级联的持久性用于指定如果一个实体被持久化, 那么其所有关联的子实体也将被持久化。以下语法用于执行级联持久操作：-
@OneToOne(cascade=CascadeType.PERSIST)JPA级联持久化示例 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。
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.PERSIST) 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。
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>
<
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="http://www.srcmini.com/jdbc:mysql://localhost:3306/studentdata"/>
<
property name="javax.persistence.jdbc.user" value="http://www.srcmini.com/root"/>
<
property name="javax.persistence.jdbc.password" value=""/>
<
property name="javax.persistence.jdbc.driver" value="http://www.srcmini.com/com.mysql.jdbc.Driver"/>
<
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.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。
package com.srcmini.jpa.cascade;
import javax.persistence.*;
import com.srcmini.jpa.student.*;
import com.srcmini.jpa.subject.Subject;
public class StudentCascade {public static void main( String[ ] args ) {EntityManagerFactory emf = Persistence.createEntityManagerFactory( "Student_details" );
EntityManager em = emf.createEntityManager( );
em.getTransaction().begin();
StudentEntity s1=new StudentEntity();
s1.setS_id(101);
s1.setS_name("Vipul");
s1.setS_age(20);
StudentEntity s2=new StudentEntity();
s2.setS_id(102);
s2.setS_name("Aman");
s2.setS_age(22);
Subject sb1=new Subject();
sb1.setName("ENGLISH");
sb1.setMarks(80);
sb1.setS_id(s1.getS_id());
Subject sb2=new Subject();
sb2.setName("Maths");
sb2.setMarks(75);
sb2.setS_id(s2.getS_id());
s1.setSub(sb1);
s2.setSub(sb2);
em.persist( s1 );
//No need to perform persist operation separately for different entities.em.persist(s2);
em.getTransaction().commit();
em.close( );
emf.close( );
} }注意-学生表中的主键s_id将被视为主题表中的外键, 以维护两个表之间的关系。 输出：
【JPA级联持久化】程序执行后, 将在MySQL工作台下生成以下表格。
- 学生表-要获取数据, 请在MySQL中运行select * from student。
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- 主题表-要获取数据, 请在MySQL中运行select * from subject。
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