服务器生成一个包含随机的字符串的图片发给客户端，客户端提交数据时需要填写字符串作为验证。在服务器端，字符串保存在 session 中，当获取到客户端上传的字符串参数时，对比是否相等。
运行结果图：
生成图片验证码的 Servlet 类
package cn.joker.servlet;import java.awt.Color;
import java.awt.Font;
import java.awt.Graphics2D;
import java.awt.image.BufferedImage;
import java.io.IOException;
import java.util.Random;import javax.imageio.ImageIO;
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 com.dec.jpeg.JPEGCodec;
import com.dec.jpeg.JPEGImageEncoder;/*** Servlet implementation class IdentityServlet*/
@WebServlet(description = "response生成图片验证码", urlPatterns = { "/IdentityServlet" })
public class IdentityServlet extends HttpServlet {private static final long serialVersionUID = 1L;//随机字符private static final char[] CHARS = {'2','3','4','5','6','7','8','9','A','B','C','D','E','F','G','H','I','J'};//随机数private Random random = new Random();/*** @see HttpServlet#HttpServlet()*/public IdentityServlet() {super();// TODO Auto-generated constructor stub}/*** @see HttpServlet#doGet(HttpServletRequest request, HttpServletResponse response)*/protected void doGet(HttpServletRequest request, HttpServletResponse response) throws ServletException, IOException {// TODO Auto-generated method stub//Writer().append("Served at: ").ContextPath());// 设置输出类型response.setContentType("image/jpeg");response.setDateHeader("expries", -1); response.setHeader("Cache-Control", "no-cache"); response.setHeader("Pragma", "no-cache"); // 获取随机字符串String randomString = getRandomString();//System.out.println(randomString);// 放到session 中Session(true).setAttribute("randomString", randomString);// 图片的宽度、高度int width = 100,height=30;//用于背景色Color color = getRandomColor();// 用于前景色Color reverse = getReverseColor(color);// 创建一个彩色图片BufferedImage bi = new BufferedImage(width,height,BufferedImage.TYPE_INT_RGB);// 获取绘图对象Graphics2D g = bi.createGraphics();g.setFont(new Font(Font.SANS_SERIF ,Font.BOLD,16));g.setColor(color);g.fillRect(0, 0, width, height);g.setColor(reverse);g.drawString(randomString, 18, 20);// 绘制噪声点for(int i = 0,n = Int(100);i<n;i++) {g.Int(width), Int(height), 1, 1);}/* 方法一：* ServletOutputStream out = OutputStream();// 转成JPEG格式JPEGImageEncoder encoder = ateJPEGEncoder(out);// 对图片进行编码de(bi);out.flush();*/ImageIO.write(bi, "jpg", OutputStream());}/*** @see HttpServlet#doPost(HttpServletRequest request, HttpServletResponse response)*/protected void doPost(HttpServletRequest request, HttpServletResponse response) throws ServletException, IOException {// TODO Auto-generated method stubdoGet(request, response);}/*** 获取六位随机字符*/public String getRandomString() {StringBuffer buffer = new StringBuffer();for(int i = 0; i<6;i++) {buffer.append(Int(18)]);}String();}/*** 获取随机颜色*/public Color getRandomColor() {return new Int(255),Int(255),Int(255));}/*** 获取某颜色的反色*/public Color getReverseColor(Color c) {return new Red(),Green(),Blue());}}
显示界面（作为首页使用）
<!DOCTYPE html>
<html>
<head>
<meta charset="UTF-8">
<title>图片验证</title>
<script type="text/javascript">function reloadImage(){ElementById('btn').disabled = true;//实时更新,保证每次都从服务器重新请求数据ElementById('identity').src = '/ServletStudy/IdentityServlet?ts='+new Date().getTime();}
</script>
</head>
<body>
<img alt="验证码" src="/ServletStudy/IdentityServlet" id="identity" onload="btn.disabled = false;" />
<input type="button" id="btn" value="换个图片" onclick="reloadImage()">
<form action="/ServletStudy/CheckIdentity" method="post">验证码： <input type="text" name="check"><input type="submit" value="提交">
</form>
</body>
</html>校验验证码的 Servlet
package cn.joker.servlet;import java.io.IOException;
import javax.servlet.ServletException;
import javax.servlet.annotation.WebServlet;
import javax.servlet.http.HttpServlet;
import javax.servlet.http.HttpServletRequest;
import javax.servlet.http.HttpServletResponse;/*** Servlet implementation class CheckIdentity*/
@WebServlet(description = "校验验证码", urlPatterns = { "/CheckIdentity" })
public class CheckIdentity extends HttpServlet {private static final long serialVersionUID = 1L;/*** @see HttpServlet#HttpServlet()*/public CheckIdentity() {super();// TODO Auto-generated constructor stub}/*** @see HttpServlet#doGet(HttpServletRequest request, HttpServletResponse response)*/protected void doGet(HttpServletRequest request, HttpServletResponse response) throws ServletException, IOException {// TODO Auto-generated method stub//request.setCharacterEncoding("UTF-8");response.setCharacterEncoding("UTF-8");// 获取客户端上传的数据String ch = Parameter("check");// 获取session 中保存的数据String id = (String) Session().getAttribute("randomString");if(ch.equals(id))Writer().append("<!DOCTYPE html>rn" + "<html>rn" + "<head>rn" + "<meta charset="UTF-8"></head>rn" + "<body>").append("验证码通过").append("</body>rn" + "</html>");Writer().append("<!DOCTYPE html>rn" + "<html>rn" + "<head>rn" + "<meta charset="UTF-8"></head>rn" + "<body>").append("验证码错误").append("</body>rn" + "</html>");}/*** @see HttpServlet#doPost(HttpServletRequest request, HttpServletResponse response)*/protected void doPost(HttpServletRequest request, HttpServletResponse response) throws ServletException, IOException {// TODO Auto-generated method stubdoGet(request, response);}}
只要正确设定 response 输出类型及输出流，就能输出很多的格式数据，例如Excel、PDF、Word、MP3等。不同的输出类型需要声明不同的 Content-Type 属性，例如JPG图片是 “image/jpeg”，Word 则是 “application/msword”。
代码及内容大部分来源于Java Web 整合开发 王者归来
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