hibernate的继承映射

本文简单讲述hibernate的继承映射相关知识，以备不时之需。继承映射，顾名思义就是有继承关系的几个实体之间的映射关系。

1.首先看看annotation的API中关于继承映射的描述

EJB3支持三种类型的继承映射:

你可以用 @Inheritance注解来定义所选择的策略. 这个注解需要在每个类层次结构(class hierarchy) 最顶端的实体类上使用.

@Entity
@Inheritance(strategy = InheritanceType.TABLE_PER_CLASS)
public class Flight implements Serializable {
            

@Entity
@Inheritance(strategy=InheritanceType.SINGLE_TABLE)
@DiscriminatorColumn(
    name="planetype",
    discriminatorType=DiscriminatorType.STRING
)
@DiscriminatorValue("Plane")
public class Plane { ... }

@Entity
@DiscriminatorValue("A320")
public class A320 extends Plane { ... }
            

@Entity
@Inheritance(strategy=InheritanceType.JOINED)
public class Boat implements Serializable { ... }

@Entity
public class Ferry extends Boat { ... }

@Entity
@PrimaryKeyJoinColumn(name="BOAT_ID")
public class AmericaCupClass  extends Boat { ... }
            

@MappedSuperclass
public class BaseEntity {
    @Basic
    @Temporal(TemporalType.TIMESTAMP)
    public Date getLastUpdate() { ... }
    public String getLastUpdater() { ... }
    ...
}

@Entity class Order extends BaseEntity {
    @Id public Integer getId() { ... }
    ...
}

@MappedSuperclass
public class FlyingObject implements Serializable {

    public int getAltitude() {
        return altitude;
    }

    @Transient
    public int getMetricAltitude() {
        return metricAltitude;
    }

    @ManyToOne
    public PropulsionType getPropulsion() {
        return metricAltitude;
    }
    ...
}

@Entity
@AttributeOverride( name="altitude", column = @Column(name="fld_altitude") )
@AssociationOverride( name="propulsion", joinColumns = @JoinColumn(name="fld_propulsion_fk") )
public class Plane extends FlyingObject {
    ...
}

 <hibernate-mapping>
     <subclass name="DomesticCat" extends="Cat" discriminator-value="D">
          <property name="name" type="string"/>
     </subclass>
 </hibernate-mapping>

<class name="Payment" table="PAYMENT">
    <id name="id" type="long" column="PAYMENT_ID">
        <generator class="native"/>
    </id>
    <discriminator column="PAYMENT_TYPE" type="string"/>
    <property name="amount" column="AMOUNT"/>
    ...
    <subclass name="CreditCardPayment" discriminator-value="CREDIT">
        <property name="creditCardType" column="CCTYPE"/>
        ...
    </subclass>
    <subclass name="CashPayment" discriminator-value="CASH">
        ...
    </subclass>
    <subclass name="ChequePayment" discriminator-value="CHEQUE">
        ...
    </subclass>
</class>

<class name="Payment" table="PAYMENT">
    <id name="id" type="long" column="PAYMENT_ID">
        <generator class="native"/>
    </id>
    <property name="amount" column="AMOUNT"/>
    ...
    <joined-subclass name="CreditCardPayment" table="CREDIT_PAYMENT">
        <key column="PAYMENT_ID"/>
        <property name="creditCardType" column="CCTYPE"/>
        ...
    </joined-subclass>
    <joined-subclass name="CashPayment" table="CASH_PAYMENT">
        <key column="PAYMENT_ID"/>
        ...
    </joined-subclass>
    <joined-subclass name="ChequePayment" table="CHEQUE_PAYMENT">
        <key column="PAYMENT_ID"/>
        ...
    </joined-subclass>
</class>

