SpringBoot集成MySQL启动报错找不到StudentRepository类型Bean如何解决？

问题现象 #

开发Spring Boot集成MySQL功能时，项目启动失败，错误日志如下：

2021-09-23 01:28:31.193  INFO 1196 --- [           main] com.example.demo.DemoApplication         : Starting DemoApplication using Java 1.8.0_144 on DESKTOP-PFH9867 with PID 1196 (C:\Users\Admin\IdeaProjects\demo\target\classes started by Admin in C:\Users\Admin\IdeaProjects\demo)
2021-09-23 01:28:31.196  INFO 1196 --- [           main] com.example.demo.DemoApplication         : No active profile set, falling back to default profiles: default
2021-09-23 01:28:33.014  INFO 1196 --- [           main] o.s.b.w.embedded.tomcat.TomcatWebServer  : Tomcat initialized with port(s): 8080 (http)
2021-09-23 01:28:33.027  INFO 1196 --- [           main] o.apache.catalina.core.StandardService   : Starting service [Tomcat]
2021-09-23 01:28:33.027  INFO 1196 --- [           main] org.apache.catalina.core.StandardEngine  : Starting Servlet engine: [Apache Tomcat/9.0.52]
2021-09-23 01:28:33.145  INFO 1196 --- [           main] o.a.c.c.C.[Tomcat].[localhost].[/]       : Initializing Spring embedded WebApplicationContext
2021-09-23 01:28:33.146  INFO 1196 --- [           main] w.s.c.ServletWebServerApplicationContext : Root WebApplicationContext: initialization completed in 1875 ms
2021-09-23 01:28:33.218  WARN 1196 --- [           main] ConfigServletWebServerApplicationContext : Exception encountered during context initialization - cancelling refresh attempt: org.springframework.beans.factory.UnsatisfiedDependencyException: Error creating bean with name 'studentController': Unsatisfied dependency expressed through field 'studentRepository'; nested exception is org.springframework.beans.factory.NoSuchBeanDefinitionException: No qualifying bean of type 'com.example.demo.repository.StudentRepository' available: expected at least 1 bean which qualifies as autowire candidate. Dependency annotations: {@org.springframework.beans.factory.annotation.Autowired(required=true)}
2021-09-23 01:28:33.221  INFO 1196 --- [           main] o.apache.catalina.core.StandardService   : Stopping service [Tomcat]
2021-09-23 01:28:33.242  INFO 1196 --- [           main] ConditionEvaluationReportLoggingListener : 

Error starting ApplicationContext. To display the conditions report re-run your application with 'debug' enabled.
2021-09-23 01:28:33.269 ERROR 1196 --- [           main] o.s.b.d.LoggingFailureAnalysisReporter   : 

***************************
APPLICATION FAILED TO START
***************************

Description:

Field studentRepository in com.example.demo.cotroller.StudentController required a bean of type 'com.example.demo.repository.StudentRepository' that could not be found.

The injection point has the following annotations:
    - @org.springframework.beans.factory.annotation.Autowired(required=true)


Action:

Consider defining a bean of type 'com.example.demo.repository.StudentRepository' in your configuration.


Process finished with exit code 1

按照Spring Boot官方规则，Repository接口创建后Spring会自动生成首字母小写的同名Bean，已排查application.properties配置和项目目录结构，均无异常。

相关代码 #

DemoApplication.java #

package com.example.demo;

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

@SpringBootApplication
public class  DemoApplication {

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

}

Student.java #

package com.example.demo.entity;

import javax.persistence.Entity;

@Entity // 告知Hibernate将该类映射为数据库表
public class Student {
    private Integer studentID;

    private String name;

    private String department;

    private String major;

    public Integer getStudentID() {
        return studentID;
    }

    public void setStudentID(Integer studentID) {
        this.studentID = studentID;
    }

    public String getName() {
        return name;
    }

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

    public String getDepartment() {
        return department;
    }

    public void setDepartment(String department) {
        this.department = department;
    }

    public String getMajor() {
        return major;
    }

    public void setMajor(String major) {
        this.major = major;
    }
}

StudentController.java #

package com.example.demo.cotroller;

import com.example.demo.entity.Student;
import com.example.demo.repository.StudentRepository;
import org.springframework.beans.factory.annotation.Autowired;
import org.springframework.stereotype.Controller;
import org.springframework.web.bind.annotation.*;

@Controller // 标记该类为Controller层组件
@RequestMapping(path="/api/v1/student") // 接口公共前缀
public class StudentController {
    // 注入Spring自动生成的studentRepository Bean
    @Autowired
    private StudentRepository studentRepository;

    @PostMapping(path="/add") // 仅接收POST请求
    public @ResponseBody String addNewUser (@RequestBody Student student) {
        studentRepository.save(student);
        return "Saved";
    }
}

StudentRepository.java #

package com.example.demo.repository;

import com.example.demo.entity.Student;
import org.springframework.data.repository.CrudRepository;

// Spring会自动实现该接口为对应的Bean，内置CRUD操作能力
public interface StudentRepository extends CrudRepository<Student, Integer> {
}

项目目录结构 #

java
|--com.example.demo
   |--controller
      |--StudentController.java
   |--entity
      |--Student.java
   |--repository
      |--StudentRepository.java
   DemoApplication.java   

根因与解决方案 #

问题根因 #

创建Spring Boot项目时漏选了必需的Spring Data JPA依赖，导致Spring无法识别Repository接口、自动生成对应的Bean。

解决步骤 #

• 在IDEA中重新创建Spring Boot项目，勾选Spring Web、Spring Data JPA、MySQL Driver三个必要依赖
• 将原有代码文件迁移到新项目中即可正常启动

内容的提问来源于stack exchange，提问作者prettypig