JPA Map映射
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小牛编辑
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2023-03-14
Map
是一个接口,其中一个唯一键与每个值对象相关联。 因此,搜索,更新,删除等操作都是基于键来进行的。
完整的项目目录结构如下所示 -
集合映射实例
在这个例子中,我们将一个对象嵌入到一个实体类中,并将其定义为一个集合类型 - Map
。
private Map<Integer,Address> map=new HashMap<Integer,Address>();
这个例子包含以下步骤 -
第1步: 在com.yiibai.jpa
包下创建一个实体类Employee.java
,这个类包含员工id
,name
和嵌入对象(员工地址)。注解 @ElementCollection
表示嵌入对象。
文件:Employee.java 的代码如下所示 -
package com.yiibai.jpa;
import java.util.*;
import javax.persistence.*;
@Entity
public class Employee {
@Id
@GeneratedValue(strategy = GenerationType.AUTO)
private int e_id;
private String e_name;
@ElementCollection
private Map<Integer, Address> map = new HashMap<Integer, 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 Map<Integer, Address> getMap() {
return map;
}
public void setMap(Map<Integer, Address> map) {
this.map = map;
}
}
第2步: 在com.yiibai.jpa
包下创建一个嵌入对象Address.java
类。 注解@Embeddable
表示可嵌入对象。
文件:Address.java 的代码如下所示 -
package com.yiibai.jpa;
import javax.persistence.*;
@Embeddable
public 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;
}
}
第3步: 将实体类和其他数据库配置映射到persistence.xml
文件中。
文件:persistence.xml 的代码如下所示 -
<?xml version="1.0" encoding="UTF-8"?>
<persistence version="2.1"
xmlns="http://xmlns.jcp.org/xml/ns/persistence"
xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance"
xsi:schemaLocation="http://xmlns.jcp.org/xml/ns/persistence http://xmlns.jcp.org/xml/ns/persistence/persistence_2_1.xsd">
<persistence-unit name="Collection_Type">
<class>com.yiibai.jpa.Employee</class>
<class>com.yiibai.jpa.Address</class>
<properties>
<property name="javax.persistence.jdbc.driver"
value="com.mysql.jdbc.Driver" />
<property name="javax.persistence.jdbc.url"
value="jdbc:mysql://localhost:3306/testdb?serverTimezone=UTC" />
<property name="javax.persistence.jdbc.user" value="root" />
<property name="javax.persistence.jdbc.password"
value="123456" />
<property name="eclipselink.logging.level" value="SEVERE" />
<property name="eclipselink.ddl-generation"
value="create-or-extend-tables" />
</properties>
</persistence-unit>
</persistence>
在com.yiibai.collection
包下创建一个持久化类MapMapping.java
,以便将实体对象与数据保持一致。
文件:MapMapping.java 的代码如下所示 -
package com.yiibai.collection;
import javax.persistence.*;
import com.yiibai.jpa.*;
public class MapMapping {
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(511000);
a1.setE_city("Guangzhou");
a1.setE_state("Guangdong");
Address a2 = new Address();
a2.setE_pincode(202001);
a2.setE_city("Nanjing");
a2.setE_state("Jiangsu");
Address a3 = new Address();
a3.setE_pincode(333301);
a3.setE_city("Chengdu");
a3.setE_state("Shichuan");
Address a4 = new Address();
a4.setE_pincode(80001);
a4.setE_city("Haikou");
a4.setE_state("Hainan");
Employee e1 = new Employee();
e1.setE_id(1);
e1.setE_name("Maxsu");
Employee e2 = new Employee();
e2.setE_id(2);
e2.setE_name("Leeze");
Employee e3 = new Employee();
e3.setE_id(3);
e3.setE_name("William");
Employee e4 = new Employee();
e4.setE_id(4);
e4.setE_name("Curry");
e1.getMap().put(1, a1);
e2.getMap().put(2, a2);
e3.getMap().put(3, a3);
e4.getMap().put(4, a4);
em.persist(e1);
em.persist(e2);
em.persist(e3);
em.persist(e4);
em.getTransaction().commit();
em.close();
emf.close();
}
}
程序输出结果
程序执行完成后,下面的表格在MySQL工作台下生成。
- Employee表 - 此表包含员工详细信息。要获取数据,请在MySQL中运行
select * from employee
查询。结果如下所示 -
mysql> select * from employee;
+------+---------+
| E_ID | E_NAME |
+------+---------+
| 2 | Leeze |
| 3 | William |
| 4 | Curry |
| 1 | Maxsu |
+------+---------+
4 rows in set
- employee_address表 - 此表表示员工和地址表之间的映射。 要获取数据,请在MySQL中运行
select * from employee_address
查询。结果如下所示 -
mysql> select * from employee_address;
+-----------+-----------+-----------+---------------+
| E_CITY | E_PINCODE | E_STATE | Employee_E_ID |
+-----------+-----------+-----------+---------------+
| Hangzhou | 533300 | Zhejiang | 1003 |
| Nanjing | 780000 | Jiangsu | 1004 |
| Guangzhou | 501000 | GuangDong | 1000 |
| Haikou | 571100 | Hainan | 1002 |
+-----------+-----------+-----------+---------------+
4 rows in set