Hibernate继承映射
2015-05-26 16:41
204 查看
1.每棵类继承树一张表
因为类继承肯定是对应多个类,要把多个类的信息存放在一张表中,必须有某种机制来区分哪些记录是属于哪个类的。因此在表中添加一个字段,用这个字段的值来进行区分。父类用普通的<class>标签定义
在父类中定义一个discriminator,即指定这个区分的字段的名称和类型如:<discriminator column=”XXX” type=”string”/>
子类使用<subclass>标签定义,Subclass标签的name属性是子类的全路径名,在Subclass标签中,用discriminator-value属性来标明本子类的discriminator字段(用来区分不同类的字段)的值Subclass标签。
Subclass标签既可以被class标签所包含(这种包含关系正是表明了类之间的继承关系),也可以与class标签平行。 当subclass标签的定义与class标签平行的时候,需要在subclass标签中,添加extends属性,里面的值是父类的全路径名称。
子类的其它属性,像普通类一样,定义在subclass标签的内部。
get和hql支持多态查询,即hibernate在加载数据的时候,能够采用instancof鉴别出其真正的类型。但是load默认支持lazy,即<class>标签中lazy=“true”不能多态查询。
如Animal a = (Animal)session.load(Animal.class, 1);得的是Animal的代理,采用instanceof无法鉴别出真正的类型。
而load在lazy设置为false的情况下支持多态查询,此时load得的不是代理。
示例:
Animal父类:
public class Animal { private int id; private String name; public int getId() { return id; } public void setId(int id) { this.id = id; } public String getName() { return name; } public void setName(String name) { this.name = name; } }Bird子类:
public class Bird extends Animal { private int height; public int getHeight() { return height; } public void setHeight(int height) { this.height = height; } }Pig子类:
public class Pig extends Animal { private int weight; public int getWeight() { return weight; } public void setWeight(int weight) { this.weight = weight; } }Extends.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 package="com.zero.hibernate.vo"> <class name="Animal" table="t_animal" lazy="false"> <id name="id"> <generator class="native" /> </id> <discriminator column="type" type="string" /> <!-- 鉴别标签,会在 t_animal表加type字段 --> <property name="name" /> <property name="sex" /> <subclass name="Pig" discriminator-value="Pig"> <!-- discriminator-value 鉴别值 --> <property name="weight" /> </subclass> <subclass name="Bird" discriminator-value="Bird"> <!-- discriminator-value 鉴别值 --> <property name="height" /> </subclass> </class> </hibernate-mapping>生成的建表语句:
drop table if exists t_animal create table t_animal ( id integer not null auto_increment, type varchar(255) not null, name varchar(255), weight integer, height integer, primary key (id) )测试:
Bird bird = new Bird(); bird.setName("鸟"); bird.setHeight(10); session.save(bird); Pig pig = new Pig(); pig.setName("猪"); pig.setWeight(100); session.save(pig);
Animal animal = (Animal)session.load(Animal.class, 1); if (animal instanceof Pig) { Pig pig = (Pig) animal; System.out.println(pig.getWeight()); }else { Bird bird = (Bird) animal; System.out.println(bird.getHeight()); }
List animalList = session.createQuery("from Animal").list(); for (Iterator iter=animalList.iterator(); iter.hasNext();) { Animal animal = (Animal)iter.next(); //采用hql查询返回的是真正的类型,所以hql支持多态查询 if (animal instanceof Pig) { System.out.println(animal.getName()); }else if (animal instanceof Bird) { System.out.println(animal.getName()); } }
2.每个子类一张表
这种策略是使用union-subclass标签来定义子类的。每个子类对应一张表,而且这个表的信息是完备的。此时父类的class标签中abstract设置为true。但是,在保存对象的时候id不能重复(不能使用数据库的自增方式生成主键)。
示例:
Extends.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 package="com.zero.hibernate.vo"> <class name="Animal" table="t_animal" abstract="true"><!-- abstract="true"将不会生成表 --> <id name="id"> <generator class="assigned" /><!-- 手动分配 id --> </id> <property name="name" /> <union-subclass name="Pig" table="t_pig"> <property name="weight" /> </union-subclass> <union-subclass name="Bird" table="t_bird"> <property name="height" /> </union-subclass> </class> </hibernate-mapping>生成的建表语句:
drop table if exists t_bird drop table if exists t_pig create table t_bird ( id integer not null, name varchar(255), height integer, primary key (id) ) create table t_pig ( id integer not null, name varchar(255), weight integer, primary key (id) )测试:
Pig pig = new Pig(); pig.setId(1); pig.setName("猪"); pig.setWeight(100); session.save(pig); Bird bird = new Bird(); bird.setId(2); bird.setName("鸟"); bird.setHeight(10); session.save(bird); //Hibernate: insert into t_pig (name, weight, id) values (?, ?, ?) //Hibernate: insert into t_bird (name, height, id) values (?, ?, ?)
3.每个类一张表
这种策略是使用joined-subclass标签来定义子类的。父类、子类,每个类都对应一张数据库表。在父类对应的数据库表中,实际上会存储所有的记录,包括父类和子类的记录;在子类对应的数据库表中,这个表只定义了子类中所特有的属性映射的字段。子类与父类,通过相同的主键值来关联。父类用普通的<class>标签,不再需要定义discriminator字段
子类用<joined-subclass>标签定义,在定义joined-subclass的时候,Joined-subclass标签的name属性是子类的全路径名,Joined-subclass标签需要包含一个key标签,这个标签指定了子类和父类之间是通过哪个字段来关联的。
示例:
Extends.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 package="com.zero.hibernate.vo"> <class name="Animal" table="t_animal"> <id name="id"> <generator class="native" /> </id> <property name="name" /> <joined-subclass name="Pig" table="t_pig"> <key column="pigId" /> <property name="weight" /> </joined-subclass> <joined-subclass name="Bird" table="t_bird"> <key column="birdId" /> <property name="height" /> </joined-subclass> </class> </hibernate-mapping>生成的建表语句:
alter table t_bird drop foreign key FK_ihcyneoc7iub1oyax1p781ly1 alter table t_pig drop foreign key FK_2cjrm0mijyj0mul5hqeck0v36 drop table if exists t_animal drop table if exists t_bird drop table if exists t_pig create table t_animal ( id integer not null auto_increment, name varchar(255), primary key (id) ) create table t_bird ( birdId integer not null, height integer, primary key (birdId) ) create table t_pig ( pigId integer not null, weight integer, primary key (pigId) ) alter table t_bird add constraint FK_ihcyneoc7iub1oyax1p781ly1 foreign key (birdId) references t_animal (id) alter table t_pig add constraint FK_2cjrm0mijyj0mul5hqeck0v36 foreign key (pigId) references t_animal (id)测试:
Pig pig = new Pig(); pig.setName("猪"); pig.setWeight(100); session.save(pig); Bird bird = new Bird(); bird.setName("鸟"); bird.setHeight(10); session.save(bird); //Hibernate: insert into t_animal (name) values (?) //Hibernate: insert into t_pig (weight, pigId) values (?, ?) //Hibernate: insert into t_animal (name) values (?) //Hibernate: insert into t_bird (height, birdId) values (?, ?)
相关文章推荐
- 【详解】Hibernate 继承映射
- hibernate映射(五)——继承映射
- Hibernate 映射继承关系
- hibernate映射关系——继承映射
- 【Hibernate】组件映射与继承映射
- Hibernate继承映射与多态查询
- 解读Hibernate继承映射策略
- Hibernate继承映射
- hibernate的继承关系映射(一)
- 精通Hibernate之映射继承关系二
- HIbernate继承映射
- Hibernate继承映射简单入门
- Hibernate继承映射的简单示例
- hibernate继承映射[映射单独一个tablexml版本]实例【与19相同,方式不同,未验证】(二十)
- 8.0 hibernate_映射继承关系
- Hibernate 简化继承映射
- 关于HIbernate继承映射中WrongClassException的解决方案
- Hibernate学习之---继承映射
- Hibernate继承映射