Hibernate的一对一映射

一、创建Java工程，新建Lib文件夹，加入Hibernate和数据库（如MySql、Oracle、SqlServer等）的Jar包，创建 hibernate.cfg.xml 文件，并配置，配置项如下：

 <?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>

         <!-- 配置连接数据库的基本信息 -->
         <property name="connection.username">root</property>
         <property name="connection.password"></property>
         <property name="connection.driver_class">com.mysql.jdbc.Driver</property>
         <!-- <property name="connection.url">jdbc:mysql:///mis</property> -->
         <property name="connection.url">
             <!-- 解决Mysql插入中文乱码的问题 -->
             <![CDATA[jdbc:mysql://localhost:3306/mis?useUnicode=true&characterEncoding=utf8]]>
         </property>

         <!-- 配置 hibernate 的基本信息 -->
         <!-- hibernate 所使用的数据库方言 -->
         <property name="dialect">org.hibernate.dialect.MySQL5Dialect</property>

         <!-- 执行操作时是否在控制台打印 SQL -->
         <property name="show_sql">true</property>

         <!-- 是否对 SQL 进行格式化 -->
         <property name="format_sql">true</property>

         <!-- 指定自动生成数据表的策略 -->
         <property name="hbm2ddl.auto">update</property>

         <!-- 设置 Hibernate 的事务隔离级别 :读已提交的记录-->
         <property name="connection.isolation"></property>

         <!-- 删除对象后, 使其 OID 置为 null -->
         <property name="use_identifier_rollback">true</property>

         <!-- 配置 C3P0 数据源 -->
         <property name="hibernate.c3p0.max_size"></property>
         <property name="hibernate.c3p0.min_size"></property>
         <property name="c3p0.acquire_increment"></property>

         <property name="c3p0.idle_test_period"></property>
         <property name="c3p0.timeout"></property>

         <property name="c3p0.max_statements"></property>

         <!-- 设定 JDBC 的 Statement 读取数据的时候每次从数据库中取出的记录条数 -->
         <property name="hibernate.jdbc.fetch_size"></property>

         <!-- 设定对数据库进行批量删除，批量更新和批量插入的时候的批次大小 -->
         <property name="jdbc.batch_size"></property>

         <!-- 指定关联的 .hbm.xml 文件 -->
         <!--
         <mapping resource="com/mcs/hibernate/entities/News.hbm.xml" />
         <mapping resource="com/mcs/hibernate/entities/Worker.hbm.xml" />

         <mapping resource="com/mcs/hibernate/entities/n21/Customer.hbm.xml" />
         <mapping resource="com/mcs/hibernate/entities/n21/Order.hbm.xml" />
          -->

         <mapping resource="com/mcs/hibernate/entities/n21/both/Customer.hbm.xml" />
         <mapping resource="com/mcs/hibernate/entities/n21/both/Order.hbm.xml" />

     </session-factory>

 </hibernate-configuration>

二、基于外键的一对一映射

1、创建Java实体类

 package com.mcs.hibernate.entities.onetoone.foreign;

 public class Department {

     private Integer departmentId;
     private String departmentName;

     private Manager manager;

     public Integer getDepartmentId() {
         return departmentId;
     }

     public void setDepartmentId(Integer departmentId) {
         this.departmentId = departmentId;
     }

     public String getDepartmentName() {
         return departmentName;
     }

     public void setDepartmentName(String departmentName) {
         this.departmentName = departmentName;
     }

     public Manager getManager() {
         return manager;
     }

     public void setManager(Manager manager) {
         this.manager = manager;
     }

 }

 package com.mcs.hibernate.entities.onetoone.foreign;

 public class Manager {

     private Integer managerId;
     private String managerName;

     private Department department;

     public Integer getManagerId() {
         return managerId;
     }

     public void setManagerId(Integer managerId) {
         this.managerId = managerId;
     }

     public String getManagerName() {
         return managerName;
     }

     public void setManagerName(String managerName) {
         this.managerName = managerName;
     }

     public Department getDepartment() {
         return department;
     }

     public void setDepartment(Department department) {
         this.department = department;
     }

 }

2、根据实体类创建对应的 hbm.xml文件

 <?xml version="1.0"?>
 <!DOCTYPE hibernate-mapping PUBLIC "-//Hibernate/Hibernate Mapping DTD 3.0//EN"
 "http://hibernate.sourceforge.net/hibernate-mapping-3.0.dtd">
 <!-- Generated -- :: by Hibernate Tools 3.5..Final -->
 <hibernate-mapping>
     <class name="com.mcs.hibernate.entities.onetoone.foreign.Department" table="DEPARTMENTS">
         <id name="departmentId" type="java.lang.Integer">
             <column name="DEPARTMENT_ID" />
             <generator class="native" />
         </id>
         <property name="departmentName" type="java.lang.String">
             <column name="DEPARTMENT_NAME" />
         </property>

         <!-- 使用 many-to-one 的方式来映射 - 关联关系 -->
         <many-to-one name="manager"
             class="com.mcs.hibernate.entities.onetoone.foreign.Manager"
             column="MANAGER_ID"
             unique="true"></many-to-one>
     </class>
 </hibernate-mapping>

