MyBatis学习记录-1-ResultMap（association与collection子标签）

domain代码

Classes.java 班级实体

package domain;

import lombok.AllArgsConstructor;
import lombok.Data;
import lombok.NoArgsConstructor;

import java.util.List;
@Data
@AllArgsConstructor
@NoArgsConstructor
public class Classes {
    private Integer id;
    private String name;
    private String classNum;

    private List<Student> studentList;

    @Override
    public String toString() {
        return "Classes{" +
                "ClassName='" + name + '\'' +
                ", classNum='" + classNum + '\'' +
                '}';
    }
}

Student.java 学生实体

package domain;

import lombok.AllArgsConstructor;
import lombok.Data;
import lombok.NoArgsConstructor;

import java.util.Date;
import java.util.Map;

@Data
@NoArgsConstructor
@AllArgsConstructor
public class Student {
    private Integer id;
    private String studentNum;
    private String classNum;
    private String name;
    private String phone;
    private String email;
    private Byte sex;
    private Date createTime;
    private Date modifyTime;

    private Map<String,Double> scoreMap;//学科-成绩
    private Classes classes;

    @Override
    public String toString() {
        return "Student{" +
                "studentNum='" + studentNum + '\'' +
                ", classNum='" + classNum + '\'' +
                ", name='" + name + '\'' +
                ", phone='" + phone + '\'' +
                ", email='" + email + '\'' +
                ", sex=" + sex +
                ", createTime=" + createTime +
                ", modifyTime=" + modifyTime +
                '}';
    }
}

Score.java 成绩实体

package domain;

import lombok.AllArgsConstructor;
import lombok.Data;
import lombok.NoArgsConstructor;

@Data
@NoArgsConstructor
@AllArgsConstructor
public class Score {
    private Integer id;
    private String studentNum;
    private Double score;
    private String subjectName;

    private Student student;

    @Override
    public String toString() {
        return "Score{" +
                "studentNum='" + studentNum + '\'' +
                ", score=" + score +
                ", subjectName='" + subjectName + '\'' +
                '}';
    }
}

级联属性

resultMap中可以设置result的级联属性(例如student.name),示例代码如下

<sql id="MyColumnList">
        score.id,score.student_num,score.score,score.subject_name,
        student.name,student.phone,student.email,student.sex,student.create_time,student.modify_time,student.class_num
sql>
<resultMap id="MyResMap1" type="domain.Score">
        <id column="id" property="id"/>
        <result column="student_num" property="studentNum"/>
        <result column="score" property="score"/>
        <result column="subject_name" property="subjectName"/>
        
        <result column="student_num" property="student.studentNum"/>
        <result column="name" property="student.name"/>
        <result column="phone" property="student.phone"/>
        <result column="email" property="student.email"/>
        <result column="sex" property="student.sex"/>
        <result column="class_num" property="student.classNum"/>
        <result column="create_time" property="student.createTime"/>
        <result column="modify_time" property="student.modifyTime"/>
resultMap>
<select id="cascadeSelect" resultMap="MyResMap1">
        select
        <include refid="MyColumnList"/>
        from mybatis.score
        left join mybatis.student
        on score.student_num=student.student_num
select>

association标签

association标签主要用来封装返回结果中的单个对象

使用association定义关联的单个对象的封装规则

其实可以理解为将级联属性换了一种更加简便的方式进行封装
示例代码如下

<sql id="MyColumnList">
        score.id,score.student_num,score.score,score.subject_name,
        student.name,student.phone,student.email,student.sex,student.create_time,student.modify_time,student.class_num
sql>
<resultMap id="MyResMap2" type="domain.Score">
        <id column="id" property="id"/>
        <result column="student_num" property="studentNum"/>
        <result column="score" property="score"/>
        <result column="subject_name" property="subjectName"/>
        
        
        <association property="student" javaType="domain.Student">
            <result column="student_num" property="studentNum"/>
            <result column="name" property="name"/>
            <result column="phone" property="phone"/>
            <result column="email" property="email"/>
            <result column="sex" property="sex"/>
            <result column="class_num" property="classNum"/>
            <result column="create_time" property="createTime"/>
            <result column="modify_time" property="modifyTime"/>
        association>
resultMap>
<select id="assoSelect" resultMap="MyResMap2">
    select
    <include refid="MyColumnList"/>
    from mybatis.score
    left join mybatis.student
    on score.student_num=student.student_num
select>

