Hibernate的一對一關聯執行個體
Hibernate中的表的關聯有一對一，一對多和多對多三種關聯方式，在這篇筆記和接下來的筆記中，我将用我自己的實際例子來說明如何具體實施。
我使用的Hibernate版本是2.1.8，在Hibernate的網站2.1.6版本的中文文檔中有關一對一的關聯有下面一段表述：
5.1.11. 一對一
持久化對象之間一對一的關聯關系是通過one-to-one元素定義的。
< 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: 屬性的名字。
(2) class (可選 - 預設是通過反射得到的屬性類型)：被關聯的類的名字。
(3) cascade(級聯) (可選) 表明操作是否從父對象級聯到被關聯的對象。
(4) constrained（限制） (可選) 表明該類對應的表對應的資料庫表，和被關聯的對象所對應的資料庫表之間，通過一個外鍵引用對主鍵進行限制。這個選項影響save()和delete()在級聯執行時的先後順序（也在schema export tool中被使用）。
(5) outer-join（外連接配接） (可選 - 預設為 自動): 當設定hibernate.use_outer_join的時候，對這個關聯允許外連接配接抓取。
(6) property-ref: (可選) 指定關聯類的一個屬性，這個屬性将會和本外鍵相對應。如果沒有指定，會使用對方關聯類的主鍵。
(7) access (可選 - 預設是 property): Hibernate用來通路屬性的政策。
有兩種不同的一對一關聯：
主鍵關聯
惟一外鍵關聯
主鍵關聯不需要額外的表字段;兩行是通過這種一對一關系相關聯的，那麼這兩行就共享同樣的主關鍵字值。是以如果你希望兩個對象通過主鍵一對一關聯，你必須确認它們被賦予同樣的辨別值！
比如說，對下面的Employee和Person進行主鍵一對一關聯:
< one-to-one name ="person" class ="Person" />
< one-to-one name ="employee" class ="Employee" constrained ="true" />
現在我們必須確定PERSON和EMPLOYEE中相關的字段是相等的。我們使用一個特别的稱為foreign的Hibernate辨別符生成器政策：
< class name ="person" table ="PERSON" >
< id name ="id" column ="PERSON_ID" >
< generator class ="foreign" >
< param name ="property" > employee </ param >
</ generator >
</ id >
< one-to-one name ="employee"
class ="Employee"
constrained ="true" />
</ class >
一個剛剛儲存的Person執行個體被賦予和該Person的employee屬性所指向的Employee執行個體同樣的關鍵字值。
另一種方式是一個外鍵和一個惟一關鍵字對應，上面的Employee和Person的例子，如果使這種關聯方式，應該表達成：
< many-to-one name ="person" class ="Person" column ="PERSON_ID" unique ="true" />
如果在Person的映射加入下面幾句，這種關聯就是雙向的：
< one-to-one name"employee" class ="Employee" property-ref ="person" />
下面是我的一個一對一主鍵關聯的例子，使用的資料庫是MySQL 4.1.11：
我有兩個表：UserBasic和UserInfo，UserBasic記錄的是使用者的基本注冊資訊，UserInfo表記錄的是使用者的詳細資訊。表的結構如下：
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CREATE TABLE IF NOT EXISTS UserBasic
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(
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Guid INT NOT NULL AUTO_INCREMENT,
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Account VARCHAR ( 64 ) NOT NULL ,
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Password VARCHAR ( 16 ) NOT NULL ,
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Email VARCHAR ( 128 ) NOT NULL ,
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PRIMARY KEY (Guid)
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) TYPE = InnoDB;
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CREATE TABLE IF NOT EXISTS UserInfo
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(
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Guid INT NOT NULL ,
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Username VARCHAR ( 128 ),
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Gender CHAR ( 1 ),
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Birthday DATETIME ,
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PRIMARY KEY (Guid)
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) TYPE = InnoDB;
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ALTER TABLE UserInfo ADD CONSTRAINT UserInfoRFUserBasic FOREIGN KEY (Guid)
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REFERENCES UserBasic (Guid) ON DELETE CASCADE ON UPDATE RESTRICT ;
UserInfo的主鍵值和UserBasic的主鍵值是一樣的，兩個表是單向的一對一關系。UserBasic為主要方，UserInfo是被動方。
用Middlegen生成的UserBasic.hbm.xml檔案，修改後的内容如下：
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<? xml version="1.0" ?>
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<! DOCTYPE hibernate-mapping PUBLIC
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"-//Hibernate/Hibernate Mapping DTD 2.0//EN"
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"http://hibernate.sourceforge.net/hibernate-mapping-2.0.dtd" >
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< hibernate-mapping >
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<!--
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Created by the Middlegen Hibernate plugin 2.1
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http://boss.bekk.no/boss/middlegen/
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http://www.hibernate.org/
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-->
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< class
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name ="com.xxx.hibernate.UserBasic"
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table ="UserBasic"
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dynamic-update ="true"
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dynamic-insert ="true"
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>
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< meta attribute ="class-description" inherit ="false" >
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@hibernate.class
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table="UserBasic"
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dynamic-update="true"
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dynamic-insert="true"
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</ meta >
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< id
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name ="guid"
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type ="int"
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column ="Guid"
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>
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< meta attribute ="field-description" >
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@hibernate.id
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generator-class="native"
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type="int"
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column="Guid"
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</ meta >
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< generator class ="native" />
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</ id >
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< property
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name ="account"
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type ="java.lang.String"
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column ="Account"
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not-null ="true"
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length ="64"
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>
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< meta attribute ="field-description" >
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@hibernate.property
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column="Account"
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length="64"
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not-null="true"
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</ meta >
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</ property >
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< property
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name ="password"
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type ="java.lang.String"
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column ="Password"
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not-null ="true"
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length ="16"
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>
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< meta attribute ="field-description" >
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@hibernate.property
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column="Password"
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length="16"
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not-null="true"
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</ meta >
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</ property >
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< property
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name ="email"
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type ="java.lang.String"
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column ="Email"
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not-null ="true"
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length ="128"
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>
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< meta attribute ="field-description" >
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@hibernate.property
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column="Email"
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length="128"
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not-null="true"
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</ meta >
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</ property >
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<!-- Associations -->
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<!-- bi-directional one-to-one association to UserInfo -->
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< one-to-one
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name ="userInfo"
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class ="com.xxx.hibernate.UserInfo"
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cascade ="save-update"
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>
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< meta attribute ="field-description" >
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@hibernate.one-to-one
96 class="com.xxx.hibernate.UserInfo"
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cascade="save-update"
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</ meta >
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</ one-to-one >
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</ class >
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</ hibernate-mapping >
由于在建立外鍵的時候就聲明了ON DELETE CASCADE，是以在xml的配置檔案中第97行聲明為save-update。如果聲明為all，那麼在删除UserBasic表的資料時，會無謂的多出一條删除UserInfo的delete語句出來。
UserInfo.hbm.xml檔案的内容如下：
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<? xml version = " 1.0 " ?>
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<! DOCTYPE hibernate - mapping PUBLIC
