protected void doGet(HttpServletRequest req, HttpServletResponse resp) throws ServletException, IOException {

        // 1. 要获取下载文件的路径

//        String realPath = "E:\\amixi.jpg";   //在网页上显示图片

        String realPath = "E:\\note.txt";  //在网页上显示文件的内容

        System.out.println("下载文件的路径："+realPath);

// 2. 下载的文件名是啥？

        String fileName = realPath.substring(realPath.lastIndexOf("\\") + 1);

// 3. 设置想办法让浏览器能够支持(Content-Disposition)自动下载我们需要的东西,中文文件

//        名URLEncoder.encode编码，否则有可能乱码

        resp.setHeader("Content-Disposition","attachment;filename="+ URLEncoder.encode(fileName,"UTF-8"));

// 4. 获取下载文件的输入流

        FileInputStream in = new FileInputStream(realPath);

// 5. 创建缓冲区

        int len = 0;

        byte[] buffer = new byte[1024];

// 6. 获取OutputStream对象

        ServletOutputStream out = resp.getOutputStream();

// 7. 将FileOutputStream流写入到buffer缓冲区,使用OutputStream将缓冲区中的数据输出到客户端！

        while ((len=in.read(buffer))>0){

            out.write(buffer,0,len);

        }

        in.close();

        out.close();

    }

package com.ljm.oo;

import javax.imageio.ImageIO;

import javax.servlet.ServletContext;

import javax.servlet.ServletException;

import javax.servlet.http.HttpServlet;

import javax.servlet.http.HttpServletRequest;

import javax.servlet.http.HttpServletResponse;

import java.awt.*;

import java.awt.image.BufferedImage;

import java.io.IOException;

import java.util.Random;

public class Captcha_ extends HttpServlet {

    @Override

    protected void doGet(HttpServletRequest req, HttpServletResponse resp) throws ServletException, IOException {

        //如何让浏览器3秒自动刷新一次;

        resp.setHeader("refresh","3");

        //在内存中创建一个图片

        BufferedImage image = new

                BufferedImage(80,20,BufferedImage.TYPE_INT_RGB);

        //得到图片

        Graphics2D g = (Graphics2D) image.getGraphics(); //笔

        //设置图片的背景颜色

        g.setColor(Color.white);

        g.fillRect(0,0,80,20);

        //给图片写数据

        g.setColor(Color.BLUE);

        g.setFont(new Font(null,Font.BOLD,20));

        g.drawString(makeNum(),0,20);

        //告诉浏览器，这个请求用图片的方式打开

        resp.setContentType("image/jpeg");

        //网站存在缓存，不让浏览器缓存

        resp.setDateHeader("expires",-1);

        resp.setHeader("Cache-Control","no-cache");

        resp.setHeader("Pragma","no-cache");

        //把图片写给浏览器

        ImageIO.write(image,"jpg", resp.getOutputStream());

    }

    @Override

    protected void doPost(HttpServletRequest req, HttpServletResponse resp) throws ServletException, IOException {

        doGet(req,resp);

    }

    private String makeNum(){

        Random random = new Random();

        String num = random.nextInt(9999999) + "";

        StringBuffer sb = new StringBuffer();

        for (int i = 0; i < 7-num.length() ; i++) {

            sb.append("0");

        }

        num = sb.toString() + num;

        return num;

    }

}

    protected void doGet(HttpServletRequest req, HttpServletResponse resp) throws ServletException, IOException {

//        resp.setHeader("Location","/servlet02_war/gethello");

//        resp.setStatus(302);

        resp.sendRedirect("/servlet02_war/baidu.jsp");

      //   /项目名字/web.xml中注册的url 丨 .jsp文件

    }

  String username = req.getParameter("username");

        String password = req.getParameter("password");

        System.out.println(username+":"+password);

        resp.sendRedirect("/servlet02_war/baidu.jsp");

<%@page contentType="text/html;" pageEncoding="UTF-8" %>

<%--{找到项目路径}/web.xml注册的url--%>

<form action="${pageContext.request.contextPath}/login" method="get">

    用户名：<input type="text" name="username"> <br>

    密码：<input type="password" name="password"> <br>

    <input type="submit">

</form>

后面在web.xml中注册/login

protected void doGet(HttpServletRequest req, HttpServletResponse resp)

throws ServletException, IOException {

req.setCharacterEncoding("utf-8");

resp.setCharacterEncoding("utf-8");

String username = req.getParameter("username");

String password = req.getParameter("password");

String[] hobbys = req.getParameterValues("hobbys");

System.out.println("=============================");

//后台接收中文乱码问题

System.out.println(username);

System.out.println(password);

System.out.println(Arrays.toString(hobbys));

System.out.println("=============================");

System.out.println(req.getContextPath());

//通过请求转发

//这里的 / 代表当前的web应用

req.getRequestDispatcher("/success.jsp").forward(req,resp);

}