MyBatis关联查询：多对多（XML实现）

数据表

注：

• 借助中间表的时候，需要创建中间表，不借助中间的话就不需要创建中间表了
• 建议使用不借助中间表的方式

项目框架搭建 第一步：创建Maven项目

• 添加Maven依赖：

        junit
        junit
        4.12
        test
    

    
        log4j
        log4j
        1.2.17
    

    
        org.projectlombok
        lombok
        1.18.10
    

    
        mysql
        mysql-connector-java
        8.0.18
    
    
        org.mybatis
        mybatis
        3.5.4
    

• 指定Mapper XML文件在mybatis-config.xml中的配置有效，强制部署resources目录到classes

    
        src/main/resources
        
            **/*.xml
        
    
    
        ${basedir}/src/main/resources
    

第二步：配置文件

• log4j.properties

log4j.rootLogger=DEBUG, Console  

#Console
log4j.appender.Console=org.apache.log4j.ConsoleAppender  
log4j.appender.Console.layout=org.apache.log4j.PatternLayout  
log4j.appender.Console.layout.ConversionPattern=%d [%t] %-5p [%c] - %m%n  

log4j.logger.java.sql.ResultSet=INFO  
log4j.logger.org.apache=INFO  
log4j.logger.java.sql.Connection=DEBUG  
log4j.logger.java.sql.Statement=DEBUG  
log4j.logger.java.sql.PreparedStatement=DEBUG  

• mysql.properties

driver=com.mysql.cj.jdbc.Driver
url=jdbc:mysql://127.0.0.1/db_test?useSSL=false&serverTimezone=CST
username=root
password=root

第三步：工具类MyBatisUtil

public class MyBatisUtil {
    private static String CONFIG_FILE_LOCATION = "mybatis-config.xml";
    private static final ThreadLocal threadLocal = new ThreadLocal();
    private static SqlSessionFactory sessionFactory;

    static {
        try {
            Reader reader = Resources.getResourceAsReader(CONFIG_FILE_LOCATION);
            SqlSessionFactoryBuilder builder = new SqlSessionFactoryBuilder();
            sessionFactory = builder.build(reader);
        } catch (Exception e) {
            e.printStackTrace();
        }
    }

    public static SqlSession getSession() {
        SqlSession session = threadLocal.get();
        if (session == null) {
            session = (sessionFactory != null) ? sessionFactory.openSession() : null;
            threadLocal.set(session);
        }
        return session;
    }

    public static void closeSession() {
        SqlSession session = threadLocal.get();
        if (session != null) {
            session.close();
            threadLocal.set(null);
        }
    }
}

借助中间表实现 实体类

• Course.javat

@Data
@ToString
@AllArgsConstructor
@NoArgsConstructor
public class Course implements Serializable {
    
    private static final long serialVersionUID = -6376901343663894314L;
    
    private Integer id;
    private String name;
    private String intro;

}

• Stu

@Data
@ToString
@AllArgsConstructor
@NoArgsConstructor
public class Stu implements Serializable {

    private static final long serialVersionUID = 1765918887138569106L;

    private Integer id;
    private String name;
    private  Boolean gender;

}

第一步：创建mybatis-config.xml

DOCTYPE configuration PUBLIC "-//mybatis.org//DTD Config 3.0//EN"
        "http://mybatis.org/dtd/mybatis-3-config.dtd">

    
    

    
        
        
    

    
        
            
              
                
                
                
                
            
        
    
 
    
        
        
    
    

注意：mappers标签中使用的是resouce

第二步：Dao接口

• CourseDao

public interface CourseDao {

    /**
     * 查询某个学生选择的所有课程
     */
    List selectCourseByStuName(String name);
}

• StuDao

public interface StuDao {

    /**
     * 查询选择了某门课程的学生
     */
    List selectStuByCourseName(String name);
}

第三步：映射文件

• CourseMapper.xml

DOCTYPE mapper PUBLIC "-//mybatis.org//DTD Mapper 3.0//EN"
        "http://mybatis.org/dtd/mybatis-3-mapper.dtd">

    
        select c.id,c.name,c.intro
        from tb_stu s
        inner join tb_stu_course sc on s.id = sc.stu_id
        inner join tb_course c  on c.id = sc.course_id
        where s.name=#{name}
    

• StuMapper.xml

DOCTYPE mapper PUBLIC "-//mybatis.org//DTD Mapper 3.0//EN"
        "http://mybatis.org/dtd/mybatis-3-mapper.dtd">

    
        select s.id,s.name,s.gender
        from tb_stu s
        inner join tb_stu_course sc on s.id = sc.stu_id
        inner join tb_course c  on c.id = sc.course_id
        where c.name=#{name}
    

测试代码

public class CourseStuDaoTest {

    @Test
    public  void  selectStuByCourseName(){
        SqlSession session = MyBatisUtil.getSession();
        CourseDao courseDao = session.getMapper(CourseDao.class);
        List courseList = courseDao.selectCourseByStuName("zhangsan");
        courseList.forEach(System.out::println);
    }

    @Test
    public  void  selectStuByCourseName(){
        SqlSession session = MyBatisUtil.getSession();
        StuDao stuDao = session.getMapper(StuDao.class);
        List stuList = stuDao.selectStuByCourseName("english");
        stuList.forEach(System.out::println);
    }

}

不借助不间表实现 实体类

• Course.javat

@Data
@ToString
@AllArgsConstructor
@NoArgsConstructor
public class Course implements Serializable {
    
    private static final long serialVersionUID = -6376901343663894314L;
    
    private Integer id;
    private String name;
    private String intro;

}

• Stu

@Data
@ToString(exclude = "courses")
@AllArgsConstructor
@NoArgsConstructor
public class Stu {
    private Integer id;
    private String name;
    private  Boolean gender;
    
    private List courses ;

}

第一步：创建mybatis-config.xml

DOCTYPE configuration PUBLIC "-//mybatis.org//DTD Config 3.0//EN"
        "http://mybatis.org/dtd/mybatis-3-config.dtd">


    
    

    
        
        
    

    
        
            
              
                
                
                
                
            
        
    
    
    
        
    

第二步：创建StuDao接口

public interface StuDao {
    List selectStuWithCouseByStuName(String name);
}

第三步：创建映射文件StuMapper.xml

DOCTYPE mapper PUBLIC "-//mybatis.org//DTD Mapper 3.0//EN"
        "http://mybatis.org/dtd/mybatis-3-mapper.dtd">


    
        
        
        
        
            
            
            
        
    

    
        SELECT
        tb_stu.id sid,tb_stu.name sname,tb_stu.gender sgender,tb_course.id cid,tb_course.name cname,tb_course.intro cintro
        FROM
        tb_stu
        INNER JOIN tb_stu_course ON tb_stu_course.stu_id = tb_stu.id
        INNER JOIN tb_course ON tb_stu_course.course_id = tb_course.id
        WHERE tb_stu.name=#{name}
    

测试代码

public class StuDaoTest {

    @Test
    public void selectStuWithCouseByStuName() {
        SqlSession session = MyBatisUtil.getSession();
        StuDao stuDao = session.getMapper(StuDao.class);
        List stuList = stuDao.selectStuWithCouseByStuName("lisi");
        System.out.println(stuList);
        for (Stu stu : stuList) {
            stu.getCourses().forEach(System.out::println);
        }
    }


}