Mybatis工具类和关联操作

墨蓝 2021-08-30 13:42 267阅读 0赞

### 文章目录 ###

*  MyBatis工具类和关联操作
 *   *  MyBatis工具类
     *  映射细节-resultmap使用
     *   *  resultmap的代码实现
     *  MyBatis处理关联关系-多表连接
     *   *  一对一关联关系的处理案列
         *  一对多关联关系的处理案例
         *  多对多关联关系处理案例

# MyBatis工具类和关联操作 #

## MyBatis工具类 ##

由于我们基础的Mybatis在操作数据库时,虽然不需要写Dao接口的实体类,但是仍然需要**重复的操作**:通过Mybatis来加载配置文件.构建SqlSession的工厂来创建SqlSession的对象.在通过SqlSession的对象来获取实体类的对象,甚至是对事务的提交,这些操作都是重复的,因此我们设计一个工具类,对Mybatis的操作进行一个封装,将必须要做的事情,将其封装到静态代码块,在类加载时进行执行,将可能会执行的操作封装为一个静态方法,在使用时进行调用就可以了.`简化我们的操作,去掉了很多冗余的代码`

下面给出MyBatis工具类的详细代码:

package per.util;
    
    
    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.IOException;
    import java.io.InputStream;
    
    
    
    /** * @author 雷雨 * @date 2020/6/16 15:06 * * mybatis工具类 * 1.加载配置 * 2.创建SqlSession工厂 * 3.创建Session * 4.事务的管理 * 5.Mapper获取 */
    public class MybatisUtil { 
        //创建线程工厂
        private static SqlSessionFactory sqlSessionFactory;
        //创建ThreadLocal绑定当前线程中的SqlSession
        private static final ThreadLocal<SqlSession> t1=new ThreadLocal<SqlSession>();
        static{ //配置加载信息,构建session工厂
            try { 
                //1.加载配置文件
                InputStream inputStream= Resources.getResourceAsStream("mybatis-config.xml");
                sqlSessionFactory = new SqlSessionFactoryBuilder().build(inputStream);
            } catch (IOException e) { 
                e.printStackTrace();
            }
        }
        public  static  SqlSession openSession(){ 
            //注意这里的SqlSession和我们普通的jdbc操作中的Connection对象很相似,是线程唯一,全局不唯一的
            //因此我们需要先看看在当前线程池中是否有没有释放的sqlsession对象
            SqlSession sqlSession = t1.get();
            if (sqlSession ==null){ 
                sqlSession = sqlSessionFactory.openSession();
                t1.set(sqlSession);
            }
            return sqlSession;
        }
        public  static  void commit(){ 
            //获取了当前线程中的SqlSession对象
            SqlSession sqlSession = openSession();
            sqlSession.commit();
            close();
        }
        public static void rollback(){ 
            SqlSession sqlSession = openSession();
            sqlSession.rollback();
            close();
        }
        public  static  void close(){ 
            SqlSession sqlSession = openSession();
            sqlSession.close();
        }
        public static <T> T getMapper(Class<T> mapper){ 
            SqlSession sqlSession = openSession();
            return sqlSession.getMapper(mapper);
    
        }
    
    }

## 映射细节-resultmap使用 ##

<table> 
 <thead> 
  <tr> 
   <th align="center">数据库中的列名和程序中的属性名不同时</th> 
  </tr> 
 </thead> 
 <tbody> 
  <tr> 
   <td align="center"><img src="https://imgconvert.csdnimg.cn/aHR0cHM6Ly9naXRlZS5jb20vbGVpeXVqaWEvY3Nkbi9yYXcvbWFzdGVyL2ltYWdlLTIwMjAwNjE2MTUyNzU0MDU3LnBuZw?x-oss-process=image/format,png" alt="image-20200616152754057"></td> 
  </tr> 
 </tbody> 
</table>

数据库中的列名和程序中的属性名不同时,因为MyBatis框架使用的是同名映射,所以我们之前的操作是**给数据库中的列名起一个别名的方式**来执行我们响应的sql语句

