How do you create and handle custom exceptions in a Spring Boot application?

In a Spring Boot application, custom exceptions can be created and handled using Java’s standard exception handling mechanisms, along with Spring’s exception handling features.

To create a custom exception, you can create a new class that extends Exception or one of its subclasses, such as RuntimeException. Here is an example of a custom exception:

    public class UserNotFoundException extends RuntimeException {
        public UserNotFoundException(String message) {
            super(message);
        }
    }

In this example, we have created a custom exception called UserNotFoundException, which extends RuntimeException. This exception can be thrown when a user is not found in the database.

To handle the custom exception, we can create an exception handler method in a @ControllerAdvice class. Here is an example:

    @ControllerAdvice
    public class ExceptionHandlerAdvice {
        @ExceptionHandler(UserNotFoundException.class)
        public ResponseEntity<String> handleUserNotFoundException(UserNotFoundException ex) {
            return ResponseEntity.status(HttpStatus.NOT\_FOUND).body(ex.getMessage());
        }
    }

In this example, we have created an exception handler method called handleUserNotFoundException that handles UserNotFoundException. The method returns a ResponseEntity object with a status code of 404 NOT_FOUND and the exception message as the response body.

To throw the custom exception, we can simply create a new instance of the exception and throw it in the appropriate place in our code. Here is an example:

    @Service
    public class UserService {
        @Autowired
        private UserRepository userRepository;
        
        public User getUserById(Long id) {
            Optional<User> user = userRepository.findById(id);
            if (user.isPresent()) {
                return user.get();
            } else {
                throw new UserNotFoundException("User not found with ID: " + id);
            }
        }
    }

In this example, we have a UserService class that retrieves a user by ID from a UserRepository. If the user is not found, we throw a UserNotFoundException with an appropriate error message.

Overall, creating and handling custom exceptions in a Spring Boot application is a powerful and flexible way to handle error conditions in your code. By creating custom exceptions and handling them with @ControllerAdvice and @ExceptionHandler, developers can provide meaningful error messages to users and gracefully handle exceptions in their code.