ireport参数问题

public class JasperPrintServlet
    extends HttpServlet {
  static Log log = LogFactory.getLog(JasperPrintServlet.class);

  public void service(
      HttpServletRequest request,
      HttpServletResponse response
      ) throws IOException, ServletException {
    ServletContext context = this.getServletConfig().getServletContext();

     File reportFile = null;

    String fileName = request.getParameter("file_name");
    System.out.println();
    if (fileName != null)
      reportFile = new File(context.getRealPath("/monapp/report/" + fileName+
                                                ".jasper"));
    else
      throw new ServletException("无效的文件名 ：" + fileName);

    Map parameters = new HashMap();

    if (log.isDebugEnabled())
      log.debug("fileName=" + fileName) ;// " reportId=" + reportId);

    JasperPrint jasperPrint = null;

    Connection conn = null;
    JasperReport jasperReport = null;

    try {
      if (log.isDebugEnabled())
        log.debug("reportFile's real path is " + reportFile.getPath());

       jasperReport = (JasperReport) JRLoader.loadObject(reportFile.getPath());

       conn = DataBaseConnection.getConnection();
    
        jasperPrint =
          JasperFillManager.fillReport(
          jasperReport,
          parameters,
          conn           );
        System.out.println(jasperPrint+"====");
    }
    catch (JRException e) {
      response.setContentType("text/html");
      PrintWriter out = response.getWriter();
      out.println("");
      out.println("");
      out.println("");
      out.println(" <link title="\" href="\" type="\" rel="\" text="">");
      out.println("");

      out.println("");

      out.println(
          "Empty response.");

      out.println("");
      out.println("");
    }
  }

}

 

以上是我的sevlet源码，当执行完        jasperPrint =       JasperFillManager.fillReport(    jasperReport,    parameters,   conn           );时返回null。不知是何原因？（其中 jasperReport,    parameters,   conn 裙可以在调试状态中看到值）