现在我们通过`配置文件resultmap`的方式能够完成`高级映射`,不仅`不需要在sql语句中起别名还可以完成级联查询(多表参与的查询)`

### resultmap的代码实现 ###

<mapper namespace="per.leiyu.dao.UserDao">
    	<resultMap id="user_resultmap" type="User">
            
            <id column="id" property="id"></id>
            <result column="username" property="username"></result>
            <result column="passward" property="passward"/>
            <result column="gender" property="gender"/>
            <result column="regist_time" property="registTime"/>
            
        </resultMap>
        
        
            <select id="queryUserByUsernameAndPassward" resultMap="user_resultmap">
            select id,username,passward,gender,regist_Time 
            from t_user
            where username=#{arg0} and passward=#{arg1}
        </select>
        
    </mapper>

其实:`如果使用resultmap只是为了实现简单的关系映射,那么是费事费力的`,我们这里使用`reslultmap的原因是`:提醒`在遇到简单的同名映射关系没办法实现其功能时,我们就使用resultmap的关系映射`

另外resultmap的关系映射在关联关系中应用非常广泛

## MyBatis处理关联关系-多表连接 ##

> 实体间的关系:关联关系(拥有has 属于belong)
> 
>  *  OneToOne:一对一(Passenger–Passport)
>  *  OneToMany:一对多(Employee-Department)
>  *  ManyToMany:多对多(Student-Subject)

<table> 
 <thead> 
  <tr> 
   <th align="center">Table建立外键关系</th> 
  </tr> 
 </thead> 
 <tbody> 
  <tr> 
   <td align="center"><img src="https://imgconvert.csdnimg.cn/aHR0cHM6Ly9naXRlZS5jb20vbGVpeXVqaWEvY3Nkbi9yYXcvbWFzdGVyL2ltYWdlLTIwMjAwNjE2MTU1MjIyMjczLnBuZw?x-oss-process=image/format,png" alt="image-20200616155222273"></td> 
  </tr> 
 </tbody> 
</table>

### 一对一关联关系的处理案列 ###

*  创建数据库
 *  编写实体类
 *  创建Dao接口,写查询的方法
 *  编写关系映射文件
 *  测试类
 *  结果展示

**创建数据库数据**

CREATE TABLE t_passengers(
    	id INT PRIMARY KEY AUTO_INCREMENT,
    	NAME VARCHAR(50),
    	sex VARCHAR(50),
    	birthday DATE
    )DEFAULT CHARSET = utf8;
    CREATE TABLE t_passports(
    	id INT PRIMARY KEY AUTO_INCREMENT,
    	nationality VARCHAR(50),
    	expire DATE,
    	passenger_id INT UNIQUE,
    	FOREIGN KEY (passenger_id) REFERENCES t_passengers(id)
    )DEFAULT CHARSET =utf8;
    
    INSERT INTO t_passengers VALUES(NULL,'leiyu_1','f','2019-11-11');
    INSERT INTO t_passengers VALUES(NULL,'leiyu_2','m','2019-12-12');
    
    INSERT INTO t_passports VALUES(NULL,'China','2050-11-11',1);
    INSERT INTO t_passports VALUES(NULL,'America','2050-12-12',2);

<table> 
 <thead> 
  <tr> 
   <th align="center">数据库表建立完成</th> 
  </tr> 
 </thead> 
 <tbody> 
  <tr> 
   <td align="center"><img src="https://imgconvert.csdnimg.cn/aHR0cHM6Ly9naXRlZS5jb20vbGVpeXVqaWEvY3Nkbi9yYXcvbWFzdGVyL2ltYWdlLTIwMjAwNjE2MTYxNDAyOTk0LnBuZw?x-oss-process=image/format,png" alt="image-20200616161402994"></td> 
  </tr> 
 </tbody> 
</table>

**开始编写实体类**

package per.Dao;
    
    import java.util.Date;
    
    /** * @author 雷雨 * @date 2020/6/16 16:05 */
    public class Passenger { 
    
        private Integer id;
        private String name ;
        private String sex ;
        private Date birthday;
    
