多对多关系
创建实体类和对应映射文件
Student.java
1 package com.qf.entity;
2
3 import java.util.HashSet;
4 import java.util.Set;
5
6 public class Student {
7
8 private Long sid;
9 private String sname;
10 //一个学生可以有很多个老师
11 private Set<Teacher> teachs = new HashSet<>();
12
13 public Long getSid() {
14 return sid;
15 }
16 public void setSid(Long sid) {
17 this.sid = sid;
18 }
19
20 public String getSname() {
21 return sname;
22 }
23 public void setSname(String sname) {
24 this.sname = sname;
25 }
26
27 public Set<Teacher> getTeachs() {
28 return teachs;
29 }
30 public void setTeachs(Set<Teacher> teachs) {
31 this.teachs = teachs;
32 }
33 @Override
34 public String toString() {
35 return "Student [sid=" + sid + ", sname=" + sname + ", teachs=" + teachs + "]";
36 }
37
38 }
Student.hbm.xml
<?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>
<!-- 配置表与实体的映射关系 -->
<class name="com.qf.entity.Student" table="student">
<id name="sid" column="sid">
<generator class="native"></generator>
</id>
<property name="sname" column="sname"/>
<set name="teachs" table="teach_stu" >
<key column="sid"/>
<many-to-many class="com.qf.entity.Teacher" column="tid" />
</set>
</class>
</hibernate-mapping>
Teacher.java
1 package com.qf.entity;
2
3 import java.util.HashSet;
4 import java.util.Set;
5
6 public class Teacher {
7 private long tid;
8 private String tname;
9 private String course;
10 //一个教师可以有多个学生
11 private Set<Student> stus = new HashSet<Student>();
12
13 public Set<Student> getStus() {
14 return stus;
15 }
16 public void setStus(Set<Student> stus) {
17 this.stus = stus;
18 }
19 public long getTid() {
20 return tid;
21 }
22 public void setTid(long tid) {
23 this.tid = tid;
24 }
25 public String getTname() {
26 return tname;
27 }
28 public void setTname(String tname) {
29 this.tname = tname;
30 }
31 public String getCourse() {
32 return course;
33 }
34 public void setCourse(String course) {
35 this.course = course;
36 }
37 @Override
38 public String toString() {
39 return "Teacher [tid=" + tid + ", tname=" + tname + ", course=" + course + "]";
40 }
41
42
43 }
Teacher.hbm.xml
<?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>
<!-- 配置表与实体的映射关系 -->
<class name="com.qf.entity.Teacher" table="teacher">
<id name="tid" column="tid">
<generator class="native"/>
</id>
<property name="tname" column="tname"/>
<property name="course" column="course"/>
<set name="stus" table="teach_stu">
<key column="tid"></key>
<many-to-many class="com.qf.entity.Student" column="sid"/>
</set>
</class>
</hibernate-mapping>
测试
@Test
public void test() {
Session session = SessionFactoryUtil.getSession();
Transaction tx = session.beginTransaction();
//创建两个教师对象，三个学生对象
//学生选择老师
Teacher t1 = new Teacher();
t1.setTname("张老师");
Teacher t2 = new Teacher();
t2.setTname("李老师");
Student s1 = new Student();
s1.setSname("张三");
Student s2 = new Student();
s2.setSname("李四");
Student s3 = new Student();
s3.setSname("王五");
//学生和老师之间建立双向关系
//多对多关系建立双向关系后，双方必须有一方放弃维护外键关系权（inverse），否则会因为中间表主键重复抛出异常
s1.getTeachs().add(t1);
s1.getTeachs().add(t2);
s3.getTeachs().add(t1);
s2.getTeachs().add(t2);
t1.getStus().add(s1);
t1.getStus().add(s3);
t2.getStus().add(s2);
t2.getStus().add(s1);
session.save(s1);
session.save(s2);
session.save(s3);
session.save(t1);
session.save(t2);
tx.commit();
}
-------------------------------------console-------------------------------------
Hibernate: alter table teach_stu drop foreign key FKsrw3nh8oe5xhmqqxm2qng952t Hibernate: alter table teach_stu drop foreign key FKtc48sy6cweqkufd4c777i8vsp Hibernate: drop table if exists student Hibernate: drop table if exists teach_stu Hibernate: drop table if exists teacher Hibernate: create table student (sid bigint not null auto_increment, sname varchar(255), primary key (sid)) Hibernate: create table teach_stu (tid bigint not null, sid bigint not null, primary key (sid, tid)) Hibernate: create table teacher (tid bigint not null auto_increment, tname varchar(255), course varchar(255), primary key (tid)) Hibernate: alter table teach_stu add constraint FKsrw3nh8oe5xhmqqxm2qng952t foreign key (sid) references student (sid) Hibernate: alter table teach_stu add constraint FKtc48sy6cweqkufd4c777i8vsp foreign key (tid) references teacher (tid) 11:21:37,135 INFO SchemaExport:464 - HHH000230: Schema export complete Hibernate: insert into student (sname) values (?) Hibernate: insert into student (sname) values (?) Hibernate: insert into student (sname) values (?) Hibernate: insert into teacher (tname, course) values (?, ?) Hibernate: insert into teacher (tname, course) values (?, ?) Hibernate: insert into teach_stu (sid, tid) values (?, ?) Hibernate: insert into teach_stu (sid, tid) values (?, ?) Hibernate: insert into teach_stu (sid, tid) values (?, ?) Hibernate: insert into teach_stu (sid, tid) values (?, ?) Hibernate: insert into teach_stu (tid, sid) values (?, ?) 