Why Handling not found exceptions in Spring Boot? - Purpose & Use Cases

The Big Idea

Discover how to stop your app from crashing when data is missing and keep users happy!

The Scenario

Imagine building a web app where users request data by ID, and you manually check if the data exists every time before responding.

If the data is missing, you have to write extra code to handle that case and send a proper error message.

The Problem

Manually checking for missing data everywhere leads to repeated code and mistakes.

You might forget to handle the missing case, causing confusing errors or crashes.

This makes your app unreliable and hard to maintain.

The Solution

Handling not found exceptions lets you centralize the missing data logic.

Spring Boot can automatically catch when data is missing and send a clear error response.

This keeps your code clean and your app stable.

Before vs After

✗ Before

if (data == null) { return ResponseEntity.status(404).body("Not found"); }

✓ After

@ResponseStatus(HttpStatus.NOT_FOUND)
public class NotFoundException extends RuntimeException {}

What It Enables

You can build apps that gracefully handle missing data without cluttering your main logic.

Real Life Example

When a user requests a product that doesn't exist, your app automatically returns a friendly 404 error page instead of crashing.

Key Takeaways

Manual checks for missing data are repetitive and error-prone.

Exception handling centralizes missing data responses.

Spring Boot makes it easy to send clear 404 errors automatically.

Practice

1. What is the main purpose of handling not found exceptions in a Spring Boot application?

easy

A. To improve database performance

B. To log all user activities

C. To automatically retry failed requests

D. To return a clear 404 status when a requested resource is missing

Solution

1. Step 1: Understand the role of not found exceptions

   Not found exceptions indicate that a requested resource does not exist on the server.

2. Step 2: Identify the correct response behavior

   Returning a 404 HTTP status code clearly informs the client that the resource is missing.

3. Final Answer:

   To return a clear 404 status when a requested resource is missing -> Option D

4. Quick Check:

   Not found exceptions = 404 status [OK]

Hint: Not found means resource missing, so send 404 status [OK]

Common Mistakes:

• Confusing not found with server errors
• Thinking it improves database speed
• Assuming it retries requests automatically

2. Which annotation is used in Spring Boot to mark a custom exception class to return HTTP 404 status automatically?

easy

A. @ResponseStatus(HttpStatus.NOT_FOUND)

B. @ExceptionHandler(HttpStatus.NOT_FOUND)

C. @NotFoundException

D. @HttpStatusCode(404)

Solution

1. Step 1: Recall Spring Boot annotations for HTTP status

   The @ResponseStatus annotation sets the HTTP status code for exceptions or controller methods.

2. Step 2: Match the annotation to 404 status

   Using @ResponseStatus(HttpStatus.NOT_FOUND) on a custom exception class sends 404 automatically.

3. Final Answer:

   @ResponseStatus(HttpStatus.NOT_FOUND) -> Option A

4. Quick Check:

   @ResponseStatus + NOT_FOUND = 404 [OK]

Hint: Use @ResponseStatus with NOT_FOUND for 404 exceptions [OK]

Common Mistakes:

• Using @ExceptionHandler instead of @ResponseStatus
• Assuming @NotFoundException is a valid annotation
• Using incorrect annotation names

3. Given this controller method in Spring Boot:

 @GetMapping("/items/{id}")
 public Item getItem(@PathVariable Long id) {
     return itemRepository.findById(id)
         .orElseThrow(() -> new ItemNotFoundException(id));
 }

What happens if the item with the given id does not exist?

medium

A. The method returns null and HTTP 200 status

B. An ItemNotFoundException is thrown and HTTP 404 status is returned

C. The server crashes with a NullPointerException

D. The method returns an empty Item object

Solution

1. Step 1: Analyze the use of orElseThrow

   If the item is not found, orElseThrow triggers the custom ItemNotFoundException.

2. Step 2: Understand exception handling with @ResponseStatus

   If ItemNotFoundException is annotated with @ResponseStatus(HttpStatus.NOT_FOUND), Spring returns 404 status.

3. Final Answer:

   An ItemNotFoundException is thrown and HTTP 404 status is returned -> Option B

4. Quick Check:

   orElseThrow + @ResponseStatus = 404 error [OK]

Hint: orElseThrow triggers exception, annotated exception sends 404 [OK]

Common Mistakes:

• Assuming null is returned instead of exception
• Confusing 404 with 500 server error
• Thinking empty object is returned

4. Consider this code snippet:

 @ResponseStatus(HttpStatus.NOT_FOUND)
 public class ProductNotFoundException extends RuntimeException {
     public ProductNotFoundException(Long id) {
         super("Product " + id + " not found");
     }
 }

 @GetMapping("/products/{id}")
 public Product getProduct(@PathVariable Long id) {
     return productRepo.findById(id).orElseThrow(() -> new ProductNotFoundException(id));
 }

What is the likely problem if the client always receives HTTP 500 instead of 404 when a product is missing?

medium

A. The exception is thrown but Spring does not map it to 404

B. The exception is not annotated with @ResponseStatus

C. The controller method does not handle the exception explicitly

D. The exception class is missing a no-argument constructor

Solution

1. Step 1: Verify annotation presence

   The exception class is correctly annotated with @ResponseStatus(HttpStatus.NOT_FOUND).

2. Step 2: Understand Spring's exception mapping

   If Spring returns 500, it means it does not recognize the exception mapping to 404, possibly due to package scanning or configuration issues.

3. Final Answer:

   The exception is thrown but Spring does not map it to 404 -> Option A

4. Quick Check:

   Annotated exception but 500 means mapping failed [OK]

Hint: 500 instead of 404 means Spring missed exception mapping [OK]

Common Mistakes:

• Assuming missing no-arg constructor causes 500
• Thinking @ResponseStatus is missing when it is present
• Believing explicit handler is always required

5. You want to create a global exception handler in Spring Boot that returns a JSON error with 404 status and a message when any ResourceNotFoundException occurs. Which approach correctly achieves this?

hard

A. Use @RestController on the exception class and return 404 status

B. Annotate ResourceNotFoundException with @ResponseStatus(HttpStatus.NOT_FOUND) only, no handler needed

C. Create a class annotated with @ControllerAdvice and a method with @ExceptionHandler(ResourceNotFoundException.class) returning ResponseEntity with status 404 and error body

D. Throw ResponseStatusException with 404 inside every controller method manually

Solution

1. Step 1: Understand global exception handling

   Using @ControllerAdvice with @ExceptionHandler allows centralized handling of exceptions across controllers.

2. Step 2: Return ResponseEntity with 404 and JSON body

   The handler method can build a ResponseEntity with HttpStatus.NOT_FOUND and a JSON error message for clients.

3. Final Answer:

   Create a class annotated with @ControllerAdvice and a method with @ExceptionHandler(ResourceNotFoundException.class) returning ResponseEntity with status 404 and error body -> Option C

4. Quick Check:

   Global handler + ResponseEntity = custom 404 JSON [OK]

Hint: Use @ControllerAdvice + @ExceptionHandler for global JSON 404 [OK]

Common Mistakes:

• Relying only on @ResponseStatus without custom JSON body
• Annotating exception class as controller
• Manually throwing exceptions in every method