Exception:Field studentRepository in com.example.demo.cotroller.StudentController required a bean of-CodePudding

I followed https://spring.io/guides/gs/accessing-data-mysql/#initial to satrt learning springboot with MySQL. And I met a bug as follows.

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

Obviously, the main problem is @Autowired in Controller. Therefore, I searched a lot of methods to deal with it. And I don't wanna add new code or new file. As the website said when the interface Repository is created, Spring automatically implements this repository interface in a bean that has the same name (with a change in the case — it is called userRepository). Therefore, I don't wanna add new code or file. I thought the issue is in my application.properties or the relative location of directories. But they don't work at all.

And my code is as follows.

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;

// https://spring.io/guides/gs/accessing-data-mysql/

@Entity // This tells Hibernate to make a table out of this class
public class Student {
    // 2147483647
    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.http.MediaType;
import org.springframework.stereotype.Controller;
import org.springframework.web.bind.annotation.*;

@Controller // This means that this class is a Controller
@RequestMapping(path="/api/v1/student") // This means URL's start with /demo (after Application path)
public class StudentController {
    // This means to get the bean called studentRepository
    // Which is auto-generated by Spring, we will use it to handle the data
    @Autowired
    private StudentRepository studentRepository;

    @PostMapping(path="/add") // Map ONLY POST Requests
    public @ResponseBody
    String addNewUser (@RequestBody Student student) {
        // @ResponseBody means the returned String is the response, not a view name
        // @RequestParam means it is a parameter from the GET or POST request

        studentRepository.save(student);
        return "Saved";
    }
}

StudentRepository.java

package com.example.demo.repository;

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

// This will be AUTO IMPLEMENTED by Spring into a Bean called userRepository
// CRUD refers Create, Read, Update, Delete
public interface StudentRepository extends CrudRepository<Student, Integer> {
}

My directories are organized as follows.

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

CodePudding user response：

You do not need to implement StudentRepository. You just have to annotated with @Repository:

@Repository
public interface StudentRepository extends CrudRepository<Student, Integer> {
}

CodePudding user response：

Currently, I found a question from https://blog.csdn.net/a532672728/article/details/77702772. The author mentioned that Normally classes annotated with @Component are automatically scanned by Spring to generate beans that are registered with the Spring container(正常情况下加上@Component注解的类会自动被Spring扫描到生成Bean注册到spring容器中), which made me understand that the process is unfinished. Therefore, I created a file named StudentRepositoryImpl in directory com.example.demo/repository/impl.

The code is as follows.

package com.example.demo.repository.impl;

import com.example.demo.entity.Student;
import com.example.demo.repository.StudentRepository;
import org.springframework.stereotype.Component;

import java.util.Optional;
@Component
public class StudentRepositoryImpl implements StudentRepository {

    @Override
    public <S extends Student> S save(S s) {
        return null;
    }

    @Override
    public <S extends Student> Iterable<S> saveAll(Iterable<S> iterable) {
        return null;
    }

    @Override
    public Optional<Student> findById(Integer integer) {
        return Optional.empty();
    }

    @Override
    public boolean existsById(Integer integer) {
        return false;
    }

    @Override
    public Iterable<Student> findAll() {
        return null;
    }

    @Override
    public Iterable<Student> findAllById(Iterable<Integer> iterable) {
        return null;
    }

    @Override
    public long count() {
        return 0;
    }

    @Override
    public void deleteById(Integer integer) {

    }

    @Override
    public void delete(Student student) {

    }

    @Override
    public void deleteAll(Iterable<? extends Student> iterable) {

    }

    @Override
    public void deleteAll() {

    }
}

And if without @Component, the spring-boot can't start as well. With the annotation, it can start successfully.

Finally, I found the true reason! I am too careless to follow the tutorial. It said Click Dependencies and select Spring Web, Spring Data JPA, and MySQL Driver. but I didn't click Spring Data JPA!