Hibernate的继承策略

节 1.01   每个带有联合的具体类一张表 – union-class

    <!-- 注：抽象的超类或者接口必须声明为 abstract ，否则超类实例需要一张单独表 -->

    < class name = "hibernate.hierarchical.BillingDetails" abstract = "true" >

< union-subclass name = "hibernate.hierarchical.CreditCard" table = "CREDIT_CARD" >

           < property name = "number" column = "NUMBER" />

           < property name = "expMonth" column = "EXP_MONTH" />

           < property name = "expYear" column = "EXP_YEAR" />

       </ union-subclass >

       < union-subclass name = "hibernate.hierarchical.BankAccount" table = "BANK_ACCOUNT" >

           < property name = "account" column = "ACCOUNT" />

           < property name = "bankname" column = "BANKNAME" />

           < property name = "swift" column = "SWIFT" />

       </ union-subclass >

    </ class >

public class BillingDetails { private Long id ; private String owner ;}

public class BankAccount extends BillingDetails {

private String account ; private String bankname ; private String swift ;}

public class CreditCard extends BillingDetails {

private String number ; private String expMonth ; private String expYear ;}

 

查询 BillingDetails 时生成的 SQL 如下：

Select BILLING_DETAIL_ID,OWNER… FROM

(

       Select BILLING_DETAIL_ID_OWNER,NUMBER,EXP_MONTH,EXMP_YEAR,null as ACCOUNT,null AS BANKNAME,null as SWIFT 1 as CLAZZ_

FROM CREDIT_CARD

       UNION

Select BILLING_DETAIL_ID_OWNER,null as NUMBER,null as EXP_MONTH,null as EXP_YEAR,ACCOUNT,BANKNAME,SWIFT,2 as CLAZZ_

       FROM BANK_ACCOUNT

)

这样使用的效率最高

节 1.02   每个类层次结构一张表 – SubClass

注：

1、  在表设计时添加辨别标志 (BILLING_DETAILS_TYPE) 用于在持久化类之间区分不同对象。

2、  在列名称设计上需要加前缀（比如 CC/BA ）等，用于让 Hibernate 自动识别不同对象。

 

    < class name = "hibernate.hierarchical.BillingDetails" table = "Billing_Details" >

       < property name = "owner" column = "OWNER" type = "string" />

       <!-- 添加一个辨别标志以便在持久化类之间进行区分 -->

       < discriminator column = "BILLING_DETAILS_TYPE" type = "string" />

       < subclass name = "hibernate.hierarchical.CreditCard" discriminator-value = "CC" >

           < property name = "number" column = "CC_NUMBER" />

           < property name = "expMonth" column = "CC_EXP_MONTH" />

           < property name = "expYear" column = "CC_EXP_YEAR" />

       </ subclass >

       < subclass name = "hibernate.hierarchical.BankAccount" discriminator-value = "BA" >

           < property name = "account" column = "BA_ACCOUNT" />

           < property name = "bankname" column = "BA_BANKNAME" />

           < property name = "swift" column = "BA_SWIFT" />

       </ subclass >

    </ class >

public class BillingDetails { private Long id ; private String owner ;}

public class BankAccount extends BillingDetails {

private String account ; private String bankname ; private String swift ;}

public class CreditCard extends BillingDetails {

private String number ; private String expMonth ; private String expYear ;}

 

当查询父表 BillingDetails 时将 BILLING_DETAILS 表字段全部查询。

Select BILLING_DETAILS_ID,CC_NUMBER,CC_EXP_MONTH… FROM BILLING_DETAILS

当查询子表 BankAccount/CreditCard 表时通过标识字段进行限制。

Select BILLING_DETAILS_ID,CC_NUMBER,CC_EXP_MONTH FROM BILLING_DETAILS WHERE BILLING_DETAILS_TYPE=’CC’

注：当类继承关系比较多时，存在性能问题

 

节 1.03   每个子类一个表 – JoinSubClass

将一个表示逻辑的大表拆分成多个子表，每个子表通过主键 / 外键的关系同父表进行关联，从而形成继承关系。每个子表的主键同时时父表的外键

< class name = "hibernate.hierarchical.BillingDetails" table = "BillingDetails" >

< joined-subclass name = "hibernate.hierarchical.CreditCard" table = "CREDIT_CARD" >

<!-- 通过 key 指定的字段将父表和子表关联。 CREDIT_CARD_ID 即使子表的主键同时也是父表的外键 -->

           < key column = "CREDIT_CARD_ID" />

           < property name = "number" column = "NUMBER" />

           < property name = "expMonth" column = "EXP_MONTH" />

           < property name = "expYear" column = "EXP_YEAR" />

       </ joined-subclass >

       < joined-subclass name = "hibernate.hierarchical.BankAccount" table = "BANK_ACCOUNT" >

           < key column = "BANK_ACCOUNT_ID" />

           <