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.