如何从JSF受管bean调用servlet？

Question

我试图从JSF托管bean调用servlet。我尝试了以下代码，但未调用servlet。

托管bean代码

FacesContext fc = FacesContext.getCurrentInstance();
ServletContext sc = (ServletContext) fc.getExternalContext().getContext();
sc.getRequestDispatcher("/download?file=#{attachments.filePath}");

Servlet代码

    @WebServlet("/download")
    ......
    ServletOutputStream servletOutputStream = null;
    BufferedInputStream bufferedInputStream = null;
    BufferedOutputStream bufferedOutputStream = null;
    try {

        // file to download
        String filePath = request.getParameter("file").trim();
        // Check also Windows style path (for development)
        int index = filePath.lastIndexOf("/") != -1 ? filePath.lastIndexOf("/") : filePath.lastIndexOf("\\");
        String fileName = filePath.substring(index + 1);            
        bufferedInputStream = new BufferedInputStream(new FileInputStream(filePath));

        byte[] data = new byte[bufferedInputStream.available()];
        bufferedInputStream.read(data);

        response.setContentType("application/octet-stream");
        response.setHeader("Content-Disposition", "attachment; filename=" + fileName);

        servletOutputStream = response.getOutputStream();
        bufferedOutputStream = new BufferedOutputStream(servletOutputStream);
        bufferedOutputStream.write(data);           
    }