在JPA中,实体继承关系的映射策略共有三种:单表继承策略(SINGLE_TABLE)、Joined策略和Table_PER_Class策略。
1、单表继承策略
单表继承策略,父类实体和子类实体共用一张数据库表,在表中通过一列辨别字段来区别不同类别的实体。具体做法如下:
a.在父类实体的@Entity注解下添加如下的注解:
@Inheritance(Strategy=InheritanceType.SINGLE_TABLE)
@DiscriminatorColumn(name=”辨别字段列名”)
@DiscriminatorValue(父类实体辨别字段列值)
b.在子类实体的@Entity注解下添加如下的注解:
@DiscriminatorValue(子类实体辨别字段列值)
定义了一个父类
1 @Entity 2 @Inheritance(strategy = InheritanceType.SINGLE_TABLE) 3 @Table(name = "WINDOW_FILE") 4 @DiscriminatorColumn(name = "DISCRIMINATOR", discriminatorType = DiscriminatorType.STRING, length = 30) 5 @DiscriminatorValue("WindowFile") 6 public class WindowFile { 7 8 @Id 9 @GeneratedValue(strategy = GenerationType.AUTO) 10 private Integer id; 11 12 @Basic 13 @Column(name = "NAME") 14 private String name; 15 16 @Basic 17 @Column(name = "TYPE") 18 private String type; 19 20 @Basic 21 @Column(name = "DATE") 22 private Date date; 23 //省略get set 24 }
后定义2个子类
1 @Entity 2 @DiscriminatorValue("Folder") 3 public class Folder extends WindowFile { 4 5 @Basic 6 @Column(name = "FILE_COUNT") 7 private Integer fileCount; 8 //省略get set 9 }
1 @Entity 2 @DiscriminatorValue("Document") 3 public class Document extends WindowFile { 4 5 @Basic 6 @Column(name = "SIZE") 7 private String size; 8 //省略get set 9 }
以上通过列DISCRIMINATOR的不同,区分具体父子实体。
实际表结构如下:
WINDOW_FILE DISCRIMINATOR,ID,NAME,DATE,TYPE,SIZE,FILE_COUNT
当你使用WindowFile实体时,实际表的字段为DISCRIMINATOR='WindowFile',SIZE与FILE_COUNT永远是空
当使用Folder实体时,DISCRIMINATOR='Folder',SIZE永远是空,FILE_COUNT为实际值。
Document同理,与Folder类似。
如果使用xml配置文件则配置如下:
<hibernate-mapping> <class name="com.baobaotao.domain.WindowFile" table="WINDOW_FILE" catalog="sampledb" discriminator-value="WindowFile"> <id name="id" type="java.lang.Integer"> <column name="id" /> <generator class="identity" /> </id> <!-- 加入鉴别标签,且必须放在id后面 --> <discriminator column="DISCRIMINATOR" type="string"> </discriminator> <property name="name" type="java.lang.String"> <column name="NAME" length="50" not-null="true" /> </property> <property name="date" type="java.util.Date"> <column name="DATE" not-null="true"> </column> </property> <subclass name="com.baobaotao.domain.MainPost" discriminator-value="Folder"> <property name="fileCount"> <column name="FILE_COUNT"></column> </property> </subclass> <subclass name="com.baobaotao.domain.MainPost" discriminator-value="Document"> <property name="size"> <column name="SIZE"></column> </property> </subclass> </class> </hibernate-mapping>
2、Joined策略
父类实体和子类实体分别对应数据库中不同的表,子类实体的表中只存在其扩展的特殊属性,父类的公共属性保存在父类实体映射表中。具体做法:
@Inheritance(Strategy=InheritanceType.JOINED)
子类实体不需要特殊说明。
父类代码:
1 @Entity 2 @Table(name = "T_ANIMAL") 3 @Inheritance(strategy = InheritanceType.JOINED) 4 public class Animal { 5 6 @Id 7 @Column(name = "ID") 8 @GeneratedValue(strategy = GenerationType.AUTO) 9 private Integer id; 10 11 @Column(name = "NAME") 12 private String name; 13 14 @Column(name = "COLOR") 15 private String color; 16 //省略get set 17 }
子类代码:
1 @Entity 2 @Table(name = "T_BIRD") 3 @PrimaryKeyJoinColumn(name = "BIRD_ID") 4 public class Bird extends Animal { 5 6 @Column(name = "SPEED") 7 private String speed; 8 //省略get set 9 }
1 @Entity 2 @Table(name = "T_DOG") 3 @PrimaryKeyJoinColumn(name = "DOG_ID") 4 public class Dog extends Animal { 5 6 @Column(name = "LEGS") 7 private Integer legs; 8 //省略get set 9 }
实际表结构如下:
T_ANIMAL ID,COLOR,NAME
T_BIRD SPEED,BIRD(既是外键,也是主键)
T_DOG LEGS,DOG_ID(既是外键,也是主键)
3、Table_PER_Class策略:
Table_PER_Class策略,父类实体和子类实体每个类分别对应一张数据库中的表,子类表中保存所有属性,包括从父类实体中继承的属性。具体做法:
只需在父类实体的@Entity注解下添加如下注解:
@Inheritance(Strategy=InheritanceType.TABLE_PER_CLASS)
父类代码:
1 @Entity 2 @Inheritance(strategy = InheritanceType.TABLE_PER_CLASS) 3 @Table(name = "T_VEHICLE") 4 public class Vehicle { // 基类 5 6 @Id 7 // @GeneratedValue 8 @Column(name = "ID") 9 private Integer id; 10 11 @Column(name = "SPEED") 12 private Integer speed;// 速度 13 //省略get set 14 }
子类代码:
1 @Entity 2 @Table(name = "T_CAR") 3 public class Car extends Vehicle { 4 5 @Column(name = "ENGINE") 6 private String engine;// 发动机 7 //省略get set 8 }
一旦使用这种策略就意味着你不能使用AUTO generator 和IDENTITY generator,即主键值不能采用数据库自动生成。
实际表结构如下:
T_VEHICLE ID,SPEED
T_CAR ID,SPEED,ENGINE