通过浏览器客户端访问Servlet，实现存储于Tomcat服务器上的资源文件的下载效果，主要结合ServletResponse与IO流文件操作实现。原理图示如下：
指定文件目录首先新建项目servletDemo2，指定用于存放文件的路径为：
servletDemo2/files/
编写Servlet类通过继承HttpServlet，重写doGet()和doPost()方法，实现文件下载功能，此处约定使用GET请求，请求参数为：fname。
package com.xwd.demo;import jdk.internal.util.xml.impl.Input;import javax.servlet.ServletContext;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.*;import java.lang.reflect.Field;public class FileDownLoad extends HttpServlet { private static final String FILE_DIR="files"; //methods @Override protected void doGet(HttpServletRequest req, HttpServletResponse resp) throws ServletException, IOException { //super.doGet(req, resp); this.doPost(req, resp); } @Override protected void doPost(HttpServletRequest request, HttpServletResponse response) throws ServletException, IOException { //super.doPost(req, resp); request.setCharacterEncoding("UTF-8"); //解析参数 String fname = request.getParameter("fname"); System.out.println(fname); //获取ServletContext对象 ServletContext servletContext = request.getServletContext(); //获取相对路径 String realPath = servletContext.getRealPath(FILE_DIR+ File.separator + fname); System.out.println(realPath); //获取媒体类型 String mimeType = servletContext.getMimeType(fname); //设置响应头 response.setHeader("Content-Disposition","attachment;filename="+realPath); //设置响应体编码格式 response.setCharacterEncoding("UTF-8"); //设置媒体类型 response.setContentType(mimeType); System.out.println(mimeType); //以字节流输出文件 InputStream is = new FileInputStream(realPath); ServletOutputStream os = response.getOutputStream(); byte[] buffer=new byte[1024]; int len=-1; while ((len = is.read(buffer)) != -1) { os.write(buffer,0,len); } //关闭流 os.close(); is.close(); }}
配置web.xml文件使用Servlet3.0以上的版本，也可直接使用@WebServlet注解配置
web.xml文件配置
@WebServlet( name = "filedownload", value = {"/filedownload"})public class FileDownLoad extends HttpServlet {}
启动项目，测试文件下载