Hibernate的双向关系映射

一、双向1 - N的关系映射
对于1 - N的关联，Hibernate推荐使用双向关联，但不用1的一端来控制关联关系，而使用N
的一端来控制关联关系。N的一端直接访问关联类属性；1的一端增加集合属性的访问。
看下面2个POJO
public   class  Person  implements  Serializable
... {
    private int personid;
    priavte String name;
    private int age;
    private Set addresses = new HashSet();
    ……
}
public   class  Address  implements  Serializable
... {
    private int addressid;
    private String addressdetail;
    private Person person;
    public Address() ...{}
    public Address(Sring addressdetail) ...{
        this.addressdetail = addressdetail;
    }
}

1 .无连接表的双向1 - N关联
两个持久化类的配置文件都需要指定外键列的列名
<? xml version = " 1.0 " ?>
<! DOCTYPE hibernate - mapping
    PUBLIC  " -//Hibernate/Hibernate Mapping DTD 3.0//EN "
     " http://hibernate.sourceforge.net/hibernate-mapping-3.0.dtd " >
< hibernate - mapping  package = " prolove " >
     < class  name = " Address " >
         < id name = " addressid " >
             < generator  class = " identity " />
         </ id >
         < property name = " addressdetail " />
         <!-- 增加many - to - one映射关联属性，column属性指定外键列名 -->
         < many - to - one name = " person "  column = " personid " />
     </ class >
     < class  name = " Person " >
         < id name = " personid " >
             < generator  class = " identity " />
         </ id >
         < property name = " name " />
         < property name = " age " />

         < set name = " addresss " >
             < key column = " personid " />
             < one - to - many  class = " Address " />
         </ set >
     </ class >
</ hibernate - mapping >

2 .有连接表的双向1 - N关联
两边确定连接表的table属性值应该相同，且不能省略。
<? xml version = " 1.0 " ?>
<! DOCTYPE hibernate - mapping
    PUBLIC  " -//Hibernate/Hibernate Mapping DTD 3.0//EN "
     " http://hibernate.sourceforge.net/hibernate-mapping-3.0.dtd " >
< hibernate - mapping  package = " prolove " >
     < class  name = " Address " >
         < id name = " addressid " >
             < generator  class = " identity " />
         </ id >
         < property name = " addressdetail " />
         <!-- 显式使用join元素确定连接表 -->
         < join table = " PersonAddress "  inverse = " true "  optional = " true " >
             < key column = " addressid " />
             < many - to - one name = " person "  column = " personid "  not - null = " true " />
         </ join >
     </ class >
     < class  name = " Person " >
         < id name = " personid " >
             < generator  class = " identity " />
         </ id >
         < property name = " name " />
         < property name = " age " />
         <!-- 映射集合属性，关联到持久化类，注明table属性 -->
         < set name = " addresses "  table = " PersonAddress " >
             < key column = " personid " />
             <!-- one - to - many元素映射关联属性，为保证为1，增加unique = " true " -->
             < one - to - many column = " addressid "  unique = " true "   class = " Address " />
         </ set >
     </ class >
</ hibernate - mapping >

二、双向N - N关联
public   class  Person
... {
    private int personid;
    private String name;
    private int age;
    private Set addresses = new HashSet();
    ……
}
public   class  Address
... {
    private int addressid;
    private String addressdetail;
    private Set persons = new HashSet();
    public Address() ...{}
    public Address(String addressdetail) ...{
        this.addressdetail = addressdetail;
    }
}

<? xml version = " 1.0 " ?>
<! DOCTYPE hibernate - mapping
    PUBLIC  " -//Hibernate/Hibernate Mapping DTD 3.0//EN "
     " http://hibernate.sourceforge.net/hibernate-mapping-3.0.dtd " >
< hibernate - mapping  package = " prolove " >
     < class  name = " Address " >
         < id name = " addressid " >
             < generator  class = " identity " />
         </ id >
         < property name = " addressdetail " />
         <!-- 映射集合属性，使用指定连接表 -->
         < set name = " persons "  inverse = " true "  table = " jointable " >
             <!-- 指定本持久化类在连接表中的外键列名 -->
             < key column = " addressid " />
             <!-- 映射多对多关联类 -->
             < many - to - many column = " personid "   class = " Person " />
         </ set >
     </ class >
     < class  name = " Person " >
         < id name = " personid " >
             < generator  class = " identity " />
         </ id >
         < property name = " name " />
         < property name = " age " />
         <!-- 映射集合属性，关联到持久化类，注明table属性 -->
         < set name = " addresses "  table = " jointable " >
             < key column = " personid " />
             < many - to - many column = " addressid "   class = " Address " />
         </ set >
     </ class >
</ hibernate - mapping >

