Spring Boot Internationalization — i18n

Configurations

One of the basic requirements for any web application is to be able to support multiple languages and have localized messages. To achieve this by default, Spring Boot looks for the presence of a messages.properties resource bundle at the root of the classpath.

These files contain a set of key-value rows for each text that needed to be supported in multiple languages just like the following example:

message.properties

welcome=Hello! this is a test message!
lang.change=Change the language
lang.eng=English
lang.fr=French

message_fr.properties

welcome=Bonjour! Ceci est un message test!
lang.change=Changer la langue
lang.eng=English
lang.fr=French

Of course, there are several attributes like basenamethat you can use to change the resource bundle configuration using spring.messages namespace in application.properties or application.yaml.

spring.messages.basename=messages, i18n.list
spring.messages.fallback-to-system-locale=false

Message Retrieve

They are a couple of ways to get the proper localized message using the key provided in the resource bundle files. You can explicitly mention the locale type and use ResourceBundleMessageSource or you can utilize a locale LocaleResolver which is more efficient and functional.

ResourceBundleMessageSource

The easiest way to get the related messages from the resource bundle is using the ResourceBundleMessageSource.

@Component
public class ApplicationConfiguration {

    @Autowired
    ResourceBundleMessageSource messageSource;

    @PostConstruct
    public void init(){
        String welcome = messageSource.getMessage("welcome", null, Locale.FRANCE));
    }

}

The above code will result in setting the french version of the welcome key message in the welcome string.

LocaleResolver

Another way to determine which locale is currently being used is to use LocaleResolver. This interface has different implementations that determine the current locale based on the session, cookies, the Accept-Language header, or a fixed value.

FixedLocaleResolver: This implementation is used mostly for debugging purposes and you will always have a single fixed language set up in the project application properties file

@Bean
public LocaleResolver localeResolver() {
   FixedLocaleResolver lr = new FixedLocaleResolver();
   lr.setDefaultLocale(Locale.US);
   return lr;
}

AcceptHeaderLocaleResolver: For web applications, one of the options is to use the “accept-language” HTTP header on the request. This implementation will take the value from the HTTP status header and apply it.

@Bean
public LocaleResolver localeResolver() {
   AcceptHeaderLocaleResolver lr = new AcceptHeaderLocaleResolver();
   lr.setDefaultLocale(Locale.US);
   return lr;
}

SessionLocaleResolver: Another approach for web applications is to use Session to get the local for the user visiting the application. In this implementation, the value will store inside the session and will be available during the session’s lifetime.

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

CookieLocaleResolver: Cookies are also another option that has an implementation available. They are stored on the user’s machine and as long as browser cookies aren’t cleared by the user, once resolved the resolved locale data will last even between sessions.

@Bean
public LocaleResolver localeResolver() {
    CookieLocaleResolver lr = new CookieLocaleResolver();
    lr.setDefaultLocale(Locale.US);
    return lr;
}

Note: For SessionLocaleResolver and CookieLocaleResolver we need to have an interceptor to get the parameter sent from the user for the preferred language and store it in session or cookie. Thanks to spring this has been done already don and you can use LocaleChangeInterceptorand just pass the name of the property to set the locale.

@Bean
public WebMvcConfigurer configurer(){
     return new WebMvcConfigurerAdapter() {
     @Override
     public void addInterceptors (InterceptorRegistry registry) {
        LocaleChangeInterceptor l = new LocaleChangeInterceptor();
        l.setParamName("localeParmName");
        registry.addInterceptor(l);
       }
    };
}

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