        //存储旅客的护照信息 :关系属性 对应着数据库中的数据
        private Passenger passenger;
    
        public Passenger() { 
            super();
        }
    
        @Override
        public String toString() { 
            return "Passenger{" +
                    "id=" + id +
                    ", name='" + name + '\'' +
                    ", sex='" + sex + '\'' +
                    ", birthday=" + birthday +
                    '}';
        }
    
        public Passenger(Integer id, String name, String sex, Date birthday) { 
            this.id = id;
            this.name = name;
            this.sex = sex;
            this.birthday = birthday;
        }
    
        public Integer getId() { 
            return id;
        }
    
        public void setId(Integer id) { 
            this.id = id;
        }
    
        public String getName() { 
            return name;
        }
    
        public void setName(String name) { 
            this.name = name;
        }
    
        public String getSex() { 
            return sex;
        }
    
        public void setSex(String sex) { 
            this.sex = sex;
        }
    
        public Date getBirthday() { 
            return birthday;
        }
    
        public void setBirthday(Date birthday) { 
            this.birthday = birthday;
        }
    }

package per.Dao;
    
    import java.util.Date;
    
    /** * @author 雷雨 * @date 2020/6/16 16:08 */
    public class Passport { 
        private Integer id;
        private String nationality;
        private Date expire;
    
        //护照存储了旅客的信息:关系属性
        private Passenger passenger;
        public Passport(Integer id, String nationality, Date expire) { 
            this.id = id;
            this.nationality = nationality;
            this.expire = expire;
        }
    
        public Passport() { 
            super();
        }
    
        @Override
        public String toString() { 
            return "Passport{" +
                    "id=" + id +
                    ", nationality='" + nationality + '\'' +
                    ", expire=" + expire +
                    '}';
        }
    
        public Integer getId() { 
            return id;
        }
    
        public void setId(Integer id) { 
            this.id = id;
        }
    
        public String getNationality() { 
            return nationality;
        }
    
        public void setNationality(String nationality) { 
            this.nationality = nationality;
        }
    
        public Date getExpire() { 
            return expire;
        }
    
        public void setExpire(Date expire) { 
            this.expire = expire;
        }
    }

**创建Dao接口,写查询方法**

**PassengerDao.java**

package per.Dao;
    
    import org.apache.ibatis.annotations.Param;
    import per.leiyu.Passenger;
    
    /** * @author 雷雨 * @date 2020/6/16 16:16 */
    public interface PassengerDao { 
        //通过旅客的id查询旅客的信息和护照的信息 --->关联查询 级联查询
        Passenger queryPassengerById(@Param("id") Integer id);
    
    }

**编写关系映射文件**

**PassengerDaoMapper.xml编写对象关系映射文件**

<?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="per.Dao.PassengerDao">
    
        <resultMap id="Passenger_resultmap" type="Passenger">
            <id column="id" property="id"/>
            <result column="name" property="name"/>
            <result column="sex" property="sex"/>
            <result column="birthday" property="birthday"/>
            <association property="passport" javaType="Passport">
                <id column="PassId" property="id"/>
                <result column="nationality" property="nationality"/>
                <result column="expire" property="expire"/>
    
            </association>
        </resultMap>
    
        <select id="queryPassengerById" resultMap="Passenger_resultmap">
            select t_passengers.id,t_passengers.name,t_passengers.sex,t_passengers.birthday,
            t_passports.id passId,t_passports.nationality,t_passports.expire
            from t_passengers join t_passports
            on  t_passengers.id=t_passports.id
            where t_passengers.id=#{id}
        </select>
    
    </mapper>

*  要注意
    
     *  property写的还是实体类中属性
     *  javaType写的是该实体类对应了那个实体类(该关系属性对应了那个实体类)
 *  要注意中使用的是我们写好的resultmap
 *  最后要在mybatis-config.xml中把我们新写好的mapper映射文件注册使用

**编写测试类(使用了我们之前写好的工具类)**

使用了工具类代码量感觉瞬间减少了很多

package per.leiyu.UserDao;
    
    import per.Dao.PassengerDao;
    import per.leiyu.Passenger;
    import per.util.MybatisUtil;
    
    /** * @author 雷雨 * @date 2020/6/16 16:31 */
    public class TesPassengerDao { 
        public static void main(String[] args) { 
            PassengerDao mapper = MybatisUtil.getMapper(PassengerDao.class);
            Passenger passenger = mapper.queryPassengerById(1);
            System.out.println(passenger);
            System.out.println(passenger.getPassport());;
    
