本文共 6414 字，大约阅读时间需要 21 分钟。

一.文件上传下载的实现

1.maven依赖

该功能需要添加三个jar，分别如下:

               
    
     javax.servlet
                
    
     javax.servlet-api
                
    
     3.0.1
            
           
               
    
     commons-fileupload
                
    
     commons-fileupload
                
    
     1.3.1
            
           
           
               
    
     commons-io
                
    
     commons-io
                
    
     2.4
            
   

2.前端代码

<%@ page contentType="text/html;charset=UTF-8" language="java"%>    
   
    
        
    
   
${message}${fileName}<%--https://www.baidu.com/s?wd=nofollow%E6%A0%87%E7%AD%BE&tn=SE_PcZhidaonwhc_ngpagmjz&rsv_dl=gh_pc_zhidao--%><%--nofollow不要追踪此特定链接--%>

action的内容为后端接收的地址，即为后端代码中@WebServlet("/fileUpload")的地址。

3.后端接收代码

package com.hly.jsp.file;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 javax.servlet.ServletException;import javax.servlet.annotation.WebServlet;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.io.PrintWriter;import java.util.List;/** * @author :hly * @github :github.com/SiriusHly * @blog :blog.csdn.net/Sirius_hly * @date :2018/8/20 */@WebServlet("/fileUpload")public class FileUpload extends HttpServlet {    //临界值，超过临界值，上传过程中会存到临时目录    private static final int BOUNDARY_MEMORY_SIDE = 1024 * 1024 * 10;    //文件最大值    private static final int MAX_MEMORY_SIDE = 1024 * 1024 * 300;    //请求值    private static final int MAX_REQUEST_MEMORY_SIDE = 1024 * 1024 * 1000;    public void doPost(HttpServletRequest request, HttpServletResponse response) throws IOException, ServletException {        //判断普通表单还是文件上传表单，enctype属性是否是"multipart/form-data"。        if (!ServletFileUpload.isMultipartContent(request)) {            System.out.println("不是多媒体上传");            PrintWriter out = response.getWriter();            out.println("不是多媒体上传");            out.flush();            return;        }        //配置上传参数        DiskFileItemFactory factory = new DiskFileItemFactory();        //设置临界值,超过部分存在临时目录        factory.setSizeThreshold(BOUNDARY_MEMORY_SIDE);        //设置临时存储目录,上传过程中超过大小存到临时目录中，也可以设置成系统临时存储目录，https://www.cnblogs.com/nbjin/p/7392541.html        File tempFile = new File("d:/FileUploadTemp");        if(!tempFile.exists())            tempFile.mkdirs();        factory.setRepository(tempFile);        ServletFileUpload upload = new ServletFileUpload(factory);        //编码        upload.setHeaderEncoding("UTF-8");        //设置文件最大上传值        upload.setFileSizeMax(MAX_MEMORY_SIDE);        //设置文件最大请求值,包括所有form        upload.setSizeMax(MAX_REQUEST_MEMORY_SIDE);        //设置相对路径        String path = getServletContext().getRealPath("/") + File.separator + "uploadFile";        File uploadDir = new File(path);        if (!uploadDir.exists()) {            uploadDir.mkdirs();        }        try {            //解析请求内容获取文件数据            List
   
     fileItems = upload.parseRequest(request);            if (fileItems != null && fileItems.size() > 0) {                for (FileItem item : fileItems) {                    //判断是否为普通类型，否则为file类型                    if (!item.isFormField()) {                        //得到前端的name                        String fileName = item.getName();                        String filePath = path + File.separator + fileName;                        File storeFile = new File(filePath);                        System.out.println("上传路径为:" + filePath);                        //保存文件到硬盘                        item.write(storeFile);                        request.setAttribute("message", "上传成功");                        request.setAttribute("fileName", fileName);                       // request.setAttribute();                    }                }            }else{                request.setAttribute("message", "上传失败");            }        } catch (FileUploadException e) {            e.printStackTrace();        } catch (Exception e) {            e.printStackTrace();        }        getServletContext().getRequestDispatcher("/file/fileUpload.jsp").forward(request, response);        System.out.println("上传成功");    }}
   

4.后端下载代码

package com.hly.jsp.file;import javax.servlet.ServletContext;import javax.servlet.ServletOutputStream;import javax.servlet.annotation.WebServlet;import javax.servlet.http.HttpServlet;import javax.servlet.http.HttpServletRequest;import javax.servlet.http.HttpServletResponse;import java.io.*;/** * @author :hly * @github :github.com/SiriusHly * @blog :blog.csdn.net/Sirius_hly * @date :2018/8/22 */@WebServlet("/FileDownload")public class FileDownload extends HttpServlet {    public void doGet(HttpServletRequest request, HttpServletResponse response) throws IOException {        String fileName = request.getParameter("fileName");        System.out.println("下载路径为:"+fileName);        //设置mime类型,getMimeType获得文档类型，这里是text/plain        response.setContentType(getServletContext().getMimeType(fileName));        System.out.println("mime类型:"+getServletContext().getMimeType(fileName));        //设置Context-Disposition        //Content-disposition：HTTP相应头的属性，允许用户下载软件到磁盘        //attachment表示以附件方式下载。如果要在页面中打开，则改为inline。        response.setHeader("Content-Disposition", "attachment;filename=" + fileName);        //输入流        String filePath = getServletContext().getRealPath("/")+ File.separator +"uploadFile"+File.separator +fileName;        InputStream in = new FileInputStream(filePath);        ServletOutputStream os = response.getOutputStream();        //缓冲区        int len =1;        byte[] b = new byte[1024];        while ((len = in.read(b))!=-1){            os.write(b,0,len);        }        in.close();        os.close();    }    public void doPost(HttpServletRequest request, HttpServletResponse response) throws IOException {        doGet(request, response);    }}

前端代码的文件目录为:

上传的文件可以在target里面找到

5.代码分析

1.关于代码中的getName()方法

源码如下，我们可以发现他返回的是前端的name，当不是普通类型的表单，是file时，则进行保存

@Override    public String toString() {        return format("name=%s, StoreLocation=%s, size=%s bytes, isFormField=%s, FieldName=%s",                      getName(), getStoreLocation(), Long.valueOf(getSize()),                      Boolean.valueOf(isFormField()), getFieldName());    }