（十）Hibernate的一对一关联关系

一、概述

 

 

我们以Company与Address类为例，介绍映射一对一关联关系的方法。 
一对一的关联方法有两种 
-按照外键映射：两个表任意一个表定义一个外键，来关联另一个表。 
-按照主键映射：一个表的主键同时作为外键，和另一个表的主键保持一致。

 

二、按照外键映射

（1）创建实体类

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public class Company {     private Integer id;     private String name;     private Address address;     public Integer getId() {         return id;     }     public void setId(Integer id) {         this.id = id;     }     public String getName() {         return name;     }     public void setName(String name) {         this.name = name;     }     public Address getAddress() {         return address;     }     public void setAddress(Address address) {         this.address = address;     }   }   public class Address {     private Integer id;     private String name;     private Company company;     public Integer getId() {         return id;     }     public void setId(Integer id) {         this.id = id;     }     public String getName() {         return name;     }     public void setName(String name) {         this.name = name;     }     public Company getCompany() {         return company;     }     public void setCompany(Company company) {         this.company = company;     }   }

　　（2）我们配置映射文件，我们在Company这一方设置外键来关联Address 有外键的一方要使用<many-to-one>元素来配置。

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Company.hbm.xml  <hibernate-mapping >     <class name="com.cad.domain.Company" table="company">         <id name="id" column="id">             <generator class="native"></generator>         </id>         <property name="name" column="name"></property>         <!--column指定外键，unique指定外键唯一约束，设为true，就可以表达Company和Address对象之间的一对一关联-->         <many-to-one name="address" class="com.cad.domain.Address" column="aid" unique="true"></many-to-one>     </class>  </hibernate-mapping>      Address.hbm.xml  <hibernate-mapping >     <class name="com.cad.domain.Address" table="address">         <id name="id" column="id">             <generator class="native"></generator>         </id>         <property name="name" column="name"></property>         <!--property-ref属性指定通过从Company的address属性来查找自己-->         <one-to-one name="Company" class="com.cad.domain.Company" property-ref="address"></one-to-one>     </class>  </hibernate-mapping>

　　测试：

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public class Demo {           private Session session;           @Test         public void test() {             //读取配置文件             Configuration conf=new Configuration().configure();               //根据配置创建factory             SessionFactory sessionfactory=conf.buildSessionFactory();             session = sessionfactory.openSession();              Transaction ts=session.beginTransaction();             Company c=new Company();             c.setName("百度");             Address a=new Address();             a.setName("深圳");             c.setAddress(a);             a.setCompany(c);               session.save(a);             session.save(c);               ts.commit();             session.close();             sessionfactory.close();         }       }   默认情况下，一对一关联采用迫切左外连接检索策略。

　　

三、按照主键映射

 

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实体类不变，我们编写一下配置文件 address的表的id字段既是主键又是外键.       编写配置文件     Company.hbm.xml      <hibernate-mapping >         <class name="com.cad.domain.Company" table="company">             <id name="id" column="id">                 <generator class="native"></generator>             </id>             <property name="name" column="name"></property>             <one-to-one name="address" class="com.cad.domain.Address" ></one-to-one>         </class>      </hibernate-mapping>       Address.hbm.xml      <hibernate-mapping >         <class name="com.cad.domain.Address" table="address">             <id name="id" column="id">                 <!--必须使用foreign标识符生成策略，还要指定哪个对象共享OID-->                 <generator class="foreign">                     <param name="property">company</param>                 </generator>             </id>             <property name="name" column="name"></property>             <!--constrained属性设为true，说明主键同时作为外键-->             <one-to-one name="company" class="com.cad.domain.Company" constrained="true"></one-to-one>         </class>      </hibernate-mapping>