java.lang.NullPointerException: Cannot invoke "com.example.repository.StudentRepository.save(Object)" because "this.studentRepository" is null – Mysql

`HTTP Status 500 – Internal Server Error
Type Exception Report

Message Request processing failed: java.lang.NullPointerException: Cannot invoke "com.example.repository.StudentRepository.save(Object)" because "this.studentRepository" is null

Description The server encountered an unexpected condition that prevented it from fulfilling the request.

Exception

jakarta.servlet.ServletException: Request processing failed: java.lang.NullPointerException: Cannot invoke "com.example.repository.StudentRepository.save(Object)" because "this.studentRepository" is null
org.springframework.web.servlet.FrameworkServlet.processRequest(FrameworkServlet.java:1018)
org.springframework.web.servlet.FrameworkServlet.doPost(FrameworkServlet.java:913)
jakarta.servlet.http.HttpServlet.service(HttpServlet.java:731)
org.springframework.web.servlet.FrameworkServlet.service(FrameworkServlet.java:884)
jakarta.servlet.http.HttpServlet.service(HttpServlet.java:814)
org.apache.tomcat.websocket.server.WsFilter.doFilter(WsFilter.java:53)
org.springframework.web.filter.RequestContextFilter.doFilterInternal(RequestContextFilter.java:100)
org.springframework.web.filter.OncePerRequestFilter.doFilter(OncePerRequestFilter.java:116)
org.springframework.web.filter.FormContentFilter.doFilterInternal(FormContentFilter.java:93)
org.springframework.web.filter.OncePerRequestFilter.doFilter(OncePerRequestFilter.java:116)
org.springframework.web.filter.CharacterEncodingFilter.doFilterInternal(CharacterEncodingFilter.java:201)
org.springframework.web.filter.OncePerRequestFilter.doFilter(OncePerRequestFilter.java:116)
Root Cause

java.lang.NullPointerException: Cannot invoke "com.example.repository.StudentRepository.save(Object)" because "this.studentRepository" is null
com.example.service.impl.StudentServiceImpl.saveStudent(StudentServiceImpl.java:37)
com.example.controller.StudentController.saveStudent(StudentController.java:45)
java.base/jdk.internal.reflect.NativeMethodAccessorImpl.invoke0(Native Method)
java.base/jdk.internal.reflect.NativeMethodAccessorImpl.invoke(NativeMethodAccessorImpl.java:77)
java.base/jdk.internal.reflect.DelegatingMethodAccessorImpl.invoke(DelegatingMethodAccessorImpl.java:43)
java.base/java.lang.reflect.Method.invoke(Method.java:568)
org.springframework.web.method.support.InvocableHandlerMethod.doInvoke(InvocableHandlerMethod.java:207)
org.springframework.web.method.support.InvocableHandlerMethod.invokeForRequest(InvocableHandlerMethod.java:152)
org.springframework.web.servlet.mvc.method.annotation.ServletInvocableHandlerMethod.invokeAndHandle(ServletInvocableHandlerMethod.java:117)
org.springframework.web.servlet.mvc.method.annotation.RequestMappingHandlerAdapter.invokeHandlerMethod(RequestMappingHandlerAdapter.java:884)
org.springframework.web.servlet.mvc.method.annotation.RequestMappingHandlerAdapter.handleInternal(RequestMappingHandlerAdapter.java:797)
org.springframework.web.servlet.mvc.method.AbstractHandlerMethodAdapter.handle(AbstractHandlerMethodAdapter.java:87)
org.springframework.web.servlet.DispatcherServlet.doDispatch(DispatcherServlet.java:1080)
org.springframework.web.servlet.DispatcherServlet.doService(DispatcherServlet.java:973)
org.springframework.web.servlet.FrameworkServlet.processRequest(FrameworkServlet.java:1010)
org.springframework.web.servlet.FrameworkServlet.doPost(FrameworkServlet.java:913)
jakarta.servlet.http.HttpServlet.service(HttpServlet.java:731)
org.springframework.web.servlet.FrameworkServlet.service(FrameworkServlet.java:884)
jakarta.servlet.http.HttpServlet.service(HttpServlet.java:814)
org.apache.tomcat.websocket.server.WsFilter.doFilter(WsFilter.java:53)
org.springframework.web.filter.RequestContextFilter.doFilterInternal(RequestContextFilter.java:100)
org.springframework.web.filter.OncePerRequestFilter.doFilter(OncePerRequestFilter.java:116)
org.springframework.web.filter.FormContentFilter.doFilterInternal(FormContentFilter.java:93)
org.springframework.web.filter.OncePerRequestFilter.doFilter(OncePerRequestFilter.java:116)
org.springframework.web.filter.CharacterEncodingFilter.doFilterInternal(CharacterEncodingFilter.java:201)
org.springframework.web.filter.OncePerRequestFilter.doFilter(OncePerRequestFilter.java:116)
Note The full stack trace of the root cause is available in the server logs.`

