Hibernate 超简单的一对多和多对一查询
这里使用的Teacher类和Student类（假设一个Teacher对应多个学生，一个学生对应一个老师）
所需jar包
开始建表
1（表名 teacher）
2（表名 student）
主键都为自增长
创建实体类
Teacher类
package com.bright.po;
import java.util.Set;
public class Teacher {
private Integer id;
private String name;
private Integer age;
private Set<Student> students;
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 Integer getAge() {
return age;
}
public void setAge(Integer age) {
this.age = age;
}
public Set<Student> getStudents() {
return students;
}
public void setStudents(Set<Student> students) {
this.students = students;
}
@Override
public String toString() {
return "Teacher [id=" + id + ", name=" + name + ", age=" + age
+ ", students=" + students + "]";
}
}
Student类
package com.bright.po;
public class Student {
private Integer id;
private String name;
private Teacher teacher;
public Student() {
super();
}
public Student(String name, Teacher teacher) {
super();
this.name = name;
this.teacher = teacher;
}
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 Teacher getTeacher() {
return teacher;
}
public void setTeacher(Teacher teacher) {
this.teacher = teacher;
}
@Override
public String toString() {
return "Student [id=" + id + ", name=" + name + "]";
}
}
配置Teacher类的映射文件
<?xml version="1.0" encoding="UTF-8"?>
<!DOCTYPE hibernate-mapping PUBLIC
"-//Hibernate/Hibernate Mapping DTD 3.0//EN"
"http://www.hibernate.org/dtd/hibernate-mapping-3.0.dtd">
<hibernate-mapping package="com.bright.po">
<!--
name：即实体类的全名
table：映射到数据库里面的那个表的名称
catalog：数据库的名称
-->
<class name="Teacher" table="teacher" catalog="me">
<!-- class下必须要有一个id的子元素 -->
<!-- id是用于描述主键的 -->
<id name="id" column="id">
<!-- 主键生成策略 -->
<generator class="native"></generator>
</id>
<!--
使用property来描述属性与字段的对应关系
如果length忽略不写，且你的表是自动创建这种方案，那么length的默认长度是255
-->
<property name="name" column="name" length="20"></property>
<property name="age" column="age" length="20"></property>
<!-- 一对多关联映射配置（通过部门管理到员工）
Teacher映射关键点:
1.指定映射的集合属性：students
2.集合属性对应的集合表：student
3.集合表的外键字段：t_id
4.集合元素的类型-->
<set name = "students" table = "student">
<key column="t_id"></key>
<one-to-many class="Student"></one-to-many>
</set>
</class>
</hibernate-mapping>
配置Student类的映射文件
<?xml version="1.0" encoding="UTF-8"?>
<!DOCTYPE hibernate-mapping PUBLIC
"-//Hibernate/Hibernate Mapping DTD 3.0//EN"
"http://www.hibernate.org/dtd/hibernate-mapping-3.0.dtd">
<hibernate-mapping package="com.bright.po">
<!--
name：即实体类的全名
table：映射到数据库里面的那个表的名称
catalog：数据库的名称
-->
<class name="Student" table="student" catalog="me">
<!-- class下必须要有一个id的子元素 -->
<!-- id是用于描述主键的 -->
<id name="id" column="id">
<!-- 主键生成策略 -->
<generator class="native"></generator>
</id>
<!--
使用property来描述属性与字段的对应关系
如果length忽略不写，且你的表是自动创建这种方案，那么length的默认长度是255
-->
<property name="name" column="name" length="20"></property>
<!-- 一对多关联映射配置（通过部门管理到员工）
Student映射关键点:
1.指定映射的属性：teacher
2.集合表的外键字段：t_id
3.元素的类型:teacher-->
<many-to-one name="teacher" column="t_Id" class="Teacher"></many-to-one>
</class>
</hibernate-mapping>
配置Hibrnate的配置文件
<?xml version="1.0" encoding="UTF-8"?>
<!DOCTYPE hibernate-configuration PUBLIC
"-//Hibernate/Hibernate Configuration DTD 3.0//EN"
"http://www.hibernate.org/dtd/hibernate-configuration-3.0.dtd">
<hibernate-configuration>
<session-factory>
<!-- 配置关于数据库连接的四个项：driverClass url username password -->
<property name="hibernate.connection.driver_class">com.mysql.jdbc.Driver</property>
<property name="hibernate.connection.url">jdbc:mysql:///me</property>
<property name="hibernate.connection.username">root</property>
<property name="hibernate.connection.password">root</property>
<!-- 可以将向数据库发送的SQL语句显示出来 -->
<property name="hibernate.show_sql">true</property>
<!-- 格式化SQL语句 -->
<property name="hibernate.format_sql">true</property>
<!-- hibernate的方言 -->
<property name="hibernate.dialect">org.hibernate.dialect.MySQLDialect</property>
<!-- 配置hibernate的映射文件所在的位置 -->
<mapping resource="com/bright/po/Teacher.hbm.xml" />
<mapping resource="com/bright/po/Student.hbm.xml" />
</session-factory>
</hibernate-configuration>
表中插入几条数据
teacher表
student表
开始测试
首先写个获取Session的工具类
package com.bright.utils;
import org.hibernate.Session;
import org.hibernate.SessionFactory;
import org.hibernate.cfg.Configuration;
public class SessionUtils {
public static Session getSession(){
Configuration cfg = new Configuration().configure();
SessionFactory factory = cfg.buildSessionFactory();
Session session = factory.openSession();
session.beginTransaction();
return session;
}
public static void closeSession(Session session){
if(session!=null){
session.getTransaction().commit();
session.close();
}
}
}
具体测试
package com.bright.test;
import java.util.List;
import org.hibernate.Session;
import com.bright.po.Student;
import com.bright.po.Teacher;
import com.bright.utils.SessionUtils;
public class TestTeacher {
public static void main(String[] args) {
Session session = SessionUtils.getSession();
List<Teacher> list = session.createQuery("from Teacher").list();
for(Teacher t:list){
System.out.println(t);
}
SessionUtils.closeSession(session);
}
}
输出结果
Hibernate:
select
teacher0_.id as id1_1_,
teacher0_.name as name2_1_,
teacher0_.age as age3_1_
from
me.teacher teacher0_
Hibernate:
select
students0_.t_id as t_id3_0_0_,
students0_.id as id1_0_0_,
students0_.id as id1_0_1_,
students0_.name as name2_0_1_,
students0_.t_Id as t_Id3_0_1_
from
me.student students0_
where
students0_.t_id=?
