ResponseEntityExceptionHandler in Spring Boot - Step-by-Step Execution

Concept Flow - ResponseEntityExceptionHandler

Exception Thrown in Controller

↓

Exception Handler Intercepts

↓

Check Exception Type

↓

Match

↓

Handle Exception

↓

Build ResponseEntity

↓

Send HTTP Response with Error Info

When an exception happens in a controller, ResponseEntityExceptionHandler catches it, checks its type, handles it if matched, builds a response, and sends it back to the client.

Execution Sample

Spring Boot

public class MyExceptionHandler extends ResponseEntityExceptionHandler {
  @Override
  protected ResponseEntity<Object> handleMethodArgumentNotValid(MethodArgumentNotValidException ex,
      HttpHeaders headers, HttpStatusCode status, WebRequest request) {
    return ResponseEntity.status(HttpStatus.BAD_REQUEST).body("Invalid input");
  }
}

This code overrides a method to handle validation errors and returns a 400 Bad Request with a message.

Execution Table

Step	Action	Exception Type Checked	Match Result	Response Built
1	Controller throws MethodArgumentNotValidException	MethodArgumentNotValidException	N/A	N/A
2	Exception handler intercepts exception	MethodArgumentNotValidException	Yes	Prepare 400 Bad Request with message
3	handleMethodArgumentNotValid called	MethodArgumentNotValidException	Yes	ResponseEntity with status 400 and body 'Invalid input'
4	Response sent to client	N/A	N/A	HTTP 400 with message 'Invalid input'
5	Execution ends	N/A	N/A	Response delivered, no further handling

💡 Response sent after matching exception and building ResponseEntity

Variable Tracker

Variable	Start	After Step 2	After Step 3	Final
ex	null	MethodArgumentNotValidException instance	Same instance	Same instance
status	null	N/A	HttpStatusCode status (passed parameter)	HttpStatusCode status (passed parameter)
responseEntity	null	null	ResponseEntity with 400 and message	ResponseEntity with 400 and message

Key Moments - 2 Insights

Why does the handler method get called only for specific exceptions?

Because ResponseEntityExceptionHandler checks the exception type and calls the matching handler method, as shown in execution_table step 2 and 3.

What happens if the exception type does not match any handler?

The exception is passed to other handlers or default handling, as shown in concept_flow branch 'No Match'.

Visual Quiz - 3 Questions

Look at the execution_table, what is the response status after step 3?

A 200 OK

B 500 Internal Server Error

C 400 Bad Request

D 404 Not Found

💡 Hint

Check the 'Response Built' column at step 3 in execution_table

At which step does the exception handler confirm the exception type matches?

A Step 2

B Step 1

C Step 4

D Step 5

💡 Hint

Look at the 'Match Result' column in execution_table

If a different exception is thrown, what happens according to the concept_flow?

A It is handled by the same method

B It is passed to other handlers

C It is ignored and no response is sent

D The application crashes immediately

💡 Hint

See the 'No Match' branch in concept_flow diagram

Concept Snapshot

ResponseEntityExceptionHandler catches exceptions from controllers.
It checks exception types and calls matching handler methods.
Handlers build ResponseEntity with status and body.
ResponseEntity is sent as HTTP response.
Unmatched exceptions pass to other handlers or default handling.

Full Transcript

ResponseEntityExceptionHandler is a Spring Boot class that helps handle exceptions thrown in controller methods. When an exception occurs, it intercepts it and checks its type. If the exception matches a known type, it calls the corresponding handler method. This method builds a ResponseEntity object containing the HTTP status code and error message. The ResponseEntity is then sent back to the client as the HTTP response. If the exception type does not match any handler, it is passed to other handlers or default error handling. This process ensures that errors are handled gracefully and clients receive meaningful error responses.

Practice

1. What is the main purpose of ResponseEntityExceptionHandler in a Spring Boot application?

easy

A. To handle user authentication and authorization

B. To centralize and customize error handling for REST controllers

C. To manage database connections automatically

D. To configure application properties at runtime

Solution

1. Step 1: Understand the role of ResponseEntityExceptionHandler

   This class provides a centralized way to handle exceptions thrown by REST controllers in Spring Boot.

2. Step 2: Compare with other options

   Options A, B, and D relate to authentication, database management, and configuration, which are not the responsibility of ResponseEntityExceptionHandler.

3. Final Answer:

   To centralize and customize error handling for REST controllers -> Option B

4. Quick Check:

   ResponseEntityExceptionHandler centralizes REST error handling [OK]

Hint: Remember: ResponseEntityExceptionHandler handles REST errors centrally [OK]

Common Mistakes:

• Confusing it with security or database management
• Thinking it configures app properties
• Assuming it handles UI rendering errors

2. Which method signature correctly overrides a method from ResponseEntityExceptionHandler to handle MethodArgumentNotValidException?

easy

A. protected ResponseEntity<Object> handleValidationException(Exception ex, HttpHeaders headers, HttpStatus status, WebRequest request)

B. public void handleMethodArgumentNotValid(Exception ex)

C. private ResponseEntity<String> handleValidationError(MethodArgumentNotValidException ex)

D. protected ResponseEntity<Object> handleMethodArgumentNotValid(MethodArgumentNotValidException ex, HttpHeaders headers, HttpStatus status, WebRequest request)

Solution

1. Step 1: Check method signature requirements

   ResponseEntityExceptionHandler requires protected method returning ResponseEntity<Object> with parameters (MethodArgumentNotValidException, HttpHeaders, HttpStatus, WebRequest).

