@ControllerAdvice for global handling in Spring Boot - Step-by-Step Execution

Concept Flow - @ControllerAdvice for global handling

Start: HTTP Request

↓

Controller Method

↓

Exception Thrown?

No→Return Normal Response

Yes↓

@ControllerAdvice Handler

↓

Handle Exception Globally

↓

Return Error Response

When a controller throws an exception, @ControllerAdvice catches it globally and returns a custom response.

Execution Sample

Spring Boot

import org.springframework.web.bind.annotation.ControllerAdvice;
import org.springframework.web.bind.annotation.ExceptionHandler;

@ControllerAdvice
public class GlobalExceptionHandler {
  @ExceptionHandler(Exception.class)
  public String handleAllExceptions(Exception ex) {
    return "error-page";
  }
}

This code catches all exceptions from any controller and returns an error page.

Execution Table

Step	Action	Exception Thrown?	Handler Invoked	Response Returned
1	HTTP request arrives at controller	No	No	Controller normal response
2	Controller method executes	Yes (e.g. NullPointerException)	Yes (@ControllerAdvice method)	Custom error page
3	Exception handled by global handler	Handled	Yes	Error response sent to client

💡 Exception handled globally by @ControllerAdvice, normal flow interrupted to return error response

Variable Tracker

Variable	Start	After Step 1	After Step 2	Final
exceptionThrown	false	false	true	true
handlerInvoked	false	false	true	true
response	null	normal response	error page	error page

Key Moments - 2 Insights

Why does the normal controller response not get sent when an exception occurs?

Because the exception interrupts normal flow and @ControllerAdvice catches it to send a custom error response instead, as shown in execution_table step 2.

Does @ControllerAdvice handle exceptions from all controllers automatically?

Yes, @ControllerAdvice applies globally to all controllers unless limited by annotations or configuration, as seen by handlerInvoked being true whenever an exception occurs.

Visual Quiz - 3 Questions

Look at the execution_table, what is the response returned at step 1?

A Custom error page

B Controller normal response

C No response yet

D Exception message

💡 Hint

Check the 'Response Returned' column at step 1 in the execution_table

At which step does the @ControllerAdvice handler get invoked?

A Step 3

B Step 1

C Step 2

D Never

💡 Hint

Look at the 'Handler Invoked' column in the execution_table

If no exception is thrown, what happens to the handlerInvoked variable?

A It remains false

B It becomes null

C It becomes true

D It throws an error

💡 Hint

Check variable_tracker for handlerInvoked values after step 1

Concept Snapshot

@ControllerAdvice catches exceptions from all controllers globally.
Use @ExceptionHandler methods inside it to define custom error responses.
When an exception occurs, normal controller response is skipped.
The global handler sends a custom response instead.
This helps centralize error handling in one place.

Full Transcript

When a user sends a request, it goes to a controller method. If the method runs fine, it returns a normal response. But if an exception happens, the normal response stops. Instead, the exception is caught by a global handler marked with @ControllerAdvice. This handler runs its method to create a custom error response, like an error page. This way, all controllers share the same error handling logic. Variables like exceptionThrown and handlerInvoked track if an error happened and if the handler ran. This makes error handling simple and consistent across the app.

Practice

1. What is the main purpose of using @ControllerAdvice in a Spring Boot application?

easy

A. To handle exceptions globally for all controllers in one place

B. To define REST endpoints for the application

C. To configure database connections

D. To create scheduled tasks

Solution

1. Step 1: Understand the role of @ControllerAdvice

   @ControllerAdvice is a special annotation in Spring Boot that allows you to write code to handle exceptions or apply logic across all controllers globally.

2. Step 2: Identify its main use

   Its main use is to centralize exception handling so you don't have to repeat error handling code in every controller.

3. Final Answer:

   To handle exceptions globally for all controllers in one place -> Option A

4. Quick Check:

   @ControllerAdvice = global exception handling [OK]

Hint: Think global error handling for all controllers [OK]

Common Mistakes:

• Confusing @ControllerAdvice with @RestController
• Thinking it configures database or tasks
• Assuming it defines endpoints

2. Which of the following is the correct way to declare a global exception handler method inside a @ControllerAdvice class?

easy

A. @Service public String handleError() { return "error"; }

B. @ExceptionHandler(Exception.class) public String handleError() { return "error"; }

C. @GetMapping public String handleError() { return "error"; }

D. @RequestMapping("/error") public String handleError() { return "error"; }

Solution

1. Step 1: Identify the annotation for exception handling

   The correct annotation to handle exceptions globally is @ExceptionHandler with the exception class specified.

2. Step 2: Check method signature and annotation usage

   @ExceptionHandler(Exception.class) public String handleError() { return "error"; } uses @ExceptionHandler(Exception.class) which is correct inside a @ControllerAdvice class to catch all exceptions.

