一、要實現Servlet的檔案上傳和下載，首先需要藉助第三方的jar包

二、Servlet的程式碼

package com.xh.web;

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.ServletOutputStream;
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.FileInputStream;
import java.io.FileOutputStream;
import java.io.IOException;
import java.text.SimpleDateFormat;
import java.util.Date;
import java.util.List;

@WebServlet(name = "DownloadServlet",urlPatterns = {"/download","/upload"})
public class DownloadServlet extends HttpServlet {
    /**
     * 檔案上傳
     * @param request
     * @param response
     * @throws ServletException
     * @throws IOException
     */
    protected void doPost(HttpServletRequest request, HttpServletResponse response) throws ServletException, IOException {
        //宣告工廠
        DiskFileItemFactory factory = new DiskFileItemFactory();
        //宣告上傳元件
        ServletFileUpload upload = new ServletFileUpload(factory);
        //指定字符集
        upload.setHeaderEncoding("UTF-8");
        //獲取專案的絕對路勁（根路徑）
        String realPath = this.getServletContext().getRealPath("./");
        //File.separator 路徑分隔符，不同系統的分隔符不同
        //建立檔案上傳的資料夾“file”,如果檔案不存在就建立
        File file = new File(realPath+File.separator+"file");
        if(!file.isDirectory()){
            file.mkdirs();
        }
        try {
            //獲取所有的表單類容
            List<FileItem> items =upload.parseRequest(request);
            for (FileItem f : items) {
                //判斷是否是一個輸入元素,false 為檔案，true 不是檔案
                if(!f.isFormField()){
                    //獲取檔名
                    String fileName = f.getName();
                    //獲取檔案字尾
                    String suffix = fileName.substring(fileName.indexOf("."));
                    //獲取當前時間
                    Date date = new Date();
                    //格式化當前時間
                    SimpleDateFormat format = new SimpleDateFormat("YYMMddHHmmss");
                    //重新命名檔名，防止重複，因為時間也有可能重複，可加入UUID等隨機資料，最大程度防止檔名重複
                    String newName = format.format(date)+suffix;  //  UUID.randomUUID()
                    //獲取檔案類容
                    byte[] bytes = f.get();
                    //建立輸出流，輸出檔案
                    FileOutputStream outputStream = new FileOutputStream(file+File.separator+newName);
                    outputStream.write(bytes);
                    outputStream.close();
                }
            }
        } catch (FileUploadException e) {
            e.printStackTrace();
        }
    }

    /**
     * 檔案下載
     * @param request
     * @param response
     * @throws ServletException
     * @throws IOException
     */
    protected void doGet(HttpServletRequest request, HttpServletResponse response) throws ServletException, IOException {
        //從前端獲取要下載的檔名
        String fileName = request.getParameter("filename");
        //獲取專案的絕對路徑
        String realPath = this.getServletContext().getRealPath("./");
        //獲得下載檔案的路徑，該路徑為之前上傳的檔案
        File file = new File(realPath+File.separator+"file");
        //拼接檔案路徑
        String fileString = file+File.separator+fileName;
        //設定字元編碼
        response.setCharacterEncoding("UTF-8");
        //String fileName_encode = URLEncoder.encode(fileName,"UTF-8"); //防止中文亂碼
        //防止中文亂碼，符號亂碼等問題
        String fileName_encode= new String(fileName.getBytes(),"ISO8859-1");
        //設定響應頭（重要）
        response.setHeader("Content-Disposition","attachment; filename="+fileName_encode);
        //建立輸出流
        FileInputStream fileInputStream = new FileInputStream(fileString);

        //建立輸出流1 （可用）
       /* byte[] bytes = new byte[2048];
        ServletOutputStream outputStream = response.getOutputStream();
        int i = 0;
        while(( i = fileInputStream.read(bytes))!=-1){
             outputStream.write(bytes,0,i);
             outputStream.flush();
        }
         outputStream.close();
        */

        //建立輸出流2
        //獲取檔案輸出流大小
        int size = fileInputStream.available();
        byte[] bytes = new byte[size];
        fileInputStream.read(bytes);
        ServletOutputStream outputStream = response.getOutputStream();
        outputStream.write(bytes);
        outputStream.close();
    }
}

三、HTML程式碼

注意：下載介面的filename為自己上傳的（處理後）的檔名，超連結的href路徑或form表單的路徑根據自己的專案設定

      1、檔案上傳的HTML程式碼

<!DOCTYPE html>
<html lang="en">
<head>
    <meta charset="UTF-8">
    <title>Title</title>
</head>
<body>
  <form action="../upload" method="post" enctype="multipart/form-data">
      <input type="file" name="myfile">
      <input type="submit">
  </form>
</body>
</html>

2、檔案下載的HTML程式碼

<!DOCTYPE html>
<html lang="en">
<head>
    <meta charset="UTF-8">
    <title>Title</title>
</head>
<body>
   <a href="../download?filename=jianli.jpg">jianli.jpg</a>
   <a href="../download?filename=Java.docx">Java.docx</a>
   <a href="../download?filename=個人簡歷.doc">個人簡歷.doc</a>
   <img src="../file/jianli.jpg">    <!-- 測試上傳圖片能否正常顯示（相對路徑）-->
</body>
</html>