How to use JPA in springboot, simple example

JPA (Java Persistence API) is a Java persistence specification officially proposed by Sun to facilitate the operation of the database.
The real work may be Hibernate, TopLink, etc. different vendors that implement the JPA specification, the default is Hibernate.
This knowledge demonstrates how to use JPA in Springboot quickly and easily.

Step 2: Create a database

Create a database, the name is how2java

create database how2java;

Step 3: create table

Create a classification table, the fields are very simple, just id and name

use how2java;

CREATE TABLE category_ (

id int(11) NOT NULL AUTO_INCREMENT,

name varchar(30),

PRIMARY KEY (id)

) DEFAULT CHARSET=UTF8;

Step 4: prepare data

Insert 4 data

insert into category_ values(null,'category 1');

insert into category_ values(null,'category 2');

insert into category_ values(null,'category 3');

insert into category_ values(null,'category 4');

Step 5: Run first, see the effect, and then learn

The old rule is to download the runnable project in the download area (click to enter) first , configure it and run it, and then learn what steps have been taken to achieve this effect.
Test address:

http://127.0.0.1:8080/listCategory

Note: The startup method is unique to Springboot, directly run the main method of the class: com.how2java.springboot.Application .

Step 6: imitation and troubleshooting

After ensuring that the runnable project can run correctly, follow the steps of the tutorial and imitate the code again.
The imitation process will inevitably have code discrepancies, resulting in the failure to obtain the expected running results. At this moment, compare the correct answer (runnable project) with your own code to locate the problem.
In this way, learning is effective and troubleshooting is efficient , which can significantly increase the speed of learning and cross all barriers on the learning path.

It is recommended to use diffmerge software for folder comparison. Compare your own project folder with my runnable project folder.
This software is very powerful, you can know which two files in the folder are wrong, and you can clearly mark them.
Here is a green installation and usage tutorial: diffmerge download and usage tutorial

Step 7: Based on the previous knowledge points

This knowledge point is developed based on error handling , so it is best to learn the previous content, otherwise some content may not be understood

Step 8: application.properties

Added required parameters for database link

spring.jpa.properties.hibernate.hbm2ddl.auto=update

Indicates that the table structure will be updated automatically, so this step of creating the table is actually unnecessary~

spring.mvc.view.prefix=/WEB-INF/jsp/

spring.mvc.view.suffix=.jsp

spring.datasource.url=jdbc:mysql://127.0.0.1:3306/how2java?characterEncoding=UTF-8

spring.datasource.username=root

spring.datasource.password=admin

spring.datasource.driver-class-name=com.mysql.jdbc.Driver

spring.jpa.properties.hibernate.hbm2ddl.auto=update

Step 9: pom.xml

Add support for mysql and jpa

<dependency>

<groupId>mysql</groupId>

<artifactId>mysql-connector-java</artifactId>

<version>5.1.21</version>

</dependency>

<dependency>

<groupId>org.springframework.boot</groupId>

<artifactId>spring-boot-starter-data-jpa</artifactId>

</dependency>

<?xml version="1.0" encoding="UTF-8"?>

<project xmlns="http://maven.apache.org/POM/4.0.0" xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance"

xsi:schemaLocation="http://maven.apache.org/POM/4.0.0 http://maven.apache.org/xsd/maven-4.0.0.xsd">

