Allow AcceptHeaderLocaleResolver to fail on unsupported locales

[michael-o]

I have configured in 5.2.0.RELEASE:

	<beans:bean id="localeResolver" class="org.springframework.web.servlet.i18n.AcceptHeaderLocaleResolver">
		<beans:property name="defaultLocale" value="de" />
		<beans:property name="supportedLocales">
			<beans:list>
				<beans:value>cs</beans:value>
				<beans:value>de</beans:value>
				<beans:value>en</beans:value>
				<beans:value>es</beans:value>
				<beans:value>fr</beans:value>
				<beans:value>pt</beans:value>
				<beans:value>ru</beans:value>
			</beans:list>
		</beans:property>
	</beans:bean>

I would like the system to use the default locale if and only if the Accept-Language header has not been provided. In the case a user sends Accept-Language: pl, da I want the resolver to return 400 because neither Polish not Danish is supported.

[rstoyanchev]

LocaleResolver has no access to to the response and can't return a 400 by itself. The contract rules out returning null as well. Its purpose is literally to "resolve" the locale for the request, if possible matching to a supported one, or falling back on a fixed default, or otherwise resolving to the one from the request.

It would have to be some other component that checks whether the resolved locale is supported or not. A HandlerInterceptor perhaps:

public class LocaleValidatingInterceptor implements HandlerInterceptor {

    @Override
    public boolean preHandle(HttpServletRequest request, HttpServletResponse response, Object handler) {
        LocaleResolver r = RequestContextUtils.getLocaleResolver(request);
        if (r instanceof AcceptHeaderLocaleResolver) {
            List<Locale> supported = ((AcceptHeaderLocaleResolver) r).getSupportedLocales();
            if (!supported.contains(LocaleContextHolder.getLocale())) {
                response.setStatus(400);
                return false;
            }
        }
        return true;
    }

}

Note that also that even if we made it configurable to force an exception, the resolver is used way before handler mappings and the usual handler exception handling would not apply.

[michael-o]

So you see no better way, but handle this either in the controller or in an controller advice? I don't want to replicate the code of AcceptHeaderLocaleResolver.

[rstoyanchev]

No I don't see any option whatsoever through the LocaleResolver. The suggested interceptor snippet above does not replicate the LocaleResolver logic.

[izeye]

Based on the "status: declined" label, this appears to be meant to be closed.

[snicoll]

Good catch!