1.Controller Class

@Controller
    public class StudentController {
    
        private StudentService studentService;
        
        public StudentController(StudentService studentService) 
        {
            super();
            this.studentService = studentService;
        }
    
        @GetMapping("/students")
        public String listStudents()
        {
            //model.addAttribute("students",studentService.getAllStudent());
            return "students";
        }
        
        @GetMapping("/students/new")
        public String createStudentForm(Model model)
        {
            Student st = new Student();
            model.addAttribute("student",st);
            
            return "create_student";    
        }
            
        @PostMapping("/students")
        public String saveStudent(@ModelAttribute("student") Student student)
        {
            System.out.println("object: " + student.getFirstName()+"="+student.getEmail()+" = "+student.getLastName());
            studentService.saveStudent(student);
            return "redirect:/students";
        }
        
        
    }


2.Entity Class

package com.example.entity;

import jakarta.persistence.Column;
import jakarta.persistence.Entity;
import jakarta.persistence.GeneratedValue;
import jakarta.persistence.GenerationType;
import jakarta.persistence.Id;
import jakarta.persistence.Table;

@Entity
@Table(name = "students")
public class Student {
    
    @Id
    @GeneratedValue(strategy = GenerationType.IDENTITY)
    private Long id;
    
    @Column(name = "first_name", nullable = false)
    private String firstName;
    
    @Column(name = "last_name")
    private String lastName;
    
    @Column(name = "email")
    private String email;
    
    public Student() {
        
    }
    
    public Student(String firstName, String lastName, String email) {
        super();
        this.firstName = firstName;
        this.lastName = lastName;
        this.email = email;
    }
    public Long getId() {
        return id;
    }
    public void setId(Long id) {
        this.id = id;
    }
    public String getFirstName() {
        return firstName;
    }
    public void setFirstName(String firstName) {
        this.firstName = firstName;
    }
    public String getLastName() {
        return lastName;
    }
    public void setLastName(String lastName) {
        this.lastName = lastName;
    }
    public String getEmail() {
        return email;
    }
    public void setEmail(String email) {
        this.email = email;
    }
}

3.Student Repository Interface

    package com.example.repository;
    
    import org.springframework.data.jpa.repository.JpaRepository;
    
    import com.example.entity.Student;
    
    public interface StudentRepository extends JpaRepository<Student, Long>{
    
    }

4. Student Service

package com.example.service;

import java.util.List;

import com.example.entity.Student;


public interface StudentService {
    List<Student> getAllStudents();
    
    Student saveStudent(Student student);
    
    Student getStudentById(Long id);
    
    Student updateStudent(Student student);
    
    void deleteStudentById(Long id);
}
4. Student Service Implementation

    package com.example.service.impl;
    import java.util.List;
    
    import org.springframework.beans.factory.annotation.Autowired;
    import org.springframework.stereotype.Service;
    
    import com.example.entity.Student;
    import com.example.repository.StudentRepository;
    import com.example.service.StudentService;
    
    
    @Service
    public class StudentServiceImpl implements StudentService{
    
        
        private StudentRepository studentRepository;
    
        public StudentServiceImpl() {
        super();
        }
    
        //@Autowired
        public StudentServiceImpl(StudentRepository studentRepository) {
            super();
            System.out.println("Student Repo:= "+this.studentRepository);
            this.studentRepository = studentRepository;
        }
    
        @Override
        public List<Student> getAllStudents() {
            return studentRepository.findAll();
        }
    
        @Override
        public Student saveStudent(Student student) {
            return studentRepository.save(student);
        }
    
        @Override
        public Student getStudentById(Long id) {
            return studentRepository.findById(id).get();
        }
    
        @Override
        public Student updateStudent(Student student) {
            return studentRepository.save(student);  //.save(student);
        }
    
        @Override
        public void deleteStudentById(Long id) {
            studentRepository.deleteById(id);   
        }
    
    }