JPA继承关系详解(2)

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虽然student与teacher有些许属性重复,但似乎传统方式在完成student与teacher没有什么问题,创建并不复杂…但如果,我们增加新的对象,医生,矿工,消防员,警察,程序员…这样看来传统方式并不适用…这就需要继承映射,Hbernate,TopLink等O-R映射框架都支持继承映射,在此,我会为大家讲解EJB3.0 JPA规范的继承映射!

学生,与老师共有的属性,属于所有人类都拥有的属性,故我们可以创建及Person类,来保存共有属性,而学生与老师继承自Person类,增加了自己的属性.

关键字 :Inheritance

对象使用引用以便关联到其它对象；关系型数据库表之间采用外键来描述表的关系。在关系型数据库中通常没有自然且有效的方法来描述类的继承关系。JPA通过Inheritance annotation提供了几种继承策略，它有以下属性：

InheritanceType strategy:用来声明继承策略。可选值是InheritanceType.SINGLE_TABLE、InheritanceType.JOINED和InheritanceType .TABLE_PER_CLASS。

缺省值是InheritanceType.SINGLE_TABLE。

关于Inheritance的更多内容，可以参考<< Hibernate实战>> by Christian Bauer, Gavin King。

一:

InheritanceType.SINGLE_TABLE 策略为类的继承体系采用同一个表。表名是基类的名称。例如：

首先创建父类:Person.java

package cn.vicky.test01;import java.io.Serializable;import javax.persistence.Basic;import javax.persistence.Column;import javax.persistence.Entity;import javax.persistence.EnumType;import javax.persistence.Enumerated;import javax.persistence.GeneratedValue;import javax.persistence.GenerationType;import javax.persistence.Id;import javax.persistence.Inheritance;import javax.persistence.InheritanceType;@Entity@Inheritance(strategy = InheritanceType.SINGLE_TABLE)// InheritanceType.SINGLE_TABLE 策略为类的继承体系采用同一个表。表名是基类的名称。public class Person implements Serializable {private static final long serialVersionUID = 1L;@Id@GeneratedValue(strategy = GenerationType.IDENTITY)private int id;/* Basic 注释表示该属性是基本属性 */@Basic@Column(length = 20, unique = true, nullable = false)private String name;@Basic@Enumerated(EnumType.STRING)@Column(length = 5, nullable = false)private Sex sex = Sex.MAN;@Basic@Column(length = 3, nullable = false)private Integer age = 1;public int getId() {return id;}public void setId(int id) {this.id = id;}public String getName() {return name;}public void setName(String name) {this.name = name;}public Sex getSex() {return sex;}public void setSex(Sex sex) {this.sex = sex;}public Integer getAge() {return age;}public void setAge(Integer age) {this.age = age;}}

Student.java

package cn.vicky.test01;import javax.persistence.Basic;import javax.persistence.Column;import javax.persistence.Entity;@Entitypublic class Student extends Person {private static final long serialVersionUID = 1L;/* Basic 注释表示该属性是基本属性 */@Basic@Column(length = 20, unique = true, nullable = true)private String study = "Java";public String getStudy() {return study;}public void setStudy(String study) {this.study = study;}}

Teacher.java

package cn.vicky.test01;import javax.persistence.Basic;import javax.persistence.Column;import javax.persistence.Entity;@Entitypublic class Teacher extends Person {private static final long serialVersionUID = 1L;/** Basic 注释表示该属性是基本属性 **/ @Basic @Column(length = 20, unique = true, nullable = true) private String teach = "大学日语";public String getTeach() {return teach;}public void setTeach(String teach) {this.teach = teach;} }

执行单元测试,生成数据库表:

sql语句是:

CREATE TABLE `person` (
`DTYPE` varchar(31) NOT NULL,
`id` int(11) NOT NULL AUTO_INCREMENT,
`age` int(11) NOT NULL,
`name` varchar(20) NOT NULL,
`sex` varchar(5) NOT NULL,
`study` varchar(20) DEFAULT NULL,
`teach` varchar(20) DEFAULT NULL,
PRIMARY KEY (`id`),
UNIQUE KEY `name` (`name`),
UNIQUE KEY `study` (`study`),
UNIQUE KEY `teach` (`teach`)
) ENGINE=InnoDB DEFAULT CHARSET=utf8;

其中的DTYPE,是记录Person的具体子类型的!我们向数据库表输入数据!

manager.getTransaction().begin(); //Student s = new Student();//s.setName("Vicky");//s.setAge(22);//s.setStudy("Flex");//s.setSex(Sex.MAN);//manager.persist(s); Teacher t = new Teacher();t.setName("GoodTeacher");t.setAge(43);t.setTeach("大学英语");t.setSex(Sex.WOMEN);manager.getTransaction().commit();

查看数据库表:

但是,这样将所有对象保存在一个表中,增加了表的容量,这就需要使用InheritanceType.JOINED