JavaWeb案例： 文件下载 基于tomcat8 默认编码为UTF-8

 package cn.itcast.download;

 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.FileInputStream;
 import java.io.IOException;

 @WebServlet("/001")
 public class DownloadServlet extends HttpServlet {
     protected void doPost(HttpServletRequest request, HttpServletResponse response) throws ServletException, IOException {
         //获取资源名
         String filename = request.getParameter("filename");
         //获取资源对应输入流
         FileInputStream fis = new FileInputStream(request.getServletContext().getRealPath("/download/" + filename));
         //告知类型
         response.setContentType(request.getServletContext().getMimeType(filename));
         //告知下载
         response.setHeader("content-disposition", "attachment;filename=" + filename);
         //获取响应输出流
         ServletOutputStream sos = response.getOutputStream();
         //写数据
         byte[] bys = new byte[1024 * 8];
         int len;
         while((len = fis.read(bys)) != -1){
             sos.write(bys, 0, len);
         }
         //释放资源
         fis.close();
     }

     protected void doGet(HttpServletRequest request, HttpServletResponse response) throws ServletException, IOException {
         doPost(request, response);
     }
 }

版本1 直接写逻辑，不考虑乱码

 package cn.itcast.download;

 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.FileInputStream;
 import java.io.IOException;
 import java.net.URLEncoder;
 import java.util.Base64;

 @WebServlet("/download")
 public class DownloadServlet2 extends HttpServlet {
     protected void doPost(HttpServletRequest request, HttpServletResponse response) throws ServletException, IOException {
         //获取资源名
         String filename = request.getParameter("filename");
         //获取资源对应输入流
         FileInputStream fis = new FileInputStream(request.getServletContext().getRealPath("/download/" + filename));
         //告知类型
         response.setContentType(request.getServletContext().getMimeType(filename));
         //告知下载
         //处理响应头乱码，浏览器兼容性问题
         String agent = request.getHeader("user-agent");
         String fileNameEnc = null;
         if(agent.contains("MSIE")){
             fileNameEnc = URLEncoder.encode(filename,"UTF-8").replace("+", " ");
         }else if(agent.contains("Firefox")){
             fileNameEnc = "=?utf-8?B?"
                     + Base64.getEncoder().encodeToString(filename.getBytes("UTF-8"))
                     + "?=";
         }else{
             fileNameEnc = URLEncoder.encode(filename,"UTF-8");
         }
         response.setHeader("content-disposition", "attachment;filename=" + fileNameEnc);
         //获取响应输出流
         ServletOutputStream sos = response.getOutputStream();
         //写数据
         byte[] bys = new byte[1024 * 8];
         int len;
         while((len = fis.read(bys)) != -1){
             sos.write(bys, 0, len);
         }
         //释放资源
         fis.close();
     }

     protected void doGet(HttpServletRequest request, HttpServletResponse response) throws ServletException, IOException {
         doPost(request, response);
     }
 }

版本2 解决响应头乱码。content-disposition:attachment;filename=xxx