Mybatis中多个对象包含同一个对象的处理操作

//老师实体类
package com.MLXH.pojo;
public class Teacher {
    private int id;
    private String 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 + '\'' +
                '}';
    }
}

//学生实体类
package com.MLXH.pojo;
public class Student  {
    private int id;
    private String name;
    private Teacher teacher;
    public Student() {
    }
    public Student(int id, String name, Teacher teacher) {
        this.id = id;
        this.name = name;
        this.teacher = teacher;
    }
    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 Teacher getTeacher() {
        return teacher;
    }
    public void setTeacher(Teacher teacher) {
        this.teacher = teacher;
    }
    @Override
    public String toString() {
        return "Student{" +
                "id=" + id +
                ", name='" + name + '\'' +
                ", teacher=" + teacher +
                '}';
    }
}

driver = com.Mysql.jdbc.Driver
url = jdbc:mysql://localhost:3306/mybatis?useSSL=true&useUnicode=true&characterEncoding=utf-8
username = root
passWord = 123456

<?xml version="1.0" encoding="UTF-8" ?>
<!DOCTYPE configuration
        PUBLIC "-//mybatis.org//DTD Config 3.0//EN"
        "Http://mybatis.org/dtd/mybatis-3-config.dtd">
<configuration>
    <!--配置文件修改-->
    <properties resource="database.properties"/>
    <!--Mybatis设置-->
    <settings>
        <!--默认日志实现-->
        <!--<setting name="logImpl" value="STDOUT_LOGGING"/>-->
        <!--Log4j实现-->
        <setting name="logImpl" value="LOG4J"/>
    </settings>
    <!--配置别名-->
    <typeAliases>
        <package name="com.MLXH.pojo"/>
    </typeAliases>
    <environments default="development">
        <environment id="development">
            <transactionManager type="JDBC"/>
            <dataSource type="POOLED">
                <property name="driver" value="${driver}"/>
                <property name="url" value="${url}"/>
                <property name="username" value="${username}"/>
                <property name="password" value="${password}"/>
            </dataSource>
        </environment>
    </environments>
    <mappers>
        <!--class对应的是一个接口类-->
        <!--resource对应的是一个接口类的映射文件-->
        <mapper resource="com/MLXH/dao/StudentMapper.xml"/>
    </mappers>
</configuration>

package com.MLXH.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;
//mybatis的工具类，重复的代码的提纯
public class MyBatisUtils {
    //类变量不需要设置默认值；
    private static SqlSessionFactory sqlSessionFactory;
    static {
        //在Maven中，所有的资源文件一般都放在resources目录下，我们可以直接拿到。
        try {
            String resource = "mybatis-config.xml";
            InputStream inputStream = Resources.getResourceAsStream(resource);
            sqlSessionFactory = new SqlSessionFactoryBuilder().build(inputStream);
        } catch (IOException e) {
            e.printStackTrace();
        }
    }
    //设置SqlSessionFactory公共的方法
    public static SqlSessionFactory getSqlSessionFactory(){
        return sqlSessionFactory;
    }
    //获得一个带事务自动提交功能的SqlSession公共的方法
    public static SqlSession getSqlSession(){
        //自动提交事务
        return sqlSessionFactory.openSession(true);
    }
}

package com.MLXH.dao;
import com.MLXH.pojo.Student;
import java.util.List;
public interface StudentDao {
    //获得全部学生的信息以及对应的老师
    List<Student> getStudents();
    //获得全部学生的信息以及对应的老师
    List<Student> getStudentsTwo();
}

<?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">
<!--namespace不能写别名-->
<mapper namespace="com.MLXH.dao.StudentDao">
    <!--遇到问题：学生类中关联老师： 多个学生对应一个老师 -->
    
    <!--解决问题方式一:按查询结果嵌套处理，模拟数据库思想；-->
    <select id="getStudents" resultMap="StudentTeacher">
        select * from mybatis.student
    </select>
    <resultMap id="StudentTeacher" type="Student">
        <id column="id" property="id"/>
        <result column="name" property="name"/>
        <!--属性和字段对应  ， 类和表对应  ， 对象和记录
        关联一个字段
        需求：拿到老师这个类的属性
        association ： 关联，多对一
            column ： 数据库对应的列名
            property ： 对应属性名
            javaType ： 多对一字段对应的Java类型
            select ： 关联一个语句
        -->
        <association column="tid" property="teacher" javaType="Teacher" select="getTeacher"/>
    </resultMap>
    <select id="getTeacher" resultType="Teacher">
        select * from mybatis.teacher where id = #{id}
    </select>
</mapper>

    @Test
    public void getStudents(){
        SqlSession sqlSession = MyBatisUtils.getSqlSession();
        StudentDao mapper = sqlSession.getMapper(StudentDao.class);
        List<Student> students = mapper.getStudents();
        for (Student student : students) {
            System.out.println("学生姓名："+student.getName()+"\t老师姓名："+student.getTeacher().getName());
        }
    }

<?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">
<!--namespace不能写别名!!!!!-->
<mapper namespace="com.MLXH.dao.StudentDao">
    <!-- 解决方式二：一个resultMap解决 ， 模拟面向对象的思想-->
    <select id="getStudentsTwo" resultMap="StudentTeacher2">
        select s.id,s.name,t.id as tid,t.name as tname
        from mybatis.student as s, mybatis.teacher as t
        where s.tid = t.id
    </select>
    <!--设置结果集映射ResultMap -->
    <resultMap id="StudentTeacher2" type="Student">
        <id property="id" column="id"/>
        <result property="name" column="name"/>
        <!--直接关联一个老师-->
        <association property="teacher" javaType="Teacher">
            <id property="id" column="tid"/>
            <result property="name" column="tname"/>
        </association>
    </resultMap>
</mapper>

@Test
public void getStudentsTwo(){
    SqlSession sqlSession = MyBatisUtils.getSqlSession();
    StudentDao mapper = sqlSession.getMapper(StudentDao.class);
    List<Student> students = mapper.getStudentsTwo();
    for (Student student : students) {
        System.out.println("学生姓名："+student.getName()+"\t老师姓名："+student.getTeacher().getName());
    }
}

//Student是对象，age是String类型。
int getPojo(@param("student") Student student, @param("age") String age );

<select id="getStudent" resultMap="BaseResultMap">
    select 
    <include refid="Base_Column_List" />
    from student
    where 1 = 1
    <!-- 使用参数一（是个自己的对象） -->
    <if test="student.id != null">
        and id = #{student.id}
    </if>
    <!-- 使用参数二 String类型 -->
    <if test="age!= null">
        and age = #{age}
    </if>
</select>