JPA继承-连接策略
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在连接策略中，为每个实体类生成一个单独的表。 每个表的属性都与主键连接。 它消除了字段字重复的可能性。
以下语法表示连接的策略: -
@Inheritance(strategy=InheritanceType.JOINED)
连接策略示例
在这个例子中，我们将员工分为活跃员工和退休员工。
因此，子类ActiveEmployees和RetiredEmployees继承父类Employee的e_id和e_name字段。
现在，按照以下步骤创建JPA项目 -
第1步: 在com.yiibai.jpa.inheritence包下创建一个根实体类Employee.java并指定所有必需的属性和注释。
文件:Employee.java -
package com.yiibai.jpa.inheritence;
import java.io.Serializable;
import javax.persistence.*;
@Entity
@Table(name = "employee_details")
@Inheritance(strategy=InheritanceType.JOINED)
public class Employee implements Serializable {
@Id
private int e_id;
private String e_name;
public Employee(int e_id, String e_name) {
super();
this.e_id = e_id;
this.e_name = e_name;
}
public Employee() {
super();
}
public int getE_id() {
return e_id;
}
public void setE_id(int e_id) {
this.e_id = e_id;
}
public String getE_name() {
return e_name;
}
public void setE_name(String e_name) {
this.e_name = e_name;
}
}
第2步: 在com.yiibai.jpa.inheritence包下创建实体类ActiveEmployee.java(它是Employee类的子类)。
文件:ActiveEmployee.java -
package com.yiibai.jpa.inheritence;
import javax.persistence.*;
@Entity
public class ActiveEmployee extends Employee {
private int e_salary;
private int e_experience;
public ActiveEmployee(int e_id, String e_name, int e_salary, int e_experience) {
super(e_id, e_name);
this.e_salary = e_salary;
this.e_experience = e_experience;
}
public ActiveEmployee() {
super();
}
public int getE_salary() {
return e_salary;
}
public void setE_salary(int e_salary) {
this.e_salary = e_salary;
}
public int getE_experience() {
return e_experience;
}
public void setE_experience(int e_experience) {
this.e_experience = e_experience;
}
}
第3步: 在com.yiibai.jpa.inheritence包下创建另一个实体类RetiredEmployee.java(它是Employee.java的子类)。
文件:RetiredEmployee.java -
package com.yiibai.jpa.inheritence;
import javax.persistence.*;
@Entity
public class RetiredEmployee extends Employee {
private int e_pension;
public RetiredEmployee(int e_id, String e_name, int e_pension) {
super(e_id, e_name);
this.e_pension = e_pension;
}
public RetiredEmployee() {
super();
}
public int getE_pension() {
return e_pension;
}
public void setE_pension(int e_pension) {
this.e_pension = e_pension;
}
}
第4步: 将实体类和其他数据库配置映射到Persistence.xml文件中。persistence.xml中
文件:persistence.xml -
<?xml version="1.0" encoding="UTF-8"?>
<persistence version="2.1"
xmlns="http://xmlns.jcp.org/xml/ns/persistence"
xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance"
xsi:schemaLocation="http://xmlns.jcp.org/xml/ns/persistence http://xmlns.jcp.org/xml/ns/persistence/persistence_2_1.xsd">
<persistence-unit name="Employee_details">
<class>com.yiibai.jpa.inheritence.ActiveEmployee</class>
<class>com.yiibai.jpa.inheritence.RetiredEmployee</class>
<class>com.yiibai.jpa.inheritence.Employee</class>
<properties>
<property name="javax.persistence.jdbc.driver"
value="com.mysql.jdbc.Driver" />
<property name="javax.persistence.jdbc.url"
value="jdbc:mysql://localhost:3306/testdb?serverTimezone=UTC&characterEncoding=utf8" />
<property name="javax.persistence.jdbc.user" value="root" />
<property name="javax.persistence.jdbc.password"
value="123456" />
<property name="eclipselink.logging.level" value="SEVERE" />
<property name="eclipselink.ddl-generation"
value="create-or-extend-tables" />
</properties>
</persistence-unit>
</persistence>
第5步:在com.yiibai.jpa.persistence包下创建持久性类EmployeePersistence.java。 这个类用于初始化一个对象并保存它。
文件: EmployeePersistence.java -
package com.yiibai.jpa.persistence;
import javax.persistence.*;
import com.yiibai.jpa.inheritence.*;
public class EmployeePersistence {
public static void main(String[] args) {
EntityManagerFactory emf = Persistence.createEntityManagerFactory("Employee_details");
EntityManager em = emf.createEntityManager();
em.getTransaction().begin();
ActiveEmployee ae1 = new ActiveEmployee(101, "李小云", 10000, 5);
ActiveEmployee ae2 = new ActiveEmployee(102, "张峰", 12000, 7);
RetiredEmployee re1 = new RetiredEmployee(103, "王四哥", 5000);
RetiredEmployee re2 = new RetiredEmployee(104, "叶问顶", 4000);
em.persist(ae1);
em.persist(ae2);
em.persist(re1);
em.persist(re2);
em.getTransaction().commit();
em.close();
emf.close();
}
}
执行程序后，在MySQL数据库中运行select * from employee_details查询，将会得到以下输出结果 -
mysql> select * from employee_details;
+------+-----------------+--------+
| E_ID | DTYPE | E_NAME |
+------+-----------------+--------+
| 101 | ActiveEmployee | 李小云 |
| 102 | ActiveEmployee | 张峰 |
| 104 | RetiredEmployee | 叶问顶 |
| 103 | RetiredEmployee | 王四哥 |
+------+-----------------+--------+
4 rows in set
查询activeemployee表，执行 Select * from activeemployee -
mysql> Select * from activeemployee;
+------+--------------+----------+
| E_ID | E_EXPERIENCE | E_SALARY |
+------+--------------+----------+
| 101 | 5 | 10000 |
| 102 | 7 | 12000 |
+------+--------------+----------+
2 rows in set
查询retired_employee表，执行 Select * from retired_employee -
mysql> Select * from retiredemployee;
+------+-----------+
| E_ID | E_PENSION |
+------+-----------+
| 104 | 4000 |
| 103 | 5000 |
+------+-----------+
2 rows in set以上是关于JPA继承-连接策略的主要内容，如果未能解决你的问题，请参考以下文章
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