MyBatis框架的一对多关联映射

红太狼 2022-06-18 02:38 262阅读 0赞

代码：

mybatis-config.xml

<?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>
    	
    	<settings>
    		<setting name="logImpl" value="LOG4J"/>
    		
    		<setting name="lazyLoadingEnabled" value="true"/>
    		<setting name="aggressiveLazyLoading" value="false"/>
    	</settings>
    	
     <environments default="mysql">
    	
        <environment id="mysql">
        
          <transactionManager type="JDBC"/>
          
          <dataSource type="POOLED">
            <property name="driver" value="com.mysql.jdbc.Driver"/>
            <property name="url" value="jdbc:mysql://localhost:3306/mybatis"/>
            <property name="username" value="root"/>
            <property name="password" value="123456"/>
          </dataSource>
        </environment>
     </environments>
      
      
      <mappers>
      	<mapper resource="com/mapper/ClazzMapper.xml"/>
      	<mapper resource="com/mapper/StudentMapper.xml"/>
      </mappers>
    </configuration>

Clazz.java

package com.bean;
    
    import java.io.Serializable;
    import java.util.List;
    
    public class Clazz implements Serializable {
    	private Integer id; // 班级id，主键
    	private String code; // 班级编号
    	private String name; // 班级名称
    	
    	// 班级和学生是一对多的关系，即一个班级可以有多个学生
    	private List<Student> students;
    
    	public Clazz() {
    		super();
    		// TODO Auto-generated constructor stub
    	}
    
    	public Integer getId() {
    		return id;
    	}
    
    	public void setId(Integer id) {
    		this.id = id;
    	}
    
    	public String getCode() {
    		return code;
    	}
    
    	public void setCode(String code) {
    		this.code = code;
    	}
    
    	public String getName() {
    		return name;
    	}
    
    	public void setName(String name) {
    		this.name = name;
    	}
    
    	public List<Student> getStudents() {
    		return students;
    	}
    
    	public void setStudents(List<Student> students) {
    		this.students = students;
    	}
    
    	@Override
    	public String toString() {
    		// TODO Auto-generated method stub
    		return "Clazz [id=" + id + ", code=" + code + ", name=" + name + "]";
    	}
    	
    }

Student.java

package com.bean;
    
    import java.io.Serializable;
    
    public class Student implements Serializable {
    	private Integer id; // 学生id，主键
    	private String name; // 姓名
    	private String sex;  // 性别
    	private Integer age; // 年龄
    	
    	// 学生和班级是多对一的关系，即一个学生只属于一个班级
    	private Clazz clazz;
    
    	public Student() {
    		super();
    		// TODO Auto-generated constructor stub
    	}
    
    	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 Integer getAge() {
    		return age;
    	}
    
    	public void setAge(Integer age) {
    		this.age = age;
    	}
    
    	public Clazz getClazz() {
    		return clazz;
    	}
    
    	public void setClazz(Clazz clazz) {
    		this.clazz = clazz;
    	}
    
    	@Override
    	public String toString() {
    		// TODO Auto-generated method stub
    		return "Student [id=" + id + ", name=" + name + ", sex=" + sex + ", age=" + age + "]";
    	}
    	
    }

ClazzMapper.java

package com.mapper;
    
    import com.bean.Clazz;
    public interface ClazzMapper {
    	// 根据id查询班级信息
    	Clazz selectClazzById(Integer id);	
    }

StudentMapper.java

package com.mapper;
    
    import com.bean.Student;
    
    public interface StudentMapper {
    	// 根据id查询学生信息
    	Student selectStudentById(Integer id);
    }

clazzMapper.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.mapper.ClazzMapper">
    
    	
    	  <select id="selectClazzById" parameterType="int" resultMap="clazzResultMap">
    	  	SELECT * FROM tb_clazz  WHERE id = #{id}
    	  </select>
    	  