<class name="Payment" table="PAYMENT">
    <id name="id" type="long" column="PAYMENT_ID">
        <generator class="native"/>
    </id>
    <discriminator column="PAYMENT_TYPE" type="string"/>
    <property name="amount" column="AMOUNT"/>
    ...
    <subclass name="CreditCardPayment" discriminator-value="CREDIT">
        <join table="CREDIT_PAYMENT">
            <key column="PAYMENT_ID"/>
            <property name="creditCardType" column="CCTYPE"/>
            ...
        </join>
    </subclass>
    <subclass name="CashPayment" discriminator-value="CASH">
        <join table="CASH_PAYMENT">
            <key column="PAYMENT_ID"/>
            ...
        </join>
    </subclass>
    <subclass name="ChequePayment" discriminator-value="CHEQUE">
        <join table="CHEQUE_PAYMENT" fetch="select">
            <key column="PAYMENT_ID"/>
            ...
        </join>
    </subclass>
</class>

<class name="Payment" table="PAYMENT">
    <id name="id" type="long" column="PAYMENT_ID">
        <generator class="native"/>
    </id>
    <discriminator column="PAYMENT_TYPE" type="string"/>
    <property name="amount" column="AMOUNT"/>
    ...
    <subclass name="CreditCardPayment" discriminator-value="CREDIT">
        <join table="CREDIT_PAYMENT">
            <property name="creditCardType" column="CCTYPE"/>
            ...
        </join>
    </subclass>
    <subclass name="CashPayment" discriminator-value="CASH">
        ...
    </subclass>
    <subclass name="ChequePayment" discriminator-value="CHEQUE">
        ...
    </subclass>
</class>

<many-to-one name="payment" column="PAYMENT_ID" class="Payment"/>

<class name="Payment">
    <id name="id" type="long" column="PAYMENT_ID">
        <generator class="sequence"/>
    </id>
    <property name="amount" column="AMOUNT"/>
    ...
    <union-subclass name="CreditCardPayment" table="CREDIT_PAYMENT">
        <property name="creditCardType" column="CCTYPE"/>
        ...
    </union-subclass>
    <union-subclass name="CashPayment" table="CASH_PAYMENT">
        ...
    </union-subclass>
    <union-subclass name="ChequePayment" table="CHEQUE_PAYMENT">
        ...
    </union-subclass>
</class>

<class name="CreditCardPayment" table="CREDIT_PAYMENT">
    <id name="id" type="long" column="CREDIT_PAYMENT_ID">
        <generator class="native"/>
    </id>
    <property name="amount" column="CREDIT_AMOUNT"/>
    ...
</class>

<class name="CashPayment" table="CASH_PAYMENT">
    <id name="id" type="long" column="CASH_PAYMENT_ID">
        <generator class="native"/>
    </id>
    <property name="amount" column="CASH_AMOUNT"/>
    ...
</class>

<class name="ChequePayment" table="CHEQUE_PAYMENT">
    <id name="id" type="long" column="CHEQUE_PAYMENT_ID">
        <generator class="native"/>
    </id>
    <property name="amount" column="CHEQUE_AMOUNT"/>
    ...
</class>

<any name="payment" meta-type="string" id-type="long">
    <meta-value value="CREDIT" class="CreditCardPayment"/>
    <meta-value value="CASH" class="CashPayment"/>
    <meta-value value="CHEQUE" class="ChequePayment"/>
    <column name="PAYMENT_CLASS"/>
    <column name="PAYMENT_ID"/>
</any>

<class name="CreditCardPayment" table="CREDIT_PAYMENT">
    <id name="id" type="long" column="CREDIT_PAYMENT_ID">
        <generator class="native"/>
    </id>
    <discriminator column="CREDIT_CARD" type="string"/>
    <property name="amount" column="CREDIT_AMOUNT"/>
    ...
    <subclass name="MasterCardPayment" discriminator-value="MDC"/>
    <subclass name="VisaPayment" discriminator-value="VISA"/>
</class>

<class name="NonelectronicTransaction" table="NONELECTRONIC_TXN">
    <id name="id" type="long" column="TXN_ID">
        <generator class="native"/>
    </id>
    ...
    <joined-subclass name="CashPayment" table="CASH_PAYMENT">
        <key column="PAYMENT_ID"/>
        <property name="amount" column="CASH_AMOUNT"/>
        ...
    </joined-subclass>
    <joined-subclass name="ChequePayment" table="CHEQUE_PAYMENT">
        <key column="PAYMENT_ID"/>
        <property name="amount" column="CHEQUE_AMOUNT"/>
        ...
    </joined-subclass>
</class>

