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MyBatis——MyBatis处理多对一，一对多

在我们日常的开发当中，难免遇到多对一和一对多的问题，比如：多个学生对应一个老师（多对一）；一个老师对应多个学生（一对多）

数据库： 学生表： 老师表：

1.多对一

方式一：按照查询结果嵌套处理，模拟数据库思想：联表查询。

对应数据库编写Student和Teacher实体类：

package com.muan.pojo;public class Student {    private int id;//学生id    private String name;//学生姓名    private int tid;//教师id    private Teacher teacher;    public Teacher getTeacher() {        return teacher;    }    public Student() {    }    public Student(int id, String name, int tid) {        this.id = id;        this.name = name;        this.tid = tid;    }    public int getId() {        return id;    }    public void setId(int id) {        this.id = id;    }    public String getName() {        return name;    }    public void setName(String name) {        this.name = name;    }    public int getTid() {        return tid;    }    public void setTid(int tid) {        this.tid = tid;    }    @Override    public String toString() {        return "Student{" +                "id=" + id +                ", name='" + name + '\'' +                ", tid=" + tid +                '}';    }}

package com.muan.pojo;public class Teacher {    private int id;//教师id    private String name;//教师name    public Teacher() {    }    public Teacher(int id, String name) {        this.id = id;        this.name = name;    }    public int getId() {        return id;    }    public void setId(int id) {        this.id = id;    }    public String getName() {        return name;    }    public void setName(String name) {        this.name = name;    }    @Override    public String toString() {        return "Teacher{" +                "id=" + id +                ", name='" + name + '\'' +                '}';    }}

编写接口：

package com.muan.dao;import com.muan.pojo.Student;import java.util.List;public interface StudentMapper {    List
   
     getStudents();}
   

编写工具类：

package com.muan.utils;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;public class MyBatisUtils {    private static SqlSession sqlSession;    static {        try {            String resource = "mybatis-config.xml";            InputStream inputStream = Resources.getResourceAsStream(resource);            SqlSessionFactory sqlSessionFactory = new SqlSessionFactoryBuilder().build(inputStream);            //openSession方法的自动提交设置为true就会自动提交了            sqlSession= sqlSessionFactory.openSession(true);        } catch (IOException e) {            e.printStackTrace();        }    }    public static SqlSession getSqlSession(){        return sqlSession;    }}

database.properties

driver = com.mysql.jdbc.Driverusername = rootpassword = 123456url = jdbc:mysql://localhost:3306/mybatis?useSSL=true&useUnicode=true&characterEncoding=UTF-8

log4j.properties

### Log4j配置 ###### 与Spring结合需要在web.xml中指定此文件位置，并添加监听器 ####定义log4j的输出级别和输出目的地（目的地可以自定义名称，和后面的对应）#[ level ] , appenderName1 , appenderName2log4j.rootLogger=DEBUG,console,file#-----------------------------------##1 定义日志输出目的地为控制台log4j.appender.console = org.apache.log4j.ConsoleAppenderlog4j.appender.console.Target = System.outlog4j.appender.console.Threshold=DEBUG####可以灵活地指定日志输出格式，下面一行是指定具体的格式 ####%c: 输出日志信息所属的类目，通常就是所在类的全名#%m: 输出代码中指定的消息,产生的日志具体信息#%n: 输出一个回车换行符，Windows平台为"/r/n"，Unix平台为"/n"输出日志信息换行log4j.appender.console.layout = org.apache.log4j.PatternLayoutlog4j.appender.console.layout.ConversionPattern=[%c]-%m%n#-----------------------------------##2 文件大小到达指定尺寸的时候产生一个新的文件log4j.appender.file = org.apache.log4j.RollingFileAppender#日志文件输出目录log4j.appender.file.File=log/tibet.log#定义文件最大大小log4j.appender.file.MaxFileSize=10mb###输出日志信息####最低级别log4j.appender.file.Threshold=ERRORlog4j.appender.file.layout=org.apache.log4j.PatternLayoutlog4j.appender.file.layout.ConversionPattern=[%p][%d{yy-MM-dd}][%c]%m%n#-----------------------------------##3 druidlog4j.logger.druid.sql=INFOlog4j.logger.druid.sql.DataSource=infolog4j.logger.druid.sql.Connection=infolog4j.logger.druid.sql.Statement=infolog4j.logger.druid.sql.ResultSet=info#4 mybatis 显示SQL语句部分log4j.logger.org.mybatis=DEBUG#log4j.logger.cn.tibet.cas.dao=DEBUG#log4j.logger.org.mybatis.common.jdbc.SimpleDataSource=DEBUG#log4j.logger.org.mybatis.common.jdbc.ScriptRunner=DEBUG#log4j.logger.org.mybatis.sqlmap.engine.impl.SqlMapClientDelegate=DEBUG#log4j.logger.java.sql.Connection=DEBUGlog4j.logger.java.sql=DEBUGlog4j.logger.java.sql.Statement=DEBUGlog4j.logger.java.sql.ResultSet=DEBUGlog4j.logger.java.sql.PreparedStatement=DEBUG

mybatis-config.xml

studentMapper.xml

   
       
            select * from mybatis.student        
            
             
             
