JavaPersistenceWithHibernate第二版笔记-第六章-Mapping inheritance-002Table per concrete class with implicit polymorphism(@MappedSuperclass、@AttributeOverride)

一、结构

二、代码

1.

 1 package org.jpwh.model.inheritance.mappedsuperclass;
 2 
 3 import javax.persistence.MappedSuperclass;
 4 import javax.validation.constraints.NotNull;
 5 
 6 @MappedSuperclass
 7 public abstract class BillingDetails {
 8 
 9     @NotNull
10     protected String owner;
11 
12     // ...
13 
14     protected BillingDetails() {
15     }
16 
17     protected BillingDetails(String owner) {
18         this.owner = owner;
19     }
20 
21     public String getOwner() {
22         return owner;
23     }
24 
25     public void setOwner(String owner) {
26         this.owner = owner;
27     }
28 }

 

2.

 1 package org.jpwh.model.inheritance.mappedsuperclass;
 2 
 3 import org.jpwh.model.Constants;
 4 
 5 import javax.persistence.AttributeOverride;
 6 import javax.persistence.Column;
 7 import javax.persistence.Entity;
 8 import javax.persistence.GeneratedValue;
 9 import javax.persistence.Id;
10 import javax.validation.constraints.NotNull;
11 
12 @Entity
13 @AttributeOverride(
14         name = "owner",
15         column = @Column(name = "CC_OWNER", nullable = false))
16 public class CreditCard extends BillingDetails {
17 
18     @Id
19     @GeneratedValue(generator = Constants.ID_GENERATOR)
20     protected Long id;
21 
22     @NotNull
23     protected String cardNumber;
24 
25     @NotNull
26     protected String expMonth;
27 
28     @NotNull
29     protected String expYear;
30 
31     // ...
32 
33     public CreditCard() {
34         super();
35     }
36 
37     public CreditCard(String owner, String cardNumber, String expMonth, String expYear) {
38         super(owner);
39         this.cardNumber = cardNumber;
40         this.expMonth = expMonth;
41         this.expYear = expYear;
42     }
43 
44     public Long getId() {
45         return id;
46     }
47 
48     public String getCardNumber() {
49         return cardNumber;
50     }
51 
52     public void setCardNumber(String cardNumber) {
53         this.cardNumber = cardNumber;
54     }
55 
56     public String getExpMonth() {
57         return expMonth;
58     }
59 
60     public void setExpMonth(String expMonth) {
61         this.expMonth = expMonth;
62     }
63 
64     public String getExpYear() {
65         return expYear;
66     }
67 
68     public void setExpYear(String expYear) {
69         this.expYear = expYear;
70     }
71 
72 }

 

3.

 1 package org.jpwh.model.inheritance.mappedsuperclass;
 2 
 3 import org.jpwh.model.Constants;
 4 
 5 import javax.persistence.Entity;
 6 import javax.persistence.GeneratedValue;
 7 import javax.persistence.Id;
 8 import javax.validation.constraints.NotNull;
 9 
10 @Entity
11 public class BankAccount extends BillingDetails {
12 
13     @Id
14     @GeneratedValue(generator = Constants.ID_GENERATOR)
15     protected Long id;
16 
17     @NotNull
18     protected String account;
19 
20     @NotNull
21     protected String bankname;
22 
23     @NotNull
24     protected String swift;
25 
26     public BankAccount() {
27         super();
28     }
29 
30     public BankAccount(String owner, String account, String bankname, String swift) {
31         super(owner);
32         this.account = account;
33         this.bankname = bankname;
34         this.swift = swift;
35     }
36 
37     public Long getId() {
38         return id;
39     }
40 
41     public String getAccount() {
42         return account;
43     }
44 
45     public void setAccount(String account) {
46         this.account = account;
47     }
48 
49     public String getBankname() {
50         return bankname;
51     }
52 
53     public void setBankname(String bankname) {
54         this.bankname = bankname;
55     }
56 
57     public String getSwift() {
58         return swift;
59     }
60 
61     public void setSwift(String swift) {
62         this.swift = swift;
63     }
64 }

 

三、存在的问题

1.it doesn’t support polymorphic associations very well。You can’t have another entity mapped with a foreign key “referencing BILLINGDETAILS ”—there is no such table. This would be problematic in the domain model, because BillingDetails is associated with User ; both the CREDITCARD and BANKACCOUNT tables would need a foreign key reference to the USERS table. None of these issues can be easily resolved, so you should consider an alternative mapping strategy.

 

2.查父类时要查每个表。Hibernate must execute a query against the superclass as several SQL SELECT s, one for each concrete subclass. The JPA query select bd from BillingDetails bd requires two SQL statements:

 

1 select
2     ID, OWNER, ACCOUNT, BANKNAME, SWIFT
3 from
4     BANKACCOUNT
5 select
6     ID, CC_OWNER, CARDNUMBER, EXPMONTH, EXPYEAR
7 from
8     CREDITCARD

 

3.several different columns, of different tables, share exactly the same semantics.

结论:We recommend this approach (only) for the top level of your class hierarchy,where polymorphism isn’t usually required, and when modification of the superclass in the future is unlikely.

 

posted @ 2016-04-07 20:49  shamgod  阅读(224)  评论(0编辑  收藏  举报
haha