hibernate 多对多(many-to-many)
多对多（many-to-many）：在操作和性能方面都不太理想，所以多对多的映射使用较少，实际使用中最好转换成一对多的对象模型；hibernate会为我们创建中间关联表，转换成两个一对多。
1. E-R图
2. 实体类：
Teacher实体类如下：
Java代码 
package com.reiyen.hibernate.domain;
import java.util.Set;
public class Teacher {
private int id;
private String name;
private Set<Student> students;
//setter和getter方法
}
Student实体类如下：
Java代码
package com.reiyen.hibernate.domain;
import java.util.Set;
public class Student {
private int id;
private String name;
private Set<Teacher> teachers;
//setter和getter方法
}
3.映射文件如下：
Teacher.hbm.xml如下：
Xml代码
<?xml version="1.0"?>
<!DOCTYPE hibernate-mapping PUBLIC
"-//Hibernate/Hibernate Mapping DTD 3.0//EN"
"http://hibernate.sourceforge.net/hibernate-mapping-3.0.dtd">
<hibernate-mapping package="com.reiyen.hibernate.domain">
<class name="Teacher">
<id name="id">
<generator class="native" />
</id>
<property name="name" />
<!-- 通过table项告诉hibernate中间表的名称 -->
<set name="students" table="teacher_student">
<!-- 通过key属性告诉hibernate在中间表里面查询teacher_id值相应的teacher记录 -->
<key column="teacher_id" />
<!-- 通过column项告诉hibernate对student表中查找student_id值相就的studnet记录 -->
<many-to-many class="Student" column="student_id" />
</set>
</class>
</hibernate-mapping>
Student.hbm.xml如下：
Xml代码
<?xml version="1.0"?>
<!DOCTYPE hibernate-mapping PUBLIC
"-//Hibernate/Hibernate Mapping DTD 3.0//EN"
"http://hibernate.sourceforge.net/hibernate-mapping-3.0.dtd">
<hibernate-mapping package="com.reiyen.hibernate.domain">
<class name="Student" >
<id name="id" >
<generator class="native" />
</id>
<property name="name" />
<set name="teachers" table="teacher_student">
<key column="student_id" />
<many-to-many class="Teacher" column="teacher_id"/>
</set>
</class>
</hibernate-mapping>
一定要注意映射文件中<many-to-many class="Teacher" column="teacher_id"/>中class的值，它必须与你另一个关联映射文件中的class属性的name值一致，其实就是与你的实体类的类名一致,如：<many-to-many class="Teacher" column="teacher_id"/>中class的值就不能写成"teacher"。如果写成这样的话，就会抛出如下异常：An association from the table teacher_student refers to an unmapped class: com.reiyen .hibernate.domain.teacher
4. 测试程序如下：
Java代码
public class Many2Many {
public static void main(String[] args) {
add();
//query(1);
}
static void query(int id) {
Session s = null;
Transaction tx = null;
try {
s = HibernateUtil.getSession();
tx = s.beginTransaction();
Teacher t = (Teacher) s.get(Teacher.class, id);
System.out.println("students:" + t.getStudents().size());
tx.commit();
} finally {
if (s != null)
s.close();
}
}
static void add() {
Session s = null;
Transaction tx = null;
try {
Set<Teacher> ts = new HashSet<Teacher>();
Teacher t1 = new Teacher();
t1.setName("t1 name");
ts.add(t1);
Teacher t2 = new Teacher();
t2.setName("t2 name");
ts.add(t2);
Set<Student> ss = new HashSet<Student>();
Student s1 = new Student();
s1.setName("s1");
ss.add(s1);
Student s2 = new Student();
s2.setName("s2");
ss.add(s2);
t1.setStudents(ss); //1
t2.setStudents(ss); //1
//
// s1.setTeachers(ts); //2
// s2.setTeachers(ts); //2
s = HibernateUtil.getSession();
tx = s.beginTransaction();
s.save(t1);
s.save(t2);
s.save(s1);
s.save(s2);
tx.commit();
} finally {
if (s != null)
s.close();
}
}
}
运行此程序后：控制台打印的sql语句如下所示：
Hibernate: insert into Teacher (name) values (?)
