JPA集映射 JPA集映射|JPA

Set是包含唯一元素的接口。这些元素不维持任何顺序。当需要以无序方式检索唯一元素时, 可以使用Set。
设置映射示例在此示例中, 我们将对象嵌入实体类中, 并将其定义为集合类型List。

private Set< Address> address=new HashSet< Address> ();

本示例包含以下步骤：-

在com.srcmini.jpa包下创建一个实体类Employee.java, 其中包含员工ID, 姓名和嵌入式对象（员工地址）。注释@ElementCollection表示嵌入式对象。

Employee.java

package com.srcmini.jpa; import java.util.*; import javax.persistence.*; @Entitypublic class Employee { @Id @GeneratedValue(strategy=GenerationType.AUTO) private int e_id; private String e_name; @ElementCollection private Set< Address> address=new HashSet< Address> (); public int getE_id() {return e_id; } public void setE_id(int e_id) {this.e_id = e_id; } public String getE_name() {return e_name; } public void setE_name(String e_name) {this.e_name = e_name; } public Set< Address> getAddress() {return address; } public void setAddress(Set< Address> address) {this.address = address; } }

现在, 在com.srcmini.jpa包下创建一个嵌入式对象Address.java类。批注@Embeddable表示可嵌入对象。

【JPA集映射】Address.java

package com.srcmini.jpa; import javax.persistence.*; @Embeddablepublic class Address { private int e_pincode; private String e_city; private String e_state; public int getE_pincode() {return e_pincode; } public void setE_pincode(int e_pincode) {this.e_pincode = e_pincode; } public String getE_city() {return e_city; } public void setE_city(String e_city) {this.e_city = e_city; } public String getE_state() {return e_state; } public void setE_state(String e_state) {this.e_state = e_state; } }

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="Collection_Type"> < class> com.srcmini.jpa.Employee< /class> < class> com.srcmini.jpa.Address< /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/collection_mapping"/> < 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.collection包下创建一个持久性类ListMapping.java, 以将实体对象与数据持久化。

SetMapping.java

package com.srcmini.collection; import javax.persistence.*; import com.srcmini.jpa.*; public class SetMapping{ public static void main(String[] args) {EntityManagerFactory emf=Persistence.createEntityManagerFactory("Collection_Type"); EntityManager em=emf.createEntityManager(); em.getTransaction().begin(); Address a1=new Address(); a1.setE_pincode(201301); a1.setE_city("Noida"); a1.setE_state("Uttar Pradesh"); Address a2=new Address(); a2.setE_pincode(302001); a2.setE_city("Jaipur"); a2.setE_state("Rajasthan"); Address a3=new Address(); a3.setE_pincode(133301); a3.setE_city("Chandigarh"); a3.setE_state("Punjab"); Address a4=new Address(); a4.setE_pincode(80001); a4.setE_city("Patna"); a4.setE_state("Bihar"); Employee e1=new Employee(); e1.setE_id(1); e1.setE_name("Vijay"); Employee e2=new Employee(); e2.setE_id(2); e2.setE_name("Vijay"); Employee e3=new Employee(); e3.setE_id(3); e3.setE_name("William"); Employee e4=new Employee(); e4.setE_id(4); e4.setE_name("Rahul"); e1.getAddress().add(a1); e2.getAddress().add(a2); e3.getAddress().add(a3); e4.getAddress().add(a4); em.persist(e1); em.persist(e2); em.persist(e3); em.persist(e4); em.getTransaction().commit(); em.close(); emf.close(); } }

输出：
程序执行后, 将在MySQL工作台下生成以下表格。