JPA 一对一关联

在一对一关联中，JPA没有规定谁为关系的维护方，OneToOne的mapped属性值指定了由另外一方实体的某个属性来维护一对一关联。以Person和IDCard为例。

Person类：

@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
}

@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. 测试保存

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();
}

当IDCard的person属性的optional为true时，上面的idcard.setPerson(person)可以省略，因为Person既设置了级联属性cascade={CascadeType.ALL}，在保存Person实体的时候也会保存IDcard，又是关系的维护方，在它们之间建立关联。如果为false，在先保存idcard时候，由于not null约束，程序报错。


2. 测试更新：

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，因为idcard已经是托管态了，对它的修改在commit的时候都会提交到数据库。如果idcard是游离态则需要该语句。


3. 测试查询：

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());
}

因为IDCard实体的person属性设置了fetch=FetchType.EAGER，jpa会使用一个SELECT连表查询出Person和IDCard。
如果fetch设置为FetchType.LAZY，jpa会使用两个SELECT语句(第一个查询出IDCard，第二个连表查询），不会产生EntityManager关闭导致的问题。


4. 测试删除：

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();
}

当IDCard的person属性的optional为true时，能删除成功；为false时删除会报错：org.hibernate.PropertyValueException: not-null property references a null or transient value: com.john.bean.Person.idcard