JPA基础（十一）：JPA中的一对一双向关联

Person.java:

 1 @Entity
 2 public class Person {
 3     @Id
 4     @GeneratedValue
 5     private Integer id;
 6     @Column(length=10,nullable=false)
 7     private String name;
 8     @OneToOne(cascade=CascadeType.ALL,optional=false)
 9     @JoinColumn(name="idcard_id")
10     private IDCard idcard;
11     public Person(){};
12     public Person(String name) {
13         super();
14         this.name = name;
15     }
16     public IDCard getIdcard() {
17         return idcard;
18     }
19     public void setIdcard(IDCard idcard) {
20         this.idcard = idcard;
21     }
22     public Integer getId() {
23         return id;
24     }
25     public void setId(Integer id) {
26         this.id = id;
27     }
28     public String getName() {
29         return name;
30     }
31     public void setName(String name) {
32         this.name = name;
33     }
34     
35 }

IDCard.java:

 1 @Entity
 2 public class IDCard {
 3     @Id
 4     @GeneratedValue
 5     private Integer id;
 6     @Column(length=18,nullable=false)
 7     private String cardno;
 8     @OneToOne(mappedBy="idcard",cascade={CascadeType.PERSIST,CascadeType.MERGE,CascadeType.REFRESH})
 9     private Person person;
10     public IDCard(){};
11     public IDCard(String cardno) {
12         super();
13         this.cardno = cardno;
14     }
15     public Person getPerson() {
16         return person;
17     }
18     public void setPerson(Person person) {
19         this.person = person;
20     }
21     public Integer getId() {
22         return id;
23     }
24     public void setId(Integer id) {
25         this.id = id;
26     }
27     public String getCardno() {
28         return cardno;
29     }
30     public void setCardno(String cardno) {
31         this.cardno = cardno;
32     }
33     
34 }

级联的定义，一定是根据你们的业务需求来定的。用不用是根据你的业务来决定的，业务需要就用，业务不需要就不用。

测试：

 1     @Test
 2     public void save(){
 3         EntityManagerFactory factory=Persistence.createEntityManagerFactory("sample");
 4         EntityManager em=factory.createEntityManager();
 5         em.getTransaction().begin();
 6         Person person=new Person("jim");
 7         person.setIdcard(new IDCard("123456789"));
 8         em.persist(person);
 9         em.getTransaction().commit();
10         em.close();
11         factory.close();
12     }

谁是关系维护端，谁就负责外键字段的更新。Person是关系维护端，IDCard是关系被维护端，怎么维护更新呢？往Person里面设置idCard,这样就相当于把关系建立起来了；如果通过IDCard设置person的话，那么这种关系是建立不起来的，因为IDCard是关系被维
护端。