Hibernate高级实体关联映射之单值的实体关联(共享的主键关联)

如果User类映射为实体，而Address类映射为值类型的组件，值类型实例的生命期限由所拥有的实体实例的生命期限决定。值类型不支持共享引用，一个值类型的实例只与一个实体关联。

如果把User和Address都映射为实体时该怎么办？这个模型的好处之一在于共享引用的可能性---另一个实体类(Shipment)也可以有一个对特定Address实例的引用。如果User有对该实例的引用，那么Address实例就必须支持共享引用，并且需要有自己的同一性。在这种情况下，User和Address类就都有一个真正的一对一关联了。

共享的主键关联

由主键关联而相关的两张表中的行共享相同的主键值。这种方法的主要困难在于，确保被关联的实例在保存对象时分配到了相同的主键值。

pom.xml:

]<project xmlns="http://maven.apache.org/POM/4.0.0" xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance"
xsi:schemaLocation="http://maven.apache.org/POM/4.0.0 http://maven.apache.org/xsd/maven-4.0.0.xsd"> <modelVersion>4.0.0</modelVersion>
<groupId>hibernateTest</groupId>
<artifactId>hibernateTest</artifactId>
<version>1.0-SNAPSHOT</version>
<packaging>jar</packaging>
<name>hibernateTest</name>
<url>http://maven.apache.org</url>
<dependencies>
<dependency>
<groupId>junit</groupId>
<artifactId>junit</artifactId>
<version>3.8.1</version>
<scope>test</scope>
</dependency>
<dependency>
<groupId>org.hibernate</groupId>
<artifactId>hibernate-core</artifactId>
<version>3.3.1.GA</version>
</dependency>
<dependency>
<groupId>org.slf4j</groupId>
<artifactId>slf4j-nop</artifactId>
<version>1.5.2</version>
</dependency>
<dependency>
<groupId>javassist</groupId>
<artifactId>javassist</artifactId>
<version>3.4.GA</version>
</dependency>
<dependency>
<groupId>c3p0</groupId>
<artifactId>c3p0</artifactId>
<version>0.9.1</version>
</dependency>
<dependency>
<groupId>com.oracle</groupId>
<artifactId>ojdbc14</artifactId>
<version>10.2.0.3.0</version>
<scope>runtime</scope>
</dependency>
</dependencies>
<build>
<finalName>hibernateTest</finalName>
<resources>
<resource>
<directory>src/main/resources</directory>
</resource>
<resource>
<directory>src/main/java</directory>
<excludes>
<exclude>**/*.java</exclude>
</excludes>
</resource>
</resources>
<plugins>
<plugin>
<artifactId>maven-compiler-plugin</artifactId>
<configuration>
<source>1.6</source>
<target>1.6</target>
<encoding>UTF-8</encoding>
</configuration>
</plugin>
</plugins>
</build>
</project>

resources/hibernate.cfg.xml:

]<?xml version="1.0" encoding="utf-8"?>
<!DOCTYPE hibernate-configuration
PUBLIC "-//Hibernate/Hibernate Configuration DTD//EN"
"http://hibernate.sourceforge.net/hibernate-configuration-3.0.dtd">
<hibernate-configuration>
<session-factory name="sessionFactory">
<!-- 指定连接数据库所用的驱动 -->
<property name="hibernate.connection.driver_class">oracle.jdbc.driver.OracleDriver</property>
<!-- 指定连接数据库的url，hibernate连接的数据库名 -->
<property name="hibernate.connection.url">jdbc:oracle:thin:@localhost:1521:XE</property>
<property name="hibernate.connection.useUnicode">true</property>
<property name="hibernate.connection.characterEncoding">gbk</property>
<!-- 指定连接数据库的用户名 -->
<property name="hibernate.connection.username">system</property>
<!-- 指定连接数据库的密码 -->
<property name="hibernate.connection.password">password</property>
<!-- 指定连接池里最大连接数 -->
<property name="hibernate.c3p0.max_size">20</property>
<!-- 指定连接池里最小连接数 -->
<property name="hibernate.c3p0.min_size">5</property>
<!-- 指定连接池里连接的超时时长,以秒为单位 -->
<property name="hibernate.c3p0.timeout">120</property>
<!-- 指定连接池里最大缓存多少个Statement对象 -->
<property name="hibernate.c3p0.max_statements">100</property>
<!-- 每隔XX秒检查连接池里的空闲连接 ，单位是秒 -->
<property name="hibernate.c3p0.idle_test_period">120</property>
<!-- 当连接池里面的连接用完的时候，C3P0一次获取的新的连接数 -->
<property name="hibernate.c3p0.acquire_increment">2</property>
<!-- 指定数据库方言 -->
<property name="dialect">org.hibernate.dialect.OracleDialect</property>
<!-- 显示Hibernate持久化操作所生成的SQL -->
<property name="show_sql">true</property>
<!-- 将SQL脚本进行格式化后再输出 -->
<property name="hibernate.format_sql">true</property>
<!-- 罗列所有的映射文件 -->
<mapping resource="pojo/User.hbm.xml" />
<mapping resource="pojo/Address.hbm.xml" />
</session-factory>
</hibernate-configuration>

pojo/User.java:

]package pojo;
public class User {
private String id;
private String userName;
private String password;
private String email;
private Address address;

public String getId() {
return id;
}
@SuppressWarnings("unused")
private void setId(String id) {
this.id = id;
}
public String getUserName() {
return userName;
}
public void setUserName(String userName) {
this.userName = userName;
}
public String getPassword() {
return password;
}
public void setPassword(String password) {
this.password = password;
}
public String getEmail() {
return email;
}
public void setEmail(String email) {
this.email = email;
}
public Address getAddress() {
return address;
}
//双向关联
public void setAddress(Address address) {
this.address = address;
address.setUser(this);
}
}

pojo/User.hbm.xml:

]<?xml version="1.0" encoding="UTF-8"?>
<!DOCTYPE hibernate-mapping PUBLIC "-//Hibernate/Hibernate Mapping DTD//EN"
"http://hibernate.sourceforge.net/hibernate-mapping-3.0.dtd">
<hibernate-mapping package="pojo">
<class name="User" table="MYUSER" dynamic-insert="true" dynamic-update="true">
<id name="id" column="ID" type="string">
<generator class="uuid.hex" />
</id>
<property name="userName" column="USERNAME" type="string"/>
<property name="password" column="PASSWORD" type="string"/>
<property name="email" column="EMAIL" type="string"/>
<!-- 注意这里一对一关联 -->
<one-to-one name="address" class="Address" cascade="save-update"/>
</class>
</hibernate-mapping>

pojo/Address.java:

]package pojo;
public class Address {
private String id;
private String street;
private String zipCode;
private String city;
private User user;

public String getId() {
return id;
}
@SuppressWarnings("unused")
private void setId(String id) {
this.id = id;
}
public String getStreet() {
return street;
}
public void setStreet(String street) {
this.street = street;
}
public String getZipCode() {
return zipCode;
}
public void setZipCode(String zipCode) {
this.zipCode = zipCode;
}
public String getCity() {
return city;
}
public void setCity(String city) {
this.city = city;
}
public User getUser() {
return user;
}
public void setUser(User user) {
this.user = user;
}
}

pojo/Address.hbm.xml:

]<?xml version="1.0" encoding="UTF-8"?>
<!DOCTYPE hibernate-mapping PUBLIC "-//Hibernate/Hibernate Mapping DTD//EN"
"http://hibernate.sourceforge.net/hibernate-mapping-3.0.dtd">
<hibernate-mapping package="pojo">
<class name="Address" table="ADDRESS" dynamic-insert="true" dynamic-update="true">
<id name="id" column="ID" type="string">
<!-- Address对象使用了特殊的foreign标识符生成器 -->
<generator class="foreign">
<!-- 表示从属性user中提取主键值 -->
<param name="property">user</param>
</generator>
</id>
<property name="street" column="STREET" type="string"/>
<property name="zipCode" column="ZIPCODE" type="string"/>
<property name="city" column="CITY" type="string"/>
<!-- 通过constrained="true",添加了把ADDRESS表的主键链接到MYUSER表主键的外键约束 -->
<one-to-one name="user" class="User" constrained="true"/>
</class>
</hibernate-mapping>

util/HibernateUtil.java:

]package util;
import org.hibernate.SessionFactory;
import org.hibernate.cfg.Configuration;
public class HibernateUtil {
private static SessionFactory sessionFactory;
static{
try {
sessionFactory = new Configuration().configure().buildSessionFactory();
} catch (Throwable e) {
throw new ExceptionInInitializerError(e);
}
}
public static SessionFactory getSessionFactory(){
return sessionFactory;
}
public static void shutdown(){
getSessionFactory().close();
}
}

util/Manager.java:

]package util;
import org.hibernate.Session;
import org.hibernate.Transaction;
import pojo.Address;
import pojo.User;
public class Manager {
public static void main(String[] args) {

Address address = new Address();
address.setCity("guangzhou");
address.setStreet("北京路");
address.setZipCode("510000");

User user = new User();
user.setUserName("fhd");
user.setPassword("000000");
user.setEmail("8888@163.com");
user.setAddress(address);

Session session = HibernateUtil.getSessionFactory().openSession();
Transaction transaction = session.beginTransaction();

session.save(user);

transaction.commit();
session.close();
}
}

表结构：

]MYUSER表：
ID,
USERNAME,
PASSWORD,
EMAIL
ADDRESS表：
ID,
STREET,
ZIPCODE,
CITY

输出sql:

]Hibernate:
insert
into
MYUSER
(USERNAME, PASSWORD, EMAIL, ID)
values
(?, ?, ?, ?)
Hibernate:
insert
into
ADDRESS
(STREET, ZIPCODE, CITY, ID)
values
(?, ?, ?, ?)

上例中，hibernate把一行插入到MYUSER表中，把另一行插入到ADDRESS表中。hibernate怎么可能知道ADDRESS表中的记录要获取与MYUSER行相同的主键值? 你需要启用一个特殊的标识符生成器。

如果Address实例被保存，它就需要获取User对象的主键值。你无法启用一个常规的标识符生成器，假设是数据库序列。用于Address的这个特殊的foreign标识符生成器，必须知道从哪里获取正确的主键值。

创建Address和User之间的这个标识符绑定，其第一步是双向关联。把一个新的user属性添加到Address属性：

public class Address{

...

private User user;

//Getters and setters

}

在Address.hbm.xml中映射Address的这个新user属性：

<one-to-one name="user" Class="User" constrained="true"/>

这个映射不仅使关联变成双向，而且通过constrained="true",添加了把ADDRESS表的主键链接到MYUSER表主键的外键约束。换句话说，数据库保证了ADDRESS行的主键引用有效的MYUSER主键。