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。

1. 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。
2. 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SQLSessionFactoryBuilder
        SqlSessionFactoryBuilder builder = new
                SqlSessionFactoryBuilder();
        SqlSessionFactory factory = builder.build(inputStream);
        //获取SqlSession
        SqlSession 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User
    public 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User
    public 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User
    public 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User
    public 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User
    public 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name
    public 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User
    public 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 cname
        from student s,stu_course sc,course c
        where 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, GPA
        from student s,stu_course sc,course c
        where s.id = sc.sid and c.id = sc.cid and c.id = 1;
    </select>
</mapper>

控制台打印结果如下：