知识大全 Hibernate的继承关系

Posted 2022-07-18 属性

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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 public abstract class Person implements 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 public class Student extends 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 publicclass Person implements java io Serializable private Long id; private String name; private Contact contact; Contact类 publicclass Contact implements 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 cha138/Article/program/Java/ky/201311/28608