Hibernate 支持三種基本的繼承映射策略：

1、單表繼承：每棵類繼承樹使用一個表
2、具體表繼承：每個子類一個表
3、類表繼承：每個具體類一個表(有一些限制)


三種方式的比較：
1、所有類映射成一張表會產生數據冗余（不過這是通常采用的方法）
2、每個類映射成一張表會有效率問題，比如是三層或四層結構時，對于查詢或更新會發出很多sql語句
3、具體類映射成表的缺點是主鍵不能自增

結論：使用第一種方式

?

Java代碼 ?

/**?動物?*/??

public?class?Animal?{??

??

????private?Integer?id;??

????private?String?name;??

????private?String?type;??

??

????//getter?and?setter??

}??

?

Java代碼 ?

/**?豬?*/??

public?class?Pig?extends?Animal?{??

??

????private?Double?weight;??

??????

????//getter?and?setter??

}??

?

Java代碼 ?

/**?鳥?*/??

public?class?Bird?extends?Animal{??

??

????private?String?color;??

??????

????//getter?and?setter??

}??

?

Xml代碼 ?

<hibernate-mapping?package="org.monday.hibernate4.domain">??

????<class?name="Animal"?table="tbl_animal">??

????????<id?name="id">??

????????????<generator?class="identity"?/>??

????????</id>??

????????<!--?辨別者列?-->??

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

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

????????<!--?子類?-->??

????????<subclass?name="Pig"?discriminator-value="p">??

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

????????</subclass>??

????????<!--?子類?-->??

????????<subclass?name="Bird"?discriminator-value="b">??

????????????<property?name="color"?/>??

????????</subclass>??

????</class>??

</hibernate-mapping>??

?下面是基于注解的

?

Java代碼 ?

/**?動物?*/??

@Entity??

@Table(name?=?"tbl_animal")??

@Inheritance(strategy?=?InheritanceType.SINGLE_TABLE)??

@DiscriminatorColumn(name?=?"type",?discriminatorType?=?DiscriminatorType.STRING)??

@DiscriminatorValue("a")??

public?class?Animal?{??

??

????@Id??

????@GeneratedValue(strategy?=?GenerationType.IDENTITY)??

????private?Integer?id;??

??

????private?String?name;??

??

????//?getter?and?setter??

}??

?

Java代碼 ?

/**?豬?*/??

@Entity??

@DiscriminatorValue("p")??

public?class?Pig?extends?Animal?{??

??

????private?Double?weight;??

??????

????//getter?and?setter??

}??

?

Java代碼 ?

/**?鳥?*/??

@Entity??

@DiscriminatorValue("b")??

public?class?Bird?extends?Animal?{??

??

????private?String?color;??

??

????//?getter?and?setter??

}?

總結

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