三、双向1 - 1关联
public   class  Person
... {
    //基本属性
    private int personid;
    private String name;
    private int age;
    //关联属性
    private Address address;
    ……
}
public   class  Address  implements  Serializable
... {
    private int addressid;
    private String addressdetail;
    private Person person;
    public Address() ...{}
    public Address(String addressdetail) ...{
        this.addressdetail = addressdetail;
    }
    ……
}

1 .基于外键的双向1 - 1关联
<? xml version = " 1.0 " ?>
<! DOCTYPE hibernate - mapping
    PUBLIC  " -//Hibernate/Hibernate Mapping DTD 3.0//EN "
     " http://hibernate.sourceforge.net/hibernate-mapping-3.0.dtd " >
< hibernate - mapping  package = " prolove " >
     < class  name = " Person " >
         < id name = " personid " >
             < generator  class = " identity " />
         </ id >
         < property name = " name " />
         < property name = " age " />
         <!-- one - to - one元素映射关联属性，外键列在对方的表内person - ref
            指向关联类的本身的属性。即在address属性所属的Address类 -->
         < one - to - one name = " address "  property - ref = " person " />
     </ class >
     < class  name = " Address " >
         < id name = " addressid " >
             < generator  class = " identity " />
         </ id >
         < property name = " addressdetail " />
         <!-- many - to - one元素映射关联属性，unique = " true " 确定为1 - 1 -->
         < many - to - one name = " person "  unique = " true " />
     </ class >
</ hibernate - mapping >

2 .基于主键的双向1 - 1关联
<? xml version = " 1.0 " ?>
<! DOCTYPE hibernate - mapping
    PUBLIC  " -//Hibernate/Hibernate Mapping DTD 3.0//EN "
     " http://hibernate.sourceforge.net/hibernate-mapping-3.0.dtd " >
< hibernate - mapping  package = " prolove " >
     < class  name = " Person " >
         < id name = " personid " >
             < generator  class = " identity " />
         </ id >
         < property name = " name " />
         < property name = " age " />
         <!-- one - to - one元素映射关联属性 -->
         < one - to - one name = " address " />
     </ class >
     < class  name = " Address " >
         < id name = " addressid " >
             < generator  class = " foreign " >
                 <!-- 指定根据主键生成的关联属性 -->
                 < param name = " property " > person </ param >
             </ generator >
         </ id >
         < property name = " addressdetail " />
         <!-- 用于映射关联属性，增加constrained = " true "
            表明主键根据关联属性生成 -->
         < one - to - one name = " person "  constrained = " true " />
     </ class >
</ hibernate - mapping >

3 .有连接表的双向1 - 1关联
本情形相当罕见，通常不推荐使用这种策略。
<? xml version = " 1.0 " ?>
<! DOCTYPE hibernate - mapping
    PUBLIC  " -//Hibernate/Hibernate Mapping DTD 3.0//EN "
     " http://hibernate.sourceforge.net/hibernate-mapping-3.0.dtd " >
< hibernate - mapping  package = " prolove " >
     < class  name = " Person " >
         < id name = " personid " >
             < generator  class = " identity " />
         </ id >
         < property name = " name " />
         < property name = " age " />
         <!-- 显式使用join元素指定连接表关联 -->
         < join table = " PersonAddress "  optional = " true " >
             <!-- key元素映射外键列的列名 -->
             < key column = " personid "  unique = " true " />
             <!-- 映射关联属性 -->
             < many - to - one name = " address "  column = " addressid "
                not - null = " true "  unique = " true " />
         </ join >
     </ class >
     <!-- 映射持久化类Address -->
     < class  name = " Address " >
         <!-- 映射标识属性 -->
         < id name = " addressid " >
             <!-- 指定主键生成器策略 -->
             < generator  class = " identity " />
         </ id >
         < property name = " addressdetail " />
         <!-- 显式使用join元素指定连接表关联 -->
         < join table = " PersonAddress "  inverse = " true "  optional = " true " >
             < key column = " addressid "  unique = " true " />
             < many - ton - one name = " person "  column = " personid "
                not - null = " true "  unique = " true " />
         </ join >
     </ class >
</ hibernate - mapping >