一個Product對應一個Category
一個Category對應多個Product

所以Product和Category是多對一的關系
本例講解如何使用Hibernate實現多對一關系

第一步 準備Category.java

 1 package com.ghw.pojo;
 2  
 3 public class Category {
 4  
 5     public int getId() {
 6         return id;
 7     }
 8     public void setId(int id) {
 9         this.id = id;
10     }
 
11     public String getName() {
12         return name;
13     }
14     public void setName(String name) {
15         this.name = name;
16     }
17     int id;
18     String name;
19 }

第二步 準備Category.hbm.xml

 1 <?xml version="1.0"?>
 2  
 3 <!DOCTYPE hibernate-mapping PUBLIC
 4         "-//Hibernate/Hibernate Mapping DTD 3.0//EN"
 
 5         "http://www.hibernate.org/dtd/hibernate-mapping-3.0.dtd">
 6 <hibernate-mapping package="com.ghw.pojo">
 7     <class name="Category" table="category_">   
 8         <id name="id" column="id">
 9             <generator class="native">
10             </generator>
 

11         </id>
12         <property name="name" />
13     </class>  
14 </hibernate-mapping>

第三步 在Product.java中添加Category屬性和它的setter和getter方法

 1 package com.ghw.pojo;
 2 public class Product {
 3     private int id;
 4     private String name;
 5     private float price;
 6     private Category category;
 7     public Category getCategory() {
 8         return category;
 9     }
10     public void setCategory(Category category) {
11         this.category = category;
12     }
13     public int getId() {
14         return id;
15     }
16     public void setId(int id) {
17         this.id = id;
18     }
19     public String getName() {
20         return name;
21     }
22     public void setName(String name) {
23         this.name = name;
24     }
25     public float getPrice() {
26         return price;
27     }
28     public void setPrice(float price) {
29         this.price = price;
30     } 
31 }

第四步 在Hibernate.hbm.xml文件中添加Category.hbm.xml的mappering映射

新增Category映射

 1 <?xml version=‘1.0‘ encoding=‘utf-8‘?>
 2 <!DOCTYPE hibernate-configuration PUBLIC
 3        "-//Hibernate/Hibernate Configuration DTD 3.0//EN"
 4 "http://www.hibernate.org/dtd/hibernate-configuration-3.0.dtd">
 5 <hibernate-configuration>
 6  
 7     <session-factory>
 8         <!-- Database connection settings -->
 9         <property name="connection.driver_class">com.mysql.jdbc.Driver</property>
10         <property name="connection.url">jdbc:mysql://localhost:3306/test?characterEncoding=GBK</property>
11         <property name="connection.username">root</property>
12         <property name="connection.password">admin</property>
13         <!-- SQL dialect -->
14         <property name="dialect">org.hibernate.dialect.MySQLDialect</property>
15         <property name="current_session_context_class">thread</property>
16         <property name="show_sql">true</property>
17         <property name="hbm2ddl.auto">update</property>
18         <mapping resource="com/ghw/pojo/Product.hbm.xml" />
19         <mapping resource="com/ghw/pojo/Category.hbm.xml" />
20     </session-factory>
21  
22 </hibernate-configuration>

第五步 在Product.hbm.xml中設置Category 多對一關系

many-to-one中

name="category" 對應Product類中的category屬性

class="Category" 表示對應Category類

column="cid" 表示指向 category_表的外鍵

 1 <?xml version="1.0"?>
 2 <!DOCTYPE hibernate-mapping PUBLIC
 3         "-//Hibernate/Hibernate Mapping DTD 3.0//EN"
 4         "http://www.hibernate.org/dtd/hibernate-mapping-3.0.dtd">
 5  
 6 <hibernate-mapping package="com.ghw.pojo">
 7     <class name="Product" table="product_">
 8         <id name="id" column="id">
 9             <generator class="native">
10             </generator>
11         </id>
12  
13         <property name="name" />
14         <property name="price" />
15         <many-to-one name="category" class="Category" column="cid" />
16     </class>
17      
18 </hibernate-mapping>

第六步 TestHibernate 測試many-to-one關系

一定要先保存c再保存p，否則報錯拋出異常。

 1 package com.how2java.test;
 2   
 3 import org.hibernate.Session;
 4 import org.hibernate.SessionFactory;
 5 import org.hibernate.cfg.Configuration;
 6  
 7 import com.ghw.pojo.Category;
 8 import com.ghw.pojo.Product;
 9  
10 public class TestHibernate {
11     public static void main(String[] args) {
12         SessionFactory sf = new Configuration().configure().buildSessionFactory();
13   
14         Session s = sf.openSession();
15         s.beginTransaction();
16          
17         Category c =new Category();
18         c.setName("c1");
19         s.save(c);
20          
21         Product p = (Product) s.get(Product.class, 8);
22         p.setCategory(c);
23         s.update(p);
24          
25         s.getTransaction().commit();
26         s.close();
27         sf.close();
28     }
29 }

HIBERNATE 實現多對一