How to configure i18n in Spring boot 2 + Webflux + Thymeleaf?

Question

I just start a new project based on Spring boot 2 + Webflux. On upgrading version of spring boot and replace spring-boot-starter-web with spring-boot-starter-webflux classes like

• WebMvcConfigurerAdapter
• LocaleResolver
• LocaleChangeInterceptor

are missing. How now can I configure defaultLocale, and interceptor to change the language?

Answer 1

Just add a WebFilter that sets the Accept-Language header from the value of a query parameter. The following example gets the language from the language query parameter on URIs like http://localhost:8080/examples?language=es:

import org.springframework.boot.context.event.ApplicationReadyEvent;
import org.springframework.context.ApplicationContext;
import org.springframework.context.event.EventListener;
import org.springframework.http.server.reactive.ServerHttpRequest;
import org.springframework.stereotype.Component;
import org.springframework.util.MultiValueMap;
import org.springframework.web.server.ServerWebExchange;
import org.springframework.web.server.WebFilter;
import org.springframework.web.server.WebFilterChain;
import org.springframework.web.server.adapter.DefaultServerWebExchange;
import org.springframework.web.server.adapter.HttpWebHandlerAdapter;
import reactor.core.publisher.Mono;

import static org.springframework.util.StringUtils.isEmpty;

@Component
public class LanguageQueryParameterWebFilter implements WebFilter {

    private final ApplicationContext applicationContext;

    private HttpWebHandlerAdapter httpWebHandlerAdapter;

    public LanguageQueryParameterWebFilter(final ApplicationContext applicationContext) {
        this.applicationContext = applicationContext;
    }

    @EventListener(ApplicationReadyEvent.class)
    public void loadHttpHandler() {
        this.httpWebHandlerAdapter = applicationContext.getBean(HttpWebHandlerAdapter.class);
    }

    @Override
    public Mono<Void> filter(final ServerWebExchange exchange, final WebFilterChain chain) {
        final ServerHttpRequest request = exchange.getRequest();
        final MultiValueMap<String, String> queryParams = request.getQueryParams();
        final String languageValue = queryParams.getFirst("language");

        final ServerWebExchange localizedExchange = getServerWebExchange(languageValue, exchange);
        return chain.filter(localizedExchange);
    }

    private ServerWebExchange getServerWebExchange(final String languageValue, final ServerWebExchange exchange) {
        return isEmpty(languageValue)
                ? exchange
                : getLocalizedServerWebExchange(languageValue, exchange);
    }

    private ServerWebExchange getLocalizedServerWebExchange(final String languageValue, final ServerWebExchange exchange) {
        final ServerHttpRequest httpRequest = exchange.getRequest()
                .mutate()
                .headers(httpHeaders -> httpHeaders.set("Accept-Language", languageValue))
                .build();

        return new DefaultServerWebExchange(httpRequest, exchange.getResponse(),
                httpWebHandlerAdapter.getSessionManager(), httpWebHandlerAdapter.getCodecConfigurer(),
                httpWebHandlerAdapter.getLocaleContextResolver());
    }
}

It uses @EventListener(ApplicationReadyEvent.class) in order to avoid cyclic dependencies.

Feel free to test it and provide feedback on this POC.

Answer 2

With spring-boot-starter-webflux, there are

• DelegatingWebFluxConfiguration
• LocaleContextResolver

For example, to use a query parameter "lang" to explicitly control the locale:

1. Implement LocaleContextResolver, so that resolveLocaleContext() returns a SimpleLocaleContext determined by a GET parameter of "lang". I name this implementation QueryParamLocaleContextResolver. Note that the default LocaleContextResolver is an org.springframework.web.server.i18n.AcceptHeaderLocaleContextResolver.

2. Create a @Configuration class that extends DelegatingWebFluxConfiguration. Override DelegatingWebFluxConfiguration.localeContextResolver() to return QueryParamLocaleContextResolver that we just created in step 1. Name this configuration class WebConfig.

3. In WebConfig, override DelegatingWebFluxConfiguration.configureViewResolvers() and add the ThymeleafReactiveViewResolver bean as a view resolver. We do this because, for some reason, DelegatingWebFluxConfiguration will miss ThymeleafReactiveViewResolver after step 2.

Also, I have to mention that, to use i18n with the reactive stack, this bean is necessary:

    @Bean
    public MessageSource messageSource() {
        final ReloadableResourceBundleMessageSource messageSource = new ReloadableResourceBundleMessageSource();
        messageSource.setBasenames("classpath:/messages");
        messageSource.setUseCodeAsDefaultMessage(true);
        messageSource.setDefaultEncoding("UTF-8");
        messageSource.setCacheSeconds(5);
        return messageSource;
}

After creating a natural template, some properties files, and a controller, you will see that:

• localhost:8080/test?lang=zh gives you the Chinese version

• localhost:8080/test?lang=en gives you the English version

Just don't forget <meta charset="UTF-8"> in <head>, otherwise you may see some nasty display of Chinese characters.