Teacher [id=1, name=白起, age=27, students=[Student [id=2, name=苏烈], Student [id=3, name=刘邦], Student [id=1, name=亚瑟]]]
Hibernate:
select
students0_.t_id as t_id3_0_0_,
students0_.id as id1_0_0_,
students0_.id as id1_0_1_,
students0_.name as name2_0_1_,
students0_.t_Id as t_Id3_0_1_
from
me.student students0_
where
students0_.t_id=?
Teacher [id=2, name=宫本武藏, age=25, students=[Student [id=6, name=兰陵王], Student [id=4, name=孙悟空], Student [id=5, name=荆轲]]]
Hibernate:
select
students0_.t_id as t_id3_0_0_,
students0_.id as id1_0_0_,
students0_.id as id1_0_1_,
students0_.name as name2_0_1_,
students0_.t_Id as t_Id3_0_1_
from
me.student students0_
where
students0_.t_id=?
Teacher [id=3, name=后羿, age=55, students=[Student [id=7, name=百里守约], Student [id=9, name=公孙离], Student [id=8, name=马可波罗]]]
可以看出，查询执行了多次，多次生成了sql语句，效率低，所以接下来采用了表的“迫切左外连接”。具体代码：
package com.bright.test;
import java.util.List;
import org.hibernate.Session;
import com.bright.po.Student;
import com.bright.po.Teacher;
import com.bright.utils.SessionUtils;
public class TestTeacher {
public static void main(String[] args) {
Session session = SessionUtils.getSession();
List<Teacher> list = session.createQuery("select t from Teacher t left outer join fetch t.students").list();
for(Teacher t:list){
System.out.println(t);
}
SessionUtils.closeSession(session);
}
}
查询结果：
Hibernate:
select
teacher0_.id as id1_1_0_,
students1_.id as id1_0_1_,
teacher0_.name as name2_1_0_,
teacher0_.age as age3_1_0_,
students1_.name as name2_0_1_,
students1_.t_Id as t_Id3_0_1_,
students1_.t_id as t_id3_0_0__,
students1_.id as id1_0_0__
from
me.teacher teacher0_
left outer join
me.student students1_
on teacher0_.id=students1_.t_id
Teacher [id=1, name=白起, age=27, students=[Student [id=3, name=刘邦], Student [id=1, name=亚瑟], Student [id=2, name=苏烈]]]
Teacher [id=1, name=白起, age=27, students=[Student [id=3, name=刘邦], Student [id=1, name=亚瑟], Student [id=2, name=苏烈]]]
Teacher [id=1, name=白起, age=27, students=[Student [id=3, name=刘邦], Student [id=1, name=亚瑟], Student [id=2, name=苏烈]]]
Teacher [id=2, name=宫本武藏, age=25, students=[Student [id=4, name=孙悟空], Student [id=5, name=荆轲], Student [id=6, name=兰陵王]]]
Teacher [id=2, name=宫本武藏, age=25, students=[Student [id=4, name=孙悟空], Student [id=5, name=荆轲], Student [id=6, name=兰陵王]]]
Teacher [id=2, name=宫本武藏, age=25, students=[Student [id=4, name=孙悟空], Student [id=5, name=荆轲], Student [id=6, name=兰陵王]]]
Teacher [id=3, name=后羿, age=55, students=[Student [id=8, name=马可波罗], Student [id=7, name=百里守约], Student [id=9, name=公孙离]]]
Teacher [id=3, name=后羿, age=55, students=[Student [id=8, name=马可波罗], Student [id=7, name=百里守约], Student [id=9, name=公孙离]]]
Teacher [id=3, name=后羿, age=55, students=[Student [id=8, name=马可波罗], Student [id=7, name=百里守约], Student [id=9, name=公孙离]]]
这样就好多了
结构图
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版权声明：本文为CSDN博主「kd_bright」的原创文章，遵循 CC 4.0 BY-SA 版权协议，转载请附上原文出处链接及本声明。
原文链接：https://blog.csdn.net/kd_bright/article/details/79528818
