Hibernate的联系关系映射单向无连接表的N-1连接
Hibernate的关联映射
客观世界中的对象很少有孤立存在的，例如人与所在的城市往往存在关联关系，如果已经得到某个人的实例，那么应该可以直接获取该人所在的城市。反之，如果已经得到一个城市的实例，也应该可以访问该城市对应的人---这种实例之间的互相访问就是关联关系。
关联关系是面向对象分析、面向对象设计最重要的知识，Hibernate完全可以理解这种关联关系，如果映射得当，Hibernate的关联映射可以大大简化持久层数据的访问。
关联关系大致分如下两类：
单向关联：只需单向访问关联端。
1 -> 1
1 -> N
N -> 1
N -> N
双向关联：关联的两端可以互相访问。
1 <-> 1
1 <-> N
N <-> N
-
单向N-1关联
为了让两个持久化类支持这种关联映射，程序应该在N的一端持久化类中增加一个属性，该属性引用1的一端的关联实体。
1.1无连接表的N-1关联
publicclass Person {
//fields
private Integer id;
private String name;
private String sex;
//引用关联实体的属性
private City city;
//Constructors
/** default Constructor */
public Person(){
}
/** full Constructor */
public Person(Integer id, String name, String sex) {
this.id = id;
this.name = name;
this.sex = sex;
}
//Property accessors
public Integer getId() {
returnid;
}
publicvoid setId(Integer id) {
this.id = id;
}
public String getName() {
returnname;
}
publicvoid setName(String name) {
this.name = name;
}
public String getSex() {
returnsex;
}
publicvoid setSex(String sex) {
this.sex = sex;
}
public City getCity() {
returncity;
}
publicvoid setCity(City city) {
this.city = city;
}
}
publicclass City {
private Integer cid;
private String cname;
private String cdesc;
public City() {
}
public City(Integer cid, String cname, String cdesc) {
this.cid = cid;
this.cname = cname;
this.cdesc = cdesc;
}
public Integer getCid() {
returncid;
}
publicvoid setCid(Integer cid) {
this.cid = cid;
}
public String getCname() {
returncname;
}
publicvoid setCname(String cname) {
this.cname = cname;
}
public String getCdesc() {
returncdesc;
}
publicvoid setCdesc(String cdesc) {
this.cdesc = cdesc;
}
}
//Person.hbm.xml
<?xml version="1.0" encoding="utf-8"?>
<!DOCTYPE hibernate-mapping PUBLIC "-//Hibernate/Hibernate Mapping DTD 3.0//EN"
"http://hibernate.sourceforge.net/hibernate-mapping-3.0.dtd">
<hibernate-mapping package="cn.jy.hibernate.domain">
<class name="Person" table="persons" catalog="hibernate">
<id name="id" >
<generator class="increment" />
</id>
<property name="name"/>
<property name="sex"/>
<many-to-one name="city" cascade="all"
class="City" column="cid"/>
</class>
</hibernate-mapping>
//City.hbm.xml
<?xml version="1.0" encoding="utf-8"?>
<!DOCTYPE hibernate-mapping PUBLIC "-//Hibernate/Hibernate Mapping DTD 3.0//EN"
"http://hibernate.sourceforge.net/hibernate-mapping-3.0.dtd">
<hibernate-mapping package="cn.jy.hibernate.domain">
<class name="City" table="cities" catalog="hibernate">
<id name="cid" >
<generator class="increment" />
</id>
<property name="cname"/>
<property name="cdesc"/>
</class>
</hibernate-mapping>
publicclass TestPerson {
privatestatic SessionFactory sessionFactory;
static{
Configuration configuration = new Configuration();
configuration.configure("/hibernate.cfg.xml");
sessionFactory = configuration.buildSessionFactory();
}
@Test
publicvoid testPerson() {
Session session = sessionFactory.openSession();
Transaction transaction = session.beginTransaction();
//创建一个临时状态的City对象
City city = new City();
city.setCname("武汉");
city.setCdesc("九省通衢");
//创建一个Person对象
Person person = new Person();
person.setName("江月");
person.setSex("男");
//设置Person对象和City对象之间的关联关系
person.setCity(city);
//持久化Person对象
session.persist(person);
//创建一个临时状态的City对象
City city1 = new City();
city1.setCname("咸宁");
city1.setCdesc("温泉之乡");
//修改持久化状态的Perosn对象
person.setCity(city1);
transaction.commit();
session.close();
}
}
<?xml version='1.0' encoding='utf-8'?>
<!DOCTYPE hibernate-configuration PUBLIC
"-//Hibernate/Hibernate Configuration DTD 3.0//EN" "http://hibernate.sourceforge.net/hibernate-configuration-3.0.dtd">
<hibernate-configuration>
<session-factory>
<property name="connection.driver_class">
com.mysql.jdbc.Driver
</property>
<property name="connection.url">
jdbc:mysql://localhost:3306/hibernate
</property>
<property name="connection.username">root</property>
<property name="connection.password">709709</property>
<property name="hbm2ddl.auto">update</property>
<property name="show_sql">true</property>
<mapping resource="cn/jy/hibernate/domain/Person.hbm.xml" />
<mapping resource="cn/jy/hibernate/domain/City.hbm.xml" />
</session-factory>
</hibernate-configuration>