使用情景

.將當前主鍵的值單獨儲存到一個數據庫的表中,主鍵的值每次都是從指定的表中查詢來獲得

.這種方法生成主鍵的策略可以適用於任何資料庫,不必擔心不同資料庫不相容造成的問題

.準備前提:

-建立一個jpa專案

-配置persistence.xml檔案:

<?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">
        <!-- 改配置檔名字和位置必須是固定的 -->
        <!-- 配置使用什麼orm產品來作為jpa的實現 -->
        <provider>org.hibernate.ejb.HibernatePersistence</provider>
        <!-- 新增持久化類 -->
        <class>com.wayne.helloworld.Customer</class>
        <properties>
            <!-- 連線資料庫基本資訊 -->
            <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"/>
        <!-- 配置JPA實現產品的基本屬性，配置hibernate的基本屬性 -->
        <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>

-新增jar包:

1.首先建立如下表:

2.實體類程式碼如下:

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;
/*
 * 如果不新增@Table註解,則表名與實體類名相同
 */
@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",
//橫向找到PK_NAME這一列
pkColumnName="PK_NAME",
pkColumnValue="CUSTOMER_ID",
//縱向找到PK_VALUE這一列
valueColumnName="PK_VALUE",
//增長值
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;
}
//若屬性名與欄位名相同,則不需要填寫註解
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;
}
//精度時間戳
@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,不需要對映為某個資料庫表的一列
@Transient
public String getInfo(){
    return "lastName:"+this.getLastName();
}
}

3.測試類程式碼如下:

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.建立EntitymanagerFactory
        //是persistence.xml的persistence-unit標籤name屬性
        String persistenceUnitName="jpa_1_helloworld";
        EntityManagerFactory entityManagerFactory=Persistence.createEntityManagerFactory(persistenceUnitName);
        //2.建立EntityManager
        EntityManager entityManager=entityManagerFactory.createEntityManager();
        //3.開啟事務
        EntityTransaction transaction = entityManager.getTransaction();
        transaction.begin();
        //4.進行持久化操作
        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.提交事務
        transaction.commit();
        //6.關閉EntityManager
        entityManager.close();
        //7.關閉EntitymanagerFactory
        entityManagerFactory.close();
    }

}

4.執行後資料庫結果: