protected static  void writeExport(HttpServletResponse response,
    ServletRequest request,
    BufferedResponseWrapper wrapper) throws IOException 
{
        if (wrapper.isOutRequested())
        {
            // data already written
            log.debug("Filter operating in unbuffered mode. Everything done, exiting");
            return;
        }
        // if you reach this point the PARAMETER_EXPORTING has been found, but the special header has never been set in
        // response (this is the signal from table tag that it is going to write exported data)
        log.debug("Filter operating in buffered mode. ");
        Map bean = (Map) request.getAttribute(TableTag.FILTER_CONTENT_OVERRIDE_BODY);
        if (log.isDebugEnabled())
        {
            log.debug(bean);
        }
        Object pageContent = bean.get(TableTagParameters.BEAN_BODY);
        if (pageContent == null)
        {
            if (log.isDebugEnabled())
            {
                log.debug("Filter is enabled but exported content has not been found. Maybe an error occurred?");
            }
            response.setContentType(wrapper.getContentType());
            PrintWriter out = response.getWriter();
            out.write(wrapper.getContentAsString());
            out.flush();
            return;
        }
        // clear headers
        if (!response.isCommitted())
        {
            response.reset();
        }
        String filename = (String) bean.get(TableTagParameters.BEAN_FILENAME);
        String contentType = (String) bean.get(TableTagParameters.BEAN_CONTENTTYPE);
        if (StringUtils.isNotBlank(filename))
        {
            response.setHeader("Content-Disposition", "attachment; filename=\"" + filename + "\"");
        }
        String characterEncoding = wrapper.getCharacterEncoding();
        String wrappedContentType = wrapper.getContentType();
        if (wrappedContentType != null && wrappedContentType.indexOf("charset")  > -1)
        {
            // charset is already specified (see #921811)
            characterEncoding = StringUtils.substringAfter(wrappedContentType, "charset=");
        }
        if (characterEncoding != null && contentType.indexOf("charset") == -1) //$NON-NLS-1$
        {
            contentType += "; charset=" + characterEncoding; //$NON-NLS-1$
        }
        response.setContentType(contentType);
        if (pageContent instanceof String)
        {
            // text content
            if (characterEncoding != null)
            {
                response.setContentLength(((String) pageContent).getBytes(characterEncoding).length);
            }
            else
            {
                response.setContentLength(((String) pageContent).getBytes().length);
            }
            PrintWriter out = response.getWriter();
            out.write((String) pageContent);
            out.flush();
        }
        else
        {
            // dealing with binary content
            byte[] content = (byte[]) pageContent;
            response.setContentLength(content.length);
            OutputStream out = response.getOutputStream();
            out.write(content);
            out.flush();
        }
}

Actually writes exported data. Extracts content from the Map stored in request with the TableTag.FILTER_CONTENT_OVERRIDE_BODY key.