在按類表策略中，為每個子實體類生成一個單獨的表。 與連線策略不同，在按類表策略中不會為父實體類生成單獨的表。
以下語法表示按類表策略 -
@Inheritance(strategy=InheritanceType.TABLE_PER_CLASS)
在這個例子中，我們將員工分為活躍員工和退休員工。
因此，子類ActiveEmployees和RetiredEmployees繼承父類別Employee的e_id和e_name欄位。
現在，按照以下步驟建立JPA專案 -
第1步: 在com.yiibai.jpa.inheritence包下建立一個根實體類Employee.java並指定所有必需的屬性和注釋。
檔案:Employee.java -
package com.yiibai.jpa.inheritence;
import java.io.Serializable;
import javax.persistence.*;
@Entity
@Table(name = "employee_details")
@Inheritance(strategy=InheritanceType.TABLE_PER_CLASS)
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包下建立實體類ActiveEmployee.java(它是Employee類的子類)。
檔案: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包下建立另一個實體類RetiredEmployee.java(它是Employee.java的子類)。
檔案: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: 將實體類和其他資料庫組態對映到Persistence.xml檔案中。persistence.xml中
檔案: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包下建立永續性類EmployeePersistence.java。 這個類用於初始化一個物件並儲存它。
檔案: 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, "李小雲", 10000, 5);
ActiveEmployee ae2 = new ActiveEmployee(102, "張峰", 12000, 7);
RetiredEmployee re1 = new RetiredEmployee(103, "王四哥", 5000);
RetiredEmployee re2 = new RetiredEmployee(104, "葉問頂", 4000);
em.persist(ae1);
em.persist(ae2);
em.persist(re1);
em.persist(re2);
em.getTransaction().commit();
em.close();
emf.close();
}
}
執行程式後，在MySQL資料庫中執行select * from activeemployee查詢，將會得到以下輸出結果 -
mysql> Select * from activeemployee;
+------+--------------+--------+----------+
| E_ID | E_EXPERIENCE | E_NAME | E_SALARY |
+------+--------------+--------+----------+
| 101 | 5 | 李小雲 | 10000 |
| 102 | 7 | 張峰 | 12000 |
+------+--------------+--------+----------+
2 rows in set
查詢retiredemployee表，執行 Select * from retiredemployee -
mysql> Select * from retiredemployee;
+------+--------+-----------+
| E_ID | E_NAME | E_PENSION |
+------+--------+-----------+
| 103 | 王四哥 | 5000 |
| 104 | 葉問頂 | 4000 |
+------+--------+-----------+
2 rows in set