@ExceptionHandler in controllers in Spring Boot - Performance & Optimization

Performance: @ExceptionHandler in controllers

MEDIUM IMPACT

This affects server response time and error handling efficiency, indirectly impacting user experience by controlling error response speed and page stability.

Handling exceptions in Spring Boot controllers to provide user-friendly error responses

Spring Boot

@Controller
public class MyController {
  @ExceptionHandler(Exception.class)
  public ResponseEntity<String> handleAllExceptions(Exception ex) {
    return ResponseEntity.status(HttpStatus.INTERNAL_SERVER_ERROR).body("Custom error message");
  }
}

Centralizes error handling, reducing duplicated code and allowing faster, consistent error responses with less server overhead.

📈 Performance GainReduces CPU cycles spent on error handling and improves response consistency

Handling exceptions in Spring Boot controllers to provide user-friendly error responses

Spring Boot

public String handleRequest() {
  try {
    // controller logic
  } catch (Exception e) {
    return "errorPage";
  }
}

Catching exceptions inside each controller method leads to duplicated code and inconsistent error handling, increasing server processing time and maintenance cost.

📉 Performance CostAdds repeated error handling logic, increasing CPU usage and response time per request

Performance Comparison

Pattern	Server Processing	Code Duplication	Response Consistency	Verdict
Try-catch in each controller method	High - repeated handling	High - duplicated code	Low - inconsistent	[X] Bad
Centralized @ExceptionHandler method	Low - single handler	Low - centralized code	High - consistent	[OK] Good

Rendering Pipeline

When an exception occurs, Spring intercepts it and routes to the @ExceptionHandler method before sending a response. This avoids full page reloads or default error pages, improving response flow.

→Server Processing

→Response Generation

⚠️ BottleneckServer Processing due to exception handling logic

Core Web Vital Affected

INP

This affects server response time and error handling efficiency, indirectly impacting user experience by controlling error response speed and page stability.

Optimization Tips

1Use centralized @ExceptionHandler methods to avoid duplicated try-catch blocks.

2Consistent error responses reduce server processing time and improve user experience.

3Efficient exception handling improves interaction responsiveness (INP) by speeding error responses.

Performance Quiz - 3 Questions

What is a performance benefit of using @ExceptionHandler in Spring Boot controllers?

A Increases server load by adding more try-catch blocks

B Delays error responses by adding extra network requests

C Centralizes error handling to reduce duplicated code and server processing

D Triggers full page reloads on every exception

DevTools: Network

How to check: Open DevTools, go to Network tab, trigger an error in the app, and inspect the response time and status code of the error response.

What to look for: Look for consistent HTTP status codes and fast error response times indicating efficient exception handling.

Practice

1. What is the main purpose of using @ExceptionHandler in a Spring Boot controller?

easy

A. To catch and handle exceptions thrown in controller methods

B. To define the URL path for a controller method

C. To inject dependencies into the controller

D. To configure database connections

Solution

1. Step 1: Understand the role of @ExceptionHandler

   @ExceptionHandler is used to catch exceptions that occur in controller methods to prevent the app from crashing.

2. Step 2: Identify what @ExceptionHandler does in controllers

   It allows sending user-friendly error messages and proper HTTP status codes when exceptions happen.

3. Final Answer:

   To catch and handle exceptions thrown in controller methods -> Option A

4. Quick Check:

   @ExceptionHandler catches exceptions [OK]

Hint: Remember: @ExceptionHandler handles errors in controllers [OK]

Common Mistakes:

• Confusing @ExceptionHandler with URL mapping annotations
• Thinking it injects dependencies
• Assuming it configures database settings

2. Which of the following is the correct way to declare an exception handler method in a Spring Boot controller?

easy

A. @RequestMapping(Exception.class) public String handleError() { return "error"; }

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

C. @Autowired(Exception.class) public String handleError() { return "error"; }

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

Solution

1. Step 1: Identify the correct annotation for exception handling

   @ExceptionHandler is the annotation used to mark methods that handle exceptions.

2. Step 2: Check the method signature

   The method should be annotated with @ExceptionHandler(Exception.class) and return a response, here a String view name.

