public boolean preHandle(HttpServletRequest request, HttpServletResponse response, Object handler)

//Returns the locale for the given request
Locale resolveLocale(HttpServletRequest request);
 
//Method to set a locale value. Depending upon the LocaleResolver,
//  the locale can be set on the request, response or on to something else altogether.
void setLocale(HttpServletRequest request, HttpServletResponse response, Locale locale);

 AcceptHeaderLocaleResolver

It creates a locale object using the "accept-language" header of the HTTP request and returns it.

    public LocaleContext resolveLocaleContext(ServerWebExchange exchange) {
        List requestLocales = null;

        try {
            requestLocales = exchange.getRequest().getHeaders().getAcceptLanguageAsLocales();
        } catch (IllegalArgumentException var4) {
            ;
        }

        return new SimpleLocaleContext(this.resolveSupportedLocale(requestLocales));
    }