hibernate学习9之extends（继承）映射

hibernate.cfg.xml：

<!DOCTYPE hibernate-configuration PUBLIC
	"-//Hibernate/Hibernate Configuration DTD 3.0//EN"
	"http://hibernate.sourceforge.net/hibernate-configuration-3.0.dtd">

<hibernate-configuration>
	<session-factory>
		<property name="hibernate.connection.url">jdbc:mysql://localhost/hibernate_extends_1</property>
		<property name="hibernate.connection.driver_class">com.mysql.jdbc.Driver</property>
		<property name="hibernate.connection.username">root</property>
		<property name="hibernate.connection.password">bjsxt</property>
		<property name="hibernate.dialect">org.hibernate.dialect.MySQLDialect</property>
		<property name="hibernate.show_sql">true</property>
		
		<mapping resource="com/bjsxt/hibernate/extends.hbm.xml"/>
	</session-factory>
</hibernate-configuration>

假定有2个子类Pig，Bird，他们都继承自Animal

public class Animal {
	private int id;
	private String name;
	private boolean sex;
	//setter，getter
}
public class Pig extends Animal {	
	private int weight;
	//setter，getter
}
public class Pig extends Animal {
	private int height;
	//setter，getter
}

他们的映射关系可以像下面那样，这样可以把Pig 和Bird存在同一张表里，以外键字段type区分存的是Pig还是Bird

<?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.bjsxt.hibernate">
	<class name="Animal" table="t_animal">
		<id name="id">
			<generator class="native"/>
		</id>
        <!--加入鉴别器，用以判断是Pig还是Bird-->
		<discriminator column="type" type="string"/>
		<property name="name"/>
		<property name="sex"/>
		<subclass name="Pig" discriminator-value="P">
			<property name="weight"/>
		</subclass>
		<subclass name="Bird" discriminator-value="B">
			<property name="height"/>
		</subclass>
	</class>
</hibernate-mapping>

测试一下：

import java.util.Iterator;
import java.util.List;

import org.hibernate.Session;

import junit.framework.TestCase;

public class ExtendsTest extends TestCase {

	public void testSave1() {
		Session session = null;
		try {
			session = HibernateUtils.getSession();
			session.beginTransaction();
			
			Pig pig = new Pig();
			pig.setName("猪猪");
			pig.setSex(true);
			pig.setWeight(100);
			session.save(pig);
			
			Bird bird = new Bird();
			bird.setName("鸟鸟");
			bird.setSex(false);
			bird.setHeight(50);
			session.save(bird);
			
			session.getTransaction().commit();
		}catch(Exception e) {
			e.printStackTrace();
			session.getTransaction().rollback();
		}finally {
			HibernateUtils.closeSession(session);
		}
	}	
	
	/**
	 * 采用load，通过Pig查询
	 */
	public void testLoad1() {
		Session session = null;
		try {
			session = HibernateUtils.getSession();
			session.beginTransaction();
			
			Pig pig = (Pig)session.load(Pig.class, 1);
			System.out.println(pig.getName());
			
			session.getTransaction().commit();
		}catch(Exception e) {
			e.printStackTrace();
			session.getTransaction().rollback();
		}finally {
			HibernateUtils.closeSession(session);
		}
	}
	
	/**
	 * 采用load，通过Animal查询
	 */
	public void testLoad2() {
		Session session = null;
		try {
			session = HibernateUtils.getSession();
			session.beginTransaction();
			
			Animal animal = (Animal)session.load(Animal.class, 1);
			System.out.println(animal.getName());
			
			session.getTransaction().commit();
		}catch(Exception e) {
			e.printStackTrace();
			session.getTransaction().rollback();
		}finally {
			HibernateUtils.closeSession(session);
		}
	}			
	
	/**
	 * 采用load，通过Animal查询
	 */
	public void testLoad3() {
		Session session = null;
		try {
			session = HibernateUtils.getSession();
			session.beginTransaction();
			
			Animal animal = (Animal)session.load(Animal.class, 1);
			
