Hibernate映射类继承之每个带有隐式多态的具体类一张表(每个子类各一张表,有各自的映射文件)

Hibernate映射类继承之每个带有隐式多态的具体类一张表(每个子类各一张表)

我们可以准确地给每个子类使用一张表。子类的所有属性，包括被继承的属性，都可以被映射到这张表的列。如图：

                        CreditCard子类   ----->  CreditCard.hbm.xml   ----->  CREDIT_CARD表

BillingDetails父类---->

                        BankAccount子类  ----->  BankAccount.hbm.xml  ----->  BANK_ACCOUNT表

                        

在Hibernate中你不必做任何特别的事来启用多态行为。对CreditCard和BankAccount的映射很简单，各自都在自己的实体<class>元素中，就像我们已经没有基类的类所做的那样。Hibernate仍然知道这个基类或者接口，因为它在启动时扫描持久化类。这种方法的主要问题在于，它不太支持多态关联。在数据库中，关联通常被表示为外键关系。如果子类全部被映射到不同的表，对它们基类(本例BillingDetails)的多态关联就无法被表示为一个简单的外键关系。这在我们的领域模型中会有问题，如果BillingDetails与一个存在的user关联，两张子类表都需要一个对User表的外键引用。或者，如果User与BillingDetails有个多对一的关系，USER表就需要单个的外键列，它必须同时引用两个具体的子类表。这用一般的外键约束是不可能的。

多态查询(它返回与被查询类的接口相匹配的所有类的对象)也有问题。针对基类的查询必须作为几个SQL SELECT执行，每个具体的子类一个。对于针对BillingDetails类的查询,hibernate使用下列SQL:

select CREDIT_CARD_ID,OWNER,NUMBER,EXP_MONTH,EXP_YEAR... from CREDIT_CARD;

select BANK_ACCOUNT_ID,OWNER,ACCOUNT,BANKNAME,... from BANK_ACCOUNT;

注意每个具体的子类都需要一个单独的查询。

这种映射策略进一步的概念问题在于，几张不同表的几个不同的列具有完全相同的语义。这使得模式演变更为复杂。例如，对一个基类属性的改变导致了多个列的变化。它也使得把应用到所有子类的数据库完整性约束实现起来变得更加困难。

我们推荐这种方法(仅仅)用于类层次结构的最顶层，那里通常不需要多态，且未来也不太可能修改基类。

---

pom.xml:

<project xmlns="http://maven.apache.org/POM/4.0.0" xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance" xsi:schemaLocation="http://maven.apache.org/POM/4.0.0 http://maven.apache.org/xsd/maven-4.0.0.xsd"> <modelVersion>4.0.0</modelVersion> <groupId>hibernateTest</groupId> <artifactId>hibernateTest</artifactId> <version>1.0-SNAPSHOT</version> <packaging>jar</packaging> <name>hibernateTest</name> <url>http://maven.apache.org</url> <dependencies> <dependency> <groupId>junit</groupId> <artifactId>junit</artifactId> <version>3.8.1</version> <scope>test</scope> </dependency> <dependency> <groupId>org.hibernate</groupId> <artifactId>hibernate-core</artifactId> <version>3.3.1.GA</version> </dependency> <dependency> <groupId>org.slf4j</groupId> <artifactId>slf4j-nop</artifactId> <version>1.5.2</version> </dependency> <dependency> <groupId>javassist</groupId> <artifactId>javassist</artifactId> <version>3.4.GA</version> </dependency> <dependency> <groupId>c3p0</groupId> <artifactId>c3p0</artifactId> <version>0.9.1</version> </dependency> <dependency> <groupId>com.oracle</groupId> <artifactId>ojdbc14</artifactId> <version>10.2.0.3.0</version> <scope>runtime</scope> </dependency> </dependencies> <build> <finalName>hibernateTest</finalName> <resources> <resource> <directory>src/main/resources</directory> </resource> <resource> <directory>src/main/java</directory> <excludes> <exclude>**/*.java</exclude> </excludes> </resource> </resources> <plugins> <plugin> <artifactId>maven-compiler-plugin</artifactId> <configuration> <source>1.6</source> <target>1.6</target> <encoding>UTF-8</encoding> </configuration> </plugin> </plugins> </build> </project>  

hibernate.cfg.xml:

