterType

parameterType 设置参数的数据类型，支持基本数据类型、包装类、String、多个参数以及 Java 对象。

1.1 基本数据类型

public interface UserMapper {//通过id查询Userpublic User findById(int id);
}

<mapper namespace="com.training.mapper.UserMapper"><select id="findById" parameterType="int" resultType="com.training.entity.User">select * from user where id=#{id};</select>
</mapper>

注：#{id} 可以替换任意名称，因为只有一个参数，所以无论映射名是什么，都可以完成映射。

1.2 包装类

public interface UserMapper {//通过score查询Userpublic User findByScore(Double score);
}

<mapper namespace="com.training.mapper.UserMapper"><select id="findByScore" parameterType="java.lang.Double" resultType="com.training.entity.User">select * from user where score=#{score}</select>
</mapper>

1.3 字符串 String

public interface UserMapper {//通过name查询Userpublic User findByName(String name);
}

<mapper namespace="com.training.mapper.UserMapper"><select id="findByName" parameterType="String" resultType="com.training.entity.User">select * from user where name=#{name}</select>
</mapper>

1.4 多个参数

多个参数的情况下，无法通过映射名来进行映射，而应该采用下标进行映射，即：[arg0, arg1, arg2, ……]或者 [param1, param2, param3, ……] 。

public interface UserMapper {//通过id和name查询Userpublic User findByIdAndName(Integer id, String name);
}

<mapper namespace="com.training.mapper.UserMapper"><select id="findByIdAndName" resultType="com.training.entity.User"><!--select * from user where id=#{arg0} and name=#{arg1};-->select * from user where id=#{param1} and name=#{param2};</select>
</mapper>

1.5 JavaBean

参数列表中的映射名就是实体类的属性名。

public interface UserMapper {//根据User封装对象查找Userpublic User findByUser(User user);
}

<mapper namespace="com.training.mapper.UserMapper"><select id="findByUser" parameterType="com.training.entity.User" resultType="com.training.entity.User">select * from user where name=#{name} and score=#{score};</select>
</mapper>

2. resultType

resultType 设置返回值的数据类型，支持基本数据类型、包装类、String、多个参数以及 Java 对象。

2.1 基本数据类型

public interface UserMapper {//查询User的总数public int getCount();
}

<mapper namespace="com.training.mapper.UserMapper"><select id="getCount" resultType="int">select count(*) from user;</select>
</mapper>

2.2 包装类

public interface UserMapper {//查询User的总数public Integer getCount();
}

<mapper namespace="com.training.mapper.UserMapper"><select id="getCount" resultType="java.lang.Integer">select count(*) from user;</select>
</mapper>

2.3 字符串 String

public interface UserMapper {//根据id查询namepublic String findNameById(Integer id);
}

<mapper namespace="com.training.mapper.UserMapper"><select id="findNameById" parameterType="int" resultType="java.lang.String">select name from user where id=#{id};</select>
</mapper>

2.4 JavaBean

public interface UserMapper {//通过id查询Userpublic User findById(Integer id);
}

<mapper namespace="com.training.mapper.UserMapper"><select id="findById" parameterType="java.lang.Integer" resultType="com.training.entity.User">select * from user where id=#{id};</select>
</mapper>

3. 级联查询

上面介绍的查询都是基于 User 的单表查询，如果是多表关联查询，比如：Student 与 Class 一对多的关联查询、Student 与 Course 多对多的级联查询，应该如何处理呢？显然仅仅使用 resultType 无法完成了。

3.1 一对多

创建数据表

create table class
(id  int auto_increment primary key,name varchar(22) null
);
create table student
(id   int auto_increment primary key,name varchar(25)  not null comment '姓名',GPA  double default 0 null comment '分数',cid  int  null
);

数据表对应的实体类

package com.training.entity;import lombok.Data;@Data
public class Student {private Integer id;private String name;private Double GPA;private Integer cid;private Class aClass;
}

package com.training.entity;import lombok.Data;@Data
public class Class {private Integer id;private String name;private List<Student> students;
}

