我要评论数据部分：
student表
set names utf8mb4; set foreign_key_checks = 0; -- ---------------------------- -- table structure for student -- ---------------------------- drop table if exists `student`; create table `student` ( `id` int(11) not null auto_increment, `sname` varchar(255) character set utf8 collate utf8_general_ci default null, `sex` varchar(255) character set utf8 collate utf8_general_ci default null, `age` int(11) default null, `t_id` int(11) default null, primary key (`id`) using btree ) engine = innodb auto_increment = 11 character set = utf8 collate = utf8_general_ci row_format = compact; -- ---------------------------- -- records of student -- ---------------------------- insert into `student` values (1, '张三', '男', 18, 1); insert into `student` values (2, '李四', '女', 18, 1); insert into `student` values (3, '王五', '男', 18, 1); insert into `student` values (4, '小白', '女', 18, 1); insert into `student` values (5, '小黑', '男', 18, 1); insert into `student` values (6, '小红', '女', 20, 2); insert into `student` values (7, '小李', '男', 20, 2); insert into `student` values (8, '小张', '女', 20, 2); insert into `student` values (9, '小赵', '男', 20, 2); insert into `student` values (10, '小王', '女', 20, 2); set foreign_key_checks = 1;
teacher表
set names utf8mb4; set foreign_key_checks = 0; -- ---------------------------- -- table structure for teacher -- ---------------------------- drop table if exists `teacher`; create table `teacher` ( `id` int(11) not null auto_increment, `tname` varchar(255) character set utf8 collate utf8_general_ci default null, primary key (`id`) using btree ) engine = innodb auto_increment = 3 character set = utf8 collate = utf8_general_ci row_format = compact; -- ---------------------------- -- records of teacher -- ---------------------------- insert into `teacher` values (1, '张老师'); insert into `teacher` values (2, '李老师'); set foreign_key_checks = 1;
查询老师的学生sql信息语句与查询结果如下：
查询学生的老师信息sql语句与查询结果如下：
直接查询
查询出来的数据用teacherstudents实体类进行封装
teacherstudents.java
package com.qcby.entity;
public class teacherstudents {
private integer id;
private string tname;
private string sname;
private string sex;
@override
public string tostring() {
return "teacherstudents{" +"id=" + id +", tname='" + tname + '\'' +", sname='" + sname + '\'' +", sex='" + sex + '\'' +'}';
}
public integer getid() {return id;}
public void setid(integer id) {this.id = id;}
public string gettname() {return tname;}
public void settname(string tname) {tname = tname;}
public string getsname() {return sname;}
public void setsname(string sname) {sname = sname;}
public string getsex() {return sex;}
public void setsex(string sex) {this.sex = sex;}
}
teacherstudentmapper.xml 内写查询语句
<?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.qcby.dao.teacherstudentsdao">
<select id="findteacherstudents" resulttype="com.qcby.entity.teacherstudents">
select teacher.*,student.sname,student.sex from teacher left join student on student.t_id = teacher.id
</select>
</mapper>
sqlmapconfig.xml
<mappers>
<mapper resource="mapper/teacherstudentsmapper.xml"></mapper>
</mappers>
teacherstudentsdao.java
package com.qcby.dao;
import com.qcby.entity.teacherstudents;
import java.util.list;
public interface teacherstudentsdao {
list<teacherstudents> findteacherstudents();
}
teacherstudentstest.java
import com.qcby.dao.teacherstudentsdao;
import com.qcby.entity.teacherstudents;
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.after;
import org.junit.before;
import org.junit.test;
import java.io.ioexception;
import java.io.inputstream;
import java.util.list;
public class teacherstudentstest {
private inputstream in = null;
private sqlsession session = null;
private teacherstudentsdao teacherstudentsdao = null;
@before //前置通知, 在方法执行之前执行
public void init() throws ioexception {
//加载主配置文件，目的是为了构建sqlsessionfactory对象
in = resources.getresourceasstream("sqlmapconfig.xml");
//创建sqlsessionfactory对象
sqlsessionfactory factory = new sqlsessionfactorybuilder().build(in);
//通过sqlsessionfactory工厂对象创建sqlsesssion对象
session = factory.opensession();
//通过session创建userdao接口代理对象
teacherstudentsdao = session.getmapper(teacherstudentsdao.class);
}
@after //@after: 后置通知, 在方法执行之后执行 。
public void destory() throws ioexception {
//释放资源
session.close();
in.close();
}
@test
public void findall(){
list<teacherstudents> list = teacherstudentsdao.findteacherstudents();
system.out.println(list.tostring());
