Hibernate 继承关系（泛化关系）

傷城~ 2022-04-18 03:40 266阅读 0赞

**一、介绍**

one-table映射继承的优劣势:  
      优点:非常简单;多态查询非常快;  
      缺点:表及其不稳定;数据分布不好;空值较多;没法实现一些非空的约束;

1,在hibernate中,继承只需要映射一个类就可以了,映射是整个继承体系的根类;  
    2,使用一张表完成继承映射,我们把这种映射方式称为ONE-TABLE;  
    3,子类都是使用<subclass>直接映射在根类的映射元素中;  
    4,子类不需要映射父类已经拥有的属性(不要再去映射id);  
    5,子类的自己的属性就是直接映射在自己的subclass之中;  
    6,需要声明一个鉴别器(discriminator),定义鉴别器列的类型和名称;  
    7,在每一个类上,要配置一个鉴别器的值;

**二、配置文件**

**      hibernate.cfg.xml**

<!DOCTYPE hibernate-configuration PUBLIC
    	"-//Hibernate/Hibernate Configuration DTD 3.0//EN"
    	"http://www.hibernate.org/dtd/hibernate-configuration-3.0.dtd">
    
    <hibernate-configuration>
    	<session-factory >
    
    		<property name="hibernate.dialect">org.hibernate.dialect.MySQLDialect</property>
    		<property name="hibernate.connection.driver_class">com.mysql.jdbc.Driver</property>
    		<property name="hibernate.connection.url">jdbc:mysql://localhost:3306/hibernatedemo</property>
    		<property name="hibernate.connection.username">root</property>
    		<property name="hibernate.connection.password">root</property>
    
    
    		<property name="hibernate.hbm2ddl.auto">create</property>
    
    		
    		<property name="hibernate.show_sql">true</property>
    		
    		
    
    
    
    		<mapping resource="com/shenzhenair/day03/extend/extend.hbm.xml"/>
    
    
    	</session-factory>
    </hibernate-configuration>

**extend.hbm.xml**

<?xml version="1.0"?>
    <!DOCTYPE hibernate-mapping SYSTEM 
     "http://www.hibernate.org/dtd/hibernate-mapping-3.0.dtd" >
    
    <hibernate-mapping  package="com.shenzhenair.day03.extend" >
    	<class name="Product" discriminator-value="1" >
    		<id name="id"  >
    			<generator class="native"/>
    		</id>
    		
    		<discriminator column="types" type="int"    />
    		<property name="name" />
    
    		<subclass name="BookProduct" discriminator-value="2" >
    			<property name="isbn" />
    			<property name="author" />
    		</subclass>
    
    		<subclass name="ClothProduct" discriminator-value="3" >
    			<property name="size" />
    			<property name="color" />
    		</subclass>
    	</class>
    
    </hibernate-mapping>

**三、实体类**

@Setter
    @Getter
    public class BookProduct extends  Product {
        private String isbn;
        private String author;
    }
    
    
    @Setter
    @Getter
    public class ClothProduct extends  Product{
        private String size;
        private String color;
    }
    
    @Setter
    @Getter
    public class Product {
        private Long id;
        private String name;
    
    }

**四、测试类**

**
     * 继承测试
     */
    public class ExtendTest {
        @Test
        public void  testSave(){
            Product p = new Product();
            p.setName("p");
    
            BookProduct bp = new BookProduct();
            bp.setName("bp");
            bp.setAuthor("author");
            bp.setIsbn("isbn");
    
            ClothProduct cp = new ClothProduct();
            cp.setColor("yellow");
            cp.setSize("50");
            cp.setName("cp");
    
    
            Session session = HibernateUtils.openSession();
            session.beginTransaction();
    
            session.save(p);
            session.save(bp);
            session.save(cp);
    
            session.getTransaction().commit();
            session.close();
    
        }
    
    
        @Test
        public  void  testGet(){
            Session session = HibernateUtils.openSession();
    
           BookProduct bp = (BookProduct) session.get(BookProduct.class, 2L);
            System.out.println(bp);
    
            session.close();
        }
    
        /**
         * 多态查询：查询的时候按照父类型查询，查出来的结果是真实的类型
         */
        @Test
        public  void  testGet2(){
            Session session = HibernateUtils.openSession();
    
            Product p = (Product) session.get(Product.class, 3L);
            System.out.println(p);
    
            session.close();
        }
    
    
    }

**五、数据库生成数据**

![20181107233214398.png][]

[20181107233214398.png]: /images/20220418/1df0048282944f60aafe64ebbd15c753.png