３.使用singleTable实现继承映射

使用Person、Student和Teacher，其中Person是父类，Student和Teacher是子类，继承Person

根据上面的API，需要在Person使用@Inheritance(strategy=InheritanceType.SINGLE_TABLE)，指定继承类型

并使用@DiscriminatorColumn(name="flag",discriminatorType=DiscriminatorType.STRING)
@DiscriminatorValue("person")

定义标志列的名称和类型，并指明本类使用的标志值

同样的，Student　extends Person，并需要使用@DiscriminatorValue("student")指明本类使用的标志值

同样的，Teacher　extends Person，并需要使用@DiscriminatorValue("teacher")指明本类使用的标志值

Person

package com.baosight.model;

import javax.persistence.DiscriminatorColumn;
import javax.persistence.DiscriminatorType;
import javax.persistence.DiscriminatorValue;
import javax.persistence.Entity;
import javax.persistence.GeneratedValue;
import javax.persistence.Id;
import javax.persistence.Inheritance;
import javax.persistence.InheritanceType;

@Entity
@Inheritance(strategy=InheritanceType.SINGLE_TABLE)
@DiscriminatorColumn(name="flag",discriminatorType=DiscriminatorType.STRING)
@DiscriminatorValue("person")
public class Person {
	private String id;
	private String name;
	@Id
	@GeneratedValue//auto
	public String getId() {
		return id;
	}
	public void setId(String id) {
		this.id = id;
	}
	public String getName() {
		return name;
	}
	public void setName(String name) {
		this.name = name;
	}
}

package com.baosight.model;

import javax.persistence.DiscriminatorValue;
import javax.persistence.Entity;

@Entity
@DiscriminatorValue("student")
public class Student extends Person{
	private String score;

	public String getScore() {
		return score;
	}

	public void setScore(String score) {
		this.score = score;
	}
}

package com.baosight.model;

import javax.persistence.DiscriminatorValue;
import javax.persistence.Entity;

@Entity
@DiscriminatorValue("teacher")
public class Teacher extends Person{
	private String title;

	public String getTitle() {
		return title;
	}

	public void setTitle(String title) {
		this.title = title;
	}

}

package com.baosight.model;

import org.hibernate.Session;
import org.hibernate.SessionFactory;
import org.hibernate.cfg.AnnotationConfiguration;
import org.hibernate.cfg.Configuration;
import org.hibernate.tool.hbm2ddl.SchemaExport;
import org.junit.AfterClass;
import org.junit.BeforeClass;
import org.junit.Test;

public class OrMappingTest {
	private static SessionFactory sf = null;
	@BeforeClass
	public static void beforeClass(){
		new SchemaExport(new AnnotationConfiguration().configure()).create(false, true);
		// 读取配置文件
		Configuration cfg = new AnnotationConfiguration();
		// 得到session工厂
		sf = cfg.configure().buildSessionFactory();
	}
	
	@Test
	public void testSave() {
		Student s = new Student();
		s.setName("学生");
		s.setScore("80");
		Teacher t = new Teacher();
		t.setName("教师");
		t.setTitle("中级");
		Session session = sf.getCurrentSession();
		session.beginTransaction();
		session.save(s);
		session.save(t);
		session.getTransaction().commit();
	}
	@Test
	public void testLoad() {
		testSave();
		Session s = sf.getCurrentSession();
		s.beginTransaction();
		Student u = (Student) s.load(Student.class, "1");
		System.out.println(u.getName());
		Person p = (Person) s.load(Person.class, "2");
		System.out.println(p.getName());
		s.getTransaction().commit();
	}
	/*@Test
	public void testSchemaExport() {
	}*/
	@AfterClass
	public static void afterClass(){
		// 关闭session工厂
		sf.close();
	}
}

3.1首先执行testSave，运行结果为

可以看到数据库只有1张表person，并有1个flag字段标识类型

3.2再来看看testLoad方法，执行结果为

当知道类型时关联person表的标识字段查询，不知道类型时，直接根据id进行查询

4.使用tablePerClass实现继承映射

使用Person、Student和Teacher，其中Person是父类，Student和Teacher是子类，继承Person

根据上面的API，需要在Person使用@Inheritance(strategy=InheritanceType.TABLE_PER_CLASS)，指定继承类型

