1.GET请求的语法
<a href="download?filename=1366768.jpg">下载图片</a><!-- get请求写法：地址?请求属性名=请求属性值-->
多个属性间用&分隔
2.通过设置响应头来实现下载功能
<!DOCTYPE html> <html> <head> <meta charset="UTF-8"> <title>Insert title here</title> </head> <body> <a href="download?filename=1366768.jpg">下载图片</a> <a href="download?filename=1366.jpg">下载图片</a> <a href="download?filename=你好.txt">下载中文名称文档</a> </body> </html>
public class DownLoadServlet extends HttpServlet { @Override public void doGet(HttpServletRequest request, HttpServletResponse response) throws ServletException, IOException { String filename = request.getParameter("filename"); File f = new File(this.getServletContext().getRealPath("download/"+filename));//获取服务器资源 if(f.exists()) {//解决文件名中文问题 String agent = request.getHeader("user-agent"); if (agent.contains("MSIE")) { // IE浏览器 filename = URLEncoder.encode(filename, "utf-8"); filename = filename.replace("+", " "); } else if (agent.contains("Firefox")) { // 火狐浏览器 BASE64Encoder base64Encoder = new BASE64Encoder(); filename = "=?utf-8?B?" + base64Encoder.encode(filename.getBytes("utf-8")) + "?="; } else { // 其它浏览器 filename = URLEncoder.encode(filename, "utf-8"); } response.setContentType(this.getServletContext().getMimeType(f.getPath()));//设置页面文件类型 response.setHeader("Content-Disposition", "attachment;filename="+filename);//以附件方式打开文件 ServletOutputStream sos = response.getOutputStream(); FileInputStream fis = new FileInputStream(f); int len = 0; byte[] b = new byte[1024]; while((len=fis.read(b))!=-1) { sos.write(b,0,len); } fis.close(); }else { response.setContentType("text/html;charset=utf-8");//解决响应中文问题 response.getWriter().println("<h2 style='color:red;'>文件不存在！</h2>"); } } @Override public void doPost(HttpServletRequest request, HttpServletResponse response) throws ServletException, IOException { doGet(request, response); } }
3.验证码生成
public class CheckImgServlet extends HttpServlet { // 集合中保存所有成语 private List<String> words = new ArrayList<String>(); @Override public void init() throws ServletException { // 初始化阶段，读取new_words.txt // web工程中读取 文件，必须使用绝对磁盘路径 String path = getServletContext().getRealPath("/WEB-INF/new_words.txt"); try { BufferedReader reader = new BufferedReader(new FileReader(path)); String line; while ((line = reader.readLine()) != null) { words.add(line); } reader.close(); } catch (IOException e) { e.printStackTrace(); } } public void doGet(HttpServletRequest request, HttpServletResponse response) throws ServletException, IOException { // 禁止缓存 // response.setHeader("Cache-Control", "no-cache"); // response.setHeader("Pragma", "no-cache"); // response.setDateHeader("Expires", -1); int width = 120; int height = 30; // 步骤一 绘制一张内存中图片 BufferedImage bufferedImage = new BufferedImage(width, height, BufferedImage.TYPE_INT_RGB); // 步骤二 图片绘制背景颜色 ---通过绘图对象 Graphics graphics = bufferedImage.getGraphics();// 得到画图对象 --- 画笔 // 绘制任何图形之前 都必须指定一个颜色 graphics.setColor(getRandColor(200, 250)); graphics.fillRect(0, 0, width, height); // 步骤三 绘制边框 graphics.setColor(Color.WHITE); graphics.drawRect(0, 0, width - 1, height - 1); // 步骤四 四个随机数字 Graphics2D graphics2d = (Graphics2D) graphics; // 设置输出字体 graphics2d.setFont(new Font("宋体", Font.BOLD, 18)); Random random = new Random();// 生成随机数 int index = random.nextInt(words.size()); String word = words.get(index);// 获得成语 // 定义x坐标 int x = 10; for (int i = 0; i < word.length(); i++) { // 随机颜色 graphics2d.setColor(new Color(20 + random.nextInt(110), 20 + random .nextInt(110), 20 + random.nextInt(110))); // 旋转 -30 --- 30度 int jiaodu = random.nextInt(60) - 30; // 换算弧度 double theta = jiaodu * Math.PI / 180; // 获得字母数字 char c = word.charAt(i); // 将c 输出到图片 graphics2d.rotate(theta, x, 20); graphics2d.drawString(String.valueOf(c), x, 20); graphics2d.rotate(-theta, x, 20); x += 