 <?xml version="1.0"?>
 <!DOCTYPE hibernate-mapping PUBLIC "-//Hibernate/Hibernate Mapping DTD 3.0//EN"
 "http://hibernate.sourceforge.net/hibernate-mapping-3.0.dtd">
 <!-- Generated -- :: by Hibernate Tools 3.5..Final -->
 <hibernate-mapping>
     <class name="com.mcs.hibernate.entities.onetoone.foreign.Manager" table="MANAGERS">
         <id name="managerId" type="java.lang.Integer">
             <column name="MANAGER_ID" />
             <generator class="native" />
         </id>
         <property name="managerName" type="java.lang.String">
             <column name="MANAGER_NAME" />
         </property>

         <!-- 映射 - 的关联关系: 在对应的数据表中已经有外键了, 当前持久化类使用 one-to-one 进行映射 -->
         <!--
             没有外键的一端需要使用one-to-one元素，该元素使用 property-ref 属性指定使用被关联实体主键以外的字段作为关联字段
          -->
         <one-to-one name="department"
             class="com.mcs.hibernate.entities.onetoone.foreign.Department"
             property-ref="manager">
         </one-to-one>
     </class>
 </hibernate-mapping>

注：

　　1、在有外键的一端需要使用many-to-one元素，该元素使用 unique="true" 属性来约束唯一值，并指定外键栏 column="MANAGER_ID"

　　2、没有外键的一端需要使用one-to-one元素，该元素使用 property-ref 属性指定使用被关联实体主键以外的字段作为关联字段

　　3、在保存操作中，建议先保存没有外键列的那个对象. 这样会减少 UPDATE 语句

　　4、默认情况下对关联属性使用懒加载，注意可能会出现懒加载异常的问题。

　　5、在查询没有外键的实体对象时, 使用的左外连接查询, 一并查询出其关联的对象，并已经进行初始化。

三、基于主键的一对一映射

1、创建Java实体类

 package com.mcs.hibernate.entities.onetoone.primary;

 public class Department {

     private Integer departmentId;
     private String departmentName;

     private Manager manager;

     public Integer getDepartmentId() {
         return departmentId;
     }

     public void setDepartmentId(Integer departmentId) {
         this.departmentId = departmentId;
     }

     public String getDepartmentName() {
         return departmentName;
     }

     public void setDepartmentName(String departmentName) {
         this.departmentName = departmentName;
     }

     public Manager getManager() {
         return manager;
     }

     public void setManager(Manager manager) {
         this.manager = manager;
     }

 }

 package com.mcs.hibernate.entities.onetoone.primary;

 public class Manager {

     private Integer managerId;
     private String managerName;

     private Department department;

     public Integer getManagerId() {
         return managerId;
     }

     public void setManagerId(Integer managerId) {
         this.managerId = managerId;
     }

     public String getManagerName() {
         return managerName;
     }

     public void setManagerName(String managerName) {
         this.managerName = managerName;
     }

     public Department getDepartment() {
         return department;
     }

     public void setDepartment(Department department) {
         this.department = department;
     }

 }

2、根据实体类创建对应的 hbm.xml文件

 <?xml version="1.0"?>
 <!DOCTYPE hibernate-mapping PUBLIC "-//Hibernate/Hibernate Mapping DTD 3.0//EN"
 "http://hibernate.sourceforge.net/hibernate-mapping-3.0.dtd">
 <!-- Generated -- :: by Hibernate Tools 3.5..Final -->
 <hibernate-mapping>
     <class name="com.mcs.hibernate.entities.onetoone.primary.Department" table="DEPARTMENTS">
         <id name="departmentId" type="java.lang.Integer">
             <column name="DEPARTMENT_ID" />
             <!-- 使用外键的方式来生成当前的主键 -->
             <generator class="foreign" >
                 <!-- property 属性指定使用当前持久化类的哪一个属性的主键作为外键 -->
                 <param name="property">manager</param>
             </generator>
         </id>
         <property name="departmentName" type="java.lang.String">
             <column name="DEPARTMENT_NAME" />
         </property>

         <!--
         采用 foreign 主键生成器策略的一端增加 one-to-one 元素映射关联属性,
         其 one-to-one 节点还应增加 constrained=true 属性, 以使当前的主键上添加外键约束
         -->
         <one-to-one name="manager"
             class="com.mcs.hibernate.entities.onetoone.primary.Manager"
             constrained="true">
         </one-to-one>
     </class>
 </hibernate-mapping>

 <?xml version="1.0"?>
 <!DOCTYPE hibernate-mapping PUBLIC "-//Hibernate/Hibernate Mapping DTD 3.0//EN"
 "http://hibernate.sourceforge.net/hibernate-mapping-3.0.dtd">
 <!-- Generated -- :: by Hibernate Tools 3.5..Final -->
 <hibernate-mapping>
     <class name="com.mcs.hibernate.entities.onetoone.primary.Manager" table="MANAGERS">
         <id name="managerId" type="java.lang.Integer">
             <column name="MANAGER_ID" />
             <generator class="native" />
         </id>
         <property name="managerName" type="java.lang.String">
             <column name="MANAGER_NAME" />
         </property>

         <one-to-one name="department"
             class="com.mcs.hibernate.entities.onetoone.primary.Department">
         </one-to-one>
     </class>
 </hibernate-mapping>

注：

　　1、其中一端会使用另外一端的主键做为主键，另外一端代码采用 foreign 主键生成器策略的一端增加 one-to-one 元素映射关联属性,其 one-to-one 节点还应增加 constrained=true 属性, 以使当前的主键上添加外键约束。　

　　2、在保存操作中，建议先保存没有外键列的那个对象. 这样会减少 UPDATE 语句

　　3、默认情况下对关联属性使用懒加载，注意可能会出现懒加载异常的问题。