判断一个请求是否是ajax

    /**
     * Prepare a request,including setting the encoding and locale.
     *
     * @param request The request
     * @param response The response
     */
    public void prepare(HttpServletRequest request,HttpServletResponse response) {
        String encoding = null;
        if (defaultEncoding != null) {
            encoding = defaultEncoding;
        }
        // check for Ajax request to use UTF-8 encoding strictly http://www.w3.org/TR/XMLHttpRequest/#the-send-method
        <strong><span style="font-size:18px;">if ("XMLHttpRequest".equals(request.getHeader("X-Requested-With"))) {
            encoding = "UTF-8";
        }</span></strong>

        Locale locale = null;
        if (defaultLocale != null) {
            locale = LocalizedTextUtil.localeFromString(defaultLocale,request.getLocale());
        }

        if (encoding != null) {
            applyEncoding(request,encoding);
        }

        if (locale != null) {
            response.setLocale(locale);
        }

        if (paramsWorkaroundEnabled) {
            request.getParameter("foo"); // simply read any parameter (existing or not) to "prime" the request
        }
    }

注：request.getHeader("X-Requested-With") （request.getHeader中传递的参数，不区分大小写的）