JPA:使用外部table来生成主键

使用情景

.将当前主键的值单独保存到一个数据库的表中,主键的值每次都是从指定的表中查询来获得

.这种方法生成主键的策略可以适用于任何数据库,不必担心不同数据库不兼容造成的问题

.准备前提:

-创建一个jpa项目

-配置persistence.xml文件:

<?xml version="1.0" encoding="UTF-8"?>
<persistence version="2.0"
   xmlns="http://java.sun.com/xml/ns/persistence" xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance"
   xsi:schemaLocation="http://java.sun.com/xml/ns/persistence http://java.sun.com/xml/ns/persistence/persistence_2_0.xsd">
   <persistence-unit name="jpa_1_helloworld" transaction-type="RESOURCE_LOCAL">
       
       
       <provider>org.hibernate.ejb.HibernatePersistence</provider>
       
       <class>com.wayne.helloworld.Customer</class>
       <properties>
           
           <property name="javax.persistence.jdbc.driver" value="com.mysql.jdbc.Driver"/>
           <property name="javax.persistence.jdbc.url" value="jdbc:mysql:///jpa"/>
           <property name="javax.persistence.jdbc.user" value="root"/>
           <property name="javax.persistence.jdbc.password" value="root"/>
       
       <property name="hibernate.format_sql" value="true"/>
       <property name="hibernate.show_sql" value="true"/>
       <property name="hibernate.hbm2ddl.auto" value="update"/>
       </properties>
   </persistence-unit>
</persistence>

-添加jar包:

1.首先创建如下表:

2.实体类代码如下:

package com.wayne.helloworld;

import java.util.Date;

import javax.persistence.Column;
import javax.persistence.Entity;
import javax.persistence.GeneratedValue;
import javax.persistence.GenerationType;
import javax.persistence.Id;
import javax.persistence.Table;
import javax.persistence.TableGenerator;
import javax.persistence.Temporal;
import javax.persistence.TemporalType;
import javax.persistence.Transient;
/*
* 如果不添加@Table注解,则表名与实体类名相同
*/
@Table(name="JPA_CUSTOMERS")
@Entity
public class Customer {
private Integer id;
private String lastName;
private String email;
private int age;
private Date createdDate;
private Date birthDay;
@Column(name="ID")
@TableGenerator(name="ID_GENERATOR",
table="JPA_ID_GENERATOR",
//横向找到PK_NAME这一列
pkColumnName="PK_NAME",
pkColumnValue="CUSTOMER_ID",
//纵向找到PK_VALUE这一列
valueColumnName="PK_VALUE",
//增长值
allocationSize=100)
@GeneratedValue(strategy=GenerationType.TABLE,generator="ID_GENERATOR")
@Id
public Integer getId() {
   return id;
}
public void setId(Integer id) {
   this.id = id;
}
@Column(name="LAST_NAME",length=50,nullable=true)
public String getLastName() {
   return lastName;
}
public void setLastName(String lastName) {
   this.lastName = lastName;
}
//若属性名与字段名相同,则不需要填写注解
public String getEmail() {
   return email;
}
public void setEmail(String email) {
   this.email = email;
}
public int getAge() {
   return age;
}
public void setAge(int age) {
   this.age = age;
}
//精度时间戳
@Temporal(TemporalType.TIMESTAMP)
public Date getCreatedDate() {
   return createdDate;
}
public void setCreatedDate(Date createdDate) {
   this.createdDate = createdDate;
}
@Temporal(TemporalType.DATE)
public Date getBirthDay() {
   return birthDay;
}
public void setBirthDay(Date birthDay) {
   this.birthDay = birthDay;
}
//工具方法,不需要映射为某个数据库表的一列
@Transient
public String getInfo(){
   return "lastName:"+this.getLastName();
}
}

3.测试类代码如下:

package com.wayne.helloworld;

import java.util.Date;

import javax.persistence.EntityManager;
import javax.persistence.EntityManagerFactory;
import javax.persistence.EntityTransaction;
import javax.persistence.Persistence;

public class Test {

   public static void main(String[] args) {
       // TODO Auto-generated method stub
       //1.创建EntitymanagerFactory
       //是persistence.xml的persistence-unit标签name属性
       String persistenceUnitName="jpa_1_helloworld";
       EntityManagerFactory entityManagerFactory=Persistence.createEntityManagerFactory(persistenceUnitName);
       //2.创建EntityManager
       EntityManager entityManager=entityManagerFactory.createEntityManager();
       //3.开启事务
       EntityTransaction transaction = entityManager.getTransaction();
       transaction.begin();
       //4.进行持久化操作
       Customer customer=new Customer();
       customer.setAge(22);
       customer.setEmail("www.wayne.com");
       customer.setLastName("wayne");
       customer.setCreatedDate(new Date());
       customer.setBirthDay(new Date());
       entityManager.persist(customer);
       //5.提交事务
       transaction.commit();
       //6.关闭EntityManager
       entityManager.close();
       //7.关闭EntitymanagerFactory
       entityManagerFactory.close();
   }

}

4.运行后数据库结果:

JPA:使用外部table来生成主键

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