级联删除用于指定如果删除父实体, 那么其所有相关实体也将被删除。以下语法用于执行级联删除操作:-
@OneToOne(cascade=CascadeType.REMOVE)
级联删除示例
在此示例中, 我们将创建两个相互关联的实体类, 但是要建立它们之间的依赖关系, 我们将执行级联操作。
本示例包含以下步骤:-
- 在com.srcmini.jpa.student包下创建一个名为StudentEntity.java的实体类, 该实体类包含s_id, s_name, s_age属性和标有级联规范的Subject类型的对象。
StudentEntity.java
package com.srcmini.jpa.student;
import javax.persistence.*;
import com.srcmini.jpa.subject.Subject;
@Entity
@Table(name="student")
public class StudentEntity {
@Id
private int s_id;
private String s_name;
private int s_age;
@OneToOne(cascade={CascadeType.REMOVE})
private Subject sub;
public Subject getSub() {
return sub;
}
public void setSub(Subject sub) {
this.sub = sub;
}
public StudentEntity(int s_id, String s_name, int s_age , Subject sub) {
super();
this.s_id = s_id;
this.s_name = s_name;
this.s_age = s_age;
this.sub=sub;
}
public StudentEntity() {
super();
}
public int getS_id() {
return s_id;
}
public void setS_id(int s_id) {
this.s_id = s_id;
}
public String getS_name() {
return s_name;
}
public void setS_name(String s_name) {
this.s_name = s_name;
}
public int getS_age() {
return s_age;
}
public void setS_age(int s_age) {
this.s_age = s_age;
}
}
- 在com.srcmini.jpa.subject包下创建另一个名为Subject.java的实体类。
Subject.java
package com.srcmini.jpa.subject;
import javax.persistence.*;
@Entity
@Table(name="subject")
public class Subject {
private String name;
private int marks;
@Id
private int s_id;
public Subject(String name, int marks, int s_id) {
super();
this.name = name;
this.marks = marks;
this.s_id=s_id;
}
public Subject()
{
super();
}
public String getName() {
return name;
}
public void setName(String name) {
this.name = name;
}
public int getMarks() {
return marks;
}
public void setMarks(int marks) {
this.marks = marks;
}
public int getS_id() {
return s_id;
}
public void setS_id(int s_id) {
this.s_id = s_id;
}
}
- 现在, 在Persistence.xml文件中映射实体类和其他数据库配置。
Persistence.xml
<persistence>
<persistence-unit name="Student_details">
<class>com.srcmini.jpa.student.StudentEntity</class>
<class>com.srcmini.jpa.subject.Subject</class>
<properties>
<property name="javax.persistence.jdbc.url" value="jdbc:mysql://localhost:3306/studentdata"/>
<property name="javax.persistence.jdbc.user" value="root"/>
<property name="javax.persistence.jdbc.password" value=""/>
<property name="javax.persistence.jdbc.driver" value="com.mysql.jdbc.Driver"/>
<property name="eclipselink.logging.level" value="SEVERE"/>
<property name="eclipselink.ddl-generation" value="create-or-extend-tables"/>
</properties>
</persistence-unit>
</persistence>
- 在com.srcmini.jpa.cascade包下创建一个名为StudentCascade.java的持久性类, 以将实体对象与数据持久化。
StudentCascade.java
package com.srcmini.jpa.cascade;
import javax.persistence.*;
import com.srcmini.jpa.student.*;
public class StudentCascade {
public static void main( String[ ] args ) {
EntityManagerFactory emf = Persistence.createEntityManagerFactory( "Student_details" );
EntityManager em = emf.createEntityManager( );
em.getTransaction().begin();
StudentEntity s=em.find(StudentEntity.class, 101);
em.remove(s);
em.getTransaction().commit();
em.close( );
emf.close( );
}
}
输出:
程序执行后, 将在MySQL工作台下生成以下表格。
- 学生表-具有s_id 101的学生的详细信息将从学生表中删除。要获取数据, 请在MySQL中从学生查询中运行select *。
- 主题表-由于级联删除, 与s_id 101相关的详细信息也将从主题表中删除。要检查结果, 请在MySQL中运行select * from subject query
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