题记：Hibernate是个好东西，可我感觉这个好东东不好驾御，一个one-to-one就让我花费了很多时间，现在终于有点理解，所以想给大家分享分享我的观点，如有差错，还请各位批评指正！
1.关于one-to-one
持久化对象之间一对一的关联关系是通过one-to-one元素定义：
<one-to-one
name="propertyName" (1)
class="ClassName" (2)
cascade="all|none|save-update|delete" (3)
constrained="true|false" (4)
outer-join="true|false|auto" (5)
property-ref="propertyNameFromAssociatedClass" (6)
access="field|property|ClassName" (7)
/>
(1)
name: 属性的名字[POJO中的]。
(2)
class (可选 - 默认是通过反射得到的属性类型)：被关联的类的名字。
(3)
cascade(级联) (可选) 表明操作是否从父对象级联到被关联的对象。
(4)
constrained（约束） (可选) 表明该类对应的表对应的数据库表，和被关联的对象所对应的数据库表之间，通过一个外键引用对主键进行约束。这个选项影响save()和delete()在级联执行时的先后顺序（也在schema export tool中被使用）。
(5)
outer-join（外连接） (可选 - 默认为 自动): 当设置hibernate.use_outer_join的时候，对这个关联允许外连接抓取。
(6)
property-ref: (可选) 指定关联类的一个属性，这个属性将会和本外键相对应。如果没有指定，会使用对方关联类的主键[POJO中POJO类的实例]。
(7)
access (可选 - 默认是 property): Hibernate用来访问属性的策略。
2.one-to-one分类
主键关联
惟一外键关联
主键关联不需要额外的表字段;两行是通过这种一对一关系相关联的，那么这两行就共享同样的主关键字值。所以如果你希望两个对象通过主键一对一关联，你必须确认它们被赋予同样的标识值！
另一种方式是一个外键和一个惟一关键字对应。
3.one-to-one中惟一外键关联操作
（1）数据库DDL
#
# Table structure for table 'author'
#
CREATE TABLE author (
author_id char(20) NOT NULL default '',
person_id char(20) default NULL,
PRIMARY KEY (author_id)
) ENGINE=InnoDB DEFAULT CHARSET=gb2312;
#
# Table structure for table 'person'
#
CREATE TABLE person (
person_id char(20) NOT NULL default '',
name char(20) default NULL,
PRIMARY KEY (person_id)
) ENGINE=InnoDB DEFAULT CHARSET=gb2312;
（2）映射文件
Author.hbm.xml
<?xml version="1.0"?>
<!DOCTYPE hibernate-mapping PUBLIC
"-//Hibernate/Hibernate Mapping DTD 2.0//EN"
"http://hibernate.sourceforge.net/hibernate-mapping-2.0.dtd" >
<!-- DO NOT EDIT: This is a generated file that is synchronized -->
<!-- by MyEclipse Hibernate tool integration. -->
<!-- Created Sat Apr 23 14:28:37 CST 2005 -->
<hibernate-mapping package="po">
<class name="Author" table="author">
<id name="authorId" column="author_id" type="java.lang.String">
<generator class="assigned"/>
</id>
<many-to-one
cascade="all"
class="po.Person"
column="person_id"
name="person"
not-null="true"
outer-join="auto"
unique="true"
/>
</class>
</hibernate-mapping>
Person.hbm.xml
<?xml version="1.0"?>
<!DOCTYPE hibernate-mapping PUBLIC
"-//Hibernate/Hibernate Mapping DTD 2.0//EN"
"http://hibernate.sourceforge.net/hibernate-mapping-2.0.dtd" >
<!-- DO NOT EDIT: This is a generated file that is synchronized -->
<!-- by MyEclipse Hibernate tool integration. -->
<!-- Created Sat Apr 23 14:28:39 CST 2005 -->
<hibernate-mapping package="po">
<class name="Person" table="person">
<id name="personId" column="person_id" type="java.lang.String">
<generator class="assigned"/>
</id>
<property name="name" column="name" type="java.lang.String" />
<one-to-one name="author" class="Author" property-ref="person"/>
</class>
</hibernate-mapping>
（2）POJO文件
AbstractAuthor.java
package po;
import java.io.Serializable;
public abstract class AbstractAuthor
implements Serializable
{
/** The cached hash code value for this instance. Settting to 0 triggers re-calculation. */
private int hashValue = 0;
/** The composite primary key value. */
private java.lang.String authorId;
/** The value of the simple personId property. */
private java.lang.String personId;
/**
* Simple constructor of AbstractAuthor instances.
*/
public AbstractAuthor()
{
}
/**
* Constructor of AbstractAuthor instances given a simple primary key.
* @param authorId
*/
public AbstractAuthor(java.lang.String authorId)
{
this.setAuthorId(authorId);
}
/**
* Return the simple primary key value that identifies this object.
* @return java.lang.String
*/
public java.lang.String getAuthorId()
{
return authorId;
}
/**
* Set the simple primary key value that identifies this object.
* @param authorId
*/
public void setAuthorId(java.lang.String authorId)
{
this.hashValue = 0;
this.authorId = authorId;
}
/**
* Return the value of the person_id column.
* @return java.lang.String
*/
public java.lang.String getPersonId()
{
return this.personId;
}
/**
* Set the value of the person_id column.
