JPA 单表策略

2024 年 8 月 29 日 | 阅读 2 分钟

单表策略是定义继承实现的最简单和有效的方法之一。在这种方法中，多个实体类的实例仅作为属性存储在单个表中。

以下语法表示单表策略：-

单表策略示例

在此示例中，我们将员工分为在职员工和退休员工。

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：-

在 com.javatpoint.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。

Employee.java

package com.javatpoint.jpa.inheritence;

import java.io.Serializable;

import javax.persistence.*;

@Entity
@Table(name="employee_details")
@Inheritance(strategy=InheritanceType.SINGLE_TABLE)
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;
}

}

在 com.javatpoint.jpa.inheritence 包下创建一个实体类 ActiveEmployee.java（Employee.java 的子类）。

ActiveEmployee.java

package com.javatpoint.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;
	}
	
	
	
}

在 com.javatpoint.jpa.inheritence 包下创建另一个实体类 RetiredEmployee.java（Employee.java 的子类）。

RetiredEmployee.java

package com.javatpoint.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;
	}

	

	
}

现在，将实体类和其他数据库配置映射到 Persistence.xml 文件中。

Persistence.xml

<persistence>
<persistence-unit name="Employee_details">
   
      <class>com.javatpoint.jpa.inheritence.ActiveEmployee</class>
      <class>com.javatpoint.jpa.inheritence.RetiredEmployee</class>
      <class>com.javatpoint.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://:3306/employee"/>
         <property name="javax.persistence.jdbc.user" value="root"/>
         <property name="javax.persistence.jdbc.password" value=""/>
         <property name="eclipselink.logging.level" value="SEVERE"/>
         <property name="eclipselink.ddl-generation" value="create-or-extend-tables"/>
      </properties>
      </persistence-unit>
      </persistence>

在 com.javatpoint.jpa.persistence 包下创建一个持久化类 EmployeePersistence.java。该类用于初始化对象并持久化它。

EmployeePersistence.java

package com.javatpoint.jpa.persistence;

import javax.persistence.*;
import com.javatpoint.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,"Karun",10000,5);
ActiveEmployee ae2=new ActiveEmployee(102,"Rishab",12000,7);

RetiredEmployee re1=new RetiredEmployee(103,"Ramesh",5000);
RetiredEmployee re2=new RetiredEmployee(104,"Raj",4000);

	em.persist(ae1);
	em.persist(ae2);
	
	em.persist(re1);
	em.persist(re2);
	
	em.getTransaction().commit();
	
	em.close();
	emf.close();
	
}}

输出

程序执行完毕后，在MySQL数据库中运行 select * from employee_details 查询以生成以下输出：-