        }
    }

**结果**

<table> 
 <thead> 
  <tr> 
   <th align="center">通过mybatis多表关联查询结果</th> 
  </tr> 
 </thead> 
 <tbody> 
  <tr> 
   <td align="center"><img src="https://imgconvert.csdnimg.cn/aHR0cHM6Ly9naXRlZS5jb20vbGVpeXVqaWEvY3Nkbi9yYXcvbWFzdGVyL2ltYWdlLTIwMjAwNjE2MTY0NzEzMjQ1LnBuZw?x-oss-process=image/format,png" alt="image-20200616164713245"></td> 
  </tr> 
 </tbody> 
</table>

**一对一关联关系总结要注意的地方:**

> 1.  编写实体类的时候,要把`关系属性`也写入到实体类中
> 2.  写关系映射文件Mapper.xml时,要注意使用的是`reslutMap`
> 3.  在`resultMap`标签的子标签,也就是我们真正要定义的关系属性时使用的标签是`association`,并且在其中的`column`标签使用的是我们在**sql语句**给起的别名,而`property`标签表示的是其真正的在类中索引的**属性名**

### 一对多关联关系的处理案例 ###

模拟员工和部门的一对多的关系

**编写数据库代码**

CREATE TABLE t_departments(
    	id INT PRIMARY KEY AUTO_INCREMENT,
    	NAME VARCHAR(50),
    	location VARCHAR(100)
    )DEFAULT CHARSET=utf8;
    CREATE TABLE t_employees(
    	id INT PRIMARY KEY AUTO_INCREMENT,
    	NAME VARCHAR(50),
    	salary DOUBLE,
    	dept_id INT ,
    	FOREIGN KEY (dept_id) REFERENCES t_departments(id)
    )DEFAULT CHARSET= utf8;
    INSERT INTO `t_departments` VALUES(1,"教学部","西安"),(2,"研发部","上海");
    INSERT INTO `t_employees` VALUES(1,"leiyu01",10000.5,1),(2,"leiyu02",20000.5,1),(3,"xiaobai01",9000.5,2),(4,"xiaobai02",20001.5,2);

<table> 
 <thead> 
  <tr> 
   <th align="center">数据库表结果图展示</th> 
  </tr> 
 </thead> 
 <tbody> 
  <tr> 
   <td align="center"><img src="https://imgconvert.csdnimg.cn/aHR0cHM6Ly9naXRlZS5jb20vbGVpeXVqaWEvY3Nkbi9yYXcvbWFzdGVyL2ltYWdlLTIwMjAwNjE2MjAyNzA3MzY5LnBuZw?x-oss-process=image/format,png" alt="image-20200616202707369"></td> 
  </tr> 
 </tbody> 
</table>

**开始编写实体类**

*  要注意:关系属性是一对多关系

package per.leiyu;
    
    import java.util.List;
    
    /** * @author 雷雨 * @date 2020/6/16 20:28 */
    public class Department { 
        private  Integer id;
        private String name ;
        private  String location;
    
        private List<Employee> employee;
    
        public Department(Integer id, String location, List<Employee> employee) { 
            this.id = id;
            this.location = location;
            this.employee = employee;
        }
    
        public Department() { 
            super();
        }
    
        public Integer getId() { 
            return id;
        }
    
        public void setId(Integer id) { 
            this.id = id;
        }
    
        public String getName() { 
            return name;
        }
    
        public void setName(String name) { 
            this.name = name;
        }
    
        public String getLocation() { 
            return location;
        }
    
        public void setLocation(String location) { 
            this.location = location;
        }
    
        public List<Employee> getEmployee() { 
            return employee;
        }
    
        public void setEmployee(List<Employee> employee) { 
            this.employee = employee;
        }
    
        @Override
        public String toString() { 
            return "Department{" +
                    "id=" + id +
                    ", name='" + name + '\'' +
                    ", location='" + location + '\'' +
                    '}';
        }
    }

package per.leiyu;
    
