JPA一对多映射 JPA一对多映射|JPA

一对多映射属于集合值关联的类别, 其中一个实体与其他实体的集合关联。因此, 在这种类型的关联中, 一个实体的实例可以与另一实体的任意数量的实例进行映射。
@OneToMany示例在这个例子中, 我们将在一个学生和图书馆之间建立一对多的关系, 这样一个学生可以得到一种以上类型的书。
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）, 学生姓名（s_name）和带有@OneToMany批注的学生名称（s_name）, 该批注包含List类型的Library类对象。

Student.java

package com.srcmini.mapping; import java.util.List; import javax.persistence.*; @Entitypublic class Student { @Id @GeneratedValue(strategy=GenerationType.AUTO) private int s_id; private String s_name; @OneToMany(targetEntity=Library.class) private List books_issued; public List getBooks_issued() {return books_issued; } public void setBooks_issued(List books_issued) {this.books_issued = books_issued; } 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.OneToOne包下创建一个持久性类OneToManyExample, 以将实体对象与数据持久化。

OneToManyExample.java

package com.srcmini.OneToMany; import java.util.ArrayList; import javax.persistence.*; import com.srcmini.mapping.Student; import com.srcmini.mapping.Library; public class OneToManyExample { public static void main(String[] args) {EntityManagerFactory emf=Persistence.createEntityManagerFactory("books_issued"); EntityManager em=emf.createEntityManager(); em.getTransaction().begin(); Library lib1=new Library(); lib1.setB_id(101); lib1.setB_name("Data Structure"); Library lib2=new Library(); lib2.setB_id(102); lib2.setB_name("DBMS"); em.persist(lib1); em.persist(lib2); ArrayList< Library> list=new ArrayList< Library> (); list.add(lib1); list.add(lib2); Student st1=new Student(); st1.setS_id(1); st1.setS_name("Vipul"); st1.setBooks_issued(list); em.persist(st1); em.getTransaction().commit(); em.close(); emf.close(); } }

输出：
【JPA一对多映射】程序执行后, 在MySQL工作台下生成了三个表。