10分钟教你学会JPA

卑微小吴励志写博客第16天。
我现在一家外包公司工作，用的框架是在谷歌上开发，图像化开发的框架。一个月都写不了几行代码，每天在跟公司的流程和文档做斗争，感觉自己离开发这条路越来越远，怕自己忘记了写代码的感觉。也害怕出去后毫无竞争力，所以每天想写点代码。今天写一个极简化的demo，带大家学习一下关于jpa的知识。

JPA

JPA：Java Persistence API：用于对象持久化 API。
在用过mybatis，mybatis plus，和jpa后，发现jpa还真的挺好用的，对象持久化的框架很对，我们可以挑选一个自己比较顺手的搭建自己的项目。

JPA Demo

今天我就搭一个最简单的demo。让jpa先跑起来。
pom文件

<?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.itheima</groupId>
    <artifactId>demo-jpa</artifactId>
    <version>1.0-SNAPSHOT</version>
    <parent>
        <groupId>org.springframework.boot</groupId>
        <artifactId>spring-boot-starter-parent</artifactId>
        <version>2.0.1.RELEASE</version>
        <relativePath/> <!-- lookup parent from repository -->
    </parent>

    <properties>
        <project.build.sourceEncoding>UTF-8</project.build.sourceEncoding>
        <project.reporting.outputEncoding>UTF-8</project.reporting.outputEncoding>
        <java.version>1.8</java.version>
    </properties>

    <dependencies>
        <dependency>
            <groupId>org.springframework.boot</groupId>
            <artifactId>spring-boot-starter-web</artifactId>
        </dependency>

        <dependency>
            <groupId>org.springframework.boot</groupId>
            <artifactId>spring-boot-starter-test</artifactId>
            <scope>test</scope>
        </dependency>

        <!-- springBoot JPA的起步依赖 -->
        <dependency>
            <groupId>org.springframework.boot</groupId>
            <artifactId>spring-boot-starter-data-jpa</artifactId>
        </dependency>

        <!-- MySQL连接驱动 -->
        <dependency>
            <groupId>mysql</groupId>
            <artifactId>mysql-connector-java</artifactId>
        </dependency>
    </dependencies>

    <build>
        <plugins>
            <plugin>
                <groupId>org.springframework.boot</groupId>
                <artifactId>spring-boot-maven-plugin</artifactId>
            </plugin>
        </plugins>
    </build>
</project>

直接copy过去就行。在这里插一句，其实程序员入门都是CV大法加上面向搜索引擎来编程的，自己写不了多少代码。慢慢的成长了再加上自己的想法去实现一些东西。
项目结构

就建这么几个文件就可以跑起来了。图中漏掉了，springboot启动类。
domain包user实体类

package com.itheima.domain;

import javax.persistence.*;

/**
 * Created by lenovo on 2020/6/3
 */
@Entity
@Table(name="user")
public class User {
    @Id
    @GeneratedValue(strategy = GenerationType.IDENTITY)
    private Long id;
    private String name;
    private String password;
    private String username;

    @Override
    public String toString() {
        return "User{" +
                "id=" + id +
                ", name='" + name + '\'' +
                ", password='" + password + '\'' +
                ", username='" + username + '\'' +
                '}';
    }

    public Long getId() {
        return id;
    }

    public void setId(Long id) {
        this.id = id;
    }

    public String getName() {
        return name;
    }

    public void setName(String name) {
        this.name = name;
    }

    public String getPassword() {
        return password;
    }

    public void setPassword(String password) {
        this.password = password;
    }

    public String getUsername() {
        return username;
    }

    public void setUsername(String username) {
        this.username = username;
    }
}

关于JPA的注解，这里就不讲解了，面向百度搜索即可。

UserRepository

package com.itheima.repository;

import com.itheima.domain.User;
import org.springframework.data.jpa.repository.JpaRepository;

/**
 * Created by lenovo on 2020/6/3.
 */
public interface UserRepository extends JpaRepository<User,Long> {
}

个人感觉这里是JPA的精髓，只用继承JpaRepository即可，简单的增删改查不用自己写了。

springboot启动类

package com.itheima;

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

/**
 * Created by lenovo on 2020/6/3.
 */
@SpringBootApplication
public class BootApplication {
    public static void main(String[] args) {
        SpringApplication.run(BootApplication.class);
    }
}

配置文件application.yml

spring:
  datasource:
    driver-class-name: com.mysql.jdbc.Driver
    url: jdbc:mysql:///test?useUnicode=true&characterEncoding=utf8
    username: root
    password: root

这里注意一点，url，用户名和密码填写自己的。url里面如果是本机，端口号默认3306，就可以不用写。

单元测试

package com.itheima;

import com.itheima.domain.User;
import com.itheima.repository.UserRepository;
import org.junit.Test;
import org.junit.runner.RunWith;
import org.springframework.beans.factory.annotation.Autowired;
import org.springframework.boot.test.context.SpringBootTest;
import org.springframework.data.domain.Example;
import org.springframework.data.domain.Page;
import org.springframework.data.domain.PageRequest;
import org.springframework.data.domain.Pageable;
import org.springframework.test.context.junit4.SpringRunner;

import java.util.List;
import java.util.Optional;

