public static void exportToExcel(HttpServlet servlet,
                                           HttpServletResponse response,
                                           String template,
                                           Map parameters,
                                           JRDataSource dataSource) throws Exception 
{
    File f = new File(servlet.getServletContext().getRealPath(template));
    if (!f.exists()) throw new JRRuntimeException("File '" + template + "' not found");

    response.setContentType("application/vnd.ms-excel;charset=Windows-1251");
    response.setHeader("Content-Disposition","attachment;filename=\"report.xls\"");
    JasperReport jasperReport = (JasperReport)JRLoader.loadObject(f);
    JasperPrint jasperPrint = JasperFillManager.fillReport(jasperReport,
                                                                          parameters,  	
                                                                          dataSource);
    JRXlsExporter exporter = new JRXlsExporter();
    exporter.setParameter(JRExporterParameter.JASPER_PRINT, jasperPrint);
    exporter.setParameter(JRExporterParameter.OUTPUT_STREAM, response.getOutputStream());
    exporter.exportReport();
}

-- место где строю отчёт
Map jrParams = new HashMap();
jrParams.put("REPORT.TITLE", "Отчёт"));

Vector data;
-- заполняю вектор
...........................

exportToExcel(servlet,
                    params.response,
                    "/WEB-INF/templates/report_template.jasper",
                    jrParams,
                    data);

OutputStream out = params.response.getOutputStream();

if (out != null) {
    out.flush();
    out.close();
}