    /** * @author 雷雨 * @date 2020/6/16 20:29 */
    public class Employee { 
        private Integer id;
        private String name;
        private Double salary;
    
        private Department department;
    
        public Employee(Integer id, String name, Double salary, Department department) { 
            this.id = id;
            this.name = name;
            this.salary = salary;
            this.department = department;
        }
    
        public Employee() { 
            super();
        }
    
        public Integer getId() { 
            return id;
        }
    
        public void setId(Integer id) { 
            this.id = id;
        }
    
        public String getName() { 
            return name;
        }
    
        public void setName(String name) { 
            this.name = name;
        }
    
        public Double getSalary() { 
            return salary;
        }
    
        public void setSalary(Double salary) { 
            this.salary = salary;
        }
    
        public Department getDepartment() { 
            return department;
        }
    
        public void setDepartment(Department department) { 
            this.department = department;
        }
    
        @Override
        public String toString() { 
            return "Employee{" +
                    "id=" + id +
                    ", name='" + name + '\'' +
                    ", salary=" + salary +
                    '}';
        }
    }

**创建Dao接口,写查询方法**

package per.Dao;
    
    import org.apache.ibatis.annotations.Param;
    import per.leiyu.Department;
    
    /** * @author 雷雨 * @date 2020/6/16 20:32 */
    public interface  DepartmentDao { 
        //根据id查询部门和属于该部门的员工
        Department queryDepartmentById(@Param("id") Integer id);
    
    }

**编写关系映射文件**

<?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="per.Dao.DepartmentDao">
    
    
    
        <resultMap id="Deparment_resultmap" type="Department">
            <id column="id" property="id"/>
            <result column="name" property="name"/>
            <result column="location" property="location"/>
            <collection property="employee" ofType="Employee">
                <id column="emp_id" property="id"/>
                <result column="emp_name" property="name"/>
                <result column="salary" property="salary"/>
            </collection>
        </resultMap>
    
        <select id="queryDepartmentById" resultMap="Deparment_resultmap">
            select t_departments.id,t_departments.name,t_departments.location,t_employees.id emp_id,t_employees.name emp_name ,t_employees.salary
            from t_departments join t_employees
            on t_departments.id = t_employees.dept_id
            where t_departments.id=#{id}
    
    
        </select>
    
    
    </mapper>

*  注意在mapresult标签的子标签中因为我们的部门和员工是一对多的关系,且我们查询的是部门,所以选择的是`collection`标签
 *  如果我们要反着做(也就是根据员工查询员工的各项信息和他所属部门的信息),那么虽然在关系仍然是一对多,但是查询之后的结果(部门信息)只有一个,因此会选择`assaciation`标签

**编写测试类**

package per.leiyu.UserDao;
    
    
    import per.Dao.DepartmentDao;
    import per.leiyu.Department;
    import per.util.MybatisUtil;
    
    /** * @author 雷雨 * @date 2020/6/16 20:46 */
    public class TestDepartment { 
        public static void main(String[] args) { 
            DepartmentDao mapper = MybatisUtil.getMapper(DepartmentDao.class);
            Department department = mapper.queryDepartmentById(1);
            System.out.println(department);
            System.out.println(department.getEmployee());
        }
    }

**查询结果**

<table> 
 <thead> 
  <tr> 
   <th align="center">数据库查询结果</th> 
  </tr> 
 </thead> 
 <tbody> 
  <tr> 
   <td align="center"><img src="https://imgconvert.csdnimg.cn/aHR0cHM6Ly9naXRlZS5jb20vbGVpeXVqaWEvY3Nkbi9yYXcvbWFzdGVyL2ltYWdlLTIwMjAwNjE2MjA1ODI5MTQ2LnBuZw?x-oss-process=image/format,png" alt="image-20200616205829146"></td> 
  </tr> 
 </tbody> 
</table>

<table> 
 <thead> 
  <tr> 
   <th align="center">我们程序查询的结果</th> 
  </tr> 
 </thead> 
 <tbody> 
  <tr> 
   <td align="center"><img src="https://imgconvert.csdnimg.cn/aHR0cHM6Ly9naXRlZS5jb20vbGVpeXVqaWEvY3Nkbi9yYXcvbWFzdGVyL2ltYWdlLTIwMjAwNjE2MjEwMzAzNjYxLnBuZw?x-oss-process=image/format,png" alt="image-20200616210303661"></td> 
  </tr> 
 </tbody> 
</table>

两者查询到的信息一致,说明我们的程序完成.

