Youtube: https://www.youtube.com/watch?v=R68OJVIXKv0&ab_channel=CaseyBui

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• The LanguageConfig class defines a list of supported locales, in this case "en" and "vi".

@Configuration
public class LanguageConfig {
    public static final List<Locale> LOCALES = Arrays.asList(
            new Locale("en"),
            new Locale("vi")
    );
}

• The MyLocaleResolver class implements the LocaleResolver interface, which is used to determine the current locale for a request. The resolveLocale method checks the "Accept-Language" header of the request, and if it is present, it attempts to match it to one of the supported locales defined in the LanguageConfig class. If a match is found, the corresponding locale is returned. If the header is not present or doesn't match any of the supported locales, the default locale "vi" is returned.

@Component
public class MyLocaleResolver implements LocaleResolver {
    @Override
    public Locale resolveLocale(HttpServletRequest request) {
        String language = request.getHeader("Accept-Language");
        if (language == null || language.isEmpty()) {
            return Locale.forLanguageTag("vi");
        }
        Locale locale = Locale.forLanguageTag(language);
        if (LanguageConfig.LOCALES.contains(locale)) 
            return locale;
        }
        // return vi if not found
        return Locale.forLanguageTag("vi");
    }

    @Override
    public void setLocale(HttpServletRequest request, HttpServletResponse response, Locale locale) {
        throw new UnsupportedOperationException("Cannot change locale - use a different locale resolution strategy");
    }
}

• The MessageConfiguration class defines a bean for a ResourceBundleMessageSource which is used to load message properties files. The files are expected to be named "messages_[locale].properties" and are stored in the classpath.

@Configuration
public class MessageConfiguration {

    @Bean
    public MessageSource messageSource() {
        ResourceBundleMessageSource messageSource = new ResourceBundleMessageSource();
        messageSource.setBasename("messages");
        messageSource.setDefaultEncoding("UTF-8");
        return messageSource;
    }
}

• The TestService class is an example of a service that uses the MyLocaleResolver and MessageSource beans to return localized messages. The message method retrieves the current locale for the request and uses the messageSource.getMessage method to get the localized message for the key "greeting".

@GetMapping("/message")
    public String message(HttpServletRequest request) {
       return testService.message(request);
    }

@Service
public class TestService {

    @Autowired
    private MessageSource messageSource;

    @Autowired
    private MyLocaleResolver myLocaleResolver;

    public String message(HttpServletRequest request) {
        throw new ApiError(400, messageSource.getMessage("greeting", null, myLocaleResolver.resolveLocale(request)));

    }
}

// messages_en.properties
greeting=hello

// messages_fr.properties
greeting=bonjour

// messages_vi.properties
greeting=xin chào