例1：根据学生 id 查询学生所在的班级信息

自定义接口 StudentMapper.java

package com.training.mapper;import com.training.entity.Student;public interface StudentMapper {//查询学生的班级信息public Student findById(Integer id);
}

SQL配置文件 StudentMapper.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.training.mapper.StudentMapper"><resultMap id="studentMap" type="com.training.entity.Student"><id column="sid" property="id"></id><result column="sname" property="name"></result><result column="GPA" property="GPA"></result><association property="aClass" javaType="com.training.entity.Class"><id column="cid" property="id"></id><result column="cname" property="name"></result></association></resultMap><select id="findById" parameterType="java.lang.Integer" resultMap="studentMap">select s.id sid, s.name sname, GPA, c.id cid, c.name cname from student s,class c where s.cid = c.id and s.id = 1;</select>
</mapper>

例2：根据班级 id 查询所在班级的学生

自定义接口 ClassMapper.java

package com.training.mapper;import com.training.entity.Class;public interface ClassMapper {public Class findById(Integer id);
}

SQL配置文件 StudentMapper.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.training.mapper.StudentMapper"><resultMap id="studentMap" type="com.training.entity.Student"><id column="sid" property="id"></id><result column="sname" property="name"></result><result column="GPA" property="GPA"></result><association property="aClass" javaType="com.training.entity.Class"><id column="cid" property="id"></id><result column="cname" property="name"></result></association></resultMap><select id="findById" parameterType="java.lang.Integer" resultMap="studentMap">select s.id sid, s.name sname, GPA, c.id cid, c.name cname from student s,class c where s.cid = c.id and s.id = 1;</select>
</mapper>

3.2 多对多

创建数据表

create table student
(id int auto_increment primary key,name varchar(25) not null comment '姓名',GPA double default 0 null comment '分数'
);
create table stu_course
(id  int auto_increment primary key,sid int null,cid int null
);
create table course
(id   int auto_increment primary key,name varchar(225) null
);

数据表对应的实体类

package com.training.entity;import lombok.Data;import java.util.List;@Data
public class Student {private Integer id;private String name;private Double GPA;private List<Course> courses;
}

package com.training.entity;import lombok.Data;import java.util.List;@Data
public class Course {private Integer id;private String name;private List<Student> students;
}

例1：通过学生 id 查询该学生的选课信息

自定义接口 StudentMapper.java

public interface StudentMapper {public Student findById(Integer id);
}

SQL配置文件 StudentMapper.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.training.mapper.StudentMapper"><resultMap id="studentMap" type="com.training.entity.Student"><id column="sid" property="id"></id><result column="sname" property="name"></result><result column="GPA" property="GPA"></result><collection property="courses" ofType="com.training.entity.Course"><id column="cid" property="id"></id><result column="cname" property="name"></result></collection></resultMap><select id="findById" parameterType="java.lang.Integer" resultMap="studentMap">select s.id sid, s.name sname, GPA, c.id cid, c.name cnamefrom student s,stu_course sc,course cwhere s.id = sc.sid and c.id = sc.cid and s.id = 1;</select>
</mapper>

控制台打印结果如下：

例2：通过课程 id 查询选过该课的学生信息

自定义接口 CourseMapper.java

public interface ClassMapper {public Class findById(Integer id);
}

SQL配置文件 CourseMapper.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.training.mapper.CourseMapper"><resultMap id="courseMap" type="com.training.entity.Course"><id column="cid" property="id"></id><result column="cname" property="name"></result><collection property="students" ofType="com.training.entity.Student"><id column="sid" property="id"></id><result column="sname" property="name"></result><result column="GPA" property="GPA"></result></collection></resultMap><select id="findById" parameterType="java.lang.Integer" resultMap="courseMap">select c.id cid, c.name cname, s.id sid, s.name sname, GPAfrom student s,stu_course sc,course cwhere s.id = sc.sid and c.id = sc.cid and c.id = 1;</select>
</mapper>

控制台打印结果如下：