Hibernate：类继承的实现方式（一）--父子类在同一个表中

Hibernate对继承关系的对象的处理分为多种形式：

第一种：继承链上的所有对象存放在一个表中

这种方式的缺陷：每次增加一个子类都要改动表结构！！而且，增加的字段不能有非空约束

domain对象：

public class Employee {
	
	private Integer id;
	private String name;
	
	public Employee(){}
	
	public Employee(String name){
		this.name = name;
	}
	
	
	public Integer getId() {
		return id;
	}
	public void setId(Integer id) {
		this.id = id;
	}
	public String getName() {
		return name;
	}
	public void setName(String name) {
		this.name = name;
	}

}

public class Skiller extends Employee {
	
	private String skill;
	
	public Skiller(){}
	
	public Skiller(String name,String skill){
		super(name);
		this.skill = skill;
	}

	public String getSkill() {
		return skill;
	}

	public void setSkill(String skill) {
		this.skill = skill;
	}
}

public class Driver extends Employee {
	
	private Integer driverYears;
	
	public Driver(){}
	
	public Driver(String name,Integer driverYears){
		super(name);
		this.driverYears = driverYears;
	}

	public Integer getDriverYears() {
		return driverYears;
	}

	public void setDriverYears(Integer driverYears) {
		this.driverYears = driverYears;
	}
}

映射文件：

<?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 package="cn.itcast.hibernate.inherit1">

	<class name="Employee" table="employee" discriminator-value="0">
		<id name="id" unsaved-value="-1">
			<generator class="native" />
		</id>
		<discriminator column="type" type="string"/>
		<property name="name"/>

		<subclass name="Skiller" discriminator-value="1">
			<property name="skill"/>
		</subclass>
		
		<subclass name="Driver" discriminator-value="2">
			<property name="driverYears"/>
		</subclass>
		
	</class>
</hibernate-mapping>

测试代码：

	public static void main(String[] args) {
		Session session = HibernateUtil.getSession();
		Transaction tx = session.beginTransaction();
		
		Employee employee = new Employee("employee name");
		Skiller skiller = new Skiller("skiller name","play game");
		Driver driver = new Driver("driver name",30);
		
		session.save(employee);
		session.save(skiller);
		session.save(driver);
		
		tx.commit();
		session.close();
		
	}

执行的sql语句：

Hibernate: insert into employee (name, type) values (?, '0')
Hibernate: insert into employee (name, skill, type) values (?, ?, '1')
Hibernate: insert into employee (name, driverYears, type) values (?, ?, '2')

查询的时候，不论你查询的是父类还是子类，都可以使用：

		Employee employee = (Employee)session.get(Employee.class, 3);
		System.out.println(employee.getName());
		
		Employee employee = (Employee)session.get(Driver.class, 3);
		System.out.println(employee.getName());

输出的语句

Hibernate: 
    select
        driver0_.id as id0_0_,
        driver0_.name as name0_0_,
        driver0_.driverYears as driverYe5_0_0_ 
    from
        employee driver0_ 
    where
        driver0_.id=? 
        and driver0_.type='2'

！！！！！！！！！！！体现出多态效果！！！！！！！！