在联合策略中, 将为每个实体类生成一个单独的表。每个表的属性都与主键结合在一起。它消除了重复的可能性。
以下语法代表了连接的策略：-
@Inheritance(strategy=InheritanceType.JOINED)连接策略实例 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.srcmini.jpa.inheritence包下创建一个根实体类Employee.java, 并指定所有必需的变量和注释。
package com.srcmini.jpa.inheritence;
import java.io.Serializable;
import javax.persistence.*;
@Entity@Table(name="employee_details")@Inheritance(strategy=InheritanceType.JOINED)public class Employee implements Serializable { @Idprivate 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.srcmini.jpa.inheritence包下创建一个实体类ActiveEmployee.java（Employee.java的子类）。
package com.srcmini.jpa.inheritence;
import javax.persistence.*;
@Entitypublic 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.srcmini.jpa.inheritence包下创建另一个实体类RetiredEmployee.java（Employee.java的子类）。
package com.srcmini.jpa.inheritence;
import javax.persistence.*;
@Entitypublic 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, 在Persistence.xml文件中映射实体类和其他数据库配置。
<
persistence>
<
persistence-unit name="Employee_details">
<
class>
com.srcmini.jpa.inheritence.ActiveEmployee<
/class>
<
class>
com.srcmini.jpa.inheritence.RetiredEmployee<
/class>
<
class>
com.srcmini.jpa.inheritence.Employee<
/class>
<
properties>
<
property name="javax.persistence.jdbc.driver" value="http://www.srcmini.com/com.mysql.jdbc.Driver"/>
<
property name="javax.persistence.jdbc.url" value="http://www.srcmini.com/jdbc:mysql://localhost:3306/employee"/>
<
property name="javax.persistence.jdbc.user" value="http://www.srcmini.com/root"/>
<
property name="javax.persistence.jdbc.password" value=""/>
<
property name="eclipselink.logging.level" value="http://www.srcmini.com/SEVERE"/>
<
property name="eclipselink.ddl-generation" value="http://www.srcmini.com/create-or-extend-tables"/>
<
/properties>
<
/persistence-unit>
<
/persistence>
- 在com.srcmini.jpa.persistence包下创建一个持久性类EmployeePersistence.java。此类用于初始化对象并将其持久化。
package com.srcmini.jpa.persistence;
import javax.persistence.*;
import com.srcmini.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();
}}程序执行后, 以下目录层次结构在MySQL工作台下生成。
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输出：
【JPA连接策略】现在, 分别从每个表中获取数据以生成输出。
- 从employee_details中选择*
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- 选择* from active_employee
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- 选择* from退休员工
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