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：

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：

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 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 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：

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。