30; } // 将验证码内容保存session request.getSession().setAttribute("checkcode_session", word); // 步骤五 绘制干扰线 graphics.setColor(getRandColor(160, 200)); int x1; int x2; int y1; int y2; for (int i = 0; i < 30; i++) { x1 = random.nextInt(width); x2 = random.nextInt(12); y1 = random.nextInt(height); y2 = random.nextInt(12); graphics.drawLine(x1, y1, x1 + x2, x2 + y2); } // 将上面图片输出到浏览器 ImageIO graphics.dispose();// 释放资源 //将图片写到response.getOutputStream()中 ImageIO.write(bufferedImage, "jpg", response.getOutputStream()); } public void doPost(HttpServletRequest request, HttpServletResponse response) throws ServletException, IOException { doGet(request, response); } /** * 取其某一范围的color * * @param fc * int 范围参数1 * @param bc * int 范围参数2 * @return Color */ private Color getRandColor(int fc, int bc) { // 取其随机颜色 Random random = new Random(); if (fc > 255) { fc = 255; } if (bc > 255) { bc = 255; } int r = fc + random.nextInt(bc - fc); int g = fc + random.nextInt(bc - fc); int b = fc + random.nextInt(bc - fc); return new Color(r, g, b); } }
使用案例：
注意，验证码中设置的Session值需要在另一个servlet中获取，利用输入的成语与生成的成语作比较，比较这一步也要放在servlet中，如果利用显示验证码的页面中定义js直接获取session值是获取不到的、或者是获取到前一个已经过期的session值，验证码图片之前是隐藏的，满足一定条件后显示，所以随着页刷新session值一直在改变。
即使在满足一定条件后再给img的src填写地址，也会导致session值被设置，但是在当前页面不刷新的情况下，直接获取必然是null，在页面再次刷新后，图片也再次刷新，此时虽然能获取到session值，但也是前一次生成的值，导致你获取到的session值永远是过期的，这里要利用ajax向验证servlet发送请求，根据返回的Text来进行操作。
究其原因，是因为页面运行在客户端，而session保存在服务端，只有客户端发起一次请求后，才能获取到session值，而servlet运行在服务端，可以随时获取到session值。
<%@ page language="java" contentType="text/html; charset=UTF-8" pageEncoding="UTF-8" isELIgnored="false"%> <%@ taglib uri="http://java.sun.com/jsp/jstl/core" prefix="c"%> <!doctype html> <html> <head> <meta http-equiv="Content-Type" content="text/html; charset=UTF-8"> <title>欢迎</title> <script src="https://cdn.bootcss.com/jquery/3.3.1/jquery.min.js"></script> <script src="https://cdn.bootcss.com/bootstrap/3.3.7/js/bootstrap.min.js"></script> <link href="https://cdn.bootcss.com/bootstrap/3.3.7/css/bootstrap.min.css" rel="stylesheet"> <style> * { margin: 0px; padding: 0px; } .form { position: absolute; top: 350px; left: 462px; height: 300px; 700px; } body { background-color: #c1c1c1; } .header { position: absolute; top: -50px; left: 230px; } </style> <script> $(document).ready(function(){ var uname = document.getElementById("uname"); var pwd = document.getElementById("pwd"); var uspan = document.getElementById("uspan"); var pspan = document.getElementById("pspan"); var message = "${message }"; if(message !=""){ var wrong = parseInt("${wrong}"); if(wrong>=2){//输错两次显示验证码 $("#code").show(); document.getElementById("lbtn").disabled=true; } uname.style.borderColor = "red"; pwd.style.borderColor = "red"; uspan.innerHTML = "×"+message; pspan.innerHTML = "×"+message; } }); </script> </head> <c:if test="${!empty cookie.logSta &&cookie.logSta.value eq '1'}"> <c:redirect url="success.jsp?uname=${cookie.uname.value }"/> </c:if> <body style="overflow:hidden;"> <div class="header"> <img src="img/1.png" alt="" height="50%" width="80%"> </div> <div class="form"> <form class="form-horizontal" role="form" action="login" method="post"> <div class="form-group"> <label for="firstname" class="col-sm-2 control-label">用户名</label> <div class="col-sm-10"> <input type="text" class="form-control" id="uname" placeholder="请输入用户名" name="user" style="200px;display:inline;"> <span style="color:red;" id="uspan"></span> </div> </div> <div class="form-group"> <label for="lastname" class="col-sm-2 control-label">密码</label> <div class="col-sm-10"> <input type="password" class="form-control" id="pwd" placeholder="请输密码" name="pwd" style="200px;display:inline;"> <span style="color:red;" id="pspan"></span> </div> </div> <div class="form-group" id="code" style="display:none;"> <label for="lastname" class="col-sm-2 control-label">验证码</label> <div class="col-sm-10"> <input type="text" class="form-control" id="checkImg" placeholder="请输验证码" style="200px;display:inline;" onBlur="check()"> <img src="checkImg" onClick="changeImg(this)" id="img"> </div> </div> <div class="form-group"> <div class="col-sm-offset-2 col-sm-10"> <div class="checkbox"> <label> <input type="checkbox" name="autologin">请记住我 </label> </div> </div> </div> <div class="form-group"> <div class="col-sm-offset-2 col-sm-10"> <button type="submit" class="btn btn-default" id="lbtn">登录</button> <a class="btn btn-primary" href="signUp.html">注册</a> </div> </div> </form> </div> <script> var count = 0; function changeImg(obj){ obj.src="checkImg?time="+count; count++; if(count==650){ count = 0; } } var xmlhttp; var checkImg = document.getElementById('checkImg'); function check(){ var val = checkImg.value; var url = "check?val="+val;//向验证地址提交输入的值 xmlhttp =new XMLHttpRequest(); xmlhttp.onreadystatechange=checkResult; xmlhttp.open("GET",url,true); xmlhttp.send(); } function checkResult(){ if (xmlhttp.readyState==4 && xmlhttp.status==200){ var result = xmlhttp.responseText;//获取返回的文本 if(result=="yes"){ checkImg.style.borderColor="green"; document.getElementById("lbtn").disabled=false; }else{ checkImg.style.borderColor="red"; document.getElementById("lbtn").disabled=true; } } } </script> </body> </html>
package com.login.controller; import java.io.IOException; import javax.servlet.ServletException; import javax.servlet.http.Cookie; import javax.servlet.http.HttpServlet; import javax.servlet.http.HttpServletRequest; import javax.servlet.http.HttpServletResponse; import com.login.model.dao.UserDAO; public class LoginServlet extends HttpServlet{ private UserDAO userDAO = new UserDAO(); private int wrong; @Override public void init() { wrong = 0; } @Override protected void doGet(HttpServletRequest request,HttpServletResponse response) throws IOException,ServletException { //doPost(request,response); } @Override public void doPost(HttpServletRequest request,HttpServletResponse response) throws IOException,ServletException { String user = request.getParameter("user"); String pwd = request.getParameter("pwd"); String autologin = request.getParameter("autologin"); if(!userDAO.login(user, pwd)) { wrong++;//设置错误次数 request.setAttribute("wrong", wrong); request.setAttribute("message", "账户名或密码错误！"); request.getRequestDispatcher("login.jsp").forward(request, response); }else { wrong = 0; if(autologin!=null) { Cookie uname = new Cookie("uname",user); Cookie logSta = new Cookie("logSta","1");//保存用户登录状态码 logSta.setMaxAge(60*60*24*7);//有效期一周 uname.setMaxAge(60*60*24*7); response.addCookie(logSta); response.addCookie(uname); } request.setAttribute("user", user); request.getRequestDispatcher("success.jsp").forward(request, response); } } }
package com.login.controller; import java.io.IOException; import javax.servlet.ServletException; import javax.servlet.http.HttpServlet; import javax.servlet.http.HttpServletRequest; import javax.servlet.http.HttpServletResponse; public class CheckServlet extends HttpServlet { protected void doGet(HttpServletRequest request, HttpServletResponse response) throws ServletException, IOException { // TODO Auto-generated method stub String word = (String) request.getSession().getAttribute("checkcode_session"); response.setContentType("text/html;charset=utf-8"); if(request.getParameter("val").equals(word)){//输入值和生成值作比较 response.getWriter().print("yes"); }else { response.getWriter().print("no"); } } protected void doPost(HttpServletRequest request, HttpServletResponse response) throws ServletException, IOException { // TODO Auto-generated method stub doGet(request, response); } }