Hibernate表关系映射之多对多映射

矫情吗；* 2022-09-28 11:38 176阅读 0赞

一、多对多的实现原理

在数据库中实现多对多的关系，必须使用连接表。也就是用一个独立的表来存入两个表的主键字段，通过遍历这张表来获取两表的关联关系。

而在我们的对象中，多对多是通过两者对象类中互相建立对方类的对象集合，类似上一篇讲到的一对多的集合类！

在Hibernate中是通过<many to many>来进行设置。同样多对多也存在双向和单向之分，表现到Hibernate中就是配置文件和POJO代码的不同。这里以单向映射为例，双向映射与之类似。

二、实例演示

1、建立数据模型

我们以学生与老师关系为例。一个学生可以有多个老师，而一个老师也一样可以有多名学生！这里我们让老师Teacher持有学生Student的集合引用。

Teacher.java

package com.chen.many2many;
    
    import java.util.HashSet;
    import java.util.Set;
    
    public class Teacher {
    	private Integer teacherID;
    	private String teacherName;
    	//持有一个student的Set集合
    	private Set<Student> studentSet=new HashSet<>();
    	
    	
    	public Integer getTeacherID() {
    		return teacherID;
    	}
    	public void setTeacherID(Integer teacherID) {
    		this.teacherID = teacherID;
    	}
    	public String getTeacherName() {
    		return teacherName;
    	}
    	public void setTeacherName(String teacherName) {
    		this.teacherName = teacherName;
    	}
    	public Set<Student> getStudentSet() {
    		return studentSet;
    	}
    	public void setStudentSet(Set<Student> studentSet) {
    		this.studentSet = studentSet;
    	}
    	
    	
    
    }

Student.java

package com.chen.many2many;
    
    import java.util.HashSet;
    import java.util.Set;
    
    public class Student {
    	private Integer studentID;
    	private String studentName;
    	//持有一个teacher的Set集合
    	//private Set<Teacher> teacherSet=new HashSet<>();
    	public Integer getStudentID() {
    		return studentID;
    	}
    	public void setStudentID(Integer studentID) {
    		this.studentID = studentID;
    	}
    	public String getStudentName() {
    		return studentName;
    	}
    	public void setStudentName(String studentName) {
    		this.studentName = studentName;
    	}
    /*	public Set<Teacher> getTeacherSet() {
    		return teacherSet;
    	}
    	public void setTeacherSet(Set<Teacher> teacherSet) {
    		this.teacherSet = teacherSet;
    	}
    	*/
    	
    	
    
    }

2、Hibernate配置文件

Student.hbm.xml

<?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>
        <class name="com.chen.many2many.Student" table="STUDENTS">
            <id name="studentID" type="java.lang.Integer">
                <column name="STUDENT_ID" />
                <generator class="native" />
            </id>
            <property name="studentName" type="java.lang.String">
                <column name="STUDENT_NAME" />
            </property>
            
        </class>
    </hibernate-mapping>

Teacher.hbm.xml

<?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>
        <class name="com.chen.many2many.Teacher" table="TEACHERS">
            <id name="teacherID" type="java.lang.Integer">
                <column name="TEACHER_ID" />
                <generator class="native" />
            </id>
            <property name="teacherName" type="java.lang.String">
                <column name="TEACHER_NAME" />
            </property>
            
            <set name="studentSet" table="TEACHERS_STUDENTS">
                <key>
                
                    <column name="T_ID" />
                </key>
                
                <many-to-many class="com.chen.many2many.Student" column="S_ID" />
            </set>
        </class>
    </hibernate-mapping>

hibernate.cfg.xml

<?xml version="1.0" encoding="UTF-8"?>
    <!DOCTYPE hibernate-configuration PUBLIC
    		"-//Hibernate/Hibernate Configuration DTD 3.0//EN"
    		"http://hibernate.sourceforge.net/hibernate-configuration-3.0.dtd">
    <hibernate-configuration>
        <session-factory>
        
        	
        	<property name="connection.username">root</property>
        	<property name="connection.password">root</property>
        	<property name="connection.driver_class">com.mysql.jdbc.Driver</property>
        	<property name="connection.url">jdbc:mysql://192.168.1.148:3306/hibernate5</property>
    
    		
    		
    		<property name="dialect">org.hibernate.dialect.MySQL5InnoDBDialect</property>
    	
    		
        	<property name="show_sql">true</property>
        
        	
        	<property name="format_sql">true</property>
        
        	
        	<property name="hbm2ddl.auto">update</property>
        	
        	
        	<property name="connection.isolation">2</property>
        	
        	
        	<property name="use_identifier_rollback">true</property>
        	
        	
        	<property name="hibernate.c3p0.max_size">10</property>
        	<property name="hibernate.c3p0.min_size">5</property>
        	<property name="c3p0.acquire_increment">2</property>
        	
