多對多（many-to-many）：在操作和性能方面都不太理想，所以多對多的映射使用較少，實際使用中最好轉換成一對多的對象模型；hibernate會為我們創建中間關聯表，轉換成兩個一對多。

1. E-R圖

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2. 實體類：

Teacher實體類如下：

Java代碼 ?

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實體類如下：

Java代碼 ?

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代碼 ?

<?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"?/>??

????????<!--?通過table項告訴hibernate中間表的名稱?-->??

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

????????????<!--?通過key屬性告訴hibernate在中間表里面查詢teacher_id值相應的teacher記錄?-->??

????????????<key?column="teacher_id"?/>??

????????????<!--?通過column項告訴hibernate對student表中查找student_id值相就的studnet記錄?-->??

????????????<many-to-many?class="Student"?column="student_id"?/>??

????????</set>??

????</class>??

</hibernate-mapping>??

?Student.hbm.xml如下：

Xml代碼 ?

<?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，它必須與你另一個關聯映射文件中的class屬性的name值一致，其實就是與你的實體類的類名一致,如：<many-to-many class="Teacher" column="teacher_id"/>中class的值就不能寫成"teacher"。如果寫成這樣的話，就會拋出如下異常：An association from the table teacher_student refers to an unmapped class: com.reiyen .hibernate.domain.teacher

4. 測試程序如下：

Java代碼 ?

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();??

????????}??

????}??

}??

?運行此程序后：控制臺打印的sql語句如下所示：

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 (?, ?)

一共在中間表里面插入了4條記錄。

中間表結構如下所示：

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;

表中插入的記錄如下所示：

mysql> select * from teacher_student;
+------------+------------+
| teacher_id | student_id |
+------------+------------+
|????????? 1 |????????? 1 |
|????????? 1 |????????? 2 |
|????????? 2 |????????? 1 |
|????????? 2 |????????? 2 |
+------------+------------+
4 rows in set (0.00 sec)

?

程序中注釋為1的語句非常重要，它是建立Teacher與Student關聯的語句，如果沒有這兩條語句，雖然程序照樣會執行，但是在中間表teacher_student沒有任何記錄，也就是Teacher與Student之間未關聯。

當然你也可以通過程序中注釋為2的語句來建立Teacher與Student之間的關聯關系，同樣會產生與注釋為1的語句的效果。但是你不能在程序中同時出現以上四句程序，否則會拋出異常（ PRIMARY KEY (`student_id`,`teacher_id`),所以會出現主鍵沖突的異常）,：

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

解決上面產生異常的辦法是設置inverse屬性。即在Tearcher一端或Student一端設置inverse="true"，即讓他們之中的某一方放棄維護關聯關系。此時，雖然上面四句程序在測試程序中同時出現了（其實就是在對象模型上相互設置了他們的關聯關系），但程序照樣能運行正常，因為了在數據庫模型上，只會有兩句程序生效，也就是沒有設置inverse="true"的那一端會去維護關聯關系。有關inverse的說細信息，可以參看我的文章hibernate級聯（cascade和inverse）.

?

執行測試程序中的查詢測試，控制臺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

從打印出的sql語句可以看出，多對多關系進行查詢時，效率是比較低的。

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