朋友们，美好的一天！

首先，对不起我的英语。这是我的问题：

我是JSF（2.0）的新手，我正尝试使用BalusC算法从托管bean下载文件。功能正常运行，浏览器中出现“另存为…”对话框。但是我不知道如何在不重新加载/重定向视图的情况下返回文件下载错误消息（后备bean方法中的异常，DB错误等）。

我在视图上的隐藏按钮：

    <h:form id="detailsDecisionMainForm">
        <h:inputHidden id="docId" value="" />
        <h:commandButton id="downloadAction" action="#{detailsDecisionGridBean.downloadForm()}" style="display: none;" />
    </h:form>

我的托管bean（我可以使用哪个范围？我尝试过请求并查看范围）方法：

public String downloadForm() {
    log.fine("downloadForm call");
    PdfService pdfService = new PdfServiceImpl();
    ArmCommonService armCommonService = new ArmCommonServiceImpl();
    String errorMessage = null;
    try {
        Long opUni = new Long(FacesContext.getCurrentInstance().getExternalContext().getRequestParameterMap().get("detailsDecisionMainForm:docId"));
        log.fine("Document opUni = " + opUni);
        DocXML docXML = armCommonService.getDocXMLById(opUni);
        if (docXML != null) {
            File pdfFile = pdfService.getPdfReport(docXML.getXml(), docXML.getType());
            if (pdfFile != null) {
                DownloadUtils.exportPdf(pdfFile);
            } else {
                log.log(Level.SEVERE, "downloadForm error: pdf generation error");
                errorMessage = "PDF-document generation error.";
            }
        } else {
            log.log(Level.SEVERE, "downloadForm error: xml not found");
            errorMessage = "XML-document not found.";
        }
    } catch (Exception ex) {
        log.log(Level.SEVERE, "downloadForm exception: " + ex.getMessage());
        errorMessage = "File download exception.";
    }
    if (errorMessage != null) {
        FacesContext.getCurrentInstance().addMessage("detailsDecisionMainForm:downloadAction", new FacesMessage(errorMessage));
    }
    return null;
}

DownloadUtils.exportPdf（）过程正确运行：

public static void exportPdf(File file) throws IOException {
    InputStream fileIS = null;
    try {
        log.fine("exportPdf call");
        fileIS = new FileInputStream(file);
        FacesContext fc = FacesContext.getCurrentInstance();
        ExternalContext ec = fc.getExternalContext();
        ec.responseReset();
        ec.setResponseContentType(APPLICATION_PDF_UTF_8);
        byte[] buffer = ByteStreams.toByteArray(fileIS);
        ec.setResponseContentLength(buffer.length);
        ec.setResponseHeader(HttpHeaders.CONTENT_DISPOSITION, String.format(CONTENT_DISPOSITION_VALUE, new String(file.getName().getBytes(StandardCharsets.UTF_8))));
        ec.getResponseOutputStream().write(buffer);
        fc.responseComplete();
    } catch (Exception ex) {
        log.log(Level.SEVERE, "exportPdf exception: " + ex.getMessage());
    } finally {
        if (fileIS != null) {
            fileIS.close();
            log.fine("exportPdf inputstream file closed");
        }
    }
}

在downloadForm（）错误/异常之后，如何防止视图重新呈现？以及如何显示带有消息文本的javascript alert（）（将来-带有错误文本的jQuery.messager面板）？

谢谢！