			//因为load默认只是lazy，因为我们看到的是Animal的代理对象
			//所以通过instanceof是反应不出正真的对象类型的
			//因此load在默认情况下是不支持多态查询的
			if (animal instanceof Pig) {
				System.out.println(animal.getName());
			}else {
				System.out.println("不是猪");
			}
			session.getTransaction().commit();
		}catch(Exception e) {
			e.printStackTrace();
			session.getTransaction().rollback();
		}finally {
			HibernateUtils.closeSession(session);
		}
	}			
	
	/**
	 * 采用load，通过Animal查询,将<class>标签上的lazy=false
	 */
	public void testLoad4() {
		Session session = null;
		try {
			session = HibernateUtils.getSession();
			session.beginTransaction();
			
			Animal animal = (Animal)session.load(Animal.class, 1);
			//可以正确的判断出Pig的类型，因为lazy=false，返回的是具体的Pig类型
			//此时load支持多态查询
			if (animal instanceof Pig) {
				System.out.println(animal.getName());
			}else {
				System.out.println("不是猪");
			}
			session.getTransaction().commit();
		}catch(Exception e) {
			e.printStackTrace();
			session.getTransaction().rollback();
		}finally {
			HibernateUtils.closeSession(session);
		}
	}		
	
	/**
	 * 采用get，通过Animal查询
	 */
	public void testLoad5() {
		Session session = null;
		try {
			session = HibernateUtils.getSession();
			session.beginTransaction();
			
			//可以正确的判断出Pig的类型，因为返回的是具体的Pig类型
			//get支持多态查询
			Animal animal = (Animal)session.get(Animal.class, 1);

			if (animal instanceof Pig) {
				System.out.println(animal.getName());
			}else {
				System.out.println("不是猪");
			}
			session.getTransaction().commit();
		}catch(Exception e) {
			e.printStackTrace();
			session.getTransaction().rollback();
		}finally {
			HibernateUtils.closeSession(session);
		}
	}	
	
	/**
	 * 采用get，通过Animal查询
	 */
	public void testLoad6() {
		Session session = null;
		try {
			session = HibernateUtils.getSession();
			session.beginTransaction();
//			List animalList = session.createQuery("from Animal").list();
//			for (Iterator iter = animalList.iterator(); iter.hasNext();) {
//				Animal a = (Animal)iter.next();
//				//能够正确的鉴别出正真的类型，hql是支持多态查询的
//				if (a instanceof Pig) {
//					System.out.println("是Pig");
//				}else if (a instanceof Bird) {
//					System.out.println("是bird");
//				} 
//			}
			
			List list = session.createQuery("from java.lang.Object").list();
			for (Iterator iter=list.iterator(); iter.hasNext();) {
				Object o = iter.next();
				if (o instanceof Pig) {
					System.out.println("是Pig");
				}else if (o instanceof Bird) {
					System.out.println("是bird");
				} 
			}
			session.getTransaction().commit();
		}catch(Exception e) {
			e.printStackTrace();
			session.getTransaction().rollback();
		}finally {
			HibernateUtils.closeSession(session);
		}
	}	
	
}

多态查询：在hibernate加载数据的时候能鉴别出正真的类型（instanceOf）

get支持多态查询
load只有在lazy=false，才支持多态查询
hql支持多态查询

例外还有一种继承映射关系可以生成3张表（每个类生成一张表，不管是父类还是子类），配置关系如下，不过效率和维护关系比较复杂，不推荐使用。

<?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.bjsxt.hibernate">
	<class name="Animal" table="t_animal">
		<id name="id">
			<generator class="native"/>
		</id>
		<property name="name"/>
		<property name="sex"/>
		<joined-subclass name="Pig" table="t_pig">
			<key column="pid"/>
			<property name="weight"/>
		</joined-subclass>
		<joined-subclass name="Bird" table="t_bird">
			<key column="bid"/>
			<property name="height"/>
		</joined-subclass>
	</class>
</hibernate-mapping>