JPA一对一映射 JPA一对一映射|JPA

一对一映射表示单值关联, 其中一个实体的实例与另一实体的实例关联。在这种类型的关联中, 可以将源实体的一个实例映射到目标实体的至少一个实例。
@OneToOne示例在此示例中, 我们将在学生与图书馆之间建立一对一关系, 以使一个学生只能获得一种类型的书。
【JPA一对一映射】本示例包含以下步骤：-

在com.srcmini.mapping包下创建包含学生ID（s_id）和学生姓名（s_name）的实体类Student.java。

Student.java

package com.srcmini.mapping; import javax.persistence.*; @Entitypublic class Student { @Id @GeneratedValue(strategy=GenerationType.AUTO) private int s_id; private String s_name; 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; } }

在com.srcmini.mapping包下创建另一个实体类Library.java, 其中包含书ID（b_id）, 书名（b_name）和标有@OneToOne注释的学生类型的对象。

Library.java

package com.srcmini.mapping; import javax.persistence.*; @Entitypublic class Library { @Id @GeneratedValue(strategy=GenerationType.AUTO)private int b_id; private String b_name; @OneToOneprivate Student stud; public Library(int b_id, String b_name, Student stud) { super(); this.b_id = b_id; this.b_name = b_name; this.stud = stud; }public Library() { super(); }public int getB_id() { return b_id; }public void setB_id(int b_id) { this.b_id = b_id; }public String getB_name() { return b_name; }public void setB_name(String b_name) { this.b_name = b_name; }public Student getStud() { return stud; }public void setStud(Student stud) { this.stud = stud; } }

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="Book_issued"> < class> com.srcmini.mapping.Student< /class> < class> com.srcmini.mapping.Library< /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/mapping_db"/> < 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.OneToOne包下创建一个持久性类OneToOneExample, 以将实体对象与数据持久化。

OneToOneExample.java

import javax.persistence.*; import com.srcmini.mapping.*; public class OneToOneExample { public static void main(String[] args) {EntityManagerFactory emf = Persistence.createEntityManagerFactory( "Book_issued" ); EntityManager em = emf.createEntityManager( ); em.getTransaction( ).begin( ); Student st1=new Student(); st1.setS_id(1); st1.setS_name("Vipul"); Student st2=new Student(); st2.setS_id(2); st2.setS_name("Vimal"); em.persist(st1); em.persist(st2); Library lib1=new Library(); lib1.setB_id(101); lib1.setB_name("Data Structure"); lib1.setStud(st1); Library lib2=new Library(); lib2.setB_id(102); lib2.setB_name("DBMS"); lib2.setStud(st2); em.persist(lib1); em.persist(lib2); em.getTransaction().commit(); em.close(); emf.close(); } }

输出：
程序执行后, 在MySQL工作台下生成了两个表。