遗赠与遗嘱继承的区别 Hibernate的继承关系

Hibernate的继承关系  

一 每个子类对应一个数据表(Table per concrete class)   学生表     create table `sample` `student`(         `id` bigint not null auto_increment        `name` varchar( ) default not null        `score` float         primary key (`id`)     );     create unique index `PRIMARY` on `sample` `student`(`id`); 教师表     create table `sample` `teacher`(         `id` bigint not null auto_increment        `name` varchar( ) default not null        `salary` float( )         primary key (`id`)     );     create unique index `PRIMARY` on `sample` `teacher`(`id`);   Person抽象基类 publicabstractclassPersonimplements java io Serializable {     private Long id;     private String name;       /**defaultconstructor*/     public Person() {     }       public Long getId() {        returnthis id;     }     publicvoid setId(Long id) {        this id = id;     }     public String getName() {        returnthis name;     }     publicvoid setName(String name) {        this name = name;     } }   子类分别实现它 并添加额外的属性和相应gettter和setter方法 如Student类 publicclassStudentextends Person {     private Float score;       public Student() {        super();     }     public Float getScore() {        returnscore;     }     publicvoid setScore(Float score) {        this score = score;     } }   hibernate cfg xml <?xml version= encoding= UTF ?> <!DOCTYPE hibernate configuration PUBLIC           //Hibernate/Hibernate Configuration DTD //EN          configuration dtd > <hibernate configuration>       <session factory>        <property name= connection username >root</property>        <property name= connection url >            jdbc:mysql://localhost: /sample        </property>        <property name= dialect >            hibernate dialect MySQLDialect        </property>        <property name= connection password > </property>        <property name= connection driver_class >            mysql jdbc Driver        </property>        <property name= show_sql >true</property>        <property name= current_session_context_class >thread</property>          <mapping resource= powerwind/bean/Student hbm xml />        <mapping resource= powerwind/bean/Teacher hbm xml />       </session factory> </hibernate configuration>   由于Person抽象类没有对应数据库的表 也没有对应的映射文件 在HQL查询中也就不支持多态查询 感觉上 Person抽象类的作用只是减少JAVA代码的编写而已 二 每个类对应一个表(Table per subclass) 人员表 create table `sample` `person`(         `id` bigint not null auto_increment        `name` varchar( )         primary key (`id`)     ); 学生表 create table `sample` `student`(         `id` bigint default not null        `score` float         primarykey (`id`)     ); 教师表 create table `sample` `teacher`(         `id` bigint default not null        `salary` float         primary key (`id`)     );   两个子类的实现和前一种完全一样 Person类也只是去掉 abstract修饰符而已 映射文件只需要Person hbm xml一个即可   <hibernate mapping>     <class name= powerwind bean Person table= person        catalog= sample >        <id name= id type= java lang Long >            <column name= id />            <generator class= native ></generator>        </id>        <property name= name type= java lang String >            <column name= name length= not null= true />        </property>               <joined subclass name= powerwind bean Student            table= student >            <key column= id />            <property name= score type= java lang Float >               <column name= score precision= scale= />            </property>        </joined subclass>               <joined subclass name= powerwind bean Teacher            table= teacher >            <key column= id />            <property name= salary type= java lang Float >               <column name= salary precision= scale= />            </property>        </joined subclass>     </class> </hibernate mapping> 这种方式是支持多态查询的 多态查询语句 Query query=sdao getSession() createQuery( from Person );   三 一个表对多个类(Table per class hierarchy) 人员表     create table `sample` `person`(         `id` bigint not null auto_increment        `name` varchar( )        `score` float        `salary` float        `type` char( )         primary key (`id`)     ); Person hbm xml <hibernate mapping>     <class name= powerwind bean Person table= person        catalog= sample >        <id name= id type= java lang Long >            <column name= id />            <generator class= native ></generator>        </id>        <discriminator column= type type= java lang String />        <property name= name type= java lang String >            <column name= name length= not null= true />        </property>               <subclass name= powerwind bean Student            discriminator value= S >            <property name= score type= java lang Float />        </subclass>               <subclass name= powerwind bean Teacher            discriminator value= T >            <property name= salary type= java lang Float />        </subclass>     </class> </hibernate mapping> 优点是单表查询 支持多态 缺点是要在表增加字段（type）用于区分子类 附加 实体粒度设计面向设计的细粒度 人员表     create table `sample` `person`(         `id` bigint not null auto_increment        `name` varchar( )        `email` varchar( )        `phone` varchar( )         primary key (`id`)     ); Person类 publicclassPersonimplements java io Serializable {     private Long id;     private String name;     private Contact contact; } Contact类 publicclassContactimplements java io Serializable {     private String email;     private String phone; } 注 以上两个类的代码省略了getter和setter方法 Person hbm xml <hibernate mapping>     <class name= powerwind bean Person table= person        catalog= sample >        <id name= id type= java lang Long >            <column name= id />            <generator class= native ></generator>        </id>        <property name= name type= string column= name />               <ponent name= contact class= powerwind bean Contact >            <property name= email type= string column= email />            <property name= phone type= string column= phone />        </ponent>     </class> </hibernate mapping> 这样的细粒度 有什么用呢？应该在处理比较复杂表结构才体现出来吧   面向性能的细粒度 假如Contact类包含的字段是重量级的数据 如图片之类 而我们一般可能只需要一些简单的信息摘要 要怎么做呢？       create table `sample` `person`(         `id` bigint not null auto_increment        `name` varchar( ) `gender` varchar( )        `email` varchar( )        `phone` varchar( )         primary key (`id`)     ); 首先定义个基类BasePerson publicclass BasePerson {     private Long id;     private String name; }   Person类继承BasePerson类 添加多一个gender属性 PersonDetail类继承Person类 添加多一个email和phone属性 Person hbm xml     <class name= powerwind bean Person table= person        catalog= sample >        <id name= id type= java lang Long >            <column name= id />            <generator class= native ></generator>        </id>        <property name= name type= string column= name />        <property name= gender type= string column= gender />     </class> PersonDetail hbm xml     <class name= powerwind bean PersonDetail table= person        catalog= sample polymorphism= explicit >        <id name= id type= java lang Long >            <column name= id />            <generator class= native ></generator>        </id>        <property name= name type= string column= name />        <property name= gender type= string column= gender />        <property name= phone type= string column= phone />        <property name= email type= string column= email />     </class>   除了polymorphism= explicit 这一句 和每个子类对应一个数据表的继承关系没有什么区别 正是这句设置 去除了对PersonDetail的隐式多态查询        Query query= getSession() createQuery( from Person );        Query query= getSession() createQuery( from PersonDetail ); 上面两句中 第一句并不会查询到PersonDetail 对象 即查询字段不包括email和phone   注 参考《深入浅出hibernate》一书 lishixinzhi/Article/program/Java/ky/201311/28608