/**
 * Created by lenovo on 2019/11/5.
 */
@RunWith(SpringRunner.class)
@SpringBootTest
public class UserTest {

    @Autowired
    private UserRepository userRepository;

    @Test
    public void test1(){
        Optional<User> optional = userRepository.findById(1l);
        if(optional.isPresent()){
            System.out.println(optional.get());
        }else{
            System.out.println("用户不存在");
        }
    }

    @Test
    public void test2(){
        User user = new User();
        user.setName("rose");
        user.setPassword("jack");
        user.setUsername("roseandjack");
//        Optional<User> optional = userRepository.findById(1l);
//        User user = optional.get();
//        user.setName("tomdady");
        User user2 = userRepository.save(user);
        System.out.println(user2.getName());
    }

    @Test
    public void test3(){
        Pageable pageable = PageRequest.of(0, 2);
        Page<User> page = userRepository.findAll(pageable);
        System.out.println("总记录数:"+page.getTotalElements());
        System.out.println("总页数:"+page.getTotalPages());
        List<User> userList = page.getContent();
        for (User user : userList) {
            System.out.println(user);
        }
    }

}

这里写单元测试，就是为了不写controller·，直接在单元测试里面测。不然还得用postman或者别的测试工具测。当然小伙伴可以多长时间一些JPA的方法，或者写上前端页面和controller去测。
注意
如果碰到以下的错误。

java.lang.IllegalStateException: Failed to load ApplicationContext

	at org.springframework.test.context.cache.DefaultCacheAwareContextLoaderDelegate.loadContext(DefaultCacheAwareContextLoaderDelegate.java:125)
	at org.springframework.test.context.support.DefaultTestContext.getApplicationContext(DefaultTestContext.java:108)
	at org.springframework.test.context.web.ServletTestExecutionListener.setUpRequestContextIfNecessary(ServletTestExecutionListener.java:190)
	at org.springframework.test.context.web.ServletTestExecutionListener.prepareTestInstance(ServletTestExecutionListener.java:132)
	at org.springframework.test.context.TestContextManager.prepareTestInstance(TestContextManager.java:246)
	at org.springframework.test.context.junit4.SpringJUnit4ClassRunner.createTest(SpringJUnit4ClassRunner.java:227)
	at org.springframework.test.context.junit4.SpringJUnit4ClassRunner$1.runReflectiveCall(SpringJUnit4ClassRunner.java:289)
	at org.junit.internal.runners.model.ReflectiveCallable.run(ReflectiveCallable.java:12)
	at org.springframework.test.context.junit4.SpringJUnit4ClassRunner.methodBlock(SpringJUnit4ClassRunner.java:291)
	at org.springframework.test.context.junit4.SpringJUnit4ClassRunner.runChild(SpringJUnit4ClassRunner.java:246)
	at org.springframework.test.context.junit4.SpringJUnit4ClassRunner.runChild(SpringJUnit4ClassRunner.java:97)
	at org.junit.runners.ParentRunner$3.run(ParentRunner.java:290)
	at org.junit.runners.ParentRunner$1.schedule(ParentRunner.java:71)
	at org.junit.runners.ParentRunner.runChildren(ParentRunner.java:288)
	at org.junit.runners.ParentRunner.access$000(ParentRunner.java:58)
	at org.junit.runners.ParentRunner$2.evaluate(ParentRunner.java:268)
	at org.springframework.test.context.junit4.statements.RunBeforeTestClassCallbacks.evaluate(RunBeforeTestClassCallbacks.java:61)
	at org.springframework.test.context.junit4.statements.RunAfterTestClassCallbacks.evaluate(RunAfterTestClassCallbacks.java:70)
	at org.junit.runners.ParentRunner.run(ParentRunner.java:363)
	at org.springframework.test.context.junit4.SpringJUnit4ClassRunner.run(SpringJUnit4ClassRunner.java:190)
	at org.junit.runner.JUnitCore.run(JUnitCore.java:137)
	at com.intellij.junit4.JUnit4IdeaTestRunner.startRunnerWithArgs(JUnit4IdeaTestRunner.java:68)
	at com.intellij.rt.execution.junit.IdeaTestRunner$Repeater.startRunnerWithArgs(IdeaTestRunner.java:47)
	at com.intellij.rt.execution.junit.JUnitStarter.prepareStreamsAndStart(JUnitStarter.java:242)
	at com.intellij.rt.execution.junit.JUnitStarter.main(JUnitStarter.java:70)
Caused by: org.springframework.beans.factory.BeanCreationException: Error creating 

把jdk版本改为1.8或者1.7就好了。不要问我为什么，搜一下就知道了。
今天的分享就到这里了哦！！！小伙伴们加油！！！