I18n support for Spring boot APIs

1. Define the Messages class

The below class will be the facade that returns the message in required locale for a given key.

@Component
public class Messages
{
    @Autowired
    private MessageSource messageSource;

    private MessageSourceAccessor accessor;

    // To let the messages accessed statically
    private static Messages ownInstance;

    // Default locale for your app.
    // app.default.locale expected to be available in application.properties/yml
    @Value("${app.default.locale}")
    private String defaultLocale;

    @PostConstruct
    public void init()
    {
        Locale.setDefault(new Locale(defaultLocale));
        accessor = new MessageSourceAccessor(messageSource);
        ownInstance = this;
    }

    private String getString(String key)
    {
        try
        {
            return accessor.getMessage(key);
        } catch (MissingResourceException e)
        {
            return '!' + key + '!';
        }
    }

    /**
     * Returns the Locale specific message from the configured
     * {@link MessageSource}
     *
     * @param theKey
     *            The key for which the message will be returned.
     * @return The message string associated with the given key.
     */
    public static String getMessage(String theKey)
    {
        return ownInstance.getString(theKey);
    }
}

2. Initialize MessageSource

The MessageSource abstracts the source from which the value of a string is being fetched.

In the @configuration class, you have to initialize a MessageSource.

Please note that the method name should be messageSource or else you have to explicitly define the id of the @Bean to be messageSource. The reason is Spring’s convention.

    @Bean
    public MessageSource messageSource()
    {
        ReloadableResourceBundleMessageSource aMessageSrc = new ReloadableResourceBundleMessageSource();
        // all files under the classpath with prefix messages
        aMessageSrc.setBasename("classpath:messages");
        return aMessageSrc;
    }

3. Initialize LocaleResolver

The LocaleResolver takes the responsibility of resolves the Locale for each request.

    @Bean
    public LocaleResolver localeResolver()
    {
        SessionLocaleResolver slr = new SessionLocaleResolver();
        return slr;
    }

4. Externalize strings

Externalize the hardcoded strings that could be responses for API. And store in message_<locale>.properties, so that the values of the properties will be your actual message for a specific locale.

Note: Eclipse users can take the help from eclipse. To use it right click on src project open "Source">"Externalize Strings…"

5. Refer the messages using keys

In your actual code, use the below call to get the respective message.

Messages.getMessage("mesage_key");

6. I18n in Filter

If you have filters(classes that extend Filter), you would be required to add the below line in the doFilter method.

LocaleContextHolder.setLocale(request.getLocale());

7 . I18n in Security Filters(untested)

If you are using Spring Security filters, you will be required to add the below initialization in the configuration.

The configuration class is expected to extend WebMvcConfigurerAdapter to make use of addInterceptors.

   @Bean
   public LocaleChangeInterceptor localeChangeInterceptor() {
       LocaleChangeInterceptor lci = new LocaleChangeInterceptor();
       return lci;
   }

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

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