Best way to add the custom exception for the spring boot code

How to display the appropriate error message when some exception occurs.

Suppose during GET methods if the data is not found, it should display the custom exception message.

Similarly if we are trying to delete the data which is not available.

Car.java

package com.car_rental_project.car_project;
import javax.persistence.Entity;
import javax.persistence.Id;
@Entity

public class Car {

    @Id
    private String id;
    private String modelname;
    private String type;
    private String year_of_registration;
    private String seating_capacity;
    private String cost_per_day;
    private String milleage;
    private String pincode;
    private String contact_number;
    private String email;

    public Car() {

    }

    public Car(String id, String modelname, String type, String year_of_registration, String seating_capacity,String cost_per_day, String milleage, String pincode, String contact_number, String email) {
        super();
        this.id = id;
        this.modelname = modelname;
        this.type = type;
        this.year_of_registration = year_of_registration;
        this.seating_capacity = seating_capacity;
        this.cost_per_day = cost_per_day;
        this.milleage = milleage;
        this.pincode = pincode;
        this.contact_number = contact_number;
        this.email = email;
    }

    public String getId() {
        return id;
    }

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

    public String getModelname() {
        return modelname;
    }

    public void setModelname(String modelname) {
        this.modelname = modelname;
    }

    public String getType() {
        return type;
    }

    public void setType(String type) {
        this.type = type;
    }

    public String getYear_of_registration() {
        return year_of_registration;
    }

    public void setYear_of_registration(String year_of_registration) {
        this.year_of_registration = year_of_registration;
    }

    public String getSeating_capacity() {
        return seating_capacity;
    }

    public void setSeating_capacity(String seating_capacity) {
        this.seating_capacity = seating_capacity;
    }

    public String getCost_per_day() {
        return cost_per_day;
    }

    public void setCost_per_day(String cost_per_day) {
        this.cost_per_day = cost_per_day;
    }

    public String getMilleage() {
        return milleage;
    }

    public void setMilleage(String milleage) {
        this.milleage = milleage;
    }

    public String getPincode() {
        return pincode;
    }

    public void setPincode(String pincode) {
        this.pincode = pincode;
    }

    public String getContact_number() {
        return contact_number;
    }

    public void setContact_number(String contact_number) {
        this.contact_number = contact_number;
    }

    public String getEmail() {
        return email;
    }

    public void setEmail(String email) {
        this.email = email;
    }
}

CarService.java

package com.car_rental_project.car_project;

import java.util.ArrayList;
import java.util.Arrays;
import java.util.List;

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

@Service
public class CarService {

    @Autowired
    private CarRepository CarRepository;

    public List<Car> getAllCars() {
        return (List<Car>) CarRepository.findAll();
    }

    public Car getCar(String id) {
        return (Car) CarRepository.findOne(id);

    }

    public void addCar(Car car) {
        this.CarRepository.save(car);
    }

    public void updateCar(String id, Car car) {
        this.CarRepository.save(car);
    }

    public void deleteCar(String id) {
        this.CarRepository.delete(id);;
    }
}

CarController.java

package com.car_rental_project.car_project;

import java.util.List;    
import org.springframework.beans.factory.annotation.Autowired;
import org.springframework.web.bind.annotation.DeleteMapping;
import org.springframework.web.bind.annotation.PathVariable;
import org.springframework.web.bind.annotation.PostMapping;
import org.springframework.web.bind.annotation.PutMapping;
import org.springframework.web.bind.annotation.RequestBody;
import org.springframework.web.bind.annotation.RequestMapping;
import org.springframework.web.bind.annotation.RequestMethod;
import org.springframework.web.bind.annotation.RestController;

@RestController
public class CarController {

    @Autowired
    private CarService carService;

    @RequestMapping("/car")
    public List<Car> getAllCars() {
        return carService.getAllCars();
    }

    @RequestMapping("/car/{id}")
    public Car getCar(@PathVariable String id) {
        return carService.getCar(id);
            }

    //@PostMapping("/car")
    @RequestMapping(method=RequestMethod.POST, value="/car")
    public String addCar(@RequestBody Car car) {
        carService.addCar(car);
        String response = "{\"success\": true, \"message\": Car has been added successfully.}";
        return response;
    }

    //@RequestMapping(method=RequestMethod.PUT, value="/car/{id}")
    @PutMapping("/car/{id}")
    public String updateCar(@RequestBody Car car, @PathVariable String id) {
        carService.updateCar(id, car);
        String response = "{\"success\": true, \"message\": Car has been updated successfully.}";
        return response;
    }

    //@RequestMapping(method=RequestMethod.DELETE, value="/topics/{id}")
    @DeleteMapping("/car/{id}")
    public String deleteCar(@PathVariable String id) {
        carService.deleteCar(id);
        String response = "{\"success\": true, \"message\": Car has been deleted successfully.}";
        return response;
    }
}

Here are some of approaches you can follow to handle your custom exceptions.

Create a POJO to handle your custom error messages and put your properties you want to return.

public class ErrorResponse {
     private String message;

     public String getMessage() {
       return message;
    }

    public void setMessage(String message) {
      this.message = message;
    }
}

Approach 1. Within your Controller method.

    @RequestMapping("/car/{id}")
    public ResponseEntity<?> getCar(@PathVariable String id) {
        Car car = carService.getCar(id);
        if (car == null) {
          ErrorResponse errorResponse = new ErrorResponse();
          errorResponse.setMessage("Record not found");
          return new ResponseEntity<>(errorResponse, HttpStatus.NOT_FOUND)
         }
       return new ResponseEntity<>(car, HttpStatus.OK); 
    }    

Approach 2: Handle exceptions globally.

Step 1: Create NotFound exception class and extend to RunTime Exception.

public class NoRecordFoundException extends RuntimeException {

    public NoRecordFoundException() {
        super();
    }
}

Step 2: Create Global Exception handler

@RestControllerAdvice
public class GlobalExceptionHandler {

@ExceptionHandler(NoRecordFoundException.class)
@ResponseStatus(HttpStatus.NOT_FOUND)
@ResponseBody
public ErrorResponse handleNoRecordFoundException(NoRecordFoundException ex) 
{

    ErrorResponse errorResponse = new ErrorResponse();
    errorResponse.setMessage("No Record Found");
    return errorResponse;
}

//same you can handle Exceptionerror for internal

@ExceptionHandler(Exception.class)
@ResponseStatus(HttpStatus.INTERNAL_SERVER_ERROR)
@ResponseBody
public ErrorResponse handleDefaultException(Exception ex) {
    ErrorResponse response = new ErrorResponse();
    response.setMessage(ex.getMessage());
    return response;
}
}

Step 3: throw Not found exception from your controller or service:

        @RequestMapping("/car/{id}")
        public ResponseEntity<?> getCar(@PathVariable String id) {
            Car car = carService.getCar(id);
            if (car == null) {
             throw new NoRecordFoundException();
             }
           return new ResponseEntity<>(car, HttpStatus.OK); 
        }    

Approach 3: Create @ExceptionHandler within controller and throw

 @ExceptionHandler(NoRecordFoundException.class)
    @ResponseStatus(HttpStatus.NOT_FOUND)
    @ResponseBody
    public ErrorResponse handleNoRecordFoundException(NoRecordFoundException ex) {

        ErrorResponse errorResponse = new ErrorResponse();
        errorResponse.setMessage("No Record Found");
        return errorResponse;
    }