             
         
        
            select * from mybatis.teacher where id=#{id}    
   

测试：

package com.muan.dao;import com.muan.pojo.Student;import com.muan.pojo.Teacher;import com.muan.utils.MyBatisUtils;import org.apache.ibatis.session.SqlSession;import org.junit.Test;import java.util.List;public class StudentMapperTest {    @Test    public void getStudents(){        SqlSession sqlSession = MyBatisUtils.getSqlSession();        StudentMapper mapper = sqlSession.getMapper(StudentMapper.class);        List
   
     students = mapper.getStudents();        for (Student student : students) {            int id = student.getId();            String name = student.getName();            String teacher = student.getTeacher().getName();            System.out.println(id+"==="+name+"==="+teacher);        }    }}
   

结果显示：

方式二：一个resultMap解决，模拟面向对象的思想

其他的配置不变，只要更改studentMapper.xml即可

   
       
            select s.id,s.name,t.name as tname from mybatis.student s,mybatis.teacher t where s.tid=t.id        
        
            
             
             
             
                 
              
         
    
   

结果展示：

2.一对多

方式一：数据库联表查询方式

编写Student，Teacher实体类

package com.muan.pojo;public class Student {    private int id;//学生id    private String name;//学生姓名    private int tid;//教师id    public Student() {    }    public Student(int id, String name, int tid) {        this.id = id;        this.name = name;        this.tid = tid;    }    public int getId() {        return id;    }    public void setId(int id) {        this.id = id;    }    public String getName() {        return name;    }    public void setName(String name) {        this.name = name;    }    public int getTid() {        return tid;    }    public void setTid(int tid) {        this.tid = tid;    }    @Override    public String toString() {        return "Student{" +                "id=" + id +                ", name='" + name + '\'' +                ", tid=" + tid +                '}';    }}

package com.muan.pojo;import java.util.List;public class Teacher {    private int id;//教师id    private String name;//教师name    private List
   
     students;    public Teacher() {    }    public Teacher(int id, String name, List
    
      students) {        this.id = id;        this.name = name;        this.students = students;    }    public int getId() {        return id;    }    public void setId(int id) {        this.id = id;    }    public String getName() {        return name;    }    public void setName(String name) {        this.name = name;    }    public List
     
       getStudents() {        return students;    }    public void setStudents(List
      
        students) {        this.students = students;    }    @Override    public String toString() {        return "Teacher{" +                "id=" + id +                ", name='" + name + '\'' +                ", students=" + students +                '}';    }}
      
     
    
   

编写dao层接口

package com.muan.dao;import com.muan.pojo.Teacher;public interface TeacherMapper {    Teacher getTecherById(int id);}

mybatis-config.xml配置

编写对应的teacherMapper.xml

   
       
        
            select * from mybatis.teacher where id=#{id}        
            
             
         
        
            select * from mybatis.student where tid=#{id}    
   

测试：

package com.muan.dao;import com.muan.pojo.Student;import com.muan.pojo.Teacher;import com.muan.utils.MyBatisUtils;import org.apache.ibatis.session.SqlSession;import org.junit.Test;import java.util.Arrays;import java.util.List;public class StudentMapperTest {    @Test    public void getStudents(){        SqlSession sqlSession = MyBatisUtils.getSqlSession();        TeacherMapper mapper = sqlSession.getMapper(TeacherMapper.class);        Teacher teacher = mapper.getTecherById(1);        System.out.println(teacher.getId()+"\n\r"+teacher.getName()+"\n\r"+teacher.getStudents());    }}

效果展示：

方式二：面向对象方法解决

编写对应的teacherMapper.xml

   
       
        
            select s.name sname,s.id sid,s.tid stid,t.id tid,t.name tname        from mybatis.student s,mybatis.teacher t        where s.tid=t.id and t.id=#{id}        
            
             
             
                 
                  
                  
              
         
    
   

测试：

package com.muan.dao;import com.muan.pojo.Student;import com.muan.pojo.Teacher;import com.muan.utils.MyBatisUtils;import org.apache.ibatis.session.SqlSession;import org.junit.Test;import java.util.Arrays;import java.util.List;public class StudentMapperTest {    @Test    public void getStudents(){        SqlSession sqlSession = MyBatisUtils.getSqlSession();        TeacherMapper mapper = sqlSession.getMapper(TeacherMapper.class);        Teacher teacher = mapper.getTecherById(1);        System.out.println(teacher.getId()+"\n\r"+teacher.getName()+"\n\r"+teacher.getStudents());    }}

效果展示：

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