hibernate继承实现

http://www.blogjava.net/xiaodu/archive/2008/07/11/214188.html

hiberate继承分为三种方式:一个表对应所有继承的类,具体类对应一个表,一个类对应一个表(父类及子类)

一个类对应一个表(父类及子类)
数据库脚本
CREATE TABLE `t_item1` (                                  
           `id` int(11) NOT NULL AUTO_INCREMENT,                   
           `manufacture` varchar(20) DEFAULT NULL,                 
           `name` varchar(20) DEFAULT NULL,                        
           PRIMARY KEY (`id`)                                      
         ) ENGINE=InnoDB DEFAULT CHARSET=latin1 ROW_FORMAT=DYNAMIC 

CREATE TABLE `t_book1` (                                                                 
           `id` int(11) NOT NULL AUTO_INCREMENT,                                                  
           `name` varchar(20) DEFAULT NULL,                                                       
           `manufacture` varchar(20) DEFAULT NULL,                                                
           `pagecount` int(11) DEFAULT NULL,                                                      
           PRIMARY KEY (`id`)                                                                     
         ) ENGINE=InnoDB DEFAULT CHARSET=latin1 ROW_FORMAT=DYNAMIC COMMENT='InnoDB free: 3072 kB' 

CREATE TABLE `t_dvd1` (                                   
          `id` int(11) NOT NULL AUTO_INCREMENT,                   
          `name` varchar(20) DEFAULT NULL,                        
          `manufacture` varchar(20) DEFAULT NULL,                 
          `regioncode` varchar(20) DEFAULT NULL,                  
          PRIMARY KEY (`id`)                                      
        ) ENGINE=InnoDB DEFAULT CHARSET=latin1 ROW_FORMAT=DYNAMIC 

类代码

public class TItem1 implements Serializable {
 private int id;
 private String name;
 private String manufacture;

 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;
 }

 public String getManufacture() {
  return manufacture;
 }

 public void setManufacture(String manufacture) {
  this.manufacture = manufacture;
 }

}

public class TBook1 extends TItem1 implements Serializable {
 private int pagecount;

 public int getPagecount() {
  return pagecount;
 }

 public void setPagecount(int pagecount) {
  this.pagecount = pagecount;
 }

}

public class TDvd1 extends TItem1 implements Serializable {
private String regioncode;

public String getRegioncode() {
 return regioncode;
}

public void setRegioncode(String regioncode) {
 this.regioncode = regioncode;
}

}

配置文件
<?xml version="1.0"?>
<!DOCTYPE hibernate-mapping PUBLIC
 "-//Hibernate/Hibernate Mapping DTD 3.0//EN"
 "http://hibernate.sourceforge.net/hibernate-mapping-3.0.dtd">
<hibernate-mapping>
<class name="mypackage.TItem1" table="t_item1">
 <id name="id" column="id">
  <generator class="identity"/>
 </id>
 <property name="name" column="name"/>
 <property name="manufacture" column="manufacture"/>
 <joined-subclass name="mypackage.TBook1" table="t_book1">
  <key column="id"/>
  <property name="pagecount" column="pagecount" type="java.lang.Integer"/>
 </joined-subclass>
 <joined-subclass name="mypackage.TDvd1" table="t_dvd1">
  <key column="id"/>
  <property name="regioncode" column="regioncode" type="java.lang.String"/>
 </joined-subclass>
</class>
 
</hibernate-mapping>
具体类对应一个表
数据库脚本
 CREATE TABLE `t_book2` (                                 
           `id` int(11) NOT NULL AUTO_INCREMENT,                  
           `name` varchar(20) DEFAULT NULL,                       
           `manufacture` varchar(20) DEFAULT NULL,                
           `pagecount` int(11) DEFAULT NULL,                      
           PRIMARY KEY (`id`)                                     
         ) ENGINE=MyISAM AUTO_INCREMENT=17 DEFAULT CHARSET=latin1
 CREATE TABLE `t_dvd2` (                                  
          `id` int(11) NOT NULL AUTO_INCREMENT,                  
          `name` varchar(23) DEFAULT NULL,                       
          `manufacture` varchar(20) DEFAULT NULL,                
          `regioncode` varchar(20) DEFAULT NULL,                 
          PRIMARY KEY (`id`)                                     
        ) ENGINE=MyISAM AUTO_INCREMENT=18 DEFAULT CHARSET=latin1 
类代码

public abstract class TItem2 implements java.io.Serializable {
 private int id;
 private String name;
 private String manufacture;

