Hibernate對映類繼承之每個帶有隱式多型的具體類一張表(每個子類各一張表) 我們可以準確地給每個子類使用一張表。子類的所有屬性，包括被繼承的屬性，都可以被對映到這張表的列。如圖：                         CreditCard子類   ----->  CreditCard.hbm.xml   ----->  CREDIT_CARD表 BillingDetails父類---->                         BankAccount子類  ----->  BankAccount.hbm.xml  ----->  BANK_ACCOUNT表 在Hibernate中你不必做任何特別的事來啟用多型行為。對CreditCard和BankAccount的對映很簡單，各自都在自己的實體<class>元素中，就像我們已經沒有基類的類所做的那樣。Hibernate仍然知道這個基類或者介面，因為它在啟動時掃描持久化類。這種方法的主要問題在於，它不太支援多型關聯。在資料庫中，關聯通常被表示為外來鍵關係。如果子類全部被對映到不同的表，對它們基類(本例BillingDetails)的多型關聯就無法被表示為一個簡單的外來鍵關係。這在我們的領域模型中會有問題，如果BillingDetails與一個存在的user關聯，兩張子類表都需要一個對User表的外來鍵引用。或者，如果User與BillingDetails有個多對一的關係，USER表就需要單個的外來鍵列，它必須同時引用兩個具體的子類表。這用一般的外來鍵約束是不可能的。 多型查詢(它返回與被查詢類的介面相匹配的所有類的物件)也有問題。針對基類的查詢必須作為幾個SQL SELECT執行，每個具體的子類一個。對於針對BillingDetails類的查詢,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; 注意每個具體的子類都需要一個單獨的查詢。 這種對映策略進一步的概念問題在於，幾張不同表的幾個不同的列具有完全相同的語義。這使得模式演變更為複雜。例如，對一個基類屬性的改變導致了多個列的變化。它也使得把應用到所有子類的資料庫完整性約束實現起來變得更加困難。 我們推薦這種方法(僅僅)用於類層次結構的最頂層，那裡通常不需要多型，且未來也不太可能修改基類。 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"> <!-- 指定連線資料庫所用的驅動 --> <property name="hibernate.connection.driver_class">oracle.jdbc.driver.OracleDriver</property> <!-- 指定連線資料庫的url，hibernate連線的資料庫名 --> <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> <!-- 指定連線資料庫的使用者名稱 --> <property name="hibernate.connection.username">system</property> <!-- 指定連線資料庫的密碼 --> <property name="hibernate.connection.password">password</property> <!-- 指定連線池裡最大連線數 --> <property name="hibernate.c3p0.max_size">20</property> <!-- 指定連線池裡最小連線數 --> <property name="hibernate.c3p0.min_size">5</property> <!-- 指定連線池裡連線的超時時長,以秒為單位 --> <property name="hibernate.c3p0.timeout">120</property> <!-- 指定連線池裡最大快取多少個Statement物件 --> <property name="hibernate.c3p0.max_statements">100</property> <!-- 每隔XX秒檢查連線池裡的空閒連線 ，單位是秒 --> <property name="hibernate.c3p0.idle_test_period">120</property> <!-- 當連線池裡面的連線用完的時候，C3P0一次獲取的新的連線數 --> <property name="hibernate.c3p0.acquire_increment">2</property> <!-- 指定資料庫方言 --> <property name="dialect">org.hibernate.dialect.OracleDialect</property> <!-- 顯示Hibernate持久化操作所生成的SQL --> <property name="show_sql">true</property> <!-- 將SQL指令碼進行格式化後再輸出 --> <property name="hibernate.format_sql">true</property> <!-- 羅列所有的對映檔案 --> <mapping resource="pojo/BankAccount.hbm.xml" /> <mapping resource="pojo/CreditCard.hbm.xml" /> </session-factory> </hibernate-configuration>   父類，BillingDetails.java: package pojo; public class BillingDetails { private String owner; public String getOwner() { return owner; } public void setOwner(String owner) { this.owner = owner; } }   子類，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; } }   子類的對映，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"> <!-- 這裡用uuid.hex生成主鍵 --> <generator class="uuid.hex" /> </id> <property name="owner" column="OWNER" type="string" /> <!-- 如果列表與資料庫關鍵字相同，如：ACCOUNT,我們可以用反引號把它括起來 ,反引號鍵就是F1下面那個鍵 --> <property name="account" column="`ACCOUNT`" type="string" /> <property name="bankName" column="BANKNAME" type="string" /> <property name="swift" column="SWIFT" type="string"/> </class> </hibernate-mapping>   子類，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; } }   子類對映，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"> <!-- 這裡用uuid.hex生成主鍵 --> <generator class="uuid.hex" /> </id> <property name="owner" column="OWNER" type="string" /> <!-- 如果列表與資料庫關鍵字相同，如：NUMBER,我們可以用反引號把它括起來,反引號鍵就是F1下面那個鍵 --> <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("個人帳戶"); ba.setBankName("付海東"); 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); //查詢 , 這裡使用父類查詢,多型的方式 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(); } }   輸出sql與日誌： 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_ 2010 2010 2010 2010 2010 付海東 付海東 付海東 付海東 付海東   注意，這裡有兩張表：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