JPA多对一映射 JPA多对一映射|JPA

多对一映射表示单值关联, 其中一组实体可以与相似实体关联。因此, 在关系数据库中, 一个实体的多于一行可以引用另一实体的相似行。
@ManyToOne示例在此示例中, 我们将以一种方式, 在一个学生和图书馆之间创建多对一关系, 以使一个以上的学生可以发行同一本书。
本示例包含以下步骤：-

在com.srcmini.mapping包下创建一个实体类Student.java, 该包包含学生ID（s_id）和学生姓名（s_name）, 并带有@ManyToOne批注, 该批注包含Library类型的对象。

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; @ManyToOne private Library lib; 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 Library getLib() {return lib; } public void setLib(Library lib) {this.lib = lib; } }

在com.srcmini.mapping包下创建另一个实体类Library.java, 其中包含书籍ID（b_id）, 书籍名称（b_name）。

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; public Library(int b_id, String b_name) { super(); this.b_id = b_id; this.b_name = b_name; }public Library() { super(); // TODO Auto-generated constructor stub}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; } }

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="books_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.ManyToOne包下创建一个持久性类ManyToOneExample以将实体对象与数据持久化。

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

输出：
【JPA多对一映射】程序执行后, 两个表在MySQL工作台下生成。