3. Final Answer:

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

4. Quick Check:

   @ExceptionHandler syntax [OK]

Hint: Use @ExceptionHandler with exception class to handle errors [OK]

Common Mistakes:

• Using @RequestMapping or @GetMapping instead of @ExceptionHandler
• Misusing @Autowired annotation
• Missing the exception class in @ExceptionHandler

3. Given the following controller code snippet, what will be the HTTP status code returned when a NullPointerException occurs?

@RestController
public class MyController {
  @GetMapping("/test")
  public String test() {
    throw new NullPointerException("Null error");
  }

  @ExceptionHandler(NullPointerException.class)
  @ResponseStatus(HttpStatus.BAD_REQUEST)
  public String handleNullPointer() {
    return "Null pointer error handled";
  }
}

medium

A. 200 OK

B. 400 Bad Request

C. 500 Internal Server Error

D. 404 Not Found

Solution

1. Step 1: Identify the exception and handler

   The controller throws NullPointerException, which is caught by the method annotated with @ExceptionHandler(NullPointerException.class).

2. Step 2: Check the response status annotation

   The handler method has @ResponseStatus(HttpStatus.BAD_REQUEST), so it sets the HTTP status to 400.

3. Final Answer:

   400 Bad Request -> Option B

4. Quick Check:

   @ResponseStatus sets HTTP code = 400 [OK]

Hint: Look for @ResponseStatus on @ExceptionHandler method [OK]

Common Mistakes:

• Assuming default 500 error without @ResponseStatus
• Confusing exception type with HTTP status
• Ignoring the @ExceptionHandler method

4. Consider this controller code snippet:

@RestController
public class DemoController {
  @GetMapping("/demo")
  public String demo() {
    throw new IllegalArgumentException("Invalid argument");
  }

  @ExceptionHandler(NullPointerException.class)
  public String handleNull() {
    return "Null error";
  }
}

What will happen when the /demo endpoint is called?

medium

A. The exception will be handled by handleNull method

B. The response will be "Null error" with HTTP 200

C. The exception will not be handled and a 500 error is returned

D. The server will return HTTP 404 Not Found

Solution

1. Step 1: Identify the thrown exception

   The method throws IllegalArgumentException, but the handler only catches NullPointerException.

2. Step 2: Check if exception is handled

   Since no handler matches IllegalArgumentException, Spring Boot returns a default 500 Internal Server Error.

3. Final Answer:

   The exception will not be handled and a 500 error is returned -> Option C

4. Quick Check:

   Handler mismatch causes 500 error [OK]

Hint: Handler must match exception type exactly [OK]

Common Mistakes:

• Assuming handler catches all exceptions
• Thinking handler method runs for different exception types
• Expecting 404 instead of 500 error

5. You want to create a global exception handler in Spring Boot that catches RuntimeException for all controllers and returns a JSON error message with HTTP status 500. Which approach correctly achieves this?

hard

A. Create a class annotated with @ControllerAdvice and define a method with @ExceptionHandler(RuntimeException.class) returning a JSON response and @ResponseStatus(HttpStatus.INTERNAL_SERVER_ERROR).

B. Add @ExceptionHandler(RuntimeException.class) method inside each controller class returning a view name.

C. Use @RestController on a class with @ExceptionHandler(Exception.class) returning a String message.

D. Configure exception handling in application.properties file.

Solution

1. Step 1: Understand global exception handling

   @ControllerAdvice allows defining exception handlers that apply to all controllers globally.

2. Step 2: Define handler method for RuntimeException

   Use @ExceptionHandler(RuntimeException.class) with @ResponseStatus(HttpStatus.INTERNAL_SERVER_ERROR) to set status 500 and return JSON response.

3. Final Answer:

   Create a class annotated with @ControllerAdvice and define a method with @ExceptionHandler(RuntimeException.class) returning a JSON response and @ResponseStatus(HttpStatus.INTERNAL_SERVER_ERROR). -> Option A

4. Quick Check:

   Global handler uses @ControllerAdvice + @ExceptionHandler [OK]

Hint: Use @ControllerAdvice for global exception handling [OK]

Common Mistakes:

• Putting handlers only inside controllers for global effect
• Using application.properties for exception handling
• Not setting proper HTTP status in handler