传智播客Hibernate视频教程学习笔记21

 

 多对多关系的关联映射：

学生教师关系为例

Student.java

package domain;import java.util.Set;public class Student {private int id;private String name;private Set<Teacher> teachers;public int getId() {return id;}public void setId(int id) {this.id = id;}public String getName() {return name;}public void setName(String name) {this.name = name;}public Set<Teacher> getTeachers() {return teachers;}public void setTeachers(Set<Teacher> teachers) {this.teachers = teachers;}}

Teacher.java

package domain;import java.util.Set;public class Teacher {private int id;private String name;private Set<Student> students;public int getId() {return id;}public void setId(int id) {this.id = id;}public String getName() {return name;}public void setName(String name) {this.name = name;}public void setStudents(Set<Student> students) {this.students = students;}public Set<Student> getStudents() {return students;}}

Student.hbm.xml

<?xml version="1.0" encoding="UTF-8"?><!DOCTYPE hibernate-mapping PUBLIC "-//Hibernate/Hibernate Mapping DTD 3.0//EN" "http://hibernate.sourceforge.net/hibernate-mapping-3.0.dtd"><hibernate-mapping package="domain"><class name="Student"><id name="id"> <generator class="native"/> </id> <property name="name"></property><set name="teachers" table="teacher_student"><key column="student_id"></key><many-to-many class="Teacher" column="teacher_id"></many-to-many></set> </class></hibernate-mapping>

Teacher.hbm.xml

<?xml version="1.0" encoding="UTF-8"?><!DOCTYPE hibernate-mapping PUBLIC "-//Hibernate/Hibernate Mapping DTD 3.0//EN" "http://hibernate.sourceforge.net/hibernate-mapping-3.0.dtd"><hibernate-mapping package="domain"><class name="Teacher"><id name="id"> <generator class="native"/> </id> <property name="name"></property><set name="students" table="teacher_student"><key column="teacher_id"></key><many-to-many class="Student" column="student_id"></many-to-many></set> </class></hibernate-mapping>

 

<set name="students" table="teacher_student">

   <key column="teacher_id"></key>
   <many-to-many class="Student" column="student_id"></many-to-many>
 </set>  
表示Teacher类中的students成员变量以Teacher类的id成员变量=teacher_student表中的teacher_id为条件，查找teacher_student表中所有符合条件的student_id，在通过student_id在Student表中查找具体信息，完成关联。

 

 

test.java

import domain.Student;import domain.Teacher;import hibernate.HibernateUtil;public class test {/** * @param args */public static void main(String[] args) {Teacher t1 = new Teacher();t1.setName("t1_name");Teacher t2 = new Teacher();t2.setName("t2_name");Student s1 = new Student();s1.setName("s1_name");Student s2 = new Student();s2.setName("s2_name");HibernateUtil.add(s1);HibernateUtil.add(s2);HibernateUtil.add(t1);HibernateUtil.add(t2);}}

一开始不设置关联，测试插入，成功。

Hibernate: insert into Student (name) values (?)
Hibernate: insert into Student (name) values (?)
Hibernate: insert into Teacher (name) values (?)
Hibernate: insert into Teacher (name) values (?)

四条插入语句。

teacher_student关联表也自动生成。关联表中teacher_id student_id均为主键 外键。

也说明Hibernate只要在满足数据库单表中符合约束的值均可正常插入，此处关联信息在生成表中，所以单表中不像一对多关系有外键约束。

 

 

Set<Student> ss = new HashSet<Student>();ss.add(s1);ss.add(s2);t1.setStudents(ss);

加入以上代码以后运行，正确生成关联，

Hibernate: insert into Student (name) values (?)
Hibernate: insert into Student (name) values (?)
Hibernate: insert into Teacher (name) values (?)
Hibernate: insert into teacher_student (teacher_id, student_id) values (?, ?)
Hibernate: insert into teacher_student (teacher_id, student_id) values (?, ?)
Hibernate: insert into Teacher (name) values (?)

 

 

但是如果同时给学生也设置老师集合，则报主键冲突错误

Set<Student> ss = new HashSet<Student>();ss.add(s1);ss.add(s2);Set<Teacher> ts = new HashSet<Teacher>();ts.add(t1);ts.add(t2);t1.setStudents(ss);s1.setTeachers(ts);

Exception in thread "main" org.hibernate.TransientObjectException: object references an unsaved transient instance - save the transient instance before flushing: domain.Teacher

其实学了关系映射以后，有一点一直不是很明白，Hibernate到底是怎么处理这种双向关系的，以前用jdbc模仿的时候也不会处理，只有一方为主来控制的，好像Hibernate也分主方从方，不明白。。。继续学习。。。