Hibernate的关联映射

时间：2017-01-05 15:12:08 阅读：211 评论：0 收藏：0 [点我收藏+]

单向N-1关联 <many-to-one>

单向N-1关系，比如多个人对应同一个住址，只需要从人实体端找到对应的住址实体，无须关系某个地址的全部住户。程序在N的一端增加一个属性，该属性引用1的一端的关联实体。

例如下面person实体中的address属性，

 1 package map.six11;
 2 
 3 public class Person {
 4     public Integer getId() {
 5         return id;
 6     }
 7     public void setId(Integer id) {
 8         this.id = id;
 9     }
10     public int getAge() {
11         return age;
12     }
13     public void setAge(int age) {
14         this.age = age;
15     }
16     public String getName() {
17         return name;
18     }
19     public void setName(String name) {
20         this.name = name;
21     }
22     public Address getAddress() {
23         return address;
24     }
25     public void setAddress(Address address) {
26         this.address = address;
27     }
28 
29     private Integer id;
30     private int age;
31     private String name;
32     private Address address;
33     
34 }

Address是一个独立的实体，

 1 package map.six11;
 2 
 3 public class Address {
 4     private Integer addressId;
 5     private String addressDetail;
 6     public Integer getAddressId() {
 7         return addressId;
 8     }
 9     public void setAddressId(Integer addressId) {
10         this.addressId = addressId;
11     }
12     public String getAddressDetail() {
13         return addressDetail;
14     }
15     public void setAddressDetail(String addressDetail) {
16         this.addressDetail = addressDetail;
17     }
18     public Address() {}
19     public Address(String addressDetail) {
20         this.addressDetail = addressDetail;
21     }
22 }

在N的一端person实体的映射文件中，用<many-to-one>标签标识关联的属性实体address。

值得注意的是 cascade="all" 这个属性，当实体类有数据更新时，关联的属性类也会更新到数据库，这叫级联行为。

 1 <?xml version="1.0"  encoding="UTF-8"?>    
 2 <!DOCTYPE hibernate-mapping PUBLIC "-//Hibernate/Hibernate Mapping DTD 3.0//EN"  
 3 "http://hibernate.sourceforge.net/hibernate-mapping-3.0.dtd">  
 4 
 5 <hibernate-mapping package="map.six11">
 6     <class name="Person" table="person_inf">
 7         <id name="id" column="person_id" type="int">
 8             <generator class="identity" />
 9         </id>
10         <property name="age" type="int" />
11         <property name="name" type="string" />
12            <many-to-one name="address" cascade="all" class="Address" column="address_id" />
13     </class>
14 </hibernate-mapping>

在1的一端，则是一个普通的映射文件，

 1 <?xml version="1.0"  encoding="UTF-8"?>    
 2 <!DOCTYPE hibernate-mapping PUBLIC "-//Hibernate/Hibernate Mapping DTD 3.0//EN"  
 3 "http://hibernate.sourceforge.net/hibernate-mapping-3.0.dtd">  
 4 
 5 <hibernate-mapping package="map.six11">
 6     <class name="Address" table="address_inf">
 7         <id name="addressId" column="address_id">
 8             <generator class="identity" />
 9         </id>
10         <property name="addressDetail"  />
11     </class>
12 </hibernate-mapping>

下面是一个测试类，

 1 package map.six11;
 2 
 3 import org.hibernate.Session;
 4 import org.hibernate.SessionFactory;
 5 import org.hibernate.Transaction;
 6 import org.hibernate.cfg.Configuration;
 7 
 8 public class PersonManager {
 9     public static void testPerson() {
10         Configuration conf = new Configuration().configure();
11         //conf.addResource("map.six11/Person.hbm.xml");
12         conf.addClass(Person.class);
13         //conf.addResource("map.six11/Address.hbm.xml");
14         conf.addClass(Address.class);
15         SessionFactory sf = conf.buildSessionFactory();
16         Session sess = sf.openSession();
17         Transaction tx = sess.beginTransaction();
18         
19         Person p = new Person();
20         Address a = new Address("广州天河");
21         p.setName("天王盖地虎");
22         p.setAge(20);
23         p.setAddress(a);
24         sess.persist(p);
25         Address a2 = new Address("上海虹口");
26         p.setAddress(a2);
27         
28         tx.commit();
29         sess.close();
30         sf.close();
31     }
32     
33     public static void main(String[] args) {
34         testPerson();
35     }
36 }

测试类中，Address只是一个瞬态的持久类，从未持久化，但是因为在Person实体类的映射文件中设置了cascade="all"属性，因此Address实体也会随着Person实体的更新而发生级联更新。

因此可以看到Hibernate不仅在Person表中插入了记录，而且还创建了Address表并且也插入了记录，hibernate日志如下，

1 Hibernate: insert into address_inf (addressDetail) values (?)
2 Hibernate: insert into person_inf (age, name, address_id) values (?, ?, ?)
3 Hibernate: insert into address_inf (addressDetail) values (?)
4 Hibernate: update person_inf set age=?, name=?, address_id=? where person_id=?

mysql数据如下，

person_inf

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