@ExceptionHandler in controllers in Spring Boot

Introduction

@ExceptionHandler helps you catch and handle errors in your web app controllers so users see friendly messages instead of crashes.

When you want to show a custom error message if a user requests a missing page.

When you want to handle invalid input errors gracefully in your API.

When you want to log errors and return a specific HTTP status code.

When you want to keep your controller methods clean by separating error handling.

When you want to return different responses for different types of exceptions.

Syntax

Spring Boot

@ExceptionHandler(ExceptionType.class)
public ResponseEntity<String> handleException(ExceptionType ex) {
    // handle error
}

Place this method inside your controller class or a @ControllerAdvice class.

The method catches exceptions of the specified type thrown in controller methods.

Examples

This handles NullPointerException and returns a 400 Bad Request with a message.

Spring Boot

@ExceptionHandler(NullPointerException.class)
public ResponseEntity<String> handleNullPointer(NullPointerException ex) {
    return ResponseEntity.status(HttpStatus.BAD_REQUEST).body("Null value found!");
}

This catches all exceptions not handled elsewhere and returns a 500 error.

Spring Boot

@ExceptionHandler(Exception.class)
public ResponseEntity<String> handleAllExceptions(Exception ex) {
    return ResponseEntity.status(HttpStatus.INTERNAL_SERVER_ERROR).body("Something went wrong.");
}

Sample Program

This controller has a /divide endpoint that causes a divide-by-zero error. The @ExceptionHandler catches that error and returns a 400 status with a friendly message.

Spring Boot

import org.springframework.http.HttpStatus;
import org.springframework.http.ResponseEntity;
import org.springframework.web.bind.annotation.ExceptionHandler;
import org.springframework.web.bind.annotation.GetMapping;
import org.springframework.web.bind.annotation.RestController;

@RestController
public class SampleController {

    @GetMapping("/divide")
    public int divide() {
        int a = 10;
        int b = 0;
        return a / b; // will cause ArithmeticException
    }

    @ExceptionHandler(ArithmeticException.class)
    public ResponseEntity<String> handleArithmeticException(ArithmeticException ex) {
        return ResponseEntity.status(HttpStatus.BAD_REQUEST).body("Cannot divide by zero.");
    }
}

Important Notes

You can have multiple @ExceptionHandler methods for different exception types.

Use @ControllerAdvice to handle exceptions globally across controllers.

Always return meaningful HTTP status codes to help clients understand the error.

Summary

@ExceptionHandler catches errors in controllers to avoid app crashes.

It lets you send friendly messages and proper HTTP status codes.

Use it to keep your controller code clean and user-friendly.

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