在每個類創(chuàng)建一張表的情況下, 表中不使用Null值的列。 這種方法的缺點是在子類表中創(chuàng)建了重復(fù)的列。
在這里,我們需要在父類中使用@Inheritance(strategy = InheritanceType.TABLE_PER_CLASS)
注釋,并在子類中使用@AttributeOverrides
注釋。@Inheritance(strategy = InheritanceType.TABLE_PER_CLASS)
指定正在使用每個具體類策略的表。它應(yīng)該僅在父類中指定。
@AttributeOverrides
定義該類中的父類屬性將被覆蓋。 在表結(jié)構(gòu)中,父類表列將被添加到子類表中。
我們來了解映射的層次結(jié)構(gòu)。
在每個具體類一張表的情況下,數(shù)據(jù)庫中將有三個表,每個表表示一個特定的類。
每個表的格結(jié)構(gòu)如下:Employee
類的表結(jié)構(gòu) -
CREATE TABLE `emp122` (
`id` int(11) NOT NULL,
`name` varchar(255) DEFAULT NULL,
PRIMARY KEY (`id`)
) ENGINE=InnoDB DEFAULT CHARSET=utf8;
Regular_Employee
類的表結(jié)構(gòu) -
CREATE TABLE `regemp122` (
`id` int(11) NOT NULL,
`name` varchar(255) DEFAULT NULL,
`salary` float DEFAULT NULL,
`bonus` int(11) DEFAULT NULL,
PRIMARY KEY (`id`)
) ENGINE=InnoDB DEFAULT CHARSET=utf8;
Contract_Employee
類的表結(jié)構(gòu) -
CREATE TABLE `contemp122` (
`id` int(11) NOT NULL,
`name` varchar(255) DEFAULT NULL,
`pay_per_hour` float DEFAULT NULL,
`contract_duration` varchar(255) DEFAULT NULL,
PRIMARY KEY (`id`)
) ENGINE=InnoDB DEFAULT CHARSET=utf8;
在這個例子中,我們創(chuàng)建了三個類,并在employee.hbm.xml
文件中提供了這些類的映射。創(chuàng)建一個項目:inheritance2annotation
, 完整的項目結(jié)構(gòu)如下 -
您需要創(chuàng)建表示繼承的持久化類。 讓我們?yōu)樯厦娴膶哟谓Y(jié)構(gòu)創(chuàng)建三個類:
文件:Employee.java
package com.yiibai;
import javax.persistence.*;
import javax.persistence.*;
@Entity
@Table(name = "employee102")
@Inheritance(strategy = InheritanceType.TABLE_PER_CLASS)
public class Employee {
@Id
@GeneratedValue(strategy = GenerationType.AUTO)
@Column(name = "id")
private int id;
@Column(name = "name")
private String name;
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;
}
// setters and getters
}
文件:Regular_Employee.java
package com.yiibai;
/**
*
* @author by maxsu
* @copyright http://www.yiibai.com
* @link download at: http://www.yiibai.com/siteinfo/download.html
*/
import javax.persistence.*;
@Entity
@Table(name = "regularemployee102")
@AttributeOverrides({
@AttributeOverride(name = "id", column = @Column(name = "id")),
@AttributeOverride(name = "name", column = @Column(name = "name")) })
public class Regular_Employee extends Employee {
@Column(name = "salary")
private float salary;
@Column(name = "bonus")
private int bonus;
public float getSalary() {
return salary;
}
public void setSalary(float salary) {
this.salary = salary;
}
public int getBonus() {
return bonus;
}
public void setBonus(int bonus) {
this.bonus = bonus;
}
// setters and getters
}
文件:Contract_Employee.java
package com.yiibai;
/**
*
* @author by maxsu
* @copyright http://www.yiibai.com
* @link download at: http://www.yiibai.com/siteinfo/download.html
*/
import javax.persistence.*;
@Entity
@Table(name = "contractemployee102")
@AttributeOverrides({
@AttributeOverride(name = "id", column = @Column(name = "id")),
@AttributeOverride(name = "name", column = @Column(name = "name")) })
public class Contract_Employee extends Employee {
@Column(name = "pay_per_hour")
private float pay_per_hour;
@Column(name = "contract_duration")
private String contract_duration;
public float getPay_per_hour() {
