Mybatis 多对一关联

小灰灰 2024-02-18 16:42 4阅读 0赞

以学生表(student) 、学校表(school) 为列 多对一的关系。

要查询多个表中的信息有多个方法：

方法一：（通过 . 输出子对象中的属性）

School.java:

public class School {
    	private int id;
    	private String name;
    
    	@Override
    	public String toString() {
    		return "School [id=" + id + ", name=" + name + "]";
    	}

SchoolMapper.xml:

Student.java:

public class Student {
    	private int id;
    	private String name;
    	private String pwd;
    	private int age;
    	private School school;
    	
    	@Override
    	public String toString() {
    		return "Student [id=" + id + ", name=" + name + ", pwd=" + pwd
    				+ ", age=" + age + ", school=" + school + "]";
    	}

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.dw.dao.StudentMapper">
      
      <resultMap type="com.dw.entity.Student" id="studentMap">
        <id property="id" column="stu_id"/>
        <result property="name" column="stu_name"/>
        <result property="pwd" column="stu_pwd"/>
        <result property="age" column="stu_age"/>
        <result property="school.id" column="s_id"/>
        <result property="school.name" column="s_name"/>
      </resultMap>
      
        <select id="selectById" parameterType="int" resultMap="studentMap">
          select stu.*,s.* from student as stu join school as s on stu.sid = s.s_id where s.s_id=#{id} 
        </select>
      </mapper>

测试：

package com.dw.dao.test;
    
    import java.io.IOException;
    import java.io.InputStream;
    
    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 org.junit.Test;
    
    import com.dw.dao.StudentMapper;
    import com.dw.dao.UserMapper;
    import com.dw.entity.Student;
    
    public class StudentTest {
    	private SqlSession session;
    	private InputStream inputStream ;
    	private SqlSessionFactory sessionFactory;	
    	private StudentMapper studentMapper ;
     @Test
     public void selectById(){
    	 init();
    	 Student student = studentMapper.selectById(1);
    	 System.out.println(student);
    	 destory();
     }
     
     
    	private void init(){
    		try {
    			inputStream = Resources.getResourceAsStream("config.xml");
    			sessionFactory = new SqlSessionFactoryBuilder().build(inputStream);
    			session = sessionFactory.openSession();
    			//userDao = session.getMapper(UserDao.class);
    			studentMapper = session.getMapper(StudentMapper.class);
    		} catch (IOException e) {
    			e.printStackTrace();
    		}
    	}
    	
    	private void destory(){
    		if(session != null){
    			session.close();
    		}
    		if(inputStream != null){
    			try {
    				inputStream.close();
    			} catch (IOException e) {
    				e.printStackTrace();
    			}
    		}
    	}   
    }

测试结果：Student [id=1, name=zs, pwd=123456, age=20, school=School [id=1, name=攀枝花大学]]

\-----------------------------------------------------------------------------------

方法二：通过association标签指定联合的javaBean

studentMapper.xml部分修改：

<resultMap type="com.dw.entity.Student" id="studentMap2">
          <id property="id" column="stu_id"/>
          <result property="name" column="stu_name"/>
          <result property="pwd" column="stu_pwd"/>
          <result property="age" column="stu_age"/>
          <association property="school" javaType="com.dw.entity.School">
             <id property="id" column="s_id"/>
             <result property="name" column="s_name"/>
          </association>
        </resultMap>
        
        <select id="selectById2" parameterType="int" resultMap="studentMap2">
          select stu.*,s.* from student as stu join school as s on stu.sid = s.s_id where s.s_id=#{id} 
        </select>

\--------------------------------------------------------------------------------------------

方法三：association定义关联对象的封装：

部分修改studentMapper.xml：

<resultMap type="com.dw.entity.Student" id="studentMap3">
          <id property="id" column="stu_id"/>
          <result property="name" column="stu_name"/>
          <result property="pwd" column="stu_pwd"/>
          <result property="age" column="stu_age"/>
          <association property="school" 
            select="com.dw.dao.SchoolMapper.selectById" 
            column="sid"> 
          </association>
        </resultMap>
        
        <select id="selectById3" parameterType="int" resultMap="studentMap3">
          select stu.* from student as stu where stu.stu_id=#{id} 
        </select>

schoolMapper.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.dw.dao.SchoolMapper">
        <select id="selectById" parameterType="int" resultType="com.dw.entity.School">
          select s_id as id,s_name as name from school as s where s.s_id=#{id} 
        </select>
      </mapper>

执行顺序：分别发出两条sql语句 第一条找到student的信息，第二条根据student中的外键sid找到school的信息

三种方法种，此种方法最为常用 ， 可以实现懒加载和立即加载：

MyBatis的Lazy Loading可以实现延迟查询Bean里的嵌套成员类,控制lazy loading的<settings>属性有

lazyLoadingEnabled: lazy loading开关,默认为true

aggressiveLazyLoading: 侵略性 lazy loading 开关, 默认为true, 这个属性比较搞笑,如果为true则当你访问任何一个属性都会加载所有的其他lazy load属性,即使你根本没有调用哪个lazy load属性,说白了就是aggressiveLazyLoading=true,则lazy load等于没用,所以要使用lazy load还是将其设为false

在全局配置文件config.xml中添加：

启动懒加载，mybatis初始化返回类型的时候，会返回一个cglib代理对象，该对象的关联对象（例如一对多，多对一）相关信息就会在loadpair里边，并且添加到loadmap中，cglib对象会过滤get，set ,is,"equals", "clone", "hashCode", "toString"触发方法，然后才会调用loadpair来加载关联对象的值。所以我们必须在进行懒加载的时候必须要导入相应的jar包，不然会报错。

1、asm.jar

2、cglib.jar