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 @GeneratedValuepublic 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 keypublic IDCard getIdcard() {return idcard;}// Setters are omitted}

@Entitypublic class IDCard {private Integer id;private String cardno;private Person person;public IDCard() {}public IDCard(String cardno) {this.cardno = cardno;}@Id @GeneratedValuepublic 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 edgepublic 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