        	<property name="c3p0.idle_test_period">2000</property>
        	<property name="c3p0.timeout">2000</property>
        	
        	<property name="c3p0.max_statements">10</property>
        	
        	
        	<property name="hibernate.jdbc.fetch_size">100</property>
        	
        	
        	<property name="jdbc.batch_size">30</property>
        	
        	
            <mapping resource="com/chen/many2many/Student.hbm.xml"/>
    		<mapping resource="com/chen/many2many/Teacher.hbm.xml"/>
    		    
        </session-factory>
    </hibernate-configuration>

3、编写测试方法

HibernateTest.java

package com.chen.many2many;
    
    import java.util.Iterator;
    import java.util.Set;
    
    import org.hibernate.Session;
    import org.hibernate.SessionFactory;
    import org.hibernate.Transaction;
    import org.hibernate.cfg.Configuration;
    import org.hibernate.service.ServiceRegistry;
    import org.hibernate.service.ServiceRegistryBuilder;
    
    public class HibernateTest {
    	private static SessionFactory sessionFactory;
    	private static Session session;
    	private static Transaction transaction;
    	
    	//定义事务初始化函数
    	public static void init(){
    		Configuration configuration = new Configuration().configure();
    		ServiceRegistry serviceRegistry = 
    				new ServiceRegistryBuilder().applySettings(configuration.getProperties())
    				                            .buildServiceRegistry();
    		sessionFactory = configuration.buildSessionFactory(serviceRegistry);
    		
    		session = sessionFactory.openSession();
    		transaction = session.beginTransaction();
    	}
    	
    	//定义事务结束时的处理函数
    	public static void doit(){
    		transaction.commit();
    		session.close();
    		sessionFactory.close();
    	}
    	
    	
    	//编写自己的处理过程
    	public static void testSave(){
    		//学生: a,b,c
    		//教师:A,B
    		//所属关系A(a,b) B(a,b,c)
    		//初始化学生
    		Student a=new Student();
    		a.setStudentName("a");
    		Student b = new Student();
    		b.setStudentName("b");
    		Student c=new Student();
    		c.setStudentName("c");
    		//初始化老师
    		Teacher A=new Teacher();
    		A.setTeacherName("A");
    		Teacher B=new Teacher();
    		B.setTeacherName("B");
    		
    		//设置关联关系
    		A.getStudentSet().add(a);
    		A.getStudentSet().add(b);
    		
    		B.getStudentSet().add(a);
    		B.getStudentSet().add(b);
    		B.getStudentSet().add(c);
    		
    		//保存
    		session.save(a);
    		session.save(b);
    		session.save(c);
    		session.save(A);
    		session.save(B);
    	}
    	public static void testGet(){
    		Teacher teacherResult=(Teacher) session.get(Teacher.class, 1);
    		System.out.println(teacherResult.getTeacherName());
    		Set<Student> set=teacherResult.getStudentSet();
    		for (Iterator iterator = set.iterator(); iterator.hasNext();) {
    			Student student = (Student) iterator.next();
    			System.out.println(student.getStudentName());
    		}
    	}
    	public static void main(String[] args) {
    		HibernateTest.init();
    		HibernateTest.testSave();
    		HibernateTest.testGet();
    		HibernateTest.doit();
    		System.out.println("完成---------");
    	}
    
    }

4、运行结果

数据表students

![20170310155501506][]

数据表teachers

![20170310155546523][]

中间表teachers\_students

![20170310155639633][]

控制台显示结果：

![20170310155741403][]

三、多对多双向映射注意项

1、双向 n-n 关联需要两端都持有对方的一个集合对象引用。  
2、双向n-n关联必须使用连接表。  
3、集合属性应增加 key 子元素用以映射外键列, 集合元素里还应增加many-to-many子元素关联实体类。  
4、在双向 n-n 关联的两边，都需指定连接表的表名及外键列的列名.。两个集合元素 set 的 table 元素的值必须指定，而且必须相同，这样就指定了同一张连接表。

5、set元素的两个子元素：key 和 many-to-many 都必须指定 column 属性，其中，key 和 many-to-many 分别指定本持久化类和关联类在连接表中的外键列名，因此两边的 key 与 many-to-many 的column属性交叉相同。也就是说，一边的set元素的key的 cloumn值为a,many-to-many 的 column 为b；则另一边的 set 元素的 key 的 column 值 b,many-to-many的 column 值为 a。  
6、对于双向 n-n 关联, 必须把其中一端的 inverse 设置为 true, 否则两端都维护关联关系可能会造成主键冲突。

[20170310155501506]: /images/20220708/cbb383a1bb924adf81c1adcefb3afebc.png
[20170310155546523]: /images/20220708/c0cd8686c5634442a4c50e89b7c44058.png
[20170310155639633]: /images/20220708/ddf4762ea654467893955b00ffcea64a.png
[20170310155741403]: /images/20220708/7599143546b94359a27758073c784850.png