使用xml文件的每个子类的表示例

对于每个子类的表, 子类映射表通过主键和外键关系与父类映射表相关。

class的<joined-subclass>元素用于使用主键和外键关系将子类与父类映射。

在此示例中, 我们将使用hb2ddl.auto属性自动生成表。因此, 我们不必担心在数据库中创建表。

让我们看一下我们要映射的类的层次结构。

让我们看看如何通过joined-subclass元素映射此层次结构：

<?xml version='1.0' encoding='UTF-8'?>
<!DOCTYPE hibernate-mapping PUBLIC

  "-//Hibernate/Hibernate Mapping DTD 5.3//EN"
  "http://hibernate.sourceforge.net/hibernate-mapping-5.3.dtd">


  <hibernate-mapping>
  <class name="com.srcmini.mypackage.Employee" table="emp123">
  <id name="id">
  <generator class="increment"></generator>
  </id>

  <property name="name"></property>

  <joined-subclass name="com.srcmini.mypackage.Regular_Employee" table="regemp123">
  <key column="eid"></key>
  <property name="salary"></property>
  <property name="bonus"></property>
  </joined-subclass>
 
  <joined-subclass name="com.srcmini.mypackage.Contract_Employee" table="contemp123">
  <key column="eid"></key>
  <property name="pay_per_hour"></property>
  <property name="contract_duration"></property>
  </joined-subclass>

  </class>
  </hibernate-mapping>

对于每个子类类有表的情况, 数据库中将有三个表, 每个表代表一个特定的类。

class的joined-subclass子元素指定子类。

joined-subclass的键子元素用于在子类映射表中生成外键。该外键将与父类映射表的主键关联。

每个表的表结构如下：

Employee类的表结构

Regular_Employee类的表结构

Contract_Employee类的表结构

每个子类类的表示例

在此示例中, 我们将创建三个类, 并在employee.hbm.xml文件中提供这些类的映射。

1）创建持久类

你需要创建表示继承的持久类。让我们为上述层次结构创建三个类：

文件：Employee.java

package com.srcmini.mypackage;

public class Employee {
private int id;
private String name;

//getters and setters
}

文件：Regular_Employee.java

package com.srcmini.mypackage;
public class Regular_Employee extends Employee{
private float salary;
private int bonus;

//getters and setters
}

文件：Contract_Employee.java

package com.srcmini.mypackage;

public class Contract_Employee extends Employee{
	private float pay_per_hour;
	private String contract_duration;

//getters and setters
}

2）为Persistent类创建映射文件

上面已经针对层次结构讨论了映射。

文件：employee.hbm.xml

<?xml version='1.0' encoding='UTF-8'?>
<!DOCTYPE hibernate-mapping PUBLIC

  "-//Hibernate/Hibernate Mapping DTD 5.3//EN"
  "http://hibernate.sourceforge.net/hibernate-mapping-5.3.dtd">


  <hibernate-mapping>
  <class name="com.srcmini.mypackage.Employee" table="emp123">
  <id name="id">
  <generator class="increment"></generator>
  </id>

  <property name="name"></property>

  <joined-subclass name="com.srcmini.mypackage.Regular_Employee" table="regemp123">
  <key column="eid"></key>
  <property name="salary"></property>
  <property name="bonus"></property>
  </joined-subclass>
 
  <joined-subclass name="com.srcmini.mypackage.Contract_Employee" table="contemp123">
  <key column="eid"></key>
  <property name="pay_per_hour"></property>
  <property name="contract_duration"></property>
  </joined-subclass>

  </class>
  </hibernate-mapping>

3）创建配置文件

打开hibernate.cgf.xml文件, 并添加如下所示的映射资源条目：

<mapping resource="employee.hbm.xml"/>

现在, 配置文件将如下所示：

文件：hibernate.cfg.xml

<?xml version='1.0' encoding='UTF-8'?>
<!DOCTYPE hibernate-configuration PUBLIC
          "-//Hibernate/Hibernate Configuration DTD 5.3//EN"
          "http://hibernate.sourceforge.net/hibernate-configuration-5.3.dtd">

<hibernate-configuration>

    <session-factory>
        <property name="hbm2ddl.auto">update</property>
        <property name="dialect">org.hibernate.dialect.Oracle9Dialect</property>
        <property name="connection.url">jdbc:oracle:thin:@localhost:1521:xe</property>
        <property name="connection.username">system</property>
        <property name="connection.password">jtp</property>
        <property name="connection.driver_class">oracle.jdbc.driver.OracleDriver</property>
    <mapping resource="employee.hbm.xml"/>
    </session-factory>

</hibernate-configuration>

定义了hbm2ddl.auto属性, 用于在数据库中创建自动表。

4）创建存储持久对象的类

在此类中, 我们仅将雇员对象存储在数据库中。

文件：StoreData.java

package com.srcmini.mypackage;

import org.hibernate.Session;
import org.hibernate.SessionFactory;
import org.hibernate.Transaction;
import org.hibernate.boot.Metadata;
import org.hibernate.boot.MetadataSources;
import org.hibernate.boot.registry.StandardServiceRegistry;
import org.hibernate.boot.registry.StandardServiceRegistryBuilder;

public class StoreData {
public static void main(String[] args) {
		
StandardServiceRegistry ssr=new StandardServiceRegistryBuilder().configure("hibernate.cfg.xml").build();
Metadata meta=new MetadataSources(ssr).getMetadataBuilder().build();

SessionFactory factory=meta.buildSessionFactory();
Session session=factory.openSession();

Transaction t=session.beginTransaction();  

Employee e1=new Employee();  
e1.setName("Gaurav Chawla");  
  
Regular_Employee e2=new Regular_Employee();  
e2.setName("Vivek Kumar");  
e2.setSalary(50000);  
e2.setBonus(5);  
  
Contract_Employee e3=new Contract_Employee();  
e3.setName("Arjun Kumar");  
e3.setPay_per_hour(1000);  
e3.setContract_duration("15 hours");  
  
session.persist(e1);  
session.persist(e2);  
session.persist(e3);  
  
t.commit();  
session.close();  
System.out.println("success");  

}
}