}
}
直接查询的查询结果：
关联映射
student 表和 teacher 表存在着一对多与多对一的关系，我们介绍 mybatis 中处理关联关系的两种关联映射查询方式，分别是直接查询与分布查询。
直接查询通过 sql 的join（如left join、inner join）关联多个表，进行联合查询获取主表和关联表的所有所需字段，然后在<resultmap>中通过<association>一对一或<collection>一对多标签，定义主表与关联表字段到对应实体属性的映射规则，mybatis 会自动将查询结果分别封装为主实体和关联实体，并将关联实体注入主实体的关联属性中。
分步查询是分阶段完成查询，先执行主查询获取主实体数据及关联字段（select * from student），再由 mybatis 自动根据关联字段调用对应 mapper 方法，执行关联查询获取关联实体数据（select * from teacher where id = #{t_id}），最终将关联实体自动封装到主实体的关联属性中。可结合延迟加载，仅在使用关联数据时触发关联查询。
下面我们来看详细代码：
sqlmapconfig.xml
<mappers>
<mapper resource="mapper/studentmapper.xml"></mapper>
<mapper resource="mapper/teachermapper.xml"></mapper>
</mappers>
student.java
package com.qcby.entity;
public class student {
private integer id;
private string sname;
private string sex;
private integer age;
private integer t_id;
private teacher teacher;
@override
public string tostring() {
return "student{" +"id=" + id +", sname='" + sname + '\'' +", sex='" + sex + '\'' +", age=" + age +", t_id=" + t_id +", teacher=" + teacher +'}';
}
public teacher getteacher() {return teacher;}
public void setteacher(teacher teacher) {this.teacher = teacher;}
public integer getid() {return id;}
public void setid(integer id) {this.id = id;}
public string getsname() {return sname;}
public void setsname(string sname) {sname = sname;}
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 integer gett_id() {return t_id;}
public void sett_id(integer t_id) {this.t_id = t_id;}
}
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.qcby.dao.studentdao">
<select id="findstudentteacher" resultmap="studentteacher">
select student.*,teacher.tname from student left join teacher on student.t_id = teacher.id
</select>
<resultmap id="studentteacher" type="com.qcby.entity.student">
<id property="id" column="id"/>
<result property="sname" column="sname"/>
<result property="sex" column="sex"/>
<result property="age" column="age"/>
<result property="t_id" column="t_id"/>
<association property="teacher" javatype="com.qcby.entity.teacher">
<result property="tname" column="tname"/>
</association>
</resultmap>
<select id="findstudentteacher1" resultmap="studentteacher1">
select * from student
</select>
<resultmap id="studentteacher1" type="com.qcby.entity.student">
<id property="id" column="id"/>
<result property="sname" column="sname"/>
<result property="sex" column="sex"/>
<result property="age" column="age"/>
<result property="t_id" column="t_id"/>
<association property="teacher" javatype="com.qcby.entity.teacher"
select="com.qcby.dao.teacherdao.findteacherbyt_id" column="t_id">
</association>
</resultmap>
<select id="findstudentbyt_id" resulttype="com.qcby.entity.student">
select * from student where t_id = #{id}
</select>
</mapper>
<select> 标签用于主查询，resultmap 属性用于指定映射规则<resultmap> 标签用于结果映射规则，其中 id 属性值要与 select 标签中的 resultmap 属性值相同，是映射规则的唯一标识，type 指定映射的目标实体类，result 映射普通字段，property=实体类中的属性名，column=数据库当中的字段，而 <association> 标签用于处理对象的关联信息，其中 property=关联对象的属性名、javatype=关联对象的类型、select=对应的查询方法、column=传递给关联查询的参数
studendao.java
package com.qcby.dao;
import com.qcby.entity.student;
import java.util.list;
public interface studentdao {
list<student> findstudentteacher();
list<student> findstudentteacher1();
student findstudentbyt_id(integer t_id);
}
studenttest.java
import com.qcby.dao.studentdao;
import com.qcby.entity.student;
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.after;
import org.junit.before;
import org.junit.test;
import java.io.ioexception;
import java.io.inputstream;
import java.util.list;
public class studenttest {
private inputstream in = null;
private sqlsession session = null;
private studentdao studentdao = null;
@before //前置通知, 在方法执行之前执行
public void init() throws ioexception {
//加载主配置文件，目的是为了构建sqlsessionfactory对象
in = resources.getresourceasstream("sqlmapconfig.xml");
//创建sqlsessionfactory对象
sqlsessionfactory factory = new sqlsessionfactorybuilder().build(in);
//通过sqlsessionfactory工厂对象创建sqlsesssion对象
session = factory.opensession();
//通过session创建userdao接口代理对象
studentdao = session.getmapper(studentdao.class);
}
@after //@after: 后置通知, 在方法执行之后执行 。
public void destory() throws ioexception {
//释放资源
session.close();
in.close();
}
@test
public void findstudentteacher(){
list<student> students = studentdao.findstudentteacher();
system.out.println(students.tostring());
}
@test
public void findstudentteacher1(){
list<student> students = studentdao.findstudentteacher();