<modelVersion>4.0.0</modelVersion>

<groupId>com.how2java</groupId>

<artifactId>springboot</artifactId>

<version>0.0.1-SNAPSHOT</version>

<name>springboot</name>

<description>springboot</description>

<packaging>war</packaging>

<parent>

<groupId>org.springframework.boot</groupId>

<artifactId>spring-boot-starter-parent</artifactId>

<version>1.5.9.RELEASE</version>

</parent>

<dependencies>

<dependency>

<groupId>org.springframework.boot</groupId>

<artifactId>spring-boot-starter-web</artifactId>

</dependency>

<dependency>

<groupId>org.springframework.boot</groupId>

<artifactId>spring-boot-starter-tomcat</artifactId>

</dependency>

<dependency>

<groupId>junit</groupId>

<artifactId>junit</artifactId>

<version>3.8.1</version>

<scope>test</scope>

</dependency>

<dependency>

<groupId>javax.servlet</groupId>

<artifactId>javax.servlet-api</artifactId>

</dependency>

<dependency>

<groupId>javax.servlet</groupId>

<artifactId>jstl</artifactId>

</dependency>

<dependency>

<groupId>org.apache.tomcat.embed</groupId>

<artifactId>tomcat-embed-jasper</artifactId>

</dependency>

<dependency>

<groupId>org.springframework.boot</groupId>

<artifactId>spring-boot-devtools</artifactId>

<optional>true</optional> 

</dependency>

<dependency>

<groupId>mysql</groupId>

<artifactId>mysql-connector-java</artifactId>

<version>5.1.21</version>

</dependency>

<dependency>

<groupId>org.springframework.boot</groupId>

<artifactId>spring-boot-starter-data-jpa</artifactId>

</dependency>

</dependencies>

<properties>

<java.version>1.8</java.version>

</properties>

<build>

<plugins>

<plugin>

<groupId>org.springframework.boot</groupId>

<artifactId>spring-boot-maven-plugin</artifactId>

</plugin>

</plugins>

</build>

</project>

Step 10: Category

增加一个包：com.how2java.springboot.pojo，然后创建实体类Category。
@Entity 注解表示这是个实体类
@Table(name = "category_") 表示这个类对应的表名是 category_ ，注意有下划线哦
@Id 表明主键
@GeneratedValue(strategy = GenerationType.IDENTITY) 表明自增长方式
@Column(name = "id") 表明对应的数据库字段名

package com.how2java.springboot.pojo;

import javax.persistence.Column;

import javax.persistence.Entity;

import javax.persistence.GeneratedValue;

import javax.persistence.GenerationType;

import javax.persistence.Id;

import javax.persistence.Table;

@Entity

@Table(name = "category_")

public class Category {

@Id

@GeneratedValue(strategy = GenerationType.IDENTITY)

@Column(name = "id")

private int id;

@Column(name = "name")

private String name;

public int getId() {

return id;

}

public void setId(int id) {

this.id = id;

}

public String getName() {

return name;

}

public void setName(String name) {

this.name = name;

}

步骤 11 : CategoryDAO

增加一个包：com.how2java.springboot.dao，然后创建dao接口CategoryDAO，继承了JpaRepository，并且提供泛型<Category,Integer> 表示这个是针对Category类的DAO,Integer表示主键是Integer类型。
JpaRepository 这个父接口，就提供了CRUD, 分页等等一系列的查询了，直接拿来用，都不需要二次开发的了。

package com.how2java.springboot.dao;

import org.springframework.data.jpa.repository.JpaRepository;

import com.how2java.springboot.pojo.Category;

public interface CategoryDAO extends JpaRepository<Category,Integer>{

}

步骤 12 : CategoryController

增加一个包：com.how2java.springboot.web，然后创建CategoryController 类。
1. 接受listCategory映射
2. 然后获取所有的分类数据
3. 接着放入Model中
4. 跳转到listCategory.jsp中

package com.how2java.springboot.web;

import java.util.List;

import org.springframework.beans.factory.annotation.Autowired;

import org.springframework.stereotype.Controller;

import org.springframework.ui.Model;

import org.springframework.web.bind.annotation.RequestMapping;

import com.how2java.springboot.dao.CategoryDAO;

import com.how2java.springboot.pojo.Category;

@Controller

public class CategoryController {

@Autowired CategoryDAO categoryDAO;

@RequestMapping("/listCategory")

public String listCategory(Model m) throws Exception {

List<Category> cs=categoryDAO.findAll();

m.addAttribute("cs", cs);

return "listCategory";

}

步骤 13 : listCategory.jsp

Use jstl to traverse the collection passed from CategoryController : cs.

<%@ page language="java" contentType="text/html; charset=UTF-8"

pageEncoding="UTF-8"%>

<%@ taglib uri="http://java.sun.com/jsp/jstl/core" prefix="c"%>

<table align='center' border='1' cellspacing='0'>

<tr>

<td>id</td>

<td>name</td>

</tr>

<c:forEach items="${cs}" var="c" varStatus="st">

<tr>

<td>${c.id}</td>

<td>${c.name}</td>

</tr>

</c:forEach>

</table>

Step 14: restart the test

Because the dependency of the jar package is added to pom.xml, only the hot deployment of Springboot itself will not work, you have to restart it manually.
Then visit the test address:

http://127.0.0.1:8080/listCategory

Observe the effect as shown in the figure

Step 15: more about JPA

There is a JPA CRUD and paging at the back , you can learn more about JPA

For more information, click to understand: https://how2j.cn/k/springboot/springboot-jpa/1648.html