hibernate映射二

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继承映射

Animal 表

Id	name	Sex	Weight	Height	Type
1	小猪	True	100		P
2	小鸟	False		20	B

Extends.hbm.xml 如下：

< hibernate-mapping package = "com.test.hibernate" >

    < class name = "Animal" table = "animal" >

       < id name = "id" >

           < generator class = "native" ></ generator >

       </ id >

        < property name = "name" ></ property >

       < property name = "sex" ></ property >

       < discriminator column = "type" type = "string" />

       < subclass name = "Pig" discriminator-value = "P" >

           < property name = "weight" ></ property >

       </ subclass >

       < subclass name = "Bird" discriminator-value = "B" >

           < property name = "height" ></ property >

       </ subclass >

    </ class >

</ hibernate-mapping >

 

Animal animal =(Animal)session.load(animal.Class, 1);

       if (animal instanceof Pig) {

           System. out .println( "Pig" );

          

       }

 

当用 Pig 去加载时是可以的，用父类 Animal 去加载也可以，但是 load 默认是 lazy 加载，用 Animal 加载的是 animal 代理类不是真正的 Pig 类

如果你把 class 中的 lazy=”false”, 则用 Animal 加载的是是真正的 Pig 类（多态）

用 get() 方法和 sql 查询支持多态查询，返回的是真正的 Pig 类

Sql 的“ from java.lang.Object ”是查到所有的实体类对象；

2 每个类一张映射表

animal 表

Id	Name	sex
1	小猪	True
2	小鸟	false

  pig 表

pid	weight
1	100

bird 表

bid	height
1	10

Extends.hbm.xml 如下：

< hibernate-mapping package = "com.test.hibernate" >

    < class name = "Animal" table = "animal" >

       < id name = "id" >

           < generator class = "native" ></ generator >

       </ id >

       < property name = "name" ></ property >

       < property name = "sex" ></ property >

       <join- subclass name = "Pig" table=”pig” >

           <key column=”pid” />

           < property name = "weight" ></ property >

       </ join- subclass >

       < join- subclass name = "Bird" table=”bird” >

           <key column=”bid” />

           < property name = "height" ></ property >

       </ join- subclass >

    </ class >

这种对象模型不变而存储模型的方法变了；缺点是建表多，效率低；

如果 User 对象的一些属性也是其他对象的属性（即他们有许多共同的属性），因此将共同的属性提取成为一个公用的类（此类包含了共同的属性）如：

public class User {

    private int id ;

    private Contact conta ct ;

 

}

//set,get 方法略

public class Contact {

private String name ;

private boolean sex ;

}

//set,get 方法略

User.hbm.xml 如下：

< hibernate-mapping package = "com.test.hibernate" >

    < class name = "User" table = "user" >

       < id name = "id" >

           < generator class = "native" ></ generator >

       </ id >

       <component name= "contact">

           < property name = "name" ></ property >

           < property name = "sex" ></ property >

       </component>

    </ class >

对关系模型没有任何影响，这样适合复用；

复合主键映射

通常将复合主键相关的属性放到一个单独的类中，这个是有约束的：

l       必须实现序列化接口

l       必须写 hashCode() 和 equals()

public class User {

    private int age ;

    private PK composite ;// 复合主键

 

}

//set,get 方法略

public class PK {

private String name ;

private boolean sex ;

}

//set,get 方法略

User.hbm.xml 如下：

< hibernate-mapping package = "com.test.hibernate" >

    < class name = "User" table = "user" >

       <composite-id name= " composite ">

           <key- property name = "name" ></key- property >

           <key- property name = "sex" ></key- property >

</composite-id>

< property name = "name" ></ property >

           </ class >

集合的映射

t_collection_mapping 表

id	name
1	xxx

t_set_values 表

set_id	set_value
1	A
1	b

t_list_values 表

list_id	list_value	list_index
1	c	0
1	d