用于在 Spring Boot 中进行验证的自定义 ExceptionHandler？

Question

In a Spring Boot app, I implemented a custom exception handling in my Spring Boot app as in this example and throw NoSuchElementFoundException without any problem as shown below:

public EmployeeDto findByEmail(String email) {
    return employeeRepository.findByEmail(email)
            .map(EmployeeDto::new)
            .orElseThrow(() -> new NoSuchElementFoundException(NO_ITEM_FOUND));
}

However, I also added another custom exception class and define its handler in my Global class as shown below:

@ExceptionHandler(NotValidFormatException.class)
@ResponseStatus(HttpStatus.UNPROCESSABLE_ENTITY)
public ResponseEntity<Object> handleNotValidFormatException(NotValidFormatException ex, WebRequest request) {
    log.error("Invalid format. Only csv files are allowed.", ex);
    return buildErrorResponse(ex, HttpStatus.UNPROCESSABLE_ENTITY, request);
}

Here is my service method where I throw this exception if the file is not valid type:

public void create(MultipartFile file) throws Exception {
    if (CsvHelper.hasCsvFormat(file)) {
        throw new NotValidFormatException("Not valid type");
    }
    // ...
}

Now, I am not sure where I should check the validity:

Should I just check it via a static method in my Util class?

or implement a custom validator as mentioned here ? In this case I am not sure if I still need my custom exception class. Any idea?

Answer 1

Creating a validator is almost always a good idea, because it is simpler to unit test this service method.

public void create(MultipartFile file) throws Exception {
    fileValidator.validate(file);
}

1. Create unit test for FileValidator
2. Mock fileValidator.validate(eq(file)) method to throw NotValidFormatException and assert that you have an exception when call create(file) .
3. Do nothing with fileValidator for other test methods for the service.

Keep in mind that sometimes it is better not to use a validator to keep code simpler. For example, if you need to validate an id .

Notes

I prefer not to create an exception for each validation check. Just to use a common class for all validation errors. You can return multiple validation errors with such approach.

public class ValidationException extends ServiceException {

    public ValidationException(ErrorResponseItem item) {
        this(Collections.singletonList(item));
    }

    public ValidationException(List<ErrorResponseItem> items) {
        super(HttpStatus.UNPROCESSABLE_ENTITY, items);
    }

}