    	   
    	<resultMap type="com.bean.Clazz" id="clazzResultMap">
    		<id property="id" column="id"/>
    		<result property="code" column="code"/>
    		<result property="name" column="name"/>
    		
    	  <collection property="students" javaType="ArrayList"
    	     column="id" ofType="com.bean.Student"
    	     select="com.mapper.StudentMapper.selectStudentByClazzId" fetchType="lazy">
    	  	<id property="id" column="id"/>
    	  	<result property="name" column="name"/>
    	  	<result property="sex" column="sex"/>
    	  	<result property="age" column="age"/>
    	  </collection>
    	</resultMap>
    </mapper>

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.mapper.StudentMapper">
      
      <select id="selectStudentById" parameterType="int" resultMap="studentResultMap">
      	   SELECT * FROM tb_clazz c,tb_student s WHERE c.id = s.clazz_id AND s.id = #{id}
      </select>
      
      
      <select id="selectStudentByClazzId" parameterType="int" resultMap="studentResultMap">
      	  SELECT * FROM tb_student WHERE clazz_id = #{id}
      </select>
      
       
    	<resultMap type="com.bean.Student" id="studentResultMap">
    		<id property="id" column="id"/>
    	  	<result property="name" column="name"/>
    	  	<result property="sex" column="sex"/>
    	  	<result property="age" column="age"/>
    		
    		<association property="clazz" javaType="com.bean.Clazz">
    			<id property="id" column="id"/>
    			<result property="code" column="code"/>
    			<result property="name" column="name"/>
    		</association>
    	</resultMap>
    </mapper>

OneToManyTest.java

package com.test;
    
    import java.io.InputStream;
    import java.util.List;
    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 com.bean.*;
    import com.mapper.*;
    
    public class OneToManyTest {
    
    	public static void main(String[] args) throws Exception {
    		// 读取mybatis-config.xml文件
    		InputStream inputStream = Resources.getResourceAsStream("mybatis-config.xml");
    		// 初始化mybatis，创建SqlSessionFactory类的实例
    		SqlSessionFactory sqlSessionFactory = new SqlSessionFactoryBuilder().build(inputStream);
    		// 创建Session实例
    		SqlSession session = sqlSessionFactory.openSession();	
    		OneToManyTest t = new OneToManyTest();	
    	    t.testSelectClazzById(session);
          //t.testSelectStudentById(session);
    		session.commit();
    		session.close();
    	}
    	
    	// 测试一对多，查询班级Clazz（一）的时候级联查询学生Student（多）  
    	public void testSelectClazzById(SqlSession session){
    		// 获得ClazzMapper接口的代理对象
    		ClazzMapper cm = session.getMapper(ClazzMapper.class);
    		// 调用selectClazzById方法
    		Clazz clazz = cm.selectClazzById(1);
    		// 查看查询到的clazz对象信息
    		System.out.println(clazz.getId() + " "+ clazz.getCode() + " "+clazz.getName());
    		// 查看clazz对象关联的学生信息
    		List<Student> students = clazz.getStudents();
    		for(Student stu : students){
    			System.out.println(stu);
    		}
    	}
    	
    	// 测试多对一，查询学生Student（多）的时候级联查询 班级Clazz（一）
    	public void testSelectStudentById(SqlSession session){
    		// 获得StudentMapper接口的代理对象
    		StudentMapper sm = session.getMapper(StudentMapper.class);
    		// 调用selectStudentById方法
    		Student stu = sm.selectStudentById(1);
    		// 查看查询到的Student对象信息
    		System.out.println(stu);
    		// 查看Student对象关联的班级信息
    		System.out.println(stu.getClazz());
    	}
    
    }

截图：

![Center][]

根据id查询学生信息

![Center 1][]

根据id查询班级信息

[Center]: /images/20220618/908415818a9f42e3951b67c1fcf502f6.png
[Center 1]: /images/20220618/7236c160f4164c67a4f605e9b8a600fb.png