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繼承映射

Animal 表

Id	name	Sex	Weight	Height	Type
1	小豬	True	100	?	P
2	小鳥	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" );

??????????

?????? }

?

當用 Pig 去加載時是可以的，用父類 Animal 去加載也可以，但是 load 默認是 lazy 加載，用 Animal 加載的是 animal 代理類不是真正的 Pig 類

如果你把 class 中的 lazy=”false”, 則用 Animal 加載的是是真正的 Pig 類（多態）

用 get() 方法和 sql 查詢支持多態查詢，返回的是真正的 Pig 類

Sql 的“ from java.lang.Object ”是查到所有的實體類對象；

2 每個類一張映射表

animal 表

Id	Name	sex
1	小豬	True
2	小鳥	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 >

如果 User 對象的一些屬性也是其他對象的屬性（即他們有許多共同的屬性），因此將共同的屬性提取成為一個公用的類（此類包含了共同的屬性）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 >

對關系模型沒有任何影響，這樣適合復用；

復合主鍵映射

通常將復合主鍵相關的屬性放到一個單獨的類中，這個是有約束的：

l ?????? 必須實現序列化接口

l ?????? 必須寫 hashCode() 和 equals()

public class User {

??? private int age ;

??? private PK composite ;// 復合主鍵

?

}

//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

hibernate映射二