hibernate 实现继承的三种方式

1、继承关系中每个类都映射成一个数据库表，基类和子类共用一个id来标识。这样相当于父类和每个子类都对应自己单独的表结构。
   优点就是完全符合面向对象思想，缺点是查询子类要级联查询多个表，效率低。

配置文件例子：<?xml version="1.0" encoding="utf-8"?> <!DOCTYPE hibernate-mapping PUBLIC "-//Hibernate/Hibernate Mapping DTD 3.0//EN" "http://hibernate.sourceforge.net/hibernate-mapping-3.0.dtd"> <hibernate-mapping> <class name="com.suteam.saas.client.crm.budget.model.Budget" table="budget"> <id name="id" type="java.lang.String"> <column name="id" /> <generator class="uuid" /> </id> <property name="companyId" type="java.lang.String"> <column name="company_id" /> </property> <property name="budgetMoney" type="java.lang.Double"> <column name="budget_money" /> </property> <property name="useMoney" type="java.lang.Double"> <column name="use_money" /> </property> <property name="adjustMoney" type="java.lang.Double"> <column name="adjust_money" /> </property> <property name="deductMoney" type="java.lang.Double"> <column name="deduct_money" /> </property> <property name="date" type="java.util.Date"> <column name="date" /> </property> <property name="ym" type="java.lang.String"> <column name="ym" /> </property> <set name="feeBudgets" table="fee_apply_budget" cascade="none"> <key column="budget_id"/> <one-to-many class="com.suteam.saas.client.crm.fee.feebudget.model.FeeBudget"/> </set> <many-to-one name="operator" class="com.suteam.saas.client.crm.model.Operator" cascade="all"> <column name="operator_id" /> </many-to-one> <joined-subclass name="com.suteam.saas.client.crm.budget.model.EmployeeSubBudget" table="budget_employee_sub"> <key column="id"></key> <property name="companyId" type="java.lang.String"> <column name="company_id" /> </property> <property name="employeeId" type="java.lang.String"> <column name="employee_id" /> </property> <many-to-one name="employee" class="com.suteam.saas.client.crm.employee.model.Employee" cascade="none" insert="false" update="false"> <column name="employee_id" /> </many-to-one> <property name="budgetSubjectId" type="java.lang.String"> <column name="subject_id" /> </property> <many-to-one name="budgetSubject" class="com.suteam.saas.client.crm.budget.subject.model.BudgetSubject" cascade="none" insert="false" update="false"> <column name="subject_id" /> </many-to-one> </joined-subclass> <joined-subclass name="com.suteam.saas.client.crm.budget.model.EmployeeBudget" table="budget_employee"> <key column="id"></key> <property name="companyId" type="java.lang.String"> <column name="company_id" /> </property> <property name="employeeId" type="java.lang.String"> <column name="employee_id" /> </property> <many-to-one name="employee" class="com.suteam.saas.client.crm.employee.model.Employee" cascade="none" insert="false" update="false"> <column name="employee_id" /> </many-to-one> </joined-subclass> <joined-subclass name="com.suteam.saas.client.crm.budget.model.OrganSubBudget" table="budget_organ_sub"> <key column="id"></key> <property name="companyId" type="java.lang.String"> <column name="company_id" /> </property> <property name="organizationId" type="java.lang.String"> <column name="organization_id" /> </property> <many-to-one name="organization" class="com.suteam.saas.client.crm.company.model.Organization" cascade="none" insert="false" update="false"> <column name="organization_id" /> </many-to-one> <property name="budgetSubjectId" type="java.lang.String"> <column name="subject_id" /> </property> <many-to-one name="budgetSubject" class="com.suteam.saas.client.crm.budget.subject.model.BudgetSubject" cascade="none" insert="false" update="false"> <column name="subject_id" /> </many-to-one> </joined-subclass> <joined-subclass name="com.suteam.saas.client.crm.budget.model.OrganBudget" table="budget_organ"> <key column="id"></key> <property name="companyId" type="java.lang.String"> <column name="company_id" /> </property> <property name="organizationId" type="java.lang.String"> <column name="organization_id" /> </property> <many-to-one name="organization" class="com.suteam.saas.client.crm.company.model.Organization" cascade="none" insert="false" update="false"> <column name="organization_id" /> </many-to-one> </joined-subclass> </class> </hibernate-mapping> 

2、将整个类层次映射为单个数据库表。即所有的类都在一个数据库表中存储。
   很显然，这种方法的缺点是浪费存储空间，每条数据都会有null的字段存在，优点就是增删查改的效率比较高。

3、将每个子类分别映射成数据库表，基类不参与映射。
   优点是操作的单个表，效率高。缺点是修改基类的时候每个子类的数据库表都要修改，不支持多态查询。

 

现在常用的javaEE的ssh开发框架中，一般都采用第一种方法，完全面向对象的思想，易于理解开发维护。

很详细的例子：http://www.ibm.com/developerworks/cn/java/j-hibernate/