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