public class VerifyCode {


private int w=70;
private int h=35;
private Random r=new Random();
//宋體，華文楷書，黑體，華文新魏，華文隸書，微軟雅黑，楷體_GB2312,Times New Roman
private String [] fontNames={"宋體","華文楷書","黑體","華文新魏","微軟雅黑","華文隸書","楷體_GB2312","Times New Roman"};
//可選的字符
private String codes="23456789abcdefghijkmnopqrstuvwxyzABCDEFGHIJKLMNPQRSTUVWXYZ";
//背景色
private Color bgColor=new Color(255,255,255);
//驗證碼上的文本
private String text;

//生成隨機顏色
private Color randomColor(){
 int red=r.nextInt(150);
 int green=r.nextInt(150);
 int blue=r.nextInt(150);
 return new Color(red,green,blue);
}

//生成隨機字體
private Font randomFont(){
int index=r.nextInt(fontNames.length);//獲取下標
String fontName=fontNames[index];//生成隨機的字體名稱
int style=r.nextInt(4);//獲取隨機樣式，0表示無樣式，1表示粗體，2表示斜體，3表示粗體加斜體
int size=r.nextInt(5)+24;//生成隨機字號，24~28
return new Font(fontName,style,size);
}

//畫干擾線
private void drawLine(BufferedImage image){
int num=3;//一共畫3條
Graphics2D g2=(Graphics2D) image.getGraphics();
for(int i=0;i<num;i++){//隨機生成坐標，即4個值
int x1=r.nextInt(w);
int y1=r.nextInt(h);
int x2=r.nextInt(w);
int y2=r.nextInt(h);
g2.setStroke(new BasicStroke(1.5F));//設置筆畫的寬度
  g2.setColor(Color.blue);//干擾線顏色
  g2.drawLine(x1, y1, x2, y2);//畫線
}
}

//隨機生成一個字符
private char randomChar(){
int index=r.nextInt(codes.length());
return codes.charAt(index);
}

//創(chuàng)建BufferedImage
private BufferedImage createImage(){
BufferedImage image=new BufferedImage(w,h,BufferedImage.TYPE_INT_RGB);
Graphics2D g2=(Graphics2D) image.getGraphics();
g2.setColor(this.bgColor);
g2.fillRect(0, 0, w, h);
return image;
}

//外界調(diào)用這個方法得到驗證碼
public BufferedImage getImage(){
BufferedImage image=createImage();//創(chuàng)建圖片緩沖區(qū)
Graphics2D g2=(Graphics2D)image.getGraphics();//得到繪制環(huán)境
StringBuilder sb=new StringBuilder();//用來裝載生成的驗證碼文本
//向圖中畫4個字符
for(int i=0;i<4;i++){//循環(huán)4次
String s=randomChar()+"";//隨機生成一個字符
sb.append(s);//把字母添加到sb中
double x=i*1.0*w/4;//設置當前字符的x軸坐標
g2.setFont(randomFont());//設置隨機字體
g2.setColor(randomColor());//設置隨機顏色
g2.drawString(s, (int)x, h-5);
}
this.text=sb.toString();//把生成的字符串賦給了this.text
drawLine(image);//添加干擾線
return image;
}

//返回驗證碼圖片上面的文本
public String getText(){
return text;
}

//保存圖片到指定的輸出流
public static void output(BufferedImage image,OutputStream out) throws IOException{
ImageIO.write(image, "JPEG", out);
}
}

<form action="/test/LoginServlet" method="post">
   用戶名：<input type="text" name="username" value="<%=uname %>"><br>
   密 碼：<input type="password" name="password"><br>
   驗證碼：<input type="text" name="verifyCode">
  <img id="img" src="/test/VerifyCodeServlet">
  <a href="JavaScript:_change()" rel="external nofollow" >看不清換一張</a><br>
  <input type="submit" value="登錄">
 </form>

   <script type="text/javascript">
      function _change(){
        /**
        *1.得到image圖片
        *2.修改其src為/test/VerifyCodeServlet 
        */
        var imgEle=document.getElementById("img");
        imgEle.src="/test/VerifyCodeServlet?a="+new Date().getTime();

      }
  </script>

public class VerifyCodeServlet extends HttpServlet {


public void doGet(HttpServletRequest request, HttpServletResponse response)
throws ServletException, IOException {


/*
* 1.生成圖片
* 2.保存圖片上的文本到session域中
* 3.把圖片響應給客戶端
*/
VerifyCode vc=new VerifyCode();
BufferedImage image=vc.getImage();
//保存圖片上的文本到session域
request.getSession().setAttribute("session_code", vc.getText());

VerifyCode.output(image, response.getOutputStream());

  }
}

public class LoginServlet extends HttpServlet {
public void doPost(HttpServletRequest request, HttpServletResponse response)
throws ServletException, IOException {


/*
* 校驗驗證碼
* 1.從session中獲取正取的驗證碼
* 2.從表單中獲取用戶填寫的驗證碼
* 3.進行比較
* 4.如果相同，向下運行，否則保存錯誤信息到request域，轉(zhuǎn)發(fā)到login.jsp
*/
String sessionCode=(String)                    

 request.getSession().getAttribute("session_code");//獲得的驗證碼里面對應的文本內(nèi)容
String paramCode=request.getParameter("verifyCode");//用戶輸入文本框里面的內(nèi)容
if(!paramCode.equalsIgnoreCase(sessionCode)){
request.setAttribute("msg", "驗證碼錯誤！");
request.getRequestDispatcher("/login.jsp").forward(request, response);//打回到登錄界面。
return;//返回，不繼續(xù)執(zhí)行下面的語句。
}
}
}