<?xml version="1.0" encoding="utf-8"?> <!DOCTYPE hibernate-configuration PUBLIC "-//Hibernate/Hibernate Configuration DTD//EN" "http://hibernate.sourceforge.net/hibernate-configuration-3.0.dtd"> <hibernate-configuration> <session-factory name="sessionFactory"> <!-- 指定连接数据库所用的驱动 --> <property name="hibernate.connection.driver_class">oracle.jdbc.driver.OracleDriver</property> <!-- 指定连接数据库的url，hibernate连接的数据库名 --> <property name="hibernate.connection.url">jdbc:oracle:thin:@localhost:1521:XE</property> <property name="hibernate.connection.useUnicode">true</property> <property name="hibernate.connection.characterEncoding">gbk</property> <!-- 指定连接数据库的用户名 --> <property name="hibernate.connection.username">system</property> <!-- 指定连接数据库的密码 --> <property name="hibernate.connection.password">password</property> <!-- 指定连接池里最大连接数 --> <property name="hibernate.c3p0.max_size">20</property> <!-- 指定连接池里最小连接数 --> <property name="hibernate.c3p0.min_size">5</property> <!-- 指定连接池里连接的超时时长,以秒为单位 --> <property name="hibernate.c3p0.timeout">120</property> <!-- 指定连接池里最大缓存多少个Statement对象 --> <property name="hibernate.c3p0.max_statements">100</property> <!-- 每隔XX秒检查连接池里的空闲连接 ，单位是秒 --> <property name="hibernate.c3p0.idle_test_period">120</property> <!-- 当连接池里面的连接用完的时候，C3P0一次获取的新的连接数 --> <property name="hibernate.c3p0.acquire_increment">2</property> <!-- 指定数据库方言 --> <property name="dialect">org.hibernate.dialect.OracleDialect</property> <!-- 显示Hibernate持久化操作所生成的SQL --> <property name="show_sql">true</property> <!-- 将SQL脚本进行格式化后再输出 --> <property name="hibernate.format_sql">true</property> <!-- 罗列所有的映射文件 --> <mapping resource="pojo/BankAccount.hbm.xml" /> <mapping resource="pojo/CreditCard.hbm.xml" /> </session-factory> </hibernate-configuration>  

父类，BillingDetails.java:

package pojo;public class BillingDetails {private String owner;public String getOwner() {return owner;}public void setOwner(String owner) {this.owner = owner;}} 

子类，BankAccount.java:

package pojo;public class BankAccount extends BillingDetails {private String id;private String account;private String bankName;private String swift;public String getId() {return id;}@SuppressWarnings("unused")private void setId(String id) {this.id = id;}public String getAccount() {return account;}public void setAccount(String account) {this.account = account;}public String getBankName() {return bankName;}public void setBankName(String bankName) {this.bankName = bankName;}public String getSwift() {return swift;}public void setSwift(String swift) {this.swift = swift;}} 

子类的映射，BankAccount.hbm.xml:

<?xml version="1.0" encoding="UTF-8"?> <!DOCTYPE hibernate-mapping PUBLIC "-//Hibernate/Hibernate Mapping DTD//EN" "http://hibernate.sourceforge.net/hibernate-mapping-3.0.dtd"> <hibernate-mapping package="pojo"> <class name="BankAccount" table="BANK_ACCOUNT" dynamic-insert="true" dynamic-update="true"> <id name="id" column="BANK_ACCOUNT_ID" type="string"> <!-- 这里用uuid.hex生成主键 --> <generator class="uuid.hex" /> </id> <property name="owner" column="OWNER" type="string" /> <!-- 如果列表与数据库关键字相同，如：ACCOUNT,我们可以用反引号把它括起来 ,反引号键就是F1下面那个键 --> <property name="account" column="`ACCOUNT`" type="string" /> <property name="bankName" column="BANKNAME" type="string" /> <property name="swift" column="SWIFT" type="string"/> </class> </hibernate-mapping>  

子类，CreditCard.java:

package pojo;public class CreditCard extends BillingDetails{private String id;private String number;private String expMonth;private String expYear;public String getId() {return id;}@SuppressWarnings("unused")private void setId(String id) {this.id = id;}public String getNumber() {return number;}public void setNumber(String number) {this.number = number;}public String getExpMonth() {return expMonth;}public void setExpMonth(String expMonth) {this.expMonth = expMonth;}public String getExpYear() {return expYear;}public void setExpYear(String expYear) {this.expYear = expYear;}} 

