Hibernate一对一双向关联（外键关联）用法小结

这几天在改一个项目源码，遇到一个问题坑了很久。场景如下（注：此处是借鉴网络上的例子，并不是自己的实验环境）：

一夫一妻制——比如夫妻关系的两张数据表，一个是wif表，一个是husban表，其数据表信息如下：

CREATE TABLE `wife` (
  `id` int(11) NOT NULL AUTO_INCREMENT,
  `name` varchar(50) NOT NULL,
  PRIMARY KEY (`id`)
) ENGINE=InnoDB AUTO_INCREMENT=15 DEFAULT CHARSET=utf8

CREATE TABLE `husband` (
  `id` int(11) NOT NULL AUTO_INCREMENT,
  `name` varchar(50) NOT NULL,
  `wifeid` int(11) DEFAULT NULL,
  PRIMARY KEY (`id`),
  KEY `fk_wife` (`wifeid`),
  CONSTRAINT `fk_wife` FOREIGN KEY (`wifeid`) REFERENCES `wife` (`id`)
) ENGINE=InnoDB AUTO_INCREMENT=20 DEFAULT CHARSET=utf8

下面是相应的POJO：

Wife：  
1.	package com.linys.model;  
2.	  
3.	/** 
4.	 * Wife entity. @author MyEclipse Persistence Tools 
5.	 */  
6.	  
7.	public class Wife implements java.io.Serializable {  
8.	  
9.	    // Fields  
10.	  
11.	    /** 
12.	     *  
13.	     */  
14.	    private static final long serialVersionUID = 1L;  
15.	    private Integer id;  
16.	    private String name;  
17.	    private Husband husband;  
18.	    // Constructors  
19.	  
20.	    /** default constructor */  
21.	    public Wife() {  
22.	    }  
23.	  
24.	    /** minimal constructor */  
25.	    public Wife(String name) {  
26.	        this.name = name;  
27.	    }  
28.	  
29.	  
30.	    // Property accessors  
31.	  
32.	    public Integer getId() {  
33.	        return this.id;  
34.	    }  
35.	  
36.	    public void setId(Integer id) {  
37.	        this.id = id;  
38.	    }  
39.	  
40.	    public String getName() {  
41.	        return this.name;  
42.	    }  
43.	  
44.	    public void setName(String name) {  
45.	        this.name = name;  
46.	    }  
47.	  
48.	    public Husband getHusband() {  
49.	        return husband;  
50.	    }  
51.	  
52.	    public void setHusband(Husband husband) {  
53.	        this.husband = husband;  
54.	    }  
55.	  
56.	  
57.	}  
 
 Husband：
   
1.	package com.linys.model;  
2.	  
3.	/** 
4.	 * Husband entity. @author MyEclipse Persistence Tools 
5.	 */  
6.	  
7.	public class Husband implements java.io.Serializable {  
8.	  
9.	    // Fields  
10.	  
11.	    /** 
12.	     *  
13.	     */  
14.	    private static final long serialVersionUID = 1L;  
15.	    private Integer id;  
16.	    private Wife wife;  
17.	    private String name;  
18.	  
19.	    // Constructors  
20.	  
21.	    /** default constructor */  
22.	    public Husband() {  
23.	    }  
24.	  
25.	    /** minimal constructor */  
26.	    public Husband(String name) {  
27.	        this.name = name;  
28.	    }  
29.	  
30.	    /** full constructor */  
31.	    public Husband(Wife wife, String name) {  
32.	        this.wife = wife;  
33.	        this.name = name;  
34.	    }  
35.	  
36.	    // Property accessors  
37.	  
38.	    public Integer getId() {  
39.	        return this.id;  
40.	    }  
41.	  
42.	    public void setId(Integer id) {  
43.	        this.id = id;  
44.	    }  
45.	  
46.	    public Wife getWife() {  
47.	        return this.wife;  
48.	    }  
49.	  
50.	    public void setWife(Wife wife) {  
51.	        this.wife = wife;  
52.	    }  
53.	  
54.	    public String getName() {  
55.	        return this.name;  
56.	    }  
57.	  
58.	    public void setName(String name) {  
59.	        this.name = name;  
60.	    }  
61.	  
62.	}  

一对一双向关联比较特殊，不像单向关联，在hbm配置文件中仅仅使用one-to-one配置即可，这种情况下需要使用may-to-one和one-to-one来模拟一一对应，既然是一一对应，所以在many-to-one的一段，要加上unique=true属性，从而保证其唯一性。不要问我为什么，这是Hibernate的机制吧，其实说到这里，我对于Hibernate还不是很了解，如果有大神读到希望可以指点一二。从项目经验来看，如果不这么做，在同时修改了这两个实体，然后保存到库中时，总是报错，错误信息为session无法同步，或者外键id为NULL。使用了这样的配置后，就没有问题了。下面看二者的配置文件：

Wife.hbm.xml
 
Java代码   
1.	<?xml version="1.0" encoding="utf-8"?>  
2.	<!DOCTYPE hibernate-mapping PUBLIC "-//Hibernate/Hibernate Mapping DTD 3.0//EN"  
3.	"http://hibernate.sourceforge.net/hibernate-mapping-3.0.dtd">  
4.	<!--   
5.	    Mapping file autogenerated by MyEclipse Persistence Tools  
6.	-->  
7.	<hibernate-mapping>  
8.	    <class name="com.linys.model.Wife" table="wife">  
9.	        <id name="id" type="java.lang.Integer">  
10.	            <column name="id" />  
11.	            <generator class="native" />  
12.	        </id>  
13.	        <property name="name" type="java.lang.String">  
14.	            <column name="name" length="50" not-null="true" />  
15.	        </property>  
16.	       <one-to-one name="husband" class="com.linys.model.Husband" property-ref="wife" cascade="all"/> 
17.	    </class>  
18.	</hibernate-mapping>  
 Husband.hbm.xml:
1.	<?xml version="1.0" encoding="utf-8"?>  
2.	<!DOCTYPE hibernate-mapping PUBLIC "-//Hibernate/Hibernate Mapping DTD 3.0//EN"  
3.	"http://hibernate.sourceforge.net/hibernate-mapping-3.0.dtd">  
4.	<!--   
5.	    Mapping file autogenerated by MyEclipse Persistence Tools  
6.	-->  
7.	<hibernate-mapping>  
8.	    <class name="com.linys.model.Husband" table="husband">  
9.	        <id name="id" type="java.lang.Integer">  
10.	            <column name="id" />  
11.	            <generator class="native" />  
12.	        </id>  
13.	        <property name="name" type="java.lang.String">  
14.	            <column name="name" length="50" not-null="true" />  
15.	        </property>  
16.	        <may-to-one name="wife" class="com.linys.model.Wife" fetch="select"/>
17.	             <column name="wifeid" unique="true" not-null="true" />  
18.	        </many-to-one>  
19.	    </class>  
20.	</hibernate-mapping>  

外键所依赖的那个表的配置文件（wife）使用one-to-one，外键所在的表（husband）用many-to-one，但是要指定unique为true从而保证其唯一性。注意many这一端中的colum，配置的应该是外键所在表的外键列名，对应这里也就是husband表中的“wifeid”，需要与数据库中的数据表表中一致，切勿弄错。

one-to-one:指定在Wife这个类中用于双向关联的属性husband

property-ref: 在关联对象中用于与本对象关联的属性。

注意：property-ref="wife"不能少，否则会造成查询时关联查询失败！

以上是实际经验的总结，如有错误，欢迎指正。