使用association分步查询

应用场景如下：我在查询学生信息的时候，需要先将学生的基本信息查询出来，在根据基本信息中的班级编号查询出班级信息，然后将班级信息封装到学生对象中的classes属性中
与全部直接查询出来在进行封装（利用级联属性或者association标签）区别在于，分步查询不需要把所有需要的数据放在一条sql语句中查询
示例代码如下：

<select id="getByStudentNum" resultMap="MyResMap2">
    select *
    from mybatis.student
    where student_num = #{studentNum};
select>
<resultMap id="MyResMap2" type="domain.Student">
        <id column="id" property="id"/>
        <result column="student_num" property="studentNum"/>
        <result column="name" property="name"/>
        <result column="phone" property="phone"/>
        <result column="email" property="email"/>
        <result column="sex" property="sex"/>
        <result column="modify_time" property="modifyTime"/>
        <result column="create_time" property="createTime"/>
        <result column="class_num" property="classNum"/>

        
        <association property="classes"
                     javaType="domain.Classes"
                     select="dao.ClassesDao.getByClassNum"
                     column="class_num"/>

    resultMap>

collection标签

使用collection定义关联集合的属性封装

应用场景如下：在查询班级信息的时候，我需要获取到该班级的所有同学信息，再将所有的同学信息封装至班级对象中的studentList属性中
resultMap如下：

<sql id="MyColumnList">
        classes.id AS classes_id, classes.name AS classes_name, classes.class_num,
        student.id AS student_id, student.student_num, student.name AS student_name,
        student.phone, student.email, student.sex, student.create_time,
        student.modify_time
sql>
<resultMap id="MyResMap2" type="domain.Classes">
        <id column="classes_id" property="id"/>
        <result column="classes_name" property="name"/>
        <result column="class_num" property="classNum"/>
        
        <collection property="studentList" ofType="domain.Student">
            <id column="student_id" property="id"/>
            <result column="student_num" property="studentNum"/>
            <result column="student_name" property="name"/>
            <result column="phone" property="phone"/>
            <result column="email" property="email"/>
            <result column="sex" property="sex"/>
            <result column="modify_time" property="modifyTime"/>
            <result column="create_time" property="createTime"/>
            <result column="class_num" property="classNum"/>
        collection>
resultMap>
<select id="getByClassNumPlus" resultMap="MyResMap2">
    select
    <include refid="MyColumnList"/>
    from mybatis.classes
    left join mybatis.student
    on classes.class_num = student.class_num
    where classes.class_num=#{classNum}
select>

使用collection分步查询

resultMap如下：

<select id="getByClassNumPlusStep" resultMap="MyResMap3">
        select * from mybatis.classes where class_num = #{classNum}
select>
<resultMap id="MyResMap3" type="domain.Classes">
        <id column="id" property="id"/>
        <result column="name" property="name"/>
        <result column="class_num" property="classNum"/>

        
        <collection property="studentList"
                    select="dao.StudentDao.getStusByClassNum"
                    column="class_num">
        collection>
resultMap>

延迟加载

延迟加载只需要在mybatis全局配置文件中设置好setting就可以了
只有在用到某个信息的时候才查询该信息

<settings>
        <setting name="logImpl" value="LOG4J"/>
        
        <setting name="lazyLoadingEnabled" value="true"/>
        <setting name="aggressiveLazyLoading" value="false"/>
settings>

实例如下：
测试代码块如下

@Test
public void test01(){
    Classes wl171 = classesDao.getByClassNumPlusStep("WL171");
    System.out.println(wl171.getName());
    for (Student student : wl171.getStudentList()){
        System.out.println(student);
    }
}

在不用延迟加载的时候，sql语句执行如下：
一次性执行所有sql语句

在使用了延迟加载的时候，sql语句执行如下
因为==wl171.getName()这句代码不会用到查询学生数据库的代码，所有先执行第一条sql语句
在打印了wl171.getName()==之后，又打印了学生列表，这时候又再执行查询学生列表的sql语句

需要注意的是，延迟加载需要跟**分步查询**一起使用

点击访问我的源代码(码云)