另外，子类主键继承自父类，需要保证子类主键的唯一，本例使用Table方式生成主键

即使用@TableGenerator(name="tableGEN",table="table_gen",pkColumnName="pk_key",valueColumnName="pk_value",pkColumnValue="teacher",initialValue=1,allocationSize=1)

并在getId上使用@GeneratedValue(strategy=GenerationType.TABLE,generator="tableGEN")

Student和Teacher需要extends Person，并需要使用@Entity

Person

package com.baosight.model;

import javax.persistence.Entity;
import javax.persistence.GeneratedValue;
import javax.persistence.GenerationType;
import javax.persistence.Id;
import javax.persistence.Inheritance;
import javax.persistence.InheritanceType;
import javax.persistence.TableGenerator;

@Entity
@Inheritance(strategy=InheritanceType.TABLE_PER_CLASS)
@TableGenerator(name="tableGEN",table="table_gen",pkColumnName="pk_key",valueColumnName="pk_value",pkColumnValue="teacher",initialValue=1,allocationSize=1)
public class Person {
	private int id;
	private String name;
	@Id
	@GeneratedValue(strategy=GenerationType.TABLE,generator="tableGEN")
	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;
	}
}

package com.baosight.model;

import javax.persistence.Entity;

@Entity
public class Student extends Person{
	private String score;

	public String getScore() {
		return score;
	}

	public void setScore(String score) {
		this.score = score;
	}
}

package com.baosight.model;

import javax.persistence.Entity;

@Entity
public class Teacher extends Person{
	private String title;
	public String getTitle() {
		return title;
	}
	public void setTitle(String title) {
		this.title = title;
	}
}

可以看到生成了3张表，并且每张表的字段都是与实体类对应的全部字段

4.2使用testLoad测试

@Test
	public void testLoad() {
		testSave();
		Session s = sf.getCurrentSession();
		s.beginTransaction();
		Student u = (Student) s.load(Student.class, 1);
		System.out.println(u.getName());
		Person p = (Person) s.load(Person.class, 2);
		System.out.println(p.getName());
		s.getTransaction().commit();
	}

可以看到通过子类查询时会直接查询对应的表，而通过父类查询时会关联查询3张表

5.使用joined实现继承映射

使用Person、Student和Teacher，其中Person是父类，Student和Teacher是子类，继承Person

根据上面的API，需要在Person使用@Inheritance(strategy=InheritanceType.JOINED)，指定继承类型

Student和Teacher需要extends Person，并需要使用@Entity

Person

package com.baosight.model;

import javax.persistence.Entity;
import javax.persistence.GeneratedValue;
import javax.persistence.Id;
import javax.persistence.Inheritance;
import javax.persistence.InheritanceType;

@Entity
@Inheritance(strategy=InheritanceType.JOINED)
public class Person {
	private String id;
	private String name;
	@Id
	@GeneratedValue//auto
	public String getId() {
		return id;
	}
	public void setId(String id) {
		this.id = id;
	}
	public String getName() {
		return name;
	}
	public void setName(String name) {
		this.name = name;
	}
}

package com.baosight.model;

import javax.persistence.Entity;

@Entity
public class Student extends Person{
	private String score;

	public String getScore() {
		return score;
	}

	public void setScore(String score) {
		this.score = score;
	}
}

package com.baosight.model;

import javax.persistence.Entity;

@Entity
public class Teacher extends Person{
	private String title;
	public String getTitle() {
		return title;
	}
	public void setTitle(String title) {
		this.title = title;
	}
}

可以看到创建了3张表，并且保存子类实体会同时向父表和子表插入数据，子表和附表是主键关联

5.2再来看看testLoad，测试结果为


可以看到当子类类型确定时会将此子类和父类进行关联查询，当直接查询父类时，会将父类和所有的子类进行关联查询

以上即为继承映射的相关内容，在实际的使用中，singleTable和joined使用的较多，当选择使用继承映射时，需要综合考虑。

hibernate的继承映射

原文：http://blog.csdn.net/u011526599/article/details/51233213