Hibernate的单向1-N关联(三)

1. 无连接表的单向1-N关联

对于无连接表的1-N单向关联而言，同样需要在N的一端增加外键列来维护关联关系，但由于程序此时只让1的一端来控制关联关系，因此直接在1的一端使用@JoinColumn修饰Set集合属性、映射外键即可。下面程序中一个Person实体可以关联多个Address实体。

@Entity@Table(name="person_inf")public class Person{// 标识属性@Id @Column(name="person_id")@GeneratedValue(strategy=GenerationType.IDENTITY)private Integer id;private String name;private int age;// 定义该Person实体所有关联的Address实体，没有指定cascade属性@OneToMany(targetEntity=Address.class)// 映射外键列，此处映射的外键列将会添加到关联实体对应的数据表中@JoinColumn(name="person_id" , referencedColumnName="person_id")private Set<Address> addresses= new HashSet<>();// id的setter和getter方法public void setId(Integer id){this.id = id;}public Integer getId(){return this.id;}// name的setter和getter方法public void setName(String name){this.name = name;}public String getName(){return this.name;}// age的setter和getter方法public void setAge(int age){this.age = age;}public int getAge(){return this.age;}// addresses的setter和getter方法public void setAddresses(Set<Address> addresses){this.addresses = addresses;}public Set<Address> getAddresses(){return this.addresses;}}

下面是Address实体的代码。

@Entity@Table(name="address_inf")public class Address{// 标识属性@Id @Column(name="address_id")@GeneratedValue(strategy=GenerationType.IDENTITY)private int addressId;// 定义地址详细信息的成员变量private String addressDetail;// 无参数的构造器public Address(){}// 初始化全部成员变量的构造器public Address(String addressDetail){this.addressDetail = addressDetail;}// addressId的setter和getter方法public void setAddressId(int addressId){this.addressId = addressId;}public int getAddressId(){return this.addressId;}// addressDetail的setter和getter方法public void setAddressDetail(String addressDetail){this.addressDetail = addressDetail;}public String getAddressDetail(){return this.addressDetail;}}

用下面的程序来保存Person和Address的对象。

public class PersonManager{public static void main(String[] args){PersonManager mgr = new PersonManager();mgr.testPerson();HibernateUtil.sessionFactory.close();}private void testPerson(){Session session = HibernateUtil.currentSession();Transaction tx = session.beginTransaction();// 创建一个Person对象Person p = new Person();// 创建一个瞬态的Address对象Address a = new Address("广州天河");// 必须先持久化Address对象session.persist(a);                  // ①// 设置Person的name为owen.org字符串p.setName("owen.org");p.setAge(21);// 设置Person和Address之间的关联关系p.getAddresses().add(a);// 持久化Person对象session.save(p);// 创建一个瞬态的Address对象Address a2 = new Address("上海虹口");// 必须先持久化Address对象session.persist(a2);                // ②// 修改持久化状态的Person对象p.getAddresses().add(a2);tx.commit();HibernateUtil.closeSession();}}

上面的代码都已经给出了注释，读者可以自己出解读。而执行的SQL语言如下：

Hibernate:     insert     into        address_inf        (addressDetail)     values        (?)Hibernate:     insert     into        person_inf        (age, name)     values        (?, ?)Hibernate:     insert     into        address_inf        (addressDetail)     values        (?)Hibernate:     update        address_inf     set        person_id=?     where        address_id=?Hibernate:     update        address_inf     set        person_id=?     where        address_id=?

执行的结果图如下。

2. 有连接表的单向1-N关联

对于有连接表的1-N关联，同样需要使用@OneToMany修饰代表表关联实体的集合。除此之外，程序还应该使用@JoinTable显式指定连接表。下面Person类使用了@OneToMany、@JoinTable修饰代表关联实体的集合属性。

 @Entity@Table(name="person_inf")public class Person{// 标识属性@Id @Column(name="person_id")@GeneratedValue(strategy=GenerationType.IDENTITY)private Integer id;private String name;private int age;// 定义该Person实体所有关联的Address实体@OneToMany(targetEntity=Address.class)// 映射连接表为person_address@JoinTable(name="person_address",// 定义连接表中名为person_id的外键列，该外键列参照当前实体对应表的主键列joinColumns=@JoinColumn(name="person_id", referencedColumnName="person_id"),// 定义连接表中名为address_id的外键列，// 该外键列参照当前实体的关联实体对应表的主键列inverseJoinColumns=@JoinColumn(name="address_id", referencedColumnName="address_id", unique=true))private Set<Address> addresses= new HashSet<>();// id的setter和getter方法public void setId(Integer id){this.id = id;}public Integer getId(){return this.id;}// name的setter和getter方法public void setName(String name){this.name = name;}public String getName(){return this.name;}// age的setter和getter方法public void setAge(int age){this.age = age;}public int getAge(){return this.age;}// addresses的setter和getter方法public void setAddresses(Set<Address> addresses){this.addresses = addresses;}public Set<Address> getAddresses(){return this.addresses;}}

用到的Address和保存对象用的执行类如下。

public class PersonManager{public static void main(String[] args){PersonManager mgr = new PersonManager();mgr.testPerson();HibernateUtil.sessionFactory.close();}private void testPerson(){Session session = HibernateUtil.currentSession();Transaction tx = session.beginTransaction();// 创建一个瞬态的Person对象Person p = new Person();// 设置Person的name为owen.org字符串p.setName("owen.org");p.setAge(21);// 创建一个瞬态的Address对象Address a = new Address("广州天河");// 设置Person和Address之间的关联关系p.getAddresses().add(a);// 创建一个瞬态的Address对象Address a2 = new Address("上海虹口");// 设置Person和Address之间的关联关系p.getAddresses().add(a2);// 由于采用了连接表来维护1-N关联关系，因此不存在主从表关系，// 程序可以随意控制先持久化哪个实体。// 持久化Address对象session.persist(a2);// 持久化Person对象session.save(p);// 持久化Address对象session.persist(a);tx.commit();HibernateUtil.closeSession();}}

后台生成的SQL语言如下：

Hibernate:     insert     into        address_inf        (addressDetail)     values        (?)Hibernate:     insert     into        person_inf        (age, name)     values        (?, ?)Hibernate:     insert     into        address_inf        (addressDetail)     values        (?)Hibernate:     insert     into        person_address        (person_id, address_id)     values        (?, ?)Hibernate:     insert     into        person_address        (person_id, address_id)     values        (?, ?)

执行的结果如下。