* @param personId
*/
public void setPersonId(java.lang.String personId)
{
this.personId = personId;
}
/**
* Implementation of the equals comparison on the basis of equality of the primary key values.
* @param rhs
* @return boolean
*/
public boolean equals(Object rhs)
{
if (rhs == null)
return false;
if (! (rhs instanceof Author))
return false;
Author that = (Author) rhs;
if (this.getAuthorId() != null && that.getAuthorId() != null)
{
if (! this.getAuthorId().equals(that.getAuthorId()))
{
return false;
}
}
return true;
}
/**
* Implementation of the hashCode method conforming to the Bloch pattern with
* the exception of array properties (these are very unlikely primary key types).
* @return int
*/
public int hashCode()
{
if (this.hashValue == 0)
{
int result = 17;
int authorIdValue = this.getAuthorId() == null ? 0 : this.getAuthorId().hashCode();
result = result * 37 + authorIdValue;
this.hashValue = result;
}
return this.hashValue;
}
}
Author.java
package po;
import java.io.Serializable;
public class Author
extends AbstractAuthor
implements Serializable
{
private Person person;
/**
* Simple constructor of Author instances.
*/
public Author()
{
}
/**
* Constructor of Author instances given a simple primary key.
* @param authorId
*/
public Author(java.lang.String authorId)
{
super(authorId);
}
/* Add customized code below */
public void setPerson(Person person){
this.person=person;
}
public Person getPerson(){
return person;
}
}
AbstractPerson.java
package po;
import java.io.Serializable;
public abstract class AbstractPerson
implements Serializable
{
/** The cached hash code value for this instance. Settting to 0 triggers re-calculation. */
private int hashValue = 0;
/** The composite primary key value. */
private java.lang.String personId;
/** The value of the simple name property. */
private java.lang.String name;
/**
* Simple constructor of AbstractPerson instances.
*/
public AbstractPerson()
{
}
/**
* Constructor of AbstractPerson instances given a simple primary key.
* @param personId
*/
public AbstractPerson(java.lang.String personId)
{
this.setPersonId(personId);
}
/**
* Return the simple primary key value that identifies this object.
* @return java.lang.String
*/
public java.lang.String getPersonId()
{
return personId;
}
/**
* Set the simple primary key value that identifies this object.
* @param personId
*/
public void setPersonId(java.lang.String personId)
{
this.hashValue = 0;
this.personId = personId;
}
/**
* Return the value of the name column.
* @return java.lang.String
*/
public java.lang.String getName()
{
return this.name;
}
/**
* Set the value of the name column.
* @param name
*/
public void setName(java.lang.String name)
{
this.name = name;
}
/**
* Implementation of the equals comparison on the basis of equality of the primary key values.
* @param rhs
* @return boolean
*/
public boolean equals(Object rhs)
{
if (rhs == null)
return false;
if (! (rhs instanceof Person))
return false;
Person that = (Person) rhs;
if (this.getPersonId() != null && that.getPersonId() != null)
{
if (! this.getPersonId().equals(that.getPersonId()))
{
return false;
}
}
return true;
}
/**
* Implementation of the hashCode method conforming to the Bloch pattern with
* the exception of array properties (these are very unlikely primary key types).
* @return int
*/
public int hashCode()
{
if (this.hashValue == 0)
{
int result = 17;
int personIdValue = this.getPersonId() == null ? 0 : this.getPersonId().hashCode();
result = result * 37 + personIdValue;
this.hashValue = result;
}
return this.hashValue;
}
}
Person.java
package po;
import java.io.Serializable;
public class Person
extends AbstractPerson
implements Serializable
{
private Author author;
/**
* Simple constructor of Person instances.
*/
public Person()
{
}
/**
* Constructor of Person instances given a simple primary key.
* @param personId
*/
public Person(java.lang.String personId)
{
super(personId);
}
/* Add customized code below */
public void setAuthor(Author author){
this.author=author;
}
public Author getAuthor(){
return author;
}
}
（3）测试文件
TestOO.java
package test;
import po.Author;
import po.Person;
import net.sf.hibernate.*;
import net.sf.hibernate.cfg.*;
public class TestOO {
private Session sesssion=null;
/**
* @param args
*/
public static void main(String[] args) {
// TODO Auto-generated method stub
try {
Configuration cfg = new Configuration().configure();
SessionFactory sessions = cfg.buildSessionFactory();
Session session = sessions.openSession();
Transaction tx = session.beginTransaction();
Person person = new Person();
person.setPersonId("1");
person.setName("Blake Stone");
Author author = new Author();
author.setAuthorId("11");
author.setPerson(person);
session.save(person);
session.save(author);
System.out.print("i will dead if i don't get success!");
tx.commit();
session.close();
} catch (Exception e) {
System.out.println(e);
}
}
}
（4）分享喜悦
运行测试类，数据正常添加进数据库。
（5）深入探讨
a.由于主键采用assigned方式，必须自己指定ID；
b.如果只采用session.save(author);将会产生级联错误[ Could not synchronize database state with session net.sf.hibernate.HibernateException: Batch update row count wrong: 0]，看看执行的SQL语句[Hibernate: insert into author (person_id, author_id) values (?, ?) Hibernate: update person set name=? where person_id=?]很明白是吧，因为person表还是空的，当然执行update操作会失败！虽然解决的方法很简单，可问题值得思考！虽然对person进行了setter方法，可不要忘了他仍旧是VO而不是PO！