3. Final Answer:

   @ExceptionHandler(Exception.class) public String handleError() { return "error"; } -> Option B

4. Quick Check:

   @ExceptionHandler = global exception method [OK]

Hint: Use @ExceptionHandler with exception class inside @ControllerAdvice [OK]

Common Mistakes:

• Using @RequestMapping or @GetMapping instead of @ExceptionHandler
• Missing exception class in @ExceptionHandler
• Using @Service annotation incorrectly

3. Given the following code snippet, what will be the HTTP response status when a NullPointerException is thrown in any controller?

@ControllerAdvice
public class GlobalExceptionHandler {
  @ExceptionHandler(NullPointerException.class)
  public ResponseEntity<String> handleNullPointer() {
    return new ResponseEntity<>("Null pointer error", HttpStatus.BAD_REQUEST);
  }
}

medium

A. 400 Bad Request with message "Null pointer error"

B. 500 Internal Server Error with default message

C. 200 OK with message "Null pointer error"

D. 404 Not Found with message "Null pointer error"

Solution

1. Step 1: Analyze the @ExceptionHandler method

   The method handles NullPointerException and returns a ResponseEntity with message "Null pointer error" and status BAD_REQUEST.

2. Step 2: Understand HTTP status returned

   HttpStatus.BAD_REQUEST corresponds to 400 status code, so the response will have status 400 with the given message.

3. Final Answer:

   400 Bad Request with message "Null pointer error" -> Option A

4. Quick Check:

   @ExceptionHandler returns 400 status [OK]

Hint: Check ResponseEntity status in @ExceptionHandler method [OK]

Common Mistakes:

• Assuming default 500 error instead of specified status
• Confusing 400 with 200 OK
• Ignoring the returned ResponseEntity status

4. Consider this @ControllerAdvice class:

@ControllerAdvice
public class GlobalHandler {
  @ExceptionHandler(IllegalArgumentException.class)
  public String handleIllegalArgument() {
    return "illegal";
  }

  @ExceptionHandler(Exception.class)
  public String handleAll() {
    return "error";
  }
}

What is the problem if a NullPointerException occurs in a controller?

medium

A. NullPointerException is not handled and causes server error

B. handleIllegalArgument() method will handle NullPointerException incorrectly

C. handleAll() method will handle NullPointerException correctly

D. Compilation error due to multiple @ExceptionHandler methods

Solution

1. Step 1: Identify exception handlers and their scopes

   handleIllegalArgument() handles IllegalArgumentException only; handleAll() handles all Exceptions as a fallback.

2. Step 2: Determine which method handles NullPointerException

   Since NullPointerException is not IllegalArgumentException, handleAll() will catch it as it handles Exception class.

3. Final Answer:

   handleAll() method will handle NullPointerException correctly -> Option C

4. Quick Check:

   More specific handlers first, then general Exception handler [OK]

Hint: General @ExceptionHandler(Exception.class) catches all others [OK]

Common Mistakes:

• Thinking NullPointerException is unhandled
• Assuming multiple handlers cause compile error
• Confusing which handler matches which exception

5. You want to create a global error handler that returns JSON with error details for all exceptions except ResourceNotFoundException, which should return 404 status with a custom message. How should you implement this using @ControllerAdvice?

hard

A. Use a single @ExceptionHandler(Exception.class) method and check exception type inside to return different responses

B. Use @ControllerAdvice only for ResourceNotFoundException and ignore other exceptions

C. Handle ResourceNotFoundException in each controller separately, and use @ControllerAdvice only for other exceptions

D. Create two @ExceptionHandler methods: one for ResourceNotFoundException returning 404 JSON, another for Exception returning generic JSON with 500 status

Solution

1. Step 1: Understand requirement for specific and general handling

   You need one handler for ResourceNotFoundException with 404 and custom JSON, and another for all other exceptions with generic 500 JSON.

2. Step 2: Implement two separate @ExceptionHandler methods in @ControllerAdvice

   One method annotated with @ExceptionHandler(ResourceNotFoundException.class) returns 404 JSON; another with @ExceptionHandler(Exception.class) returns 500 JSON.

3. Final Answer:

   Create two @ExceptionHandler methods: one for ResourceNotFoundException returning 404 JSON, another for Exception returning generic JSON with 500 status -> Option D

4. Quick Check:

   Separate handlers for specific and general exceptions [OK]

Hint: Use multiple @ExceptionHandler methods for different exceptions [OK]

Common Mistakes:

• Using one method with manual type checks inside
• Handling specific exceptions in controllers, not globally
• Ignoring general exceptions in @ControllerAdvice