JPA entity继承策略

JPA支持3种类型的继承形式:
1.Single Table Strategy ,单表策略,一张表包含基类与子类的所有数据,很多情况下都是采用这样的冗余设计,通过一个discriminator来区分。

2.Table Per Class Strategy ,每个子类对应一张表,每张表都拥有基类的属性，基类不会生成表。

3.Join Strategy ,仍然是每个子类对应一张表，但此表中不包含基类的属性,仅仅是此子类的扩展属性,共享基类的属性。

下面是SingleTable的一个小例子：
基类:

package com.kevin.ejb.extendtable;

import javax.persistence.DiscriminatorColumn;
import javax.persistence.DiscriminatorType;
import javax.persistence.Entity;
import javax.persistence.GeneratedValue;
import javax.persistence.GenerationType;
import javax.persistence.Id;
import javax.persistence.Inheritance;
import javax.persistence.InheritanceType;
import javax.persistence.Table;

@Entity
@Table(name = "t_animal")
@Inheritance(strategy = InheritanceType.SINGLE_TABLE)
@DiscriminatorColumn(name = "animalType", discriminatorType = DiscriminatorType.STRING)
public class Animal {

	@Id
	@GeneratedValue(strategy = GenerationType.AUTO)
	private int id;
	private String name;
	private String sex;
	
	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 getSex() {
		return sex;
	}
	public void setSex(String sex) {
		this.sex = sex;
	}
	
	
}

子类：

package com.kevin.ejb.extendtable;

import javax.persistence.DiscriminatorColumn;
import javax.persistence.DiscriminatorType;
import javax.persistence.DiscriminatorValue;
import javax.persistence.Entity;
import javax.persistence.Inheritance;
import javax.persistence.InheritanceType;
import javax.persistence.Table;

@Entity
@Table(name = "t_cat")
@Inheritance(strategy = InheritanceType.SINGLE_TABLE)
@DiscriminatorColumn(discriminatorType = DiscriminatorType.STRING)
@DiscriminatorValue(value = "cat")
public class Cat extends Animal {

	private String nickname;

	public String getNickname() {
		return nickname;
	}

	public void setNickname(String nickname) {
		this.nickname = nickname;
	}
	
}

子类：

package com.kevin.ejb.extendtable;

import javax.persistence.DiscriminatorColumn;
import javax.persistence.DiscriminatorType;
import javax.persistence.DiscriminatorValue;
import javax.persistence.Entity;
import javax.persistence.Inheritance;
import javax.persistence.InheritanceType;
import javax.persistence.Table;

@Entity
@Table(name = "t_dog")
@Inheritance(strategy = InheritanceType.SINGLE_TABLE)
@DiscriminatorColumn(discriminatorType = DiscriminatorType.STRING)
@DiscriminatorValue(value = "dog")
public class Dog extends Animal {

	private String address;

	public String getAddress() {
		return address;
	}

	public void setAddress(String address) {
		this.address = address;
	}
	
	
}

生成的数据库表：