Spring MVC: Fallback for unknown language code in uri parameter value

Question

Im trying to build my first Spring MVC 4 app with i18n support and was thinking how i can use a default/fallback locale in case of the user is manipulating the language uri parameter to a non existing or supported locale For example http://localhost.de?lang=abc

Im using the code

@Bean
public LocaleResolver localeResolver() {
    SessionLocaleResolver sessionLocaleResolver = new SessionLocaleResolver();
    sessionLocaleResolver.setDefaultLocale(Locale.GERMAN);
    return sessionLocaleResolver;
}

which works in general if i open the url the very first time but it seems not to work for the case i was describing. I know there is a mechanism which would use the default messages properties file but i would like to set a default/fallback locale for this case. Do i need to implement maybe a custom filter?

Answer 1

My suggestion would be to subclass the SessionLocaleResolver and override the getLocale method:

@SpringBootApplication
public class DemoApplication {

    public static void main(String[] args) {
        SpringApplication.run(DemoApplication.class, args);
    }

    private static Set<Locale> allowedLocales;

    static {
        HashSet<Locale> allowed = new HashSet<>();
        allowed.add(Locale.GERMAN);
        allowed.add(Locale.CANADA);
        allowedLocales = Collections.unmodifiableSet(allowed);

    }

    @Bean
    LocaleResolver localeResolver() {
        return new LimitedSessionLocaleResolver();
    }

    class LimitedSessionLocaleResolver extends SessionLocaleResolver {
        @Override
        public Locale resolveLocale(HttpServletRequest request) {
            Locale locale = super.resolveLocale(request);
            if (!allowedLocales.contains(locale)) {
                return determineDefaultLocale(request);
            }
            return locale;
        }
    }
}

This does not modify the Spring classes in any major way and is probably going to work without issues for the foreseeable future.

Answer 2

Probably far from being perfect but this is what i built...

I also need to say that i changed the default language select mechanism a bit because of SEO needs and i decided to change the language not by using a get parameter but using instead the first part of my uri path for the selected language. For example: http://myurl.com/en/test.html instead of http://myurl.com/test.html?lang=de

In my annotation based configuration:

@Bean
public LocaleResolver localeResolver() {
    UriLocaleResolver uriLocaleResolver = new UriLocaleResolver();
    return uriLocaleResolver;
}

The locale resolver

public class UriLocaleResolver implements LocaleResolver {

    private final Logger logger = LoggerFactory.getLogger(getClass());

    private Locale locale = null;

    @Autowired
    private LocalizationService localizationService;


    @Override
    public Locale resolveLocale(final HttpServletRequest servletRequest) {
        if (locale != null) {
            return locale;
        }

        String languageIsoCode = null;
        try {
            languageIsoCode = ((String)servletRequest.getAttribute(RequestKey.LANGUAGE_ISO_CODE)).toLowerCase();
        }
        catch (Exception e) { }
        if (StringUtils.isBlank(languageIsoCode) || !localizationService.getSupportedLocaleLanguageIsoCodes().contains(languageIsoCode)) {
            logger.trace("Couldn't find valid language iso code. Using default locale '{}'", GlobalConstant.DEFAULT_LOCALE);
            return GlobalConstant.DEFAULT_LOCALE;
        }

        logger.trace("Found language iso code '{}'", languageIsoCode);
        return new Locale(languageIsoCode);
    }

    @Override
    public void setLocale(HttpServletRequest request, HttpServletResponse response, Locale locale)     {
    this.locale = locale;
    }

}

The filter which checks for a valid language code in the uri...

@Component
public class UriLocalizationFilter extends OncePerRequestFilter {

    private final Logger logger = LoggerFactory.getLogger(getClass());

    @Autowired
    private LocalizationService localizationService;


    @Override
    protected void doFilterInternal(HttpServletRequest request, HttpServletResponse response, FilterChain filterChain) throws ServletException, IOException {
        String uri = request.getRequestURI().substring(request.getContextPath().length());
        String[] pathParts = uri.split("/");

        if (!uri.startsWith(GlobalConstant.FRONTEND_RESOURCE_PATH_PREFIX) && pathParts.length >= 1 && pathParts[1].length() == 2) {
             String originalLanguageIsoCode = pathParts[1];
             String lowerCaseLanguageIsoCode = originalLanguageIsoCode.toLowerCase();

            if (localizationService.getSupportedLocaleLanguageIsoCodes().contains(lowerCaseLanguageIsoCode)) {
                logger.debug("Found valid language iso code {}", lowerCaseLanguageIsoCode);
            }
            else {
                 logger.debug("Found invalid language iso code {}. Using default language iso code {}.", lowerCaseLanguageIsoCode, GlobalConstant.DEFAULT_LOCALE.getLanguage());
                 lowerCaseLanguageIsoCode = GlobalConstant.DEFAULT_LOCALE.getLanguage();
            }

            String newUrl = StringUtils.removeStart(uri, '/' + originalLanguageIsoCode);
            request.setAttribute(RequestKey.LANGUAGE_ISO_CODE, lowerCaseLanguageIsoCode);
            logger.debug("Dispatching to new url '{}'", newUrl);
            request.getRequestDispatcher(newUrl).forward(request, response);
        }
        else {
            filterChain.doFilter(request, response);
        }
    }

    public void setLocalizationService(LocalizationService localizationService) {
        this.localizationService = localizationService;
    }

}

}

Frontend path is "resources" in my case where all static files like js, css, fonts etc are laying. localizationService.getSupportedLocaleLanguageIsoCodes() is a Set containing currently three language codes (en, ru, de). So in case of a wrong language code like abc im doing a forward with my default language "de". For me it was/is acceptable solution cause having a wrong language code in the uri means that the uri was manipulated by the user...

Like i said its maybe not "the" solution; for example im not sure how and if it works with cookies and/or "remember me" authentications (using spring security)...just had no time to test it yet...