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" -//Hibernate/Hibernate Mapping DTD 2.0//EN "
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" http://hibernate.sourceforge.net/hibernate-mapping-2.0.dtd " >
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< hibernate - mapping >
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<!--
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Created by the Middlegen Hibernate plugin 2.1
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http: // boss.bekk.no/boss/middlegen/
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http: // www.hibernate.org/
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-->
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< class
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name = " com.xxx.hibernate.UserInfo "
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table = " UserInfo "
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dynamic - update = " true "
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dynamic - insert = " true "
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>
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< meta attribute = " class-description " inherit = " false " >
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@hibernate. class
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table = " UserInfo "
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dynamic - update = " true "
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dynamic - insert = " true "
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</ meta >
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< id
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name = " guid "
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type = " int "
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column = " Guid "
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>
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< meta attribute = " field-description " >
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@hibernate.id
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generator - class = " foreign "
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type = " int "
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column = " Guid "
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</ meta >
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< generator class = " foreign " >
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< param name = " property " > userBasic </ param >
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</ generator >
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</ id >
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< property
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name = " username "
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type = " java.lang.String "
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column = " Username "
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length = " 128 "
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>
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< meta attribute = " field-description " >
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@hibernate.property
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column = " Username "
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length = " 128 "
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</ meta >
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</ property >
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< property
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name = " gender "
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type = " java.lang.String "
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column = " Gender "
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length = " 1 "
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>
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< meta attribute = " field-description " >
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@hibernate.property
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column = " Gender "
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length = " 1 "
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</ meta >
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</ property >
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< property
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name = " birthday "
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type = " java.sql.Date "
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column = " Birthday "
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length = " 19 "
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>
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< meta attribute = " field-description " >
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@hibernate.property
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column = " Birthday "
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length = " 19 "
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</ meta >
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</ property >
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<!-- Associations -->
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<!-- bi - directional one - to - one association to UserBasic -->
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< one - to - one
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name = " userBasic "
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class = " com.xxx.hibernate.UserBasic "
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constrained = " true "
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>
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< meta attribute = " field-description " >
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@hibernate.one - to - one
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class = " com.xxx.hibernate.UserBasic "
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constrained = " true "
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</ meta >
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</ one - to - one >
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</ class >
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</ hibernate - mapping >
用hbm2java生成對應的對應的Java類：hbm2java *.xml --output=xxx。
Hibernate的配置檔案hibernate.cfg.xml内容如下：
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<? xml version="1.0" encoding="utf-8" ?>
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<! DOCTYPE hibernate-configuration
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PUBLIC "-//Hibernate/Hibernate Configuration DTD//EN"
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"http://hibernate.sourceforge.net/hibernate-configuration-2.0.dtd" >
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< hibernate-configuration >
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< session-factory >
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<!-- local connection properties -->
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< property name ="hibernate.connection.url" > jdbc:mysql://127.0.0.1/xxx?useUnicode=true & characterEncoding=UTF-8 & autoReconnect=true </ property >
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< property name ="hibernate.connection.driver_class" > com.mysql.jdbc.Driver </ property >
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< property name ="hibernate.connection.username" > root </ property >
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< property name ="hibernate.connection.password" > 123456 </ property >
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<!-- property name="hibernate.connection.pool_size"></property -->
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<!-- dialect for MySQL -->
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< property name ="dialect" > net.sf.hibernate.dialect.MySQLDialect </ property >
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< property name ="hibernate.show_sql" > true </ property >
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< property name ="hibernate.use_outer_join" > true </ property >
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< property name ="hibernate.transaction.factory_class" > net.sf.hibernate.transaction.JDBCTransactionFactory </ property >
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< mapping resource ="com/xxx/hibernate/UserBasic.hbm.xml" />
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< mapping resource ="com/xxx/hibernate/UserInfo.hbm.xml" />
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</ session-factory >
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</ hibernate-configuration >
JUnit的測試用例程式片斷如下：
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public void testInsertUser() throws Exception
{
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UserBasic user = new UserBasic();
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user.setAccount("test");
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user.setPassword("123456");
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user.setEmail("[email protected]");
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UserInfo info = new UserInfo();
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info.setUsername("George Hill");
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info.setGender("M");
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info.setBirthday(new Date());
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user.setUserInfo(info);
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info.setUserBasic(user);
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Transaction tx = session.beginTransaction();
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session.save(user);
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tx.commit();
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}
運作測試程式，可以看到輸出了兩條insert語句。