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在一對(duì)一關(guān)聯(lián)中，JPA沒有規(guī)定誰為關(guān)系的維護(hù)方，OneToOne的mapped屬性值指定了由另外一方實(shí)體的某個(gè)屬性來維護(hù)一對(duì)一關(guān)聯(lián)。以Person和IDCard為例。

Person類：

Java代碼

@Entity
@Table(name="pillow")
public class Person {
private Integer id;
private String name;
private IDCard idcard;
public Person() {
}
public Person(String name) {
this.name = name;
}
@Id @GeneratedValue
public Integer getId() {
return id;
}
@Column(length=20, nullable=false)
public String getName() {
return name;
}
@OneToOne(cascade={CascadeType.ALL},optional=false) // optional specified to false means the corresponding column couldn't be nullable
@JoinColumn(name="idcard_id") // Indicate the column name of the foreign key
public IDCard getIdcard() {
return idcard;
}
// Setters are omitted
}

Java代碼

@Entity
public class IDCard {
private Integer id;
private String cardno;
private Person person;
public IDCard() {
}
public IDCard(String cardno) {
this.cardno = cardno;
}
@Id @GeneratedValue
public Integer getId() {
return id;
}
@Column(length=18, nullable=false)
public String getCardno() {
return cardno;
}
// mappedBy specifies the property of the corresponding class that maintains the relationship. Also indicates this entity is the maintained edge.
@OneToOne(mappedBy="idcard", cascade={CascadeType.REMOVE, CascadeType.MERGE, CascadeType.REFRESH},
optional=false, fetch=FetchType.EAGER) // Maintained edge
public Person getPerson() {
return person;
}
// Setters are omitted
}

1. 測(cè)試保存

Java代碼

public void save() {
EntityManagerFactory factory = Persistence.createEntityManagerFactory("refrigerator");
EntityManager em = factory.createEntityManager();
Person person = new Person("Johnson");
IDCard idcard = new IDCard("330818198602260444");
// idcard.setPerson(person);
person.setIdcard(idcard);
em.getTransaction().begin(); // It's mandatory to open transaction to insert, update or delete data.
em.persist(person);
em.getTransaction().commit();
em.close();
factory.close();
}

當(dāng)IDCard的person屬性的optional為true時(shí)，上面的idcard.setPerson(person)可以省略，因?yàn)镻erson既設(shè)置了級(jí)聯(lián)屬性cascade={CascadeType.ALL}，在保存Person實(shí)體的時(shí)候也會(huì)保存IDcard，又是關(guān)系的維護(hù)方，在它們之間建立關(guān)聯(lián)。如果為false，在先保存idcard時(shí)候，由于not null約束，程序報(bào)錯(cuò)。

2. 測(cè)試更新：

Java代碼

EntityManagerFactory factory = Persistence.createEntityManagerFactory("refrigerator");
EntityManager em = factory.createEntityManager();
em.getTransaction().begin();
IDCard idcard = em.find(IDCard.class, 1);
idcard.setCardno("330624198802060168");
em.merge(idcard);
em.getTransaction().commit();
em.close();
factory.close();

em.merge(idcard)可以省略，因?yàn)閕dcard已經(jīng)是托管態(tài)了，對(duì)它的修改在commit的時(shí)候都會(huì)提交到數(shù)據(jù)庫。如果idcard是游離態(tài)則需要該語句。

3. 測(cè)試查詢：

Java代碼

public void queryFromMaintained() {
EntityManagerFactory factory = Persistence.createEntityManagerFactory("refrigerator");
EntityManager em = factory.createEntityManager();
IDCard idcard = em.find(IDCard.class, 1);
em.close();
factory.close();
System.out.println(idcard.getCardno());
System.out.println(idcard.getPerson().getName());
}

因?yàn)镮DCard實(shí)體的person屬性設(shè)置了fetch=FetchType.EAGER，jpa會(huì)使用一個(gè)SELECT連表查詢出Person和IDCard。
如果fetch設(shè)置為FetchType.LAZY，jpa會(huì)使用兩個(gè)SELECT語句(第一個(gè)查詢出IDCard，第二個(gè)連表查詢），不會(huì)產(chǎn)生EntityManager關(guān)閉導(dǎo)致的問題。

4. 測(cè)試刪除：

Java代碼

public void deleteFromMaintained() {
EntityManagerFactory factory = Persistence.createEntityManagerFactory("refrigerator");
EntityManager em = factory.createEntityManager();
em.getTransaction().begin();
IDCard idcard = em.find(IDCard.class, 1);
em.remove(idcard);
em.getTransaction().commit();
em.close();
factory.close();
}

當(dāng)IDCard的person屬性的optional為true時(shí)，能刪除成功；為false時(shí)刪除會(huì)報(bào)錯(cuò)：org.hibernate.PropertyValueException: not-null property references a null or transient value: com.john.bean.Person.idcard

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