Exception Handling in Spring Boot

 🧰 Common Ways to Handle Exceptions in Spring Boot

✅ 1. Using @ExceptionHandler in a Controller

This handles exceptions locally for a specific controller.

java

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@RestController

public class MyController {

    @GetMapping("/user/{id}")

    public User getUser(@PathVariable Long id) {

        return userService.findById(id).orElseThrow(() -> new UserNotFoundException(id));

    }

    @ExceptionHandler(UserNotFoundException.class)

    public ResponseEntity<String> handleUserNotFound(UserNotFoundException ex) {

        return new ResponseEntity<>(ex.getMessage(), HttpStatus.NOT_FOUND);

    }

}

✅ 2. Using @ControllerAdvice for Global Handling

This is the recommended and reusable way to handle exceptions across the entire app.

java

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@ControllerAdvice

public class GlobalExceptionHandler {

    @ExceptionHandler(UserNotFoundException.class)

    public ResponseEntity<ErrorResponse> handleUserNotFound(UserNotFoundException ex) {

        ErrorResponse error = new ErrorResponse("USER_NOT_FOUND", ex.getMessage());

        return new ResponseEntity<>(error, HttpStatus.NOT_FOUND);

    }

    @ExceptionHandler(Exception.class)

    public ResponseEntity<ErrorResponse> handleGeneral(Exception ex) {

        ErrorResponse error = new ErrorResponse("INTERNAL_ERROR", "Something went wrong");

        return new ResponseEntity<>(error, HttpStatus.INTERNAL_SERVER_ERROR);

    }

}

✅ Define your ErrorResponse DTO for consistent API responses.

✅ 3. Custom Exceptions

java

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public class UserNotFoundException extends RuntimeException {

    public UserNotFoundException(Long id) {

        super("User not found with ID: " + id);

    }

}

✅ 4. Validation Exception Handling

When using @Valid or @Validated, Spring throws MethodArgumentNotValidException.

java

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@ExceptionHandler(MethodArgumentNotValidException.class)

public ResponseEntity<ErrorResponse> handleValidationErrors(MethodArgumentNotValidException ex) {

    List<String> errors = ex.getBindingResult()

        .getFieldErrors()

        .stream()

        .map(error -> error.getField() + ": " + error.getDefaultMessage())

        .collect(Collectors.toList());

    ErrorResponse error = new ErrorResponse("VALIDATION_FAILED", String.join(", ", errors));

    return new ResponseEntity<>(error, HttpStatus.BAD_REQUEST);

}

✅ 5. ResponseStatusException

You can also throw exceptions with a response status inline:

java

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@GetMapping("/user/{id}")

public User getUser(@PathVariable Long id) {

    return userRepository.findById(id)

        .orElseThrow(() -> new ResponseStatusException(HttpStatus.NOT_FOUND, "User not found"));

}

🎯 Best Practices

Practice Description

✅ Use @ControllerAdvice Centralized exception management

✅ Use custom exception classes Clear, reusable error semantics

✅ Return meaningful HTTP status codes E.g. 404, 400, 500

✅ Create a standard error response structure Consistency for API clients

✅ Log exceptions properly Helps with debugging

🧪 Example ErrorResponse Class

java

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public class ErrorResponse {

    private String code;

    private String message;

    // constructors, getters, setters

}

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