return pay_per_hour;
}
public void setPay_per_hour(float payPerHour) {
pay_per_hour = payPerHour;
}
public String getContract_duration() {
return contract_duration;
}
public void setContract_duration(String contractDuration) {
contract_duration = contractDuration;
}
}
打開hibernate.cfg.xml文
件,并添加如下映射資源的項:
<mapping class="com.yiibai.Employee" />
<mapping class="com.yiibai.Contract_Employee" />
<mapping class="com.yiibai.Regular_Employee" />
現(xiàn)在配置文件將如下所示:
文件:hibernate.cfg.xml
<?xml version='1.0' encoding='UTF-8'?>
<!DOCTYPE hibernate-configuration PUBLIC
"-//Hibernate/Hibernate Configuration DTD 3.0//EN"
"http://hibernate.sourceforge.net/hibernate-configuration-3.0.dtd">
<!-- Generated by MyEclipse Hibernate Tools. -->
<hibernate-configuration>
<session-factory>
<property name="hbm2ddl.auto">update</property>
<property name="connection.driver_class">com.mysql.jdbc.Driver</property>
<property name="connection.url">jdbc:mysql://localhost:3306/test</property>
<property name="connection.username">root</property>
<property name="connection.password">123456</property>
<property name="dialect">org.hibernate.dialect.MySQL5InnoDBDialect</property>
<property name="show_sql">true</property>
<mapping class="com.yiibai.Employee" />
<mapping class="com.yiibai.Contract_Employee" />
<mapping class="com.yiibai.Regular_Employee" />
</session-factory>
</hibernate-configuration>
hbm2ddl.auto
屬性定義是用于在數(shù)據(jù)庫中創(chuàng)建自動表。
在這個類中,我們只是將Employee
對象存儲在數(shù)據(jù)庫表中。
文件:MainTest.java
package com.yiibai;
import org.hibernate.*;
import org.hibernate.boot.MetadataSources;
import org.hibernate.boot.registry.StandardServiceRegistry;
import org.hibernate.boot.registry.StandardServiceRegistryBuilder;
/**
*
* @author by maxsu
* @copyright http://www.yiibai.com
* @link download at: http://www.yiibai.com/siteinfo/download.html
*/
public class MainTest {
public static void main(String[] args) {
// 但在5.1.0版本匯總,hibernate則采用如下新方式獲?。? // 1. 配置類型安全的準(zhǔn)服務(wù)注冊類,這是當(dāng)前應(yīng)用的單例對象,不作修改,所以聲明為final
// 在configure("cfg/hibernate.cfg.xml")方法中,如果不指定資源路徑,默認(rèn)在類路徑下尋找名為hibernate.cfg.xml的文件
final StandardServiceRegistry registry = new StandardServiceRegistryBuilder()
.configure("hibernate.cfg.xml").build();
// 2. 根據(jù)服務(wù)注冊類創(chuàng)建一個元數(shù)據(jù)資源集,同時構(gòu)建元數(shù)據(jù)并生成應(yīng)用一般唯一的的session工廠
SessionFactory sessionFactory = new MetadataSources(registry)
.buildMetadata().buildSessionFactory();
/**** 上面是配置準(zhǔn)備,下面開始我們的數(shù)據(jù)庫操作 ******/
Session session = sessionFactory.openSession();// 從會話工廠獲取一個session
// creating transaction object
Transaction t = session.beginTransaction();
Employee e1 = new Employee();
e1.setName("用戶名-01");
Regular_Employee e2 = new Regular_Employee();
e2.setName("yiibai su");
e2.setSalary(50002);
e2.setBonus(5);
Contract_Employee e3 = new Contract_Employee();
e3.setName("Mina su");
e3.setPay_per_hour(1010);
e3.setContract_duration("15 hours");
session.persist(e1);
session.persist(e2);
session.persist(e3);
t.commit();
session.close();
System.out.println("success");
}
}
執(zhí)行上面代碼運行測試即可,應(yīng)該會自動創(chuàng)建三張表,并插入數(shù)據(jù)。