Checking whether client locale is in the list of supported languages

I want to present the website to the users in their preferred language in the case that the website supports that language.

In Spring Boot, I use the following bean to set the default language for the client:

@Bean
public LocaleResolver localeResolver() {
	SessionLocaleResolver localeResolver = new SessionLocaleResolver();
	localeResolver.setLocaleAttributeName("user_locale"); // Not strictly needed
	localeResolver.setDefaultLocale(Locale.ENGLISH);
	return localeResolver;
}

How could I modify it to take the client's language, and check whether it is in the list of supported languages, e.g.

private List<Locale> LOCALES = Arrays.asList(new Locale("en"), new Locale("de"), new Locale("fr"), new Locale("sp"), new Locale("it"));

and then set the locale using something like

return locales.contains(locale) ? locale : Locale.ENGLISH;

Notice that this should happen every time a session is initiated.

You can retrieve the currently stored Locale like this:

Locale locale = LocaleContextHolder.getLocale();
localeResolver.setDefaultLocale(locale);

A good way, if cookies are allowed, would be to implement it like that:

@Bean
public LocaleResolver localeResolver() {

    CookieLocaleResolver localeResolver = new CookieLocaleResolver("BALLROOM_LANGUAGE");

    Locale locale = LocaleContextHolder.getLocale();
    localeResolver.setDefaultLocale(locale);

    return localeResolver;
}

There is no need to check whether the automatically requested clients language is in your list, since it would always fallback to your default message implementation (which in your case would be english).

One way to let the user change the locale manually (what should always be possible) would be a custom LocaleController:

@Controller
@RequestMapping("/locale")
public class LocaleController {

    @PostMapping(path = {"", "/"}, consumes = {MediaType.APPLICATION_FORM_URLENCODED_VALUE})
    public String locale(@RequestParam("locale") String locale,
                         HttpServletRequest request,
                         HttpServletResponse response) {

        if (locale != null && !locale.isBlank()) {
            LocaleResolver localeResolver = RequestContextUtils.getLocaleResolver(request);
            if (localeResolver == null)
                throw new IllegalStateException("No LocaleResolver found");

            try {
                localeResolver.setLocale(request, response, StringUtils.parseLocale(locale));
            } catch (IllegalArgumentException e) {
                logger.debug("Ignoring invalid locale value [{}]: {}", locale, e.getMessage());
            }
        }

        String referer = request.getHeader("referer");
        return referer != null && !referer.isBlank() ? "redirect:" + referer : "redirect:/";
    }
}

You would simply send a post request containing the desired locale as String in form of e.g. en_US.

If you want to change the locale depending on the HTTP header Accept-Language you need to use AcceptHeaderLocaleResolver, for example:

@Bean
public LocaleResolver localeResolver() {
    var resolver = new AcceptHeaderLocaleResolver();
    resolver.setDefaultLocale(Locale.ENGLISH);
    resolver.setSupportedLocales(
            List.of(Locale.ENGLISH,
                    Locale.FRENCH,
                    Locale.of("es"))
    );

    return resolver;
}

It's easy to set up and I guess that's exatly what you want.

The next step is to get exatly the locale that is defined using AcceptHeaderLocaleResolver and use it, for example, to get a message from the message sources. To do this you can use LocaleContextHolder as follows:

@Autowired
private MessageSourceAccessor messageSourceAccessor;

@GetMapping(produces = MediaType.TEXT_PLAIN_VALUE)
public ResponseEntity<String> getGreeting() {
    String msg = messageSourceAccessor.getMessage("msg", LocaleContextHolder.getLocale());
    return ResponseEntity.ok(msg);
}

}

You can visit my github where I created a working example. To test the locale I used the Locale Switcher Chrome Extension.