11:21:37,270 WARN SqlExceptionHelper:127 - SQL Error: 1062, SQLState: 23000 11:21:37,271 ERROR SqlExceptionHelper:129 - Duplicate entry ‘1-1‘ for key ‘PRIMARY‘ 11:21:37,271 INFO AbstractBatchImpl:193 - HHH000010: On release of batch it still contained JDBC statements 11:21:37,272 ERROR SessionImpl:2994 - HHH000346: Error during managed flush [could not execute statement]
原因
测试中因为建立了双向关系，所以teacher和student都会去维护外键，导致相同的（1，1）会两次插入中间关系表teach_stu中
解决办法
一方放弃维护外键关系权。一般被动主体放弃维护外键权，比如学生选择老师，老师放弃维护外键权；老师选择学生，学生放弃维护外键权
修改Teacher.hbm.xml：<set name="stus" table="teach_stu" inverse="true">
-------------------------------------console-------------------------------------
Hibernate: alter table teach_stu drop foreign key FKsrw3nh8oe5xhmqqxm2qng952t Hibernate: alter table teach_stu drop foreign key FKtc48sy6cweqkufd4c777i8vsp Hibernate: drop table if exists student Hibernate: drop table if exists teach_stu Hibernate: drop table if exists teacher Hibernate: create table student (sid bigint not null auto_increment, sname varchar(255), primary key (sid)) Hibernate: create table teach_stu (tid bigint not null, sid bigint not null, primary key (sid, tid)) Hibernate: create table teacher (tid bigint not null auto_increment, tname varchar(255), course varchar(255), primary key (tid)) Hibernate: alter table teach_stu add constraint FKsrw3nh8oe5xhmqqxm2qng952t foreign key (sid) references student (sid) Hibernate: alter table teach_stu add constraint FKtc48sy6cweqkufd4c777i8vsp foreign key (tid) references teacher (tid) 11:30:43,077 INFO SchemaExport:464 - HHH000230: Schema export complete Hibernate: insert into student (sname) values (?) Hibernate: insert into student (sname) values (?) Hibernate: insert into student (sname) values (?) Hibernate: insert into teacher (tname, course) values (?, ?) Hibernate: insert into teacher (tname, course) values (?, ?) Hibernate: insert into teach_stu (sid, tid) values (?, ?) Hibernate: insert into teach_stu (sid, tid) values (?, ?) Hibernate: insert into teach_stu (sid, tid) values (?, ?) Hibernate: insert into teach_stu (sid, tid) values (?, ?)
数据成功插入数据库
级联保存
保存学生级联保存教师
主体是学生，所以修改学生的映射文件Student.hbm.xml
<hibernate-mapping> <!-- 配置表与实体的映射关系 --> <class name="com.qf.entity.Student" table="student"> <id name="sid" column="sid"> <generator class="native"></generator> </id> <property name="sname" column="sname"/> <set name="teachs" table="teach_stu" cascade="save-update" > <key column="sid"/> <many-to-many class="com.qf.entity.Teacher" column="tid" /> </set> </class> </hibernate-mapping>
测试方法
@Test
public void test() {
Session session = SessionFactoryUtil.getSession();
Transaction tx = session.beginTransaction();
//保存学生级联保存教师
Teacher t1 = new Teacher();
t1.setTname("张老师");
Student s1 = new Student();
s1.setSname("张三");
t1.getStus().add(s1);
s1.getTeachs().add(t1);
session.save(s1);
tx.commit();
}
-------------------------------------console-------------------------------------
Hibernate: create table student (sid bigint not null auto_increment, sname varchar(255), primary key (sid)) Hibernate: create table teach_stu (tid bigint not null, sid bigint not null, primary key (sid, tid)) Hibernate: create table teacher (tid bigint not null auto_increment, tname varchar(255), course varchar(255), primary key (tid)) Hibernate: alter table teach_stu add constraint FKsrw3nh8oe5xhmqqxm2qng952t foreign key (sid) references student (sid) Hibernate: alter table teach_stu add constraint FKtc48sy6cweqkufd4c777i8vsp foreign key (tid) references teacher (tid) 14:22:37,418 INFO SchemaExport:464 - HHH000230: Schema export complete Hibernate: insert into student (sname) values (?) Hibernate: insert into teacher (tname, course) values (?, ?) Hibernate: insert into teach_stu (sid, tid) values (?, ?
级联删除（一般不使用）
想以哪一方为主体删除另一方，就在哪一方映射文件中配置cascade属性：
<set name="teachs" table="teach_stu" cascade="delete" >
原文地址：https://www.cnblogs.com/qf123/p/10173799.html
时间： 2024-10-05 01:09:16