为什么我的列表不能正确显示

出现的错误：

我的服务器报错：

java.lang.NullPointerException
	at org.imooc.util.FileUtil.save(FileUtil.java:19)
	at org.imooc.service.ExcelService.imp(ExcelService.java:29)
	at org.imooc.servlet.ImportExcelServlet.doPost(ImportExcelServlet.java:38)
	at javax.servlet.http.HttpServlet.service(HttpServlet.java:650)
	at javax.servlet.http.HttpServlet.service(HttpServlet.java:731)
	at org.apache.catalina.core.ApplicationFilterChain.internalDoFilter(ApplicationFilterChain.java:303)
	at org.apache.catalina.core.ApplicationFilterChain.doFilter(ApplicationFilterChain.java:208)
	at org.apache.tomcat.websocket.server.WsFilter.doFilter(WsFilter.java:52)
	at org.apache.catalina.core.ApplicationFilterChain.internalDoFilter(ApplicationFilterChain.java:241)
	at org.apache.catalina.core.ApplicationFilterChain.doFilter(ApplicationFilterChain.java:208)
	at org.apache.catalina.core.StandardWrapperValve.invoke(StandardWrapperValve.java:219)
	at org.apache.catalina.core.StandardContextValve.invoke(StandardContextValve.java:110)
	at org.apache.catalina.authenticator.AuthenticatorBase.invoke(AuthenticatorBase.java:494)
	at org.apache.catalina.core.StandardHostValve.invoke(StandardHostValve.java:169)
	at org.apache.catalina.valves.ErrorReportValve.invoke(ErrorReportValve.java:104)
	at org.apache.catalina.valves.AccessLogValve.invoke(AccessLogValve.java:1025)
	at org.apache.catalina.core.StandardEngineValve.invoke(StandardEngineValve.java:116)
	at org.apache.catalina.connector.CoyoteAdapter.service(CoyoteAdapter.java:445)
	at org.apache.coyote.http11.AbstractHttp11Processor.process(AbstractHttp11Processor.java:1136)
	at org.apache.coyote.AbstractProtocol$AbstractConnectionHandler.process(AbstractProtocol.java:637)
	at org.apache.tomcat.util.net.AprEndpoint$SocketProcessor.doRun(AprEndpoint.java:2555)
	at org.apache.tomcat.util.net.AprEndpoint$SocketProcessor.run(AprEndpoint.java:2544)
	at java.util.concurrent.ThreadPoolExecutor.runWorker(ThreadPoolExecutor.java:1149)
	at java.util.concurrent.ThreadPoolExecutor$Worker.run(ThreadPoolExecutor.java:624)
	at org.apache.tomcat.util.threads.TaskThread$WrappingRunnable.run(TaskThread.java:61)
	at java.lang.Thread.run(Thread.java:748)

相关的代码：

package org.imooc.util;

import org.apache.commons.fileupload.FileItem;

import java.io.File;

public class FileUtil {
    /*
    *
    * 上传文件的保存路径
    * */
    public static final String SAVE_PATH = "d:/upload/";

    /*
    *
    * 保存上传的文件
    * */
    public static String save(FileItem fileItem,String path) throws Exception {
        String fileName = System.currentTimeMillis() + "_" + fileItem.getName();
        fileItem.write(new File(path + fileName));
        return fileName;
    }
}

package org.imooc.service;

import org.apache.poi.openxml4j.exceptions.InvalidFormatException;
import org.apache.poi.ss.usermodel.Row;
import org.apache.poi.ss.usermodel.Sheet;
import org.apache.poi.ss.usermodel.Workbook;
import org.apache.poi.ss.usermodel.WorkbookFactory;
import org.imooc.dto.ImportExcelParamDto;
import org.imooc.dto.ImportExcelResultDto;
import org.imooc.entity.Student;
import org.imooc.util.FileUtil;

import java.io.File;
import java.io.IOException;
import java.util.ArrayList;
import java.util.List;

public class ExcelService {
    public ImportExcelResultDto imp(ImportExcelParamDto dto){

        ImportExcelResultDto result = new ImportExcelResultDto();
        result.setTitle(dto.getTitle());

        List<Student> studentList = new ArrayList<>();
        result.setStudentList(studentList);

        String fileName = null;
        try {
            fileName = FileUtil.save(dto.getExcel(),FileUtil.SAVE_PATH);
        } catch (Exception e) {
            e.printStackTrace();
            result.setMsg("保存上传文件失败");
        }

        if(fileName != null) {
            //创建workbook，即取到一个Excel表
            try {
                Workbook workbook = WorkbookFactory.create(new File(FileUtil.SAVE_PATH + fileName));
                //然后我们取里面的sheet，我们取的是第一个sheet
                Sheet sheet = workbook.getSheetAt(0);
                //接下来，我们取行，然后我们看有效的数据最后一行在哪
                int rowNum = sheet.getLastRowNum();
                for (int i = 1; i <= rowNum; i++) {
                    //取每一行
                    Row row = sheet.getRow(i);
                    //取每一行里面的单元格
                    Student student = new Student();
                    studentList.add(student);
                    student.setName(row.getCell(0).getStringCellValue());
                    student.setAge((int)row.getCell(1).getNumericCellValue());
                    student.setTime( row.getCell(2).getDateCellValue());
                }
            } catch (Exception e) {
                e.printStackTrace();
                result.setMsg("解析Excel失败");
            }
        }

        return result;
    }
}

package org.imooc.servlet;

import org.apache.commons.fileupload.FileItem;
import org.apache.commons.fileupload.FileUploadException;
import org.apache.commons.fileupload.disk.DiskFileItemFactory;
import org.apache.commons.fileupload.servlet.ServletFileUpload;
import org.imooc.dto.ImportExcelParamDto;
import org.imooc.dto.ImportExcelResultDto;
import org.imooc.dto.ParamDto;
import org.imooc.service.ExcelService;
import org.imooc.util.RequestUtil;

import javax.servlet.ServletException;
import javax.servlet.http.HttpServlet;
import javax.servlet.http.HttpServletRequest;
import javax.servlet.http.HttpServletResponse;
import java.io.File;
import java.io.IOException;
import java.util.List;

public class ImportExcelServlet extends HttpServlet {

    @Override
    protected void doGet(HttpServletRequest req, HttpServletResponse resp) throws ServletException, IOException {
        this.doPost(req, resp);
    }

    @Override
    protected void doPost(HttpServletRequest req, HttpServletResponse resp) throws ServletException, IOException {
        //判断request流中的内容是普通的表单还是文件类型的
        if(ServletFileUpload.isMultipartContent(req)){
            ParamDto dto = RequestUtil.parseParam(req);
            ImportExcelParamDto paramDto = new ImportExcelParamDto();
            paramDto.setTitle(dto.getParamMap().get("title"));
            paramDto.setExcel(dto.getFileMap().get("excle"));

            ExcelService service = new ExcelService();
            ImportExcelResultDto resultDto = service.imp(paramDto);
            req.setAttribute("result",resultDto);
        }else{

        }
        req.getRequestDispatcher("WEB-INF/jsp/importExcelResult.jsp").forward(req,resp);
    }
}