hibernate 多對多(many-to-many)

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會爲我們創建中間關聯表，轉換成兩個一對多。

1. E-R圖

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
}

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
}

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" />
<set name="students" table="teacher_student">
<key column="teacher_id" />
<many-to-many class="Student" column="student_id" />
</set>
</class>
</hibernate-mapping>

<?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在中間表裏面查詢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>

<?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();
}
}
}

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();
		}
	}
}

運行此程序後：控制檯打印的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 (?, ?)

一共在中間表裏面插入了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)

程序中註釋爲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註釋爲2的語句來建立Teacher與Student之間的關聯關係，同樣會產生與註釋爲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`),所以會出現主鍵衝突的異常）,：

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係。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係），但程序照樣能運行正常，因爲了在數據庫模型上，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係。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，控制檯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，多對多關係進行查詢時，效率是比較低的。

fibrelin

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