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);
    }
}

慕瓜1143186 2018-09-06

源自：项目：Java导入导出 2-5 导入Excel-3

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2回答

chrismorgen 回答被采纳获得+3积分 2018-09-17 15:33:19

经测试，在ImportExcelServlet类中，你的excle应该改为excel，因为名字单词写错了，所以获取的excel为null，修订之后的代码如下图，祝学习愉快~

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chrismorgen 2018-09-06 12:03:14

FileUtil类中的第19行代码出现空指针制针异常，同学的代码是根据老师的视频一起敲的么？

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提问者慕瓜1143186 #1

是啊，是跟着老师敲的

2018-09-14 09:28:11
提问者慕瓜1143186 #2

可是老师的代码，并没有进行判断啊

2018-09-14 09:32:45
慕布斯645313 回复提问者慕瓜1143186 #3

你这代码和老师写的不一样啊，我照着老师代码敲的就可以成功导入了，要不你把这三个类和老师的代码核对一下不就知道哪里出现问题了么。

2018-09-14 14:16:09