system.out.println(students.tostring());
}
}
findstudentteacher() 直接查询结果如下：
findstudentteacher1() 分步查询结果如下：
teacher.java
package com.qcby.entity;
import java.util.list;
public class teacher {
private integer id;
private string tname;
private list<student> students;
@override
public string tostring() {
return "teacher{" +"id=" + id +", tname='" + tname + '\'' +", students=" + students +'}';
}
public list<student> getstudents() {return students;}
public void setstudents(list<student> students) {this.students = students;}
public integer getid() {return id;}
public void setid(integer id) {this.id = id;}
public string gettname() {return tname;}
public void settname(string tname) {tname = tname;}
}
teachermapper.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.qcby.dao.teacherdao">
<select id="getstudents" resultmap="teacherstudents">
select teacher.*,student.sname,student.sex from teacher left join student on student.t_id = teacher.id
</select>
<resultmap id="teacherstudents" type="com.qcby.entity.teacher">
<id property="id" column="id"/>
<result property="tname" column="tname"/>
<collection property="students" oftype="com.qcby.entity.student">
<result property="sname" column="sname"/>
<result property="sex" column="sex"/>
</collection>
</resultmap>
<select id="findteacherbyt_id" resulttype="com.qcby.entity.teacher">
select * from teacher where id = #{t_id}
</select>
<select id="getstudents1" resultmap="teacherstudents1">
select * from teacher
</select>
<resultmap id="teacherstudents1" type="com.qcby.entity.teacher">
<id property="id" column="id"/>
<result property="tname" column="tname"/>
<collection property="students" oftype="com.qcby.entity.student"
select="com.qcby.dao.studentdao.findstudentbyt_id" column="id" fetchtype="lazy"/>
</resultmap>
</mapper>
在 teacher.java 中 student 属性的类型是 list 集合，collection 标签用于处理集合映射到 student.java 中，oftype=集合泛型的类型
teacherdao.java
package com.qcby.dao;
import com.qcby.entity.teacher;
import java.util.list;
public interface teacherdao {
list<teacher> getstudents();
teacher findteacherbyt_id(integer t_id);
list<teacher> getstudents1();
}
teachertest.java
import com.qcby.dao.teacherdao;
import com.qcby.entity.teacher;
import com.qcby.entity.teacherstudents;
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.after;
import org.junit.before;
import org.junit.test;
import java.io.ioexception;
import java.io.inputstream;
import java.util.list;
public class teachertest {
private inputstream in = null;
private sqlsession session = null;
private teacherdao teacherdao = null;
@before //前置通知, 在方法执行之前执行
public void init() throws ioexception {
//加载主配置文件，目的是为了构建sqlsessionfactory对象
in = resources.getresourceasstream("sqlmapconfig.xml");
//创建sqlsessionfactory对象
sqlsessionfactory factory = new sqlsessionfactorybuilder().build(in);
//通过sqlsessionfactory工厂对象创建sqlsesssion对象
session = factory.opensession();
//通过session创建userdao接口代理对象
teacherdao = session.getmapper(teacherdao.class);
}
@after //@after: 后置通知, 在方法执行之后执行 。
public void destory() throws ioexception {
//释放资源
session.close();
in.close();
}
@test
public void getstudents(){
list<teacher> teachers = teacherdao.getstudents();
for (teacher teacher:teachers){
system.out.println(teacher);
}
}
@test
public void getstudents1(){
list<teacher> teachers = teacherdao.getstudents1();
for (teacher teacher:teachers){
system.out.println(teacher);
system.out.println(teacher.gettname());
system.out.println(teacher.getstudents());
}
}
}
getstudents() 直接查询结果如下：
getstudents1() 分步查询结果如下：
延迟加载
mybatis 的延迟加载（lazy loading）是一种优化数据库查询性能的机制，它允许在需要时才加载关联对象的数据，而不是在加载主对象时就立即加载所有关联数据。这种 “按需加载” 的方式可以减少不必要的数据库查询，提升系统性能。
延迟加载实现效果如图：
只执行了一个sql’语句，当查询该对象时，只加载该对象本身的属性，对于其关联的对象暂不加载，直到程序真正需要访问这些关联对象时，才会发起对应的数据库查询
如何配置延迟加载
- 全局配置：sqlmapconfig.xml
<setting name="lazyloadingenabled" value="true"/>
<setting name="aggressivelazyloading" value="false"/>
lazyloadingenabled 是延迟加载的全局开关，aggressivelazyloading 对 lazyloadingenabled 的补充配置，当开启延迟加载，aggressivelazyloading 再设置为 true 时，任何方式调用都会加载该对象的所有属性，设置为 false 则该属性将按需加载，若 lazyloadingenabled=false 关闭延迟加载时，此配置无效，所有关联对象都会立即加载。
- 局部配置：在映射文件中通过
<association>或<collection>标签中由 fetchtype 属性指定
<collection property="students" oftype="com.qcby.entity.student"
select="com.qcby.dao.studentdao.findstudentbyt_id" column="id" fetchtype="lazy"/>
fetchtype有两个值，一个是"lazy"为懒加载，一个是"eager"为立即加载
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