package util; 
import java.awt.Color; 
import java.awt.Font; 
import java.awt.Graphics; 
import java.awt.image.BufferedImage; 
import java.io.IOException; 
import java.io.OutputStream; 
import java.io.PrintWriter; 
import java.util.Random; 
import javax.imageio.ImageIO; 
import javax.servlet.ServletException; 
import javax.servlet.http.HttpServlet; 
import javax.servlet.http.HttpServletRequest; 
import javax.servlet.http.HttpServletResponse; 
import javax.servlet.http.HttpSession; 
import javax.servlet.jsp.PageContext; 
import javax.servlet.jsp.tagext.BodyContent; 
import com.sun.image.codec.jpeg.JPEGCodec; 
import com.sun.image.codec.jpeg.JPEGImageDecoder; 
import com.sun.image.codec.jpeg.JPEGImageEncoder; 
public class CodeUtil extends HttpServlet { 
 public void service(HttpServletRequest request, HttpServletResponse response) 
  throws ServletException, IOException { 
 //System.out.println("哈哈哈"); 
 response.setContentType("image/jpeg"); 
 //设置页面不缓存 
 response.setHeader("Pragma", "No-cache"); 
 response.setHeader("Cache-Control", "no-cache"); 
 response.setDateHeader("Expires", 0); 
 //在内存中创建图像 
 int width = 90; 
 int height = 35; 
 BufferedImage image = new BufferedImage(width,height,BufferedImage.TYPE_INT_RGB); 
 //获取图形上下文 
 Graphics g = image.getGraphics(); 
 //随机类 
 Random random = new Random(); 
 //设定背景 
 g.setColor(getRandColor(200, 250)); 
 g.fillRect(0, 0, width, height); 
 //设定字体 
 g.setFont(new Font("Times New Roman",Font.PLAIN,30)); 
 //随机产生干扰线 
 g.setColor(getRandColor(160, 200)); 
 for (int i = 0; i < 100; i++) { 
  int x = random.nextInt(width); 
  int y = random.nextInt(height); 
  int xl = random.nextInt(12); 
  int yl = random.nextInt(12); 
  g.drawLine(x, y, x + xl, y + yl); 
 } 
 //随机产生4位验证码 
 String[] codes = {"0","1","2","3","4","5","6","7","8","9","A","B","C","D","E","F","G","H","I","J","K","L","M","N","O","P","Q","R","S","T","U","V","W","X","Y","Z","a","b","c","d","e","f","g","h","i","j","k","l","m","n","o","p","q","r","s","t","u","v","w","x","y","z"}; 
 String code = ""; 
 for(int i=0;i<4;i++){ 
  String str = codes[random.nextInt(codes.length)]; 
  code += str; 
  // 将认证码显示到图象中 
  g.setColor(new Color(50 + random.nextInt(110), 20 + random.nextInt(110), 30 + random.nextInt(110))); 
  //调用函数出来的颜色相同，可能是因为种子太接近，所以只能直接生成 
  g.drawString(str, 18 * i +13, 27); //文字间距*i+距离左边距，上边距 
 } 
 HttpSession session=request.getSession(); 
 // 将认证码存入SESSION 
 session.setAttribute("code", code); 
 // 图象生效 
 g.dispose(); 
 // 输出图象到页面 
 ImageIO.write(image, "JPEG", response.getOutputStream()); 
 //加上下面代码,运行时才不会出现java.lang.IllegalStateException: getOutputStream() has already been called ..........等异常 
 response.getOutputStream().flush(); 
 response.getOutputStream().close(); 
 response.flushBuffer(); 
 } 
 //获取随机颜色 
 private Color getRandColor(int fc,int bc){ 
 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); 
 } 
} 

package util; 
import java.io.IOException; 
import java.io.PrintWriter; 
import javax.servlet.ServletException; 
import javax.servlet.http.HttpServlet; 
import javax.servlet.http.HttpServletRequest; 
import javax.servlet.http.HttpServletResponse; 
import javax.servlet.http.HttpSession; 
public class CheckCodeUtil extends HttpServlet { 
 public void service(HttpServletRequest request, HttpServletResponse response) 
  throws ServletException, IOException { 
 request.setCharacterEncoding("utf-8"); 
 response.setContentType("text/html;charset=utf-8"); 
 PrintWriter out = response.getWriter(); 
 //从session获取验证码 
 HttpSession session=request.getSession(); 
 String code=session.getAttribute("code").toString(); 
 System.out.println(code); 
 //获取用户输入验证码 
 String input=request.getParameter("code"); 
 System.out.println(input); 
 if(code.equalsIgnoreCase(input)){ 
  //转发数据 
  request.setAttribute("result", "true"); 
  request.getRequestDispatcher("test/regist.jsp").forward(request, response); 
//  response.sendRedirect(request.getContextPath()+"/regist/regist.jsp"); 
 }else{ 
  request.setAttribute("result", "false"); 
  request.getRequestDispatcher("test/regist.jsp").forward(request, response); 
 } 
 } 
} 

<%@ page session="true" pageEncoding="utf-8" contentType="text/html; charset=utf-8"%> 
<!DOCTYPE html PUBLIC "-//W3C//DTD HTML 4.01 Transitional//EN" "http://www.w3.org/TR/html4/loose.dtd"> 
<html> 
 <head> 
 <meta http-equiv="Content-Type" content="text/html; charset=UTF-8"> 
 <title>Insert title here</title> 
 <script> 
  function showResult1(){ 
  alert('输入正确'); 
  } 
  function showResult2(){ 
  alert('输入错误'); 
  } 
 </script> 
 </head> 
 <% 
 response.setContentType("text/html;charset=utf-8"); 
 request.setCharacterEncoding("utf-8"); 
 String path=request.getContextPath(); 
 if(request.getAttribute("result")!=null){ 
 String result=request.getAttribute("result").toString(); 
 out.print(result); 
 if(result.equals("true")){ 
  out.print("true"); 
  out.print("<script>showResult1();<script>"); 
 }else{ 
  out.print("<script>showResult2();<script>"); 
 } 
 } 
 %> 
 <body> 
 <br>验证码演示 
 <form action="<%=path %>/checkCodeUtil" method="post"> 
 <input type="text" size="10" name="code"/> 
 <img src="<%=path %>/codeUtil" id="img"/> 
 <a href="javascript:;" onclick="document. 
  getElementById('img').src='<%=path %>/codeUtil?'+new Date().getTime();">看不清，换一个</a> 
 <input type="submit" value="提交"> 
 </form> 
 </body> 
</html>