图形验证码的实现

1.编写生成图形的Servlet CheckCodeServlet

package com.cjg.servlet;
import java.io.*;
import javax.servlet.*;
import javax.servlet.http.*;
import java.awt.*;
import java.awt.image.*;
import javax.imageio.ImageIO;

public class CheckCodeServlet extends HttpServlet
{
	private static int WIDTH = 60;
	private static int HEIGHT = 20;
	public void doGet(HttpServletRequest request,HttpServletResponse response) 
			throws ServletException,IOException
	{		
		HttpSession session = request.getSession();
		response.setContentType("image/jpeg");
		ServletOutputStream sos = response.getOutputStream();

		//设置浏览器不要缓存此图片
		/**/
		response.setHeader("Pragma","No-cache");
		response.setHeader("Cache-Control","no-cache");
		response.setDateHeader("Expires", 0);
		
		
		//创建内存图象并获得其图形上下文
		BufferedImage image = 
			new BufferedImage(WIDTH, HEIGHT, BufferedImage.TYPE_INT_RGB); 
		Graphics g = image.getGraphics();
		
		//产生随机的认证码
		char [] rands = generateCheckCode();
		
		//产生图像
		drawBackground(g);
		drawRands(g,rands);
		
		//结束图像的绘制过程，完成图像
		g.dispose();
		
		//将图像输出到客户端
		ByteArrayOutputStream bos = new ByteArrayOutputStream();
		ImageIO.write(image, "JPEG", bos);
		byte [] buf = bos.toByteArray();
		response.setContentLength(buf.length);
		//下面的语句也可写成：bos.writeTo(sos);
		sos.write(buf);
		bos.close();
		sos.close();

		//将当前验证码存入到Session中
		session.setAttribute("check_code",new String(rands));
		//直接使用下面的代码将有问题，Session对象必须在提交响应前获得
		//request.getSession().setAttribute("check_code",new String(rands));
	}
	
	private char [] generateCheckCode()
	{
		//定义验证码的字符表
		String chars = "0123456789abcdefghijklmnopqrstuvwxyz";
		char [] rands = new char[4];
		for(int i=0; i<4; i++)
		{
			int rand = (int)(Math.random() * 36);
			rands[i] = chars.charAt(rand);
		}
		return rands;
	}
	
	private void drawRands(Graphics g , char [] rands)
	{
		g.setColor(Color.BLACK);
		g.setFont(new Font(null,Font.ITALIC|Font.BOLD,18));
		//在不同的高度上输出验证码的每个字符		
		g.drawString("" + rands[0],1,17);
		g.drawString("" + rands[1],16,15);
		g.drawString("" + rands[2],31,18);
		g.drawString("" + rands[3],46,16);
		System.out.println(rands);
	}
	
	private void drawBackground(Graphics g)
	{
 		//画背景
		g.setColor(new Color(0xDCDCDC));
		g.fillRect(0, 0, WIDTH, HEIGHT);
		//随机产生120个干扰点
		for(int i=0; i<120; i++)
		{
			int x = (int)(Math.random() * WIDTH);
			int y = (int)(Math.random() * HEIGHT);
			int red = (int)(Math.random() * 255);
			int green = (int)(Math.random() * 255);
			int blue = (int)(Math.random() * 255);
			g.setColor(new Color(red,green,blue));		
			g.drawOval(x,y,1,0);
		}
	}
}

2.编写html页面

<h3>带有验证码的登录页面</h3>
<form action="servlet/LogonFormServlet" method="post">
	用户名：<input type="text" name="name"><br>
	密 码：<input type="password" name="pass"><br>
	验证码：<input type="text" name="check_code">
	<img src="servlet/CheckCodeServlet"><br>
	<input type="submit" value="登录">
</form>

3.验证的Servlet LogonFormServlet

package com.cjg.servlet;
import java.io.*;
import javax.servlet.*;
import javax.servlet.http.*;

public class LogonFormServlet extends HttpServlet
{
	public void service(HttpServletRequest request,
		HttpServletResponse response) throws ServletException, IOException
	{
		response.setContentType("text/html;charset=GB2312");				
		PrintWriter out = response.getWriter();	
		
		HttpSession session = request.getSession(false);
		if(session == null)
		{
			out.println("验证码处理问题!");
			return;
		}
		
		String savedCode = (String)session.getAttribute("check_code");
		if(savedCode == null)
		{
			out.println("验证码处理问题!");
			return;
		}
				
		String checkCode = request.getParameter("check_code");
		if(!savedCode.equals(checkCode))
		{
			/*验证码未通过，不从Session中清除原来的验证码，
			以便用户可以后退回登录页面继续使用原来的验证码进行登录*/
			out.println("验证码无效!");
			return;
		}
		/*验证码检查通过后，从Session中清除原来的验证码，
		以防用户后退回登录页面继续使用原来的验证码进行登录*/
		session.removeAttribute("check_code");
		out.println("验证码通过，服务器正在校验用户名和密码!");
	}
}

 4.配置web.xml

	<servlet>
		<servlet-name>LogonFormServlet</servlet-name>
		<servlet-class>com.cjg.servlet.LogonFormServlet</servlet-class>
	</servlet>
	<servlet>
		<servlet-name>CheckCodeServlet</servlet-name>
		<servlet-class>com.cjg.servlet.CheckCodeServlet</servlet-class>
	</servlet>
	<servlet-mapping>
		<servlet-name>LogonFormServlet</servlet-name>
		<url-pattern>/servlet/LogonFormServlet</url-pattern>
	</servlet-mapping>
	<servlet-mapping>
		<servlet-name>CheckCodeServlet</servlet-name>
		<url-pattern>/servlet/CheckCodeServlet</url-pattern>
	</servlet-mapping>