**需要注意的:**

*  在映射文件的`resultMap`的字标签选择,要分析关系查询的结果是集合还是一个实体类的单个对象,选择合适的标签
 *  其他要注意的和在一对一查询中药注意的事项是一样的

### 多对多关联关系处理案例 ###

多对多关联关系,我们这里用学生和课程来实现.

最终实现**查询某一课程的信息和该课程下的所有学生的信息**

<table> 
 <thead> 
  <tr> 
   <th align="center">多对多关联关系的关键是建立第三张表</th> 
  </tr> 
 </thead> 
 <tbody> 
  <tr> 
   <td align="center"><img src="https://imgconvert.csdnimg.cn/aHR0cHM6Ly9naXRlZS5jb20vbGVpeXVqaWEvY3Nkbi9yYXcvbWFzdGVyL2ltYWdlLTIwMjAwNjE2MjIxMzAyNDAzLnBuZw?x-oss-process=image/format,png" alt="image-20200616221302403"></td> 
  </tr> 
 </tbody> 
</table>

**编写数据库代码**

CREATE TABLE t_students(
    	id INT PRIMARY KEY AUTO_INCREMENT,
    	NAME VARCHAR(50),
    	sex VARCHAR(1)
    )DEFAULT CHARSET = utf8;
    CREATE TABLE t_subjects(
    	id INT PRIMARY KEY AUTO_INCREMENT,
    	NAME VARCHAR(50),
    	grade INT 
    )DEFAULT CHARSET = utf8;
    
    CREATE TABLE t_stu_sub(
    	student_id INT,
    	subject_id INT ,
    	FOREIGN KEY (student_id) REFERENCES t_students(id),
    	FOREIGN KEY (subject_id) REFERENCES t_subjects(id),
    	PRIMARY KEY (student_id,subject_id)
    )DEFAULT CHARSET = utf8;
    INSERT INTO t_students VALUES(1,"leiyu01",'m'),(2,"leiyu02",'f');
    INSERT INTO t_subjects VALUES(1001,"JavaSE",1),(1002,"JavaEE",2);
    INSERT INTO t_stu_sub VALUES(1,1001),(1,1002),(2,1001),(2,1002);

**实体类**

package per.leiyu;
    
    import java.util.List;
    
    /** * @author 雷雨 * @date 2020/6/16 22:29 */
    public class Student { 
        private Integer id;
        private String name;
        private String sex;
    
        private List<Subject> subject;
    
        public Student(Integer id, String name, String sex) { 
            this.id = id;
            this.name = name;
            this.sex = sex;
        }
    
        public Student() { 
            super();
        }
    
        public Integer getId() { 
            return id;
        }
    
        public void setId(Integer id) { 
            this.id = id;
        }
    
        public String getName() { 
            return name;
        }
    
        public void setName(String name) { 
            this.name = name;
        }
    
        public String getSex() { 
            return sex;
        }
    
        public void setSex(String sex) { 
            this.sex = sex;
        }
    
        public List<Subject> getSubject() { 
            return subject;
        }
    
        public void setSubject(List<Subject> subject) { 
            this.subject = subject;
        }
    
        @Override
        public String toString() { 
            return "Student{" +
                    "id=" + id +
                    ", name='" + name + '\'' +
                    ", sex='" + sex + '\'' +
                    '}';
        }
    }

package per.leiyu;
    
    import java.util.List;
    
    /** * @author 雷雨 * @date 2020/6/16 22:30 */
    public class Subject { 
        private Integer id;
        private String name;
        private Integer grade;
    
        private List<Student> student;
    
        public Subject(Integer id, String name, Integer grade) { 
            this.id = id;
            this.name = name;
            this.grade = grade;
        }
    
        public Subject() { 
            super();
        }
    
        public Integer getId() { 
            return id;
        }
    
        public void setId(Integer id) { 
            this.id = id;
        }
    
        public String getName() { 
            return name;
        }
    
        public void setName(String name) { 
            this.name = name;
        }
    
        public Integer getGrade() { 
            return grade;
        }
    
        public void setGrade(Integer grade) { 
            this.grade = grade;
        }
    
        public List<Student> getStudent() { 
            return student;
        }
    
        public void setStudent(List<Student> student) { 
            this.student = student;
        }
    
