JFreeChart多重饼图如何显示在jsp页面中?

@RequestMapping("getPie")
public String getPie(HttpServletRequest request) throws Exception{
String checkInType[] = { "出勤", "病假", "事假", "其他" };
String clazzs[] = { "JSD1208班", "JSD1209班", "JSD1210班", "JSD1211班" };
String chartTitle = "各班级出勤情况";

      // 创建数据集
      DefaultCategoryDataset dataset = new DefaultCategoryDataset();

      // 构建数据
      int checkInData;
      for (int i = 0; i < checkInType.length; i++) {
       for (int j = 0; j < clazzs.length; j++) {
        checkInData = 1 + (int) (Math.random() * 1000);
        dataset.addValue(checkInData, checkInType[i], clazzs[j]);
       }
      }

      // 获取JFreeChart对象
      JFreeChart chart =ChartFactory.createMultiplePieChart(

      chartTitle, // 图表标题
        dataset, // 数据集
        TableOrder.BY_COLUMN, // 指定被提取数据的顺序
        false, // 是否包含图例
        true, // 是否包含提示工具
        false // 是否包含url
        );
      // 获取PiePlot对象
      MultiplePiePlot multiPlot = (MultiplePiePlot) chart.getPlot();
      JFreeChart obj = multiPlot.getPieChart();
      PiePlot plot = (PiePlot) obj.getPlot();

      //设置标签格式
      plot.setLabelGenerator(new StandardPieSectionLabelGenerator("{0} = {1} ({2})", NumberFormat.getNumberInstance(),
                    new DecimalFormat("0.00%")));

      // 分离圆弧
      for (int i = 0; i < clazzs.length; i++) {
       plot.setExplodePercent(i, 0.03);
      }

      ChartRenderingInfo info = new ChartRenderingInfo(
        new StandardEntityCollection());
      String fileName = ServletUtilities.saveChartAsPNG(chart, 500, 400, info, request.getSession());
      request.setAttribute("fileName", fileName);
    return BASE_PATH + " figure/pie";
}

String fileName = ServletUtilities.saveChartAsPNG(chart, 500, 400, info, request.getSession());这里报integer转换string异常