Locales as part of the URL in Spring MVC

Question

I'm now looking for a framework for multilingual web-applications. At the moment it seems to me that the best choice is Spring MVC. But I faced the fact that all the guidelines for developers suggests to switch languages using LocaleChangeInterceptor in such way:

http://www.somesite.com/action/?locale=en

Unfortunately, there are a number of reasons why I would like avoid this. How could I make language code to be an essential part of URL? For example:

http://www.somesite.com/en/action

Thanks.

UPD: I've found following solution. It's not complete yet, but works. Solution consists in two parts - servlet filter and locale resolver bean. It's looks little bit hackish, but I do not see other way to solve this problem.

public class LocaleFilter implements Filter
{

    ...

    private static final String DEFAULT_LOCALE = "en";
    private static final String[] AVAILABLE_LOCALES = new String[] {"en", "ru"};

    public LocaleFilter() {} 

    private List<String> getSevletRequestParts(ServletRequest request)
    {
        String[] splitedParts = ((HttpServletRequest) request).getServletPath().split("/");
        List<String> result = new ArrayList<String>();

        for (String sp : splitedParts)
        {
            if (sp.trim().length() > 0)
                result.add(sp);
        }

        return result;
    }

    private Locale getLocaleFromRequestParts(List<String> parts)
    {
        if (parts.size() > 0)
        {
            for (String lang : AVAILABLE_LOCALES)
            {
                if (lang.equals(parts.get(0)))
                {
                    return new Locale(lang);
                }
            }
        }

        return null;
    }

    @Override
    public void doFilter(ServletRequest request, ServletResponse response,
                         FilterChain chain) throws IOException, ServletException
    {
        List<String> requestParts = this.getSevletRequestParts(request);
        Locale locale = this.getLocaleFromRequestParts(requestParts);

        if (locale != null)
        {
            request.setAttribute(LocaleFilter.class.getName() + ".LOCALE", locale);

            StringBuilder sb = new StringBuilder();
            for (int i = 1; i < requestParts.size(); i++)
            {
                sb.append('/');
                sb.append((String) requestParts.get(i));
            }

            RequestDispatcher dispatcher = request.getRequestDispatcher(sb.toString());
            dispatcher.forward(request, response);
        }
        else
        {
            request.setAttribute(LocaleFilter.class.getName() + ".LOCALE", new Locale(DEFAULT_LOCALE));
            chain.doFilter(request, response);
        }
    }

    ...
}

public class FilterLocaleResolver implements LocaleResolver
{

    private Locale DEFAULT_LOCALE = new Locale("en");

    @Override
    public Locale resolveLocale(HttpServletRequest request)
    {
        Locale locale = (Locale) request.getAttribute(LocaleFilter.class.getName() + ".LOCALE");
        return (locale != null ? locale : DEFAULT_LOCALE);
    }

    @Override
    public void setLocale(HttpServletRequest request, HttpServletResponse response, Locale locale)
    {
        request.setAttribute(LocaleFilter.class.getName() + ".LOCALE", locale);
    }

}

So there is no need to map locale in each action in controllers. The following example will work fine:

@Controller
@RequestMapping("/test")
public class TestController
{

    @RequestMapping("action")
    public ModelAndView action(HttpServletRequest request, HttpServletResponse response)
    {
        ModelAndView mav = new ModelAndView("test/action");
        ...
        return mav;
    }

}

Answer 1

I implemented something very similar using a combination of Filter and Interceptor.

The filter extracts the first path variable and, if it's a valid locale it sets it as a request attribute, strips it from the beginning of the requested URI and forward the request to the new URI.

public class PathVariableLocaleFilter extends OncePerRequestFilter {
private static final Logger LOG = LoggerFactory.getLogger(PathVariableLocaleFilter.class);

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

    if (variables.length > 1 && isLocale(variables[1])) {
        LOG.debug("Found locale {}", variables[1]);
        request.setAttribute(LOCALE_ATTRIBUTE_NAME, variables[1]);
        String newUrl = StringUtils.removeStart(url, '/' + variables[1]);
        LOG.trace("Dispatching to new url \'{}\'", newUrl);
        RequestDispatcher dispatcher = request.getRequestDispatcher(newUrl);
        dispatcher.forward(request, response);
    } else {
        filterChain.doFilter(request, response);
    }
}

private boolean isLocale(String locale) {
    //validate the string here against an accepted list of locales or whatever
    try {
        LocaleUtils.toLocale(locale);
        return true;
    } catch (IllegalArgumentException e) {
        LOG.trace("Variable \'{}\' is not a Locale", locale);
    }
    return false;
}
}

The interceptor is very similar to the LocaleChangeInterceptor, it tries to get the locale from the request attribute and, if the locale is found, it sets it to the LocaleResolver.

public class LocaleAttributeChangeInterceptor extends HandlerInterceptorAdapter {
public static final String LOCALE_ATTRIBUTE_NAME = LocaleAttributeChangeInterceptor.class.getName() + ".LOCALE";

@Override
public boolean preHandle(HttpServletRequest request, HttpServletResponse response, Object handler) {

    Object newLocale = request.getAttribute(LOCALE_ATTRIBUTE_NAME);
    if (newLocale != null) {
        LocaleResolver localeResolver = RequestContextUtils.getLocaleResolver(request);
        if (localeResolver == null) {
            throw new IllegalStateException("No LocaleResolver found: not in a DispatcherServlet request?");
        }
        localeResolver.setLocale(request, response, StringUtils.parseLocaleString(newLocale.toString()));
    }
    // Proceed in any case.
    return true;
}
}

Once you have them in place you need to configure Spring to use the interceptor and a LocaleResolver.

@Override
public void addInterceptors(InterceptorRegistry registry) {
    registry.addInterceptor(new LocaleAttributeChangeInterceptor());
}

@Bean(name = "localeResolver")
public LocaleResolver getLocaleResolver() {
    return new CookieLocaleResolver();
}

And add the filter to the AbstractAnnotationConfigDispatcherServletInitializer.

@Override
protected Filter[] getServletFilters() {
    return new Filter[] { new PathVariableLocaleFilter() };
}

I haven't tested it thoroughly but it seems working so far and you don't have to touch your controllers to accept a {locale} path variable, it should just work out of the box. Maybe in the future we'll have 'locale as path variable/subfolder' Spring automagic solution as it seems more and more websites are adopting it and according to some it's the way to go.