Introduction

When building Spring Boot applications, especially those involving CRUD operations, it's crucial to handle exceptions gracefully and validate user input effectively. In this article, we'll explore how to implement exception handling and validation in Spring Boot, focusing on practical techniques you can apply to your projects.

Global Exception Handling

Spring Boot provides a powerful mechanism for handling exceptions globally using the @ControllerAdvice annotation. This allows you to centralize your exception handling logic and apply it across your entire application.

Let's create a global exception handler:

@ControllerAdvice
public class GlobalExceptionHandler {

    @ExceptionHandler(ResourceNotFoundException.class)
    public ResponseEntity<?> handleResourceNotFoundException(ResourceNotFoundException ex, WebRequest request) {
        ErrorDetails errorDetails = new ErrorDetails(new Date(), ex.getMessage(), request.getDescription(false));
        return new ResponseEntity<>(errorDetails, HttpStatus.NOT_FOUND);
    }

    @ExceptionHandler(Exception.class)
    public ResponseEntity<?> handleGlobalException(Exception ex, WebRequest request) {
        ErrorDetails errorDetails = new ErrorDetails(new Date(), ex.getMessage(), request.getDescription(false));
        return new ResponseEntity<>(errorDetails, HttpStatus.INTERNAL_SERVER_ERROR);
    }
}

In this example, we've created handlers for a custom ResourceNotFoundException and a catch-all for generic exceptions. This ensures that all exceptions are caught and returned in a consistent format.

Custom Exceptions

Creating custom exceptions allows you to provide more meaningful error messages and handle specific scenarios in your application. Here's an example of a custom exception:

public class ResourceNotFoundException extends RuntimeException {
    private static final long serialVersionUID = 1L;

    public ResourceNotFoundException(String message) {
        super(message);
    }
}

You can then throw this exception in your service layer when a resource is not found:

@Service
public class UserService {
    public User getUserById(Long id) {
        return userRepository.findById(id)
            .orElseThrow(() -> new ResourceNotFoundException("User not found with id: " + id));
    }
}

Input Validation

Spring Boot integrates seamlessly with Bean Validation API, allowing you to validate input data easily. Here's how you can add validation to your entity class:

import javax.validation.constraints.*;

@Entity
public class User {
    @Id
    @GeneratedValue(strategy = GenerationType.IDENTITY)
    private Long id;

    @NotBlank(message = "Name is required")
    @Size(min = 2, max = 50, message = "Name must be between 2 and 50 characters")
    private String name;

    @Email(message = "Email should be valid")
    @NotBlank(message = "Email is required")
    private String email;

    // getters and setters
}

To enforce these validations in your controller, use the @Valid annotation:

@PostMapping("/users")
public ResponseEntity<User> createUser(@Valid @RequestBody User user) {
    User savedUser = userService.saveUser(user);
    return new ResponseEntity<>(savedUser, HttpStatus.CREATED);
}

If the validation fails, Spring Boot will automatically throw a MethodArgumentNotValidException. You can handle this in your global exception handler:

@ExceptionHandler(MethodArgumentNotValidException.class)
public ResponseEntity<?> handleValidationExceptions(MethodArgumentNotValidException ex) {
    Map<String, String> errors = new HashMap<>();
    ex.getBindingResult().getAllErrors().forEach((error) -> {
        String fieldName = ((FieldError) error).getField();
        String errorMessage = error.getDefaultMessage();
        errors.put(fieldName, errorMessage);
    });
    return new ResponseEntity<>(errors, HttpStatus.BAD_REQUEST);
}

Putting It All Together

By combining global exception handling, custom exceptions, and input validation, you can create a robust error handling system for your Spring Boot CRUD application. This approach provides clear, informative error messages to clients, enhances the security of your application, and improves the overall user experience.

Remember to test your exception handling and validation thoroughly to ensure they work as expected in various scenarios. As you become more comfortable with these techniques, you'll find that your Spring Boot applications become more resilient and easier to maintain.