Hibernate注释中的继承？

Question

如何使用Java注释配置Hibernate继承映射？在注释中使用继承有什么好处？

Answer 1

这是一个非常普遍的问题，但我会在下面的资源劝考虑看看：

• The documentation on how inheritance is declared via Hibernate annotations.
• This PDF file（一本书上的Hibernate的第2章）。转发Hibernate注释的处理。

但是非常基本的回答你的问题是，你应该使用@Inheritance注释，像这样：

@Entity 
@Inheritance(strategy = InheritanceType.TABLE_PER_CLASS) 
public class Flight implements Serializable { 
    ... 
} 

Answer 2

我会建议使用在@MappedSuperclass的@Inheritance注解。过去我们遇到了问题，@MappedSuperclass对于我们持续的维护需求足够灵活。

Answer 3

3种可能：

1.每类层次结构战略单个表：

@Entity 
@Inheritance(strategy=InheritanceType.SINGLE_TABLE) 
@DiscriminatorColumn(  name="planetype", 
    discriminatorType=DiscriminatorType.STRING  ) 
@DiscriminatorValue("Plane") 
public class Plane { ... } 

@Entity 
@DiscriminatorValue("A320") 
public class A320 extends Plane { ... }  


<hibernate-mapping> 
<subclass name="DomesticCat" extends="Cat" discriminator-value="D"> 
     <property name="name" type="string"/> 
</subclass> 

• 优点：最简单的。不需要连接
• 缺点：不能使用空值。对象图深度增加列数。

2.连接子类策略：

数据库表

CREATE TABLE SUPER_TABLE(
id_col number primary key, 
sup_Name varchar2(20)); 

CREATE TABLE SUB_TABLE(
SUP_ID primary key, 
sub_name varchar2(20), 
constraint SUB_TABLE_fk foreign key (sup_Id) references super_table(id_col)); 


@Entity 
@Table(name= "SUPER_TABLE") 
@Inheritance(strategy= InheritanceType.JOINED) 
    public class TestSuperClass { 
@Id 
@GeneratedValue(
     strategy=GenerationType.SEQUENCE, 
     generator="SEQ_GEN") 
    @SequenceGenerator(
     name="SEQ_GEN", 
     sequenceName="hibernate_sequence" 
    ) 
    @Column(name ="id_col") 
private long idcol; 
@Column(name ="sup_name") 
private String supName; 


@Entity 
@Table(name="SUB_TABLE") 
@PrimaryKeyJoinColumn(name="SUP_ID") 

<class name="Payment" table="PAYMENT"> 
<id name="id" type="long" column="PAYMENT_ID"> 
    <generator class="native"/> 
</id> 
<property name="amount" column="AMOUNT"/> 
... 
<joined-subclass name="CreditCardPayment" table="CREDIT_PAYMENT"> 
    <key column="PAYMENT_ID"/> 
    <property name="creditCardType" column="CCTYPE"/> 
    ... 
</joined-subclass> 
<joined-subclass name="CashPayment" table="CASH_PAYMENT"> 
    <key column="PAYMENT_ID"/> 
    ... 
</joined-subclass> 

public class TestSubClass extends TestSuperClass{ 
    private String sub_name; 
} 

测试模块

TestSubClass sub = new TestSubClass("sub1"); 
sub.setSupName("supersuper"); session1.save(sub); 

SQL生成

Hibernate: insert into SUPER_TABLE (sup_name, id_col) values (?, ?)  
Hibernate: insert into SUB_TABLE (sub_name, SUP_ID) values (?, ?) 

• 优点：规范化的数据结构。
• 缺点：连接总是需要。

3.每个具体类策略表：

create table CREDIT_CARD(payment_id number primary key, amount 
number, creditCardType varchar2(2)); 


    @Entity 
    @Inheritance(strategy=InheritanceType.TABLE_PER_CLASS) 
    public abstract class Payment { 
@Id 
@GeneratedValue(
     strategy=GenerationType.SEQUENCE, 
     generator="SEQ_GEN") 
    @SequenceGenerator(
     name="SEQ_GEN", 
     sequenceName="hibernate_sequence" 
    ) 
@Column(name = "payment_id") 
private long id; 

private double amount; 



@Entity 
@Table(name="CREDIT_CARD") 
public class CreditCardPayment extends Payment { 
private String creditCardType; 


<class name="Payment"> 
<id name="id" type="long" column="PAYMENT_ID"> 
    <generator class="sequence"/> 
</id> 
<property name="amount" column="AMOUNT"/> 
... 
<union-subclass name="CreditCardPayment" table="CREDIT_PAYMENT"> 
    <property name="creditCardType" column="CCTYPE"/> 
    ... 
</union-subclass> 

测试模块

CreditCardPayment credit = new CreditCardPayment("C",1.0); 
session1.save(credit); 

SQL生成

Hibernate: insert into CREDIT_CARD (amount, creditCardType, payment_id) values (?, ?, ?) 

还有，我们已经使用了@MappedSuperclass在o你的申请。