2. Step 2: Match options to signature

   protected ResponseEntity<Object> handleMethodArgumentNotValid(MethodArgumentNotValidException ex, HttpHeaders headers, HttpStatus status, WebRequest request) matches exactly. Options B and C have wrong access modifiers or parameters. protected ResponseEntity<Object> handleValidationException(Exception ex, HttpHeaders headers, HttpStatus status, WebRequest request) uses wrong exception type and method name.

3. Final Answer:

   protected ResponseEntity<Object> handleMethodArgumentNotValid(MethodArgumentNotValidException ex, HttpHeaders headers, HttpStatus status, WebRequest request) -> Option D

4. Quick Check:

   Exact override signature required [OK]

Hint: Override with exact signature including all parameters [OK]

Common Mistakes:

• Using wrong access modifier like public or private
• Missing parameters like HttpHeaders or WebRequest
• Changing return type to something other than ResponseEntity<Object>

3. Given this code snippet inside a class extending ResponseEntityExceptionHandler:

  @Override
  protected ResponseEntity<Object> handleHttpMessageNotReadable(HttpMessageNotReadableException ex, HttpHeaders headers, HttpStatus status, WebRequest request) {
      String body = "Malformed JSON request";
      return new ResponseEntity<>(body, headers, HttpStatus.BAD_REQUEST);
  }

What will be the HTTP status code returned when a malformed JSON is sent to the API?

medium

A. 200 OK

B. 500 Internal Server Error

C. 400 Bad Request

D. 404 Not Found

Solution

1. Step 1: Analyze the overridden method

   The method returns a ResponseEntity with status HttpStatus.BAD_REQUEST when JSON is malformed.

2. Step 2: Identify the HTTP status code for BAD_REQUEST

   BAD_REQUEST corresponds to HTTP status code 400.

3. Final Answer:

   400 Bad Request -> Option C

4. Quick Check:

   BAD_REQUEST = 400 [OK]

Hint: BAD_REQUEST means 400 status code [OK]

Common Mistakes:

• Confusing BAD_REQUEST with 500 Internal Server Error
• Assuming 200 OK is returned on error
• Mixing up 404 Not Found with validation errors

4. Consider this custom exception handler class:

  @ControllerAdvice
  public class CustomExceptionHandler extends ResponseEntityExceptionHandler {

      @ExceptionHandler(NullPointerException.class)
      public ResponseEntity<Object> handleNullPointer(NullPointerException ex, WebRequest request) {
          String body = "Null value found";
          return new ResponseEntity<>(body, HttpStatus.INTERNAL_SERVER_ERROR);
      }

      @Override
      protected ResponseEntity<Object> handleMethodArgumentNotValid(MethodArgumentNotValidException ex, HttpHeaders headers, HttpStatus status, WebRequest request) {
          String body = "Validation failed";
          return new ResponseEntity<>(body, HttpStatus.BAD_REQUEST);
      }
  }

If a NullPointerException occurs, what will be the HTTP status code returned?

medium

A. 500 Internal Server Error

B. 400 Bad Request

C. 404 Not Found

D. 200 OK

Solution

1. Step 1: Identify the handler for NullPointerException

   The method annotated with @ExceptionHandler(NullPointerException.class) returns status INTERNAL_SERVER_ERROR.

2. Step 2: Map INTERNAL_SERVER_ERROR to HTTP status code

   INTERNAL_SERVER_ERROR corresponds to HTTP status 500.

3. Final Answer:

   500 Internal Server Error -> Option A

4. Quick Check:

   NullPointerException handled with 500 status [OK]

Hint: Check @ExceptionHandler annotation for exception type [OK]

Common Mistakes:

• Assuming validation error handler is called for NullPointerException
• Confusing 400 Bad Request with 500 Internal Server Error
• Ignoring @ExceptionHandler methods

5. You want to customize error responses for both validation errors and JSON parse errors in your Spring Boot REST API. Which approach correctly combines ResponseEntityExceptionHandler methods and @ExceptionHandler to achieve this?

hard

A. Override handleMethodArgumentNotValid for validation errors and add an @ExceptionHandler for HttpMessageNotReadableException

B. Use only @ExceptionHandler methods for all exceptions, no overrides needed

C. Override handleHttpMessageNotReadable for validation errors and use @ExceptionHandler for MethodArgumentNotValidException

D. Override handleExceptionInternal to catch all exceptions globally

Solution

1. Step 1: Understand which methods handle which exceptions

   handleMethodArgumentNotValid handles validation errors; handleHttpMessageNotReadable handles JSON parse errors.

2. Step 2: Combine overrides and @ExceptionHandler correctly

   Override handleMethodArgumentNotValid for validation errors and use @ExceptionHandler for HttpMessageNotReadableException to customize JSON parse error responses.

3. Final Answer:

   Override handleMethodArgumentNotValid for validation errors and add an @ExceptionHandler for HttpMessageNotReadableException -> Option A

4. Quick Check:

   Use overrides for validation, @ExceptionHandler for JSON errors [OK]

Hint: Override validation method, @ExceptionHandler for JSON parse errors [OK]

Common Mistakes:

• Mixing exception types in wrong handlers
• Using only @ExceptionHandler without overrides
• Overriding wrong methods for given exceptions