        @Override
        public String toString() { 
            return "Subject{" +
                    "id=" + id +
                    ", name='" + name + '\'' +
                    ", grade=" + grade +
                    '}';
        }
    }

**编写SubjectDao**

package per.Dao;
    
    import org.apache.ibatis.annotations.Param;
    import per.leiyu.Subject;
    
    /** * @author 雷雨 * @date 2020/6/16 22:33 */
    public interface SubjectDao { 
        Subject querySubjectById(@Param("id") Integer id);
    }

**编写文件映射**

<?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="per.Dao.SubjectDao">
    
        <resultMap id="Subject_map" type="Subject">
            <id column="id" property="id"/>
            <result column="name" property="name"/>
            <result column="grade" property="grade"/>
            <collection property="student" ofType="Student">
                <id column="stu_id" property="id"/>
                <result column="stu_name" property="name"/>
                <result column="sex" property="sex"/>
            </collection>
        </resultMap>
    
        <select id="querySubjectById" resultMap="Subject_map">
            select t_subjects.id,t_subjects.name,t_subjects.grade,t_students.id stu_id,t_students.name stu_name,t_students.sex
            from t_subjects join t_stu_sub
            on t_subjects.id = t_stu_sub.subject_id
            join t_students
            on t_students.id = t_stu_sub.student_id
            where t_subjects.id=#{id}
        </select>
    
    </mapper>

然后要在mybatis的配置文件中对我们写的mapper文件注册

**编写测试类**

package per.leiyu.UserDao;
    
    import per.Dao.SubjectDao;
    import per.leiyu.Subject;
    import per.util.MybatisUtil;
    
    /** * @author 雷雨 * @date 2020/6/16 22:52 */
    public class TestSubjectDao { 
        public static void main(String[] args) { 
            SubjectDao mapper = MybatisUtil.getMapper(SubjectDao.class);
            Subject subject = mapper.querySubjectById(1001);
            System.out.println(subject);
            System.out.println(subject.getStudent());
        }
    }

**结果**

<table> 
 <thead> 
  <tr> 
   <th align="center">程序的结果</th> 
  </tr> 
 </thead> 
 <tbody> 
  <tr> 
   <td align="center"><img src="https://imgconvert.csdnimg.cn/aHR0cHM6Ly9naXRlZS5jb20vbGVpeXVqaWEvY3Nkbi9yYXcvbWFzdGVyL2ltYWdlLTIwMjAwNjE2MjMwMzAwOTAyLnBuZw?x-oss-process=image/format,png" alt="image-20200616230300902"></td> 
  </tr> 
 </tbody> 
</table>

<table> 
 <thead> 
  <tr> 
   <th align="center">在数据库中查询的结果</th> 
  </tr> 
 </thead> 
 <tbody> 
  <tr> 
   <td align="center"><img src="https://imgconvert.csdnimg.cn/aHR0cHM6Ly9naXRlZS5jb20vbGVpeXVqaWEvY3Nkbi9yYXcvbWFzdGVyL2ltYWdlLTIwMjAwNjE2MjMwNDI3NjMwLnBuZw?x-oss-process=image/format,png" alt="image-20200616230427630"></td> 
  </tr> 
 </tbody> 
</table>

> 我是雷雨佳,一个`普本科`的学生,主要专注于Java后端和大数据开发

> 如果这篇文章有帮助到你,希望你给我一个`大大的赞`  
> 如果有什么问题,希望你能`留言`和我一起`研究`,`学习靠自觉,分享靠自愿`

> 转载注明出处  
> [https://blog.csdn.net/qq\_40742223][https_blog.csdn.net_qq_40742223]

[https_blog.csdn.net_qq_40742223]: https://blog.csdn.net/qq_40742223