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In RESTful APIs, it's a best practice to return appropriate HTTP status codes for various error scenarios. In Spring Boot, if an ID is not found in the database or repository, returning a 400 Bad Request is common when the request itself is invalid or malformed.

This article shows you how to throw a custom BadRequestException when an ID is missing, and how to handle it cleanly using @RestControllerAdvice.

🛠️ Step-by-Step Guide

✅ 1. Create a Custom Exception

java

package com.example.demo.exception;

public class BadRequestException extends RuntimeException {
    public BadRequestException(String message) {
        super(message);
    }
}

This custom exception represents an invalid request, like a missing or non-existent ID.

✅ 2. Handle the Exception with a Global Handler

java

package com.example.demo.exception;

import org.springframework.http.HttpStatus;
import org.springframework.http.ResponseEntity;
import org.springframework.web.bind.annotation.ExceptionHandler;
import org.springframework.web.bind.annotation.RestControllerAdvice;

@RestControllerAdvice
public class CustomExceptionHandler {

    @ExceptionHandler(BadRequestException.class)
    public ResponseEntity<String> handleBadRequestException(BadRequestException ex) {
        return new ResponseEntity<>(ex.getMessage(), HttpStatus.BAD_REQUEST);
    }
}

This catches any BadRequestException thrown in your app and sends a proper HTTP 400 response.

✅ 3. Throw Exception in the Service Layer

java

package com.example.demo.service;

import com.example.demo.exception.BadRequestException;
import com.example.demo.model.YourEntity;
import com.example.demo.repository.YourRepository;
import org.springframework.beans.factory.annotation.Autowired;
import org.springframework.stereotype.Service;

@Service
public class YourService {

    @Autowired
    private YourRepository repository;

    public YourEntity findById(Long id) {
        return repository.findById(id)
                .orElseThrow(() -> new BadRequestException("ID " + id + " not found"));
    }
}

The orElseThrow() method ensures that if the entity isn’t found, the exception is thrown immediately.

✅ 4. Use the Service in Your Controller

java

package com.example.demo.controller;

import com.example.demo.model.YourEntity;
import com.example.demo.service.YourService;
import org.springframework.beans.factory.annotation.Autowired;
import org.springframework.web.bind.annotation.GetMapping;
import org.springframework.web.bind.annotation.PathVariable;
import org.springframework.web.bind.annotation.RestController;

@RestController
public class YourController {

    @Autowired
    private YourService service;

    @GetMapping("/entity/{id}")
    public YourEntity getEntityById(@PathVariable Long id) {
        return service.findById(id);
    }
}

When a GET request is made for an entity with a non-existing ID, the client will receive:

http

HTTP/1.1 400 Bad Request
Content-Type: text/plain
ID 123 not found

🔚 Conclusion

GoalMethodThrow 400 if ID missingCreate custom BadRequestExceptionCatch globallyUse @RestControllerAdviceClean error messagesReturn error from ResponseEntity<String>Lightweight & scalableWorks with any RESTful controller

This approach keeps your API responses consistent and easy to debug, while giving developers clear feedback when an invalid ID is queried.