JPA:Java Persistence API是一個標準規範及接口(API)來實現ORM(object-relational mapping)框架，JPA通過註解(annotation) [如：@Entity、@Table、@Column等註解] 或XML描述物件－關係表的對映關係，並將執行期的實體物件持久化到資料庫中。

接續上一個文章:
JDBCtamplate 要新增實體類別是這樣:

@Data
public class User implements RowMapper<User> {
    private int id;
    private String username;
    private String password;
    //  必需重寫mapRow方法 新增實體類別方法
   @Override
    public User mapRow(ResultSet resultSet, int i) throws SQLException {
        User user = new User();
        user.setId(resultSet.getInt("id"));
        user.setUsername(resultSet.getString("username"));
        user.setPassword(resultSet.getString("password"));
        return user;
    }
}

JPA簡化成這樣:

@Data
@Entity
public class User{
    private int id;
    @Id
//ID 自動增加由資料庫自動管理
    @GeneratedValue(strategy = GenerationType.IDENTITY)
    private String username;
    private String password;
}

JPA的底層基本依賴設定，設定資料庫連接:

<dependencies>
    <dependency>
			<groupId>org.springframework.boot</groupId>
			<artifactId>spring-boot-starter-data-jpa</artifactId>
		</dependency>
        <dependency>
			<groupId>mysql</groupId>
			<artifactId>mysql-connector-java</artifactId>
			<scope>runtime</scope>
		</dependency>
</dependency>

#設定資料庫連接資訊:
####springboot用mysql等關聯式資料庫，需要設定連接資訊。application.properties檔案中設定

spring.datasource.url=jdbc:mysql://127.0.0.1/book?useUnicode=true&characterEncoding=utf-8&serverTimezone=UTC&useSSL=true
spring.datasource.username=root
spring.datasource.password=root
spring.datasource.driver-class-name=com.mysql.cj.jdbc.Driver
spring.datasource.driver-class-name=com.mysql.jdbc.Driver
spring.jpa.properties.hibernate.hbm2ddl.auto=update
spring.jpa.properties.hibernate.dialect=org.hibernate.dialect.MySQL5InnoDBDialect
spring.jpa.show-sql= true//主控台顯示sql敘述與否

server.port=8080

用實例介紹jpa建置物件 與 關係對應的實體模型
這裡撰寫

這是自動產生一份報紙裡面欄位有標題\圖文還有虛擬欄位(@Transient)
ID\title\enum\keywordlists

@Entity
@Data
public class Paper  implements Serializable {
    @Id
    /**
     * Description: 由数据库控制,auto是程序统一控制
     */
    @GeneratedValue(strategy = GenerationType.IDENTITY)
    private long id;

    @Column(nullable = false, unique = true)
    @NotEmpty(message = "标题不能为空")
    private String title;
    /**
     * Description: 枚举类型
     */
    @Column(columnDefinition="enum('图','图文','文')")
    private String type;//类型
    /**
     * Description:  Boolean类型默认false
     */
    private Boolean available = Boolean.FALSE;
    @Size(min=0, max=20)
    private String keyword;
    @Size(max = 255)
    private String description;
    @Column(nullable = false)
    private String body;

//如想建立虛擬欄位就在屬性上方加入這個註解@Transient。專案執行後會自動產生資料表。
   @Transient
    private List keywordlists;

    public List getKeywordlists() {

        return Arrays.asList(this.keyword.trim().split("|"));
    }
    public void setKeywordlists(List keywordlists) {
        this.keywordlists = keywordlists;

    }

}

JpaEntityDemoApplication.java

package com.example.demo;

import org.springframework.boot.SpringApplication;
import org.springframework.boot.autoconfigure.SpringBootApplication;

@SpringBootApplication
public class JpaEntityDemoApplication {

	public static void main(String[] args) {
		SpringApplication.run(JpaEntityDemoApplication.class, args);
	}

}