Hibernate学习笔记：继承映射之每个类继承结构一张表

对象之间除了拥有，使用的关系，还有继承关系。Hibernate在作继承关系映射的时候有多种做法，今天学习第一种：每个类继承结构一张表，也就是说父类和子类在一张表中， 通过额外字段区分子类型。为了演示这种映射关系的做法，本文引入了四个对象，Animal, Monkey, Tiger，Zoo. Monkey和Tiger继承自Animal. 每个Animal都归属一个动物园Zoo,关系表animal(id, name, zoo_id,age, kind, animal_type), 其中animal_type用来标识子类是Tiger还是Monkey, age是monkey的专有属性， kind是tiger的专有属性。 关系表zoo(id, name, address).
package model;public class Animal {private int id;private String name;private Zoo zoo;//getter setter ignored}

package model;public class Monkey extends Animal{private int age;public int getAge() {return age;}public void setAge(int age) {this.age = age;}public String toString(){return super.toString() + "/nage: " + age;}}

package model;public class Tiger extends Animal{private String kind;public String getKind() {return kind;}public void setKind(String kind) {this.kind = kind;}public String toString(){return super.toString() + "/nkind: " + kind;}}

public class Zoo {private int id;private String name;private String address;private Set animals;//getter and setter ignored}

Zoo.hbm.xml:

<hibernate-mapping><class name="model.Zoo" table="zoo" schema="dbo" catalog="hibernate"><id name="id" column="id"><generator class="native" /></id><property name="name" /><property name="address" /><set name="animals" fetch="join" lazy="false"><key column="zoo_id" /><one-to-many class="model.Animal" /></set></class></hibernate-mapping><!-- create table zoo(id int identity(1,1) primary key,name varchar(20) not null,address varchar(50)) -->

只有父类Animal有映射文件， 子类的配置通过subclass来标识。

Animal.hbm.xml:

<hibernate-mapping package="model"><class name="Animal" table="animal" schema="dbo" catalog="hibernate"><id name="id" column="id"><generator class="native" /></id><!-- 定义子类区分字段 --><discriminator column="animal_type" type="string" /><property name="name" /><many-to-one name="zoo" class="model.Zoo" column="zoo_id" /><!-- 映射子类 --><subclass name="Monkey" discriminator-value="MONKEY"><property name="age" /></subclass><!-- 映射子类 --><subclass name="Tiger" discriminator-value="TIGER"><property name="kind" /></subclass></class></hibernate-mapping><!-- create table animal(id int primary key identity(1,1),name varchar(20) not null,zoo_id int,age int,kind varchar(20),animal_type varchar(20))create table zoo(id int identity(1,1) primary key,name varchar(20) not null,address varchar(50))alter table animal add constraint fk_zoo_id foreign key (zoo_id) references zoo(id)insert into zoo (name,address)values('beijing zoo', 'beijing road')insert into zoo (name,address)values('shanghai zoo', 'shanghai road')insert into etl.animal(name, zoo_id, age, kind, animal_type) values('monkey1',1, 7, null,'MONKEY')insert into etl.animal(name, zoo_id, age, kind, animal_type) values('tiger9',2, null, 'south china tiger','TIGER') -->

dao和测试用例:

public interface AnimalDao {public List listTiger();public List listMonkdy();public void save(Animal a);public void delete(Animal a);public Zoo getZooById(int id);public List getAgedMonkey(int age);public List useCriteria();public List useDetachedCriteria();}

public class AnimalHibernateDao extends BaseHibernateDao implements AnimalDao{public List<Tiger> listTiger() {return getSession().createQuery("from Tiger").list();}public List<Monkey> listMonkdy() {return getSession().createQuery("from Monkey").list();}public void save(Animal a) {Transaction tx = getSession().beginTransaction();getSession().saveOrUpdate(a);tx.commit();}public void delete(Animal a) {Transaction tx = getSession().beginTransaction();getSession().delete(a);tx.commit();}public List getAgedMonkey(int age){String hql = "From Monkey m where m.age>:age";Query q = getSession().createQuery(hql);q.setInteger("age", age);return q.list();}//test casepublic static void main(String[] args){AnimalDao dao = new AnimalHibernateDao();List l = dao.list();System.out.println(l.size());}}

在向数据库中填充测试数据的时候注意animal_type的数据，本例用"MONKEY"和"TIGER"来标识子类， 所以最好对这个字段有约束。

这种做法的优点是：简单， 增加子类方便，数据存储快(因为在一张表中)，方便制作报表；

这种做法的优点是：耦合性高； 潜在的数据空间浪费。

需要注意的是： 子类专有的字段不能有非空约束。