Hibernate: insert into Teacher (name) values (?)
Hibernate: insert into Student (name) values (?)
Hibernate: insert into Student (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_student (teacher_id, student_id) values (?, ?)
Hibernate: insert into teacher_student (teacher_id, student_id) values (?, ?)
一共在中间表里面插入了4条记录。
中间表结构如下所示：
DROP TABLE IF EXISTS `test`.`teacher_student`;
CREATE TABLE `test`.`teacher_student` (
`teacher_id` int(11) NOT NULL,
`student_id` int(11) NOT NULL,
PRIMARY KEY (`student_id`,`teacher_id`),
KEY `FK2E2EF2DE6C8A2663` (`teacher_id`),
KEY `FK2E2EF2DE5BEEDBC3` (`student_id`),
CONSTRAINT `FK2E2EF2DE5BEEDBC3` FOREIGN KEY (`student_id`) REFERENCES `student` (`id`),
CONSTRAINT `FK2E2EF2DE6C8A2663` FOREIGN KEY (`teacher_id`) REFERENCES `teacher` (`id`)
) ENGINE=InnoDB DEFAULT CHARSET=utf8;
表中插入的记录如下所示：
mysql> select * from teacher_student;
+------------+------------+
| teacher_id | student_id |
+------------+------------+
| 1 | 1 |
| 1 | 2 |
| 2 | 1 |
| 2 | 2 |
+------------+------------+
4 rows in set (0.00 sec)
程序中注释为1的语句非常重要，它是建立Teacher与Student关联的语句，如果没有这两条语句，虽然程序照样会执行，但是在中间表teacher_student没有任何记录，也就是Teacher与Student之间未关联。
当然你也可以通过程序中注释为2的语句来建立Teacher与Student之间的关联关系，同样会产生与注释为1的语句的效果。但是你不能在程序中同时出现以上四句程序，否则会抛出异常（ PRIMARY KEY (`student_id`,`teacher_id`),所以会出现主键冲突的异常 ）,：
Hibernate: insert into teacher_student (teacher_id, student_id) 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_student (teacher_id, student_id) values (?, ?)
Hibernate: insert into teacher_student (student_id, teacher_id) values (?, ?)
Hibernate: insert into teacher_student (student_id, teacher_id) values (?, ?)
Hibernate: insert into teacher_student (student_id, teacher_id) values (?, ?)
Hibernate: insert into teacher_student (student_id, teacher_id) values (?, ?)
Exception in thread "main" org.hibernate.exception.ConstraintViolationException: Could not execute JDBC batch update
解决上面产生异常的办法是设置inverse属性。即在Tearcher一端或Student一端设置inverse="true"，即让他们之中的某一方放弃维护关联关系。此时，虽然上面四句程序在测试程序中同时出现了（其实就是在对象模型上相互设置了他们的关联关系），但程序照样能运行正常，因为了在数据库模型上，只会有两句程序生效，也就是没有设置inverse="true"的那一端会去维护关联关系。有关inverse的说细信息，可以参看我的文章hibernate级联（cascade和inverse）.
执行测试程序中的查询测试，控制台打印的信息如下所示：
Hibernate: select teacher0_.id as id5_0_, teacher0_.name as name5_0_ from Teacher teacher0_ where teacher0_.id=?
Hibernate: select students0_.teacher_id as teacher1_1_, students0_.student_id as student2_1_, student1_.id as id7_0_, student1_.name as name7_0_ from teacher_student students0_ left outer join Student student1_ on students0_.student_id=student1_.id where students0_.teacher_id=?
students:2
从打印出的sql语句可以看出，多对多关系进行查询时，效率是比较低的。