JPA级联持久化

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。

StudentEntity.java

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。

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.*;
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：

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。

• 主题表-要获取数据, 请在MySQL中运行select * from subject。