子类映射，CreditCard.hbm.xml:

<?xml version="1.0" encoding="UTF-8"?> <!DOCTYPE hibernate-mapping PUBLIC "-//Hibernate/Hibernate Mapping DTD//EN" "http://hibernate.sourceforge.net/hibernate-mapping-3.0.dtd"> <hibernate-mapping package="pojo"> <class name="CreditCard" table="CREDIT_CARD" dynamic-insert="true" dynamic-update="true"> <id name="id" column="CREDIT_CARD_ID" type="string"> <!-- 这里用uuid.hex生成主键 --> <generator class="uuid.hex" /> </id> <property name="owner" column="OWNER" type="string" /> <!-- 如果列表与数据库关键字相同，如：NUMBER,我们可以用反引号把它括起来,反引号键就是F1下面那个键 --> <property name="number" column="`NUMBER`" type="string" /> <property name="expMonth" column="EXP_MONTH" type="string" /> <property name="expYear" column="EXP_YEAR" type="string"/> </class> </hibernate-mapping>  

util/HibernateUtil.java:

package util;import org.hibernate.SessionFactory;import org.hibernate.cfg.Configuration;public class HibernateUtil {private static SessionFactory sessionFactory;static{try {sessionFactory = new Configuration().configure().buildSessionFactory();} catch (Throwable e) {throw new ExceptionInInitializerError(e);}}public static SessionFactory getSessionFactory(){return sessionFactory;}public static void shutdown(){getSessionFactory().close();}} 

util/Manager.java:

package util;import java.util.List;import org.hibernate.Session;import org.hibernate.Transaction;import pojo.BankAccount;import pojo.CreditCard;public class Manager {@SuppressWarnings("rawtypes")public static void main(String[] args) {BankAccount ba = new BankAccount();ba.setAccount("个人帐户");ba.setBankName("付海东");ba.setOwner("fuhaidong");ba.setSwift("aaa");CreditCard cc = new CreditCard();cc.setOwner("fuhaidong");cc.setNumber("100000000");cc.setExpYear("2010");cc.setExpMonth("10");Session session = HibernateUtil.getSessionFactory().openSession();Transaction transaction = session.beginTransaction();//插入session.save(cc);session.save(ba);//查询 , 这里使用父类查询,多态的方式 List list = session.createQuery("from pojo.BillingDetails").list();for(int i=0;i<list.size();i++){Object o = list.get(i);if (o instanceof BankAccount) {System.out.println(((BankAccount) o).getBankName());} else {System.out.println(((CreditCard)o).getExpYear());}}transaction.commit();session.close();}} 

输出sql与日志：

Hibernate: insert into CREDIT_CARD (OWNER, "NUMBER", EXP_MONTH, EXP_YEAR, CREDIT_CARD_ID) values (?, ?, ?, ?, ?)Hibernate: insert into BANK_ACCOUNT (OWNER, "ACCOUNT", BANKNAME, SWIFT, BANK_ACCOUNT_ID) values (?, ?, ?, ?, ?)Hibernate: select creditcard0_.CREDIT_CARD_ID as CREDIT1_1_, creditcard0_.OWNER as OWNER1_, creditcard0_."NUMBER" as NUMBER3_1_, creditcard0_.EXP_MONTH as EXP4_1_, creditcard0_.EXP_YEAR as EXP5_1_ from CREDIT_CARD creditcard0_Hibernate: select bankaccoun0_.BANK_ACCOUNT_ID as BANK1_0_, bankaccoun0_.OWNER as OWNER0_, bankaccoun0_."ACCOUNT" as ACCOUNT3_0_, bankaccoun0_.BANKNAME as BANKNAME0_, bankaccoun0_.SWIFT as SWIFT0_ from BANK_ACCOUNT bankaccoun0_20102010201020102010付海东付海东付海东付海东付海东 

注意，这里有两张表：CREDIT_CARD , BANK_ACCOUNT.

---

CREDIT_CARD表：

CREDIT_CARD_ID

OWNER

NUMBER

EXP_MONTH

EXP_YEAR

BANK_ACCOUNT表：

BANK_ACCOUNT_ID

OWNER

ACCOUNT

BANKNAME

SWIFT