 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;
 }

 public String getManufacture() {
  return manufacture;
 }

 public void setManufacture(String manufacture) {
  this.manufacture = manufacture;
 }

}

public class TDvd2 extends TItem2 implements Serializable {
 private String regioncode;

 public String getRegioncode() {
  return regioncode;
 }

 public void setRegioncode(String regioncode) {
  this.regioncode = regioncode;
 }

}

public class TBook2 extends TItem2 implements Serializable {
 private int pagecount;

 public int getPagecount() {
  return pagecount;
 }

 public void setPagecount(int pagecount) {
  this.pagecount = pagecount;
 }

}
配置文件
<?xml version="1.0"?>
<!DOCTYPE hibernate-mapping PUBLIC
 "-//Hibernate/Hibernate Mapping DTD 3.0//EN"
 "http://hibernate.sourceforge.net/hibernate-mapping-3.0.dtd">
<hibernate-mapping>
 <class name="mypackage.TDvd2" table="t_dvd2">
  <id name="id" column="id">
   <generator class="identity"/>
  </id>
  <property name="name" column="name"/>
  <property name="manufacture" column="manufacture"/>
  <property name="regioncode" column="regioncode"/>
 </class>
 <class name="mypackage.TBook2" table="t_book2">
  <id name="id" column="id">
   <generator class="identity"/>
  </id>
  <property name="name" column="name"/>
  <property name="manufacture" column="manufacture"/>
  <property name="pagecount" column="pagecount"/>
 </class>
</hibernate-mapping>

一个表对应所有继承的类
数据库脚本
CREATE TABLE `t_item3` (                                
           `id` int(11) NOT NULL AUTO_INCREMENT,                 
           `name` varchar(20) DEFAULT NULL,                      
           `manufacture` varchar(20) DEFAULT NULL,               
           `pagecount` int(11) DEFAULT NULL,                     
           `regioncode` varchar(20) DEFAULT NULL,                
           `category` varchar(11) DEFAULT NULL,                  
           PRIMARY KEY (`id`)                                    
         ) ENGINE=MyISAM AUTO_INCREMENT=5 DEFAULT CHARSET=latin1 
类代码

public class TItem3 implements Serializable {
 private int id;
 private String name;
 private String manufacture;
 private String category;

 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;
 }

 public String getManufacture() {
  return manufacture;
 }

 public void setManufacture(String manufacture) {
  this.manufacture = manufacture;
 }

 public String getCategory() {
  return category;
 }

 public void setCategory(String category) {
  this.category = category;
 }

}

public class TDvd3 extends TItem3 implements Serializable {
 private String regioncode;

 public String getRegioncode() {
  return regioncode;
 }

 public void setRegioncode(String regioncode) {
  this.regioncode = regioncode;
 }

}

public class TBook3 extends TItem3 implements Serializable {
 private int pagecount;

 public int getPagecount() {
  return pagecount;
 }

 public void setPagecount(int pagecount) {
  this.pagecount = pagecount;
 }

}

配置文件
<?xml version="1.0"?>
<!DOCTYPE hibernate-mapping PUBLIC
 "-//Hibernate/Hibernate Mapping DTD 3.0//EN"
 "http://hibernate.sourceforge.net/hibernate-mapping-3.0.dtd">
<hibernate-mapping>
<class name="mypackage.TItem3" table="t_item3">
 <id name="id" column="id">
  <generator class="identity"/>
 </id>
 <discriminator column="category" type="java.lang.String"/>
 <property name="name" column="name"/>
 <property name="manufacture" column="manufacture"/>
 <subclass name="mypackage.TDvd3" discriminator-value="1">
  <property name="regioncode" column="regioncode"/>
 </subclass>
 <subclass name="mypackage.TBook3" discriminator-value="0">
  <property name="pagecount" column="pagecount"/>
 </subclass>
</class>
</hibernate-mapping>
三种方式的优缺点
每个类对应一个表
缺点:创建表过多,查询需用求外连接,优点:某个表字段发生变化只需修改相对表,支持多态查询,符合关系型数据库设计
具体类对应一个表
缺点:数据表包含重复字段,如果父类发生变化子类也要变化,查询父类时要查询所有子类,不支持多态查询,优点:符合关系型数据库设计
一个表对应所有类
缺点:表中引入区分子类的字段,子类的字段不能创建为空,